# Actual effective focal length of 80-400 VR when close focused?

Discussion in 'Nikon' started by bobatkins, May 1, 2010.

1. ### bobatkins

Does anyone have any data on the effective focal length of the 80-400 VR when close focused at the 400mm setting. I'm assuming that like most close focusing internal focus telephoto zooms the focal length drops to enable close focus but I'm wondering if anyone knows of any hard data on that.
The information is for something I'm writing for photo.net right now and I want to do a comparison between several lenses.
For the uninitiated, the focal length(s) printed on lenses are the focal length(s) measured with the lens focused at infinity.

2. ### ilkka_nissila

How would you define "effective focal length"? Focal length is defined for infinity. If you focus a unit-focusing lens near, its angle of view becomes narrower in the process than it is at infinity. It is much easier and more clear to use the minimum focus distance and the corresponding magnification. With these specified, everyone can easily calculate what it means in real world terms, rather than using a term outside of the parameters used in its definition, confusing at least me, if not everyone.
The closest focusing distance of the 80-400 is according to Nikon international web site, 2.3 m, and the magnification at that distance (presumably zoomed to 400mm) is 1:4.8.

3. ### bjørn rørslett

I'm with Ilkka. Besides, the concept "effective focal length" is already defined and is what in photography we would designate "focal length [of a lens]". So using it for something else just adds to the confusion.
Use maximum magnification and the distance at which that occurs.

4. ### jose_angel

The maximum reproduction ratio is at 80mm (1:25). Looks like the minimum focusing distance remains in 2.3m.

5. ### John Crowe

With all the concern raised by the lowered close focus "effective focal length" of the new Nikon 70-200/2.8 VR2, I would say this is a very pertinent investigation. It is interesting that the 100/micro suffers from the same problem. I think stating the difference in focal length is more straight forward than putting it in terms of magnification.
While I suspect that this is an effect that one may not notice in real use, it would be nice to know when comparing lenses. I am sure that a lens that does maintain focal length at close distances would offer a very significant difference.

6. ### bobatkins

The point is that given a prime 400mm lens and a 400mm zoom, from a given close distance, say 3m, they will likely have different magnifications because most zooms close focus by both focusing and modifying focal length. Pretty much always, the 400mm prime will provide a higher magnification at that given distance, though the zoom may be able to get closer and provide a higher ultimate magnification
If you are not restricted by distance, then the higher magnification of the zoom may be more useful. You don't care what the focal length is in that case. You can just move in until you get the magnification you want, e.g. photographing a flower.
However suppose you are shooting birds from a blind and you are 3m from your subject. You can't get any closer. Lens "A" is a 400mm prime, while lens "B" is a 400mm zoom which shortens it's focal length to 300mm when focused down to 3m. In that case the prime produces an image that's 33% larger than the zoom, even though both are nominally "400mm lenses" (when focused at infinity).
So sometimes it does matter.
I would define "effective focal length" as being equivalent to the focal length of a lens which when focused by linear extension at distance "x" gives magnification "y". In practice no current telephoto primes focus by linear extension, but let's assume that internal focusing primes don't usually exhibit much of a focal length change when close focused, which is a decent assumption.
So if a close focused 400mm zoom give you the same subject magnification as a 300mm prime when focused at 2.5m (subject to focal plane), the effective focal length of the 400mm zoom under those conditions would be 300mm.

7. ### bjørn rørslett

Only magnification is "straight forward", since focal length is defined for infinity focus. Why would stating a non-existing difference clarify anything? If the lens goes to 1:6 or 1:8 on the other hand is hard fact and a reality. Together with the focused distance that is the information one needs.
A lens that "does maintain focal length at close distance" implies two aspects: firstly, that it has unit focusing,thus has a slow focusing action; and secondly, that it changes effective aperture more than with the usual. modern designs (ie., it "loses" more light). A very significant difference, yes, but perhaps not in the direction that you indicated.

8. ### oskar_ojala

Maybe it would be useful to compare lens by quantifying this to a common magnification, e.g. 1:10, and list at what working distance/focusing distance the lenses reach that magnification. Minimum focus distance and maximum magnification are of course useful, but comparing is rather straightforward when standardizing the magnification and more practical than listing just a focal length.
(no, I don't have a 80-400 to test, but the numbers Ilkka quoted should be a good starting point...)

9. ### bobatkins

Magnification at a given distance is a direct function of focal length. While focal length is defined at infinity it can nevertheless be used at shorter distances, e.g. in the simple lens equation 1/v + 1/u = 1/f. If you know v and u, you can calculate f, the effective focal length when not focused at infinity.
If it doesn't matter then tell me, which of these two lenses would you prefer (let's assume same price, size, aperture, quality, focus speed etc. for the purposes of this argument). Lens "A" can be internal focus if you wish.
A 500mm lens "A" which gives you a magnification of 0.27x at a distance of 3m (effective focal length 500mm, focal length maintained at close focus)
A 500mm zoom lens "B" which gives you a magnification of 0.16x at a distance of 3m (effective focal length reduces to 350mm when focused down to 3m)

10. ### jose_angel

IMHO we should not be debating this way, the topic is crystal clear and I believe Bob`s article could be extremely helpful.
There are many situations where one must choose between lenses and a reference point is needed. Someone could need to know if a lighter 70-200 is enough or if he should take the "next" longer lens for a given task.
To Bob`s question: Of course it matters, but it doesn`t mean that one lens should be better than the other. Everybody could say No doubt, I prefer lens "A"! because absolute reach could be the aim of a 500mm lens, but on an all purpose, portable tele-zoom it could not be as easy. There aren`t two identical lenses with the only difference in the magnification curve. For sure some people will prefer to trade some benefits in favour of others that are more useful to them; we all know a recent case.

11. ### bjørn rørslett

In the thin lens equation, v=f+e, so it's actually the extension you discuss, not focal length. We could replace that with the rear nodal distance to avoid the "focal length" dilemma.
For the first case, m=0.27, so m=e/f=0.27, f=500, then e=135 mm. You need to add 13.5 cm of extension. That gives v=e+f => 635mm. From the lens equation, 1/u=1/500-1/635 => u=2352mm or 2.35m. So this has to be a massive long-focal length lens, no telephoto (internodal spacing approx. zero), and a monster on a tripod.
In the other example, m=0.16, so m=e/f, f=500 (definition), e=80mm. An additional 8 cm of extension is needed. That gives v=e+f=580 mm. From the lens equation, 1/u=1/500-1/580 => u=3625. So the lens equation is violated unless we assume a negative internodal space, thus this could be a zoom lens or a telephoto lens with internal focusing. The lens likely is much smaller than the first case and would be far easier to handle on a tripod, thus also bringing with it the added benefit of a longer free working distance.

12. ### bobatkins

Bjorn I did say (or at least imply) "all else being equal". It's pointless to discuss this further because we are not going to agree.Whatever I come up with you'll nit-pick at.
Lots of people get quite confused over all this and some get quite upset. Whether they have cause or not is debatable.
Most users expect a 500mm lens to be 500mm and to give the magnification they'd expect from a 500mm lens whatever the focus distance. However with most zooms they'd be wrong and they wouldn't get what they expect. If you don't care that's fine. Lots of people do. Just look at the new Nikon 70-200 which gives up to 30% less magnification at close distances than the old 70-200. You may not care. Others certainly do.
I'm not going to go to the trouble of constructing rigorous examples because it wouldn't change your mind on the subject. You'd say that lens "B" would be much more convenient, but that's a red herring. It's not the issue under discussion. We're not talking about size, cost, convenience, macro capability, focusing speed or any other optical property. We're taking about magnification at a given distance. People expect a lens set to, say, 400mm to act like a 400mm lens and give them the magnification a 400mm lens would give, not 75% of that magnification. Of course that may be an unreasonable expectation, but it takes a lot of people by surprise.

13. ### bobatkins

Bjorn I did say (or at least imply) "all else being equal". It's pointless to discuss this further because we are not going to agree.Whatever I come up with you'll nit-pick at.
Lots of people get quite confused over all this and some get quite upset. Whether they have cause or not is debatable.
Most users expect a 500mm lens to be 500mm and to give the magnification they'd expect from a 500mm lens whatever the focus distance. However with most zooms they'd be wrong and they wouldn't get what they expect. If you don't care that's fine. Lots of people do. Just look at the new Nikon 70-200 which gives up to 30% less magnification at close distances than the old 70-200. You may not care. Others certainly do.
I'm not going to go to the trouble of constructing rigorous examples because it wouldn't change your mind on the subject. You'd say that lens "B" would be much more convenient, but that's a red herring. It's not the issue under discussion. We're not talking about size, cost, convenience, macro capability, focusing speed or any other optical property. We're taking about magnification at a given distance. People expect a lens set to, say, 400mm to act like a 400mm lens and give them the magnification a 400mm lens would give, not 75% of that magnification. Of course that may be an unreasonable expectation, but it takes a lot of people by surprise and it's a lens property that lens manufacturers never mention. I'm pretty sure Nikon didn't ever say "Oh, and by the way, the new 70-200 VR II will give you significantly lower magnification at a given close focus distance than the old 70-200 did". Not exactly a selling point...

14. ### lex_jenkins

Seems like a relevant question. I noticed a few years ago that the effective maximum focal length of the 18-70/3.5-4.5 DX is closer to 55mm near the closest focusing range, while the 35-70/2.8D AF appears to remain close to 70mm throughout. (Presumably it's because the 35-70/2.8D AF doesn't appear to be an IF model, as far as I can tell from Nikon specs.)
For me this was relevant because I take a lot of candid photos of people at close range. I noticed I wasn't getting the same magnification at the 70mm setting on the 18-70 DX that I was with the 35-70 at the same focal length setting. Made a difference in isolating subjects in a crowded room.
Since I seldom use a dSLR for photographing subjects at or near infinity, I'm more interested in how a lens performs at ranges typical for candid photography.

15. ### bjørn rørslett

Bob, the "red herring" here is insisting to use a fictive entity ("effective focal length") when the issue at hand is what kind of magnification (and of course field of view) one gets at a given shooting distance. Your contrived example shows this clearly.
The point in question here is that there is no free lunch. Modern lens designs, such as internal focusing, give us lighter and shorter lenses that focus much closer, and because of the optical design where the nodal points move around, they in general keep the change in effective aperture at a lower value. Image quality can hold up very well over most of the focusing range too. The price to be paid is that at a given total distance, magnification will be somewhat less than with an older (unit-focusing) design. If the actual design can be bent slightly the designers may come up with a higher magnification, but always at the price of a deteriorated image quality. So, they put the near focus limit at a point where they believe image quality hasn't suffered too much. We see clearly from the newer zoom designs that the balance between image quality decline and a shorter near limit is a delicate one. However, we are in general spared from the "macro" setting with the accompanying horrible drop in quality on the newer pro-class zoom lenses.
To come up with a real, not contrived, example, I can just have a look at the two 800 mm lenses in front of me. Both have the same specification (800/8ED). One weighs 5.5 kg, the other 3.3 kg, one focuses to 20 m, the other to 10m. Guess which has the unit-focusing design, and which has internal focusing? I might be able, by adding massive extension, to bring the unit-focusing lens to the same close focus as the IF model, but doing that with a lens already almost 1 m long is hardly practical.
Sorry, Bob. You are barking up the wrong tree.

16. ### jose_angel

I actually feel myself not qualified to write on such technical threads, but I believe I have several things to say as a long term Nikon real user.

Something like a comparison chart will be certainly helpful, in fact I thought about it several times, but,

This kind of comparisons between lenses cannot be exclusively related to magnification curves because it could get potential users or buyers to the wrong choice as well. I can imagine someone saying "hey, I`ve got a perfect curve, I`m not losing any milimeter on the screen but sharpness/corner performance/vignetting (amongst others) is far below expectations".

To be more illustrative, I`ll refer to the sparking lens: In such kind of chart, any comparison between the 70-200 VRI and VRII versions will undoubtly give the award to the VRI, taking all the mass readers to the wrong choice; what about the real useful coverage? Vignetting? Well, some could say this lens is best for DX... What? What about sharpness? The VRII is, IMHO, the clear choice for me.

Many people looking for advice only discern basic concepts. Every point on a lens design counts, and some of them looks to be absolutely irrelevant to them; probably the best example is size. After a little experience most photographers come to the conclusion that the biggest, sharpest, fastest, better and acclaimed lens, if unused, becomes the worst.

All info should be complete. If not, it is a half truth. Time ago I heard someone saying... "The problem is that there is too much information out there".

There is never too much information. If any, there is too much ignorance (and sadly, I belong to this group... ). I believe this kind of info will be useful, but perhaps could be important to add two asterisks with the sentence; "**Disclaimer: use this chart to your own risk- you could find that the best lens in this chart is a wonderful piece of cr*p... -"
---
(Edit: I`m so slow writing in english, I have seen after posting that Bjorn have overtaked mine... it was not my intention to over-emphasize his words. In fact I recall that a chart this way will be helpful (at least for me... )).

17. ### john_tran|14

I'm not sure what "focal length" you are trying to find but I believe you can calculate it from an experiment with the lens:
Take a picture of some subject at say 3m. Knowing the sizes of your sensor and the sizes of the real frame you figure out the magnification ratio r (examples 0.16 or 0.27 as you said ). Then from the equation v+u = 3 and v=r.u, you have u=3/(1+r) and v=r.u=3r/(1+r). Then I think you can calculate what you need. (your examples 0.16 gives 357mm, 0.27 gives 502mm)

18. ### bjørn rørslett

The problem is that for modern lenses the equation is undetermined, since we cannot in general assume internodal distance is zero. Then, the simple thin lens equation no longer suffices because total distance D no longer is just D=u+v. While the difference is insignificant for distant subjects, it cannot be ignored for lenses approaching their near limit.

19. ### john_tran|14

the equation is undetermined​
If the equations are undetermined than the concepts are also undetermined and the numbers like 80-400mm are merely numbers of no meaning and we are discussing about nothing. In fact that is true because if the equations are correct there is no distorsion of any kind
That is why Bob Atkins used the word "effective" which means we dont care how it really works inside the lens but when we use it the "effect" is like we have a lens that follows the equations and its focal length is ... the effective focal length.
Now if some of us dont like how the word is used for the concept, feel free to translate the question to a clearer and more precise language. But the question stays the same

20. ### bjørn rørslett

The concept "effective focal length" *is* already defined and is the realised focal length (at infinity) for a complex (photographic) lens. The lens equation *will* be determined if the internodal distance is known. I'm merely pointing out known facts and definitions. If people chose to ignore these, there is nothing I can do about that.

21. ### john_tran|14

The closest focusing distance of the 80-400 is according to Nikon international web site, 2.3 m, and the magnification at that distance (presumably zoomed to 400mm) is 1:4.8.​
This information is good but it only tells us that the "effective focal length" when focus at the closest distance 2.3m is 328mm

22. ### jose_angel

The main issue here is that a test like this is a real pain to make. Magnification at each distance should be then measured instead of calculated (for easiness). It`d be great if this data is just copied&pasted from Nikon`s literature, but not all manuals show it.
Once over the chart, what we could call a "theoretically perfect lens" reference curve is useless, at best a curiosity; the interesting thing is to compare one lens to another in the same range.

23. ### kelly_flanigan|1

BOB;
Just look at lens assembly as a black box.
Forget about unit focus; moving elements; nodes; lens groups moving; elves inside the lens.
Shoot a close image of a ruler.
The ratio the rulers markings to what is on the film/sensor gives the reproduction ratio; ie magnification.
The distance from the sensor to the ruler/object can be measured with a steel tape measure.
With these two measurements one can calculate the actual focal length that the black box has at that configuration.
With a zoom or a lens with internal groups moving around the actual focal length can vary ; one has a complex situation going on.
The entire test is very simple.
The pickle/issue is some folks may not like your answer or even understand what it means.
Lets say if the 400mm at closest setting really was like a 127mm; their brains may explode.
The beauty is the entire calc rises above the gobble gook. If the distance from sensor to ruler is 508mm; and it is exactly at 1:1; the focal length thus is 127mm. Folks with rigid mindsets might scream!

24. ### john_tran|14

The concept "effective focal length" *is* already defined and is the realised focal length (at infinity)​
So I guess Bob should have called it the "effective effective focal length" which means the "effective realised focal length at infinity", or the "realized effective focal length when the lens is focused at 3m"

25. ### kelly_flanigan|1

Bob; as a hypothetical set of data you might have something like these:

lens set to 400mm; actual calculated/measured focal length versus ratio (objects image size on sensor versus objects size in real life)

infinity; 386mm
1:50 388mm
1:20 385mm
1:10 387mm
1:5 320mm
1:4 258mm
1:3 215mm
1:2 157mm
1:1 127mm

(1) ie at a far distances the lens might "act" like 400mm lens; ie 386 due to a lenses 5 percent tolerance. Before the macro group is engaged; the focal lenght will be about constant; ie just vary due to test errors. My made up data set show the macro mode causeing the focal length to change below maybe 1:7

(2) then when the elves inside the lens start moving elements and groups in macro mode; it might "act" like a short focal length lens; and it might vary with magnification like my hypothetical data table.

A big issue is the data is real and valid; but the lay public's brains cannot fathom this.
Or worse yet they feel cheated; get a lawyer and sue! ie their 400mm lens at 1:2 might be really like a 157mm lens.

*****The way Warren Smith at Infrared Industries put this in the 1970's; is if the complex lens assembly unit was removed; what thin lens could be subsituted; so a "little man" at the film/sensor plane would have the same image size; when the same film/sensor to object distance is kept as the complex unit.
Thus if a film/sensor distance is 2 meters and the little man sees an object 1/3 life size; one "turns the crank" and calculates the focal length the black box was operating at.
I think the lay public will get confused about this; that a lens set to 400mm when super close really is a different focal length. This can happen of course with a complex lens where the elements and groups are being moved around.
This concept of looking a the issue as a black box is decades old
the lay public will get confused about the actual data; I know I did this on some zooms many decades ago and some folks got excited/

26. ### ilkka_nissila

One problem is that although the formula of calculating what the focal length of a simple lens would be to match the magnification at close focus of a real lens is simple, people will still do the experiment incorrectly. Since practically no one uses unit-focusing lenses these days for regular photography, using it as a reference is not intuitive. The data which Nikon already gives for each lens already has the answer by giving the magnification at the closest focusing distance - and it is in a format that everyone can understand and visualize without having to compare it to a 1960's lens to get an idea of how a unit focusing lens would behave in a similar situation.
In addition, for the data to have practical value, you also need to determine the image quality at MFD - since in many cases, the IF lens performs better here, the question becomes: does the improved image quality compensate for the loss in magnification compared to the imaginary simple lens, or more than compensate for it, leading to superior detail in the lens that has lower magnification and greater angle of view (best of both worlds). Without actual image quality testing, the "EFL" data can lead people to make the wrong conclusions about how the lenses compare in real-world use. What good is a lens that has high magnification at a long minimum focus distance if the image is not acceptably sharp at those settings? The 70-200 II produces pretty amazing results at 200mm, f/2.8 which arguably is a key feature of it. The image at these settings is so sharp (even at f/2.8) that in case you really need to get a tighter crop than you can by zooming alone, cropping it is possible with still excellent results. Whereas at equivalent settings, the old version was fairly soft and I always had to stop down to f/4 to get a good image at 200mm. Yet people cry foul when they actually get a wider angle of view all the while preserving fine detail in the subject. I get this feeling that the people who are so loudly complaining about this aren't really very interested in making images, but merely enjoy the argument without actually taking the time to find out how things work in the real world.
What amazes me is that Nikon manages to produce a lens that is so perfect that people who want to put it down have to resort to this kind of arguments.

27. ### kelly_flanigan|1

Ilkka; Here I am confident that Bob can figure out the focal length curves versus magnification; and plot curves for each focal length "setting".

28. ### doug herr

Kelly Flanigan wrote: "
BOB;

Just look at lens assembly as a black box.
Forget about unit focus; moving elements; nodes; lens groups moving; elves inside the lens.
Shoot a close image of a ruler."
This would require Bob to actually touch and use the lens. If he does it will give his review some credibility.

29. ### john_tran|14

What amazes me is that Nikon manages to produce a lens that is so perfect that people who want to put it down have to resort to this kind of argument​
Ohh, so the whole discussion here is just a scheme to make a great lens looks bad to ... ruin Nikon's reputation? and to counter-attack against the develish scheme to protect Nikon's name? ... I'm not interested in the fight at all. I thought the question is just "technical"

30. ### lex_jenkins

I'm seeing some rather odd comments in this thread without any apparent basis or justification.
"What amazes me is that Nikon manages to produce a lens that is so perfect that people who want to put it down have to resort to this kind of arguments."​
I'll admit I'm not feeling up to par this week, fighting off bronchitis, and might be missing some relevant post, but so far I haven't seen anything to support the assertion that there's any nefarious hidden agenda in Bob's question.
"This would require Bob to actually touch and use the lens. If he does it will give his review some credibility."​
After more than a decade as technical editor, writing articles for photo.net and his own website, Bob's credibility as a reviewer is not in doubt.
Bob's original question seemed straightforward enough. We know the nominal focal lengths. We know the magnification at minimum focus. Those are available from the manufacturer. The remaining question is, what is the magnification at a specific distance between the extremes of infinity and minimum focus range?
Bob suggested 3 meters. I'd suggest a little more distance, perhaps 5 meters or so since some older telephotos don't focus as close as many contemporary primes and zooms. A 5 meter or so testing distance would make it easier to compare contemporary lenses with older telephoto primes of the same nominal focal lengths.
It would be interesting and helpful if folks who actually own the 80-400 VR Nikkor and other zooms or primes within that focal range could photograph some test targets at a close intermediate distance for the sake of comparing magnification.
By the way, here's a relevant quote from Thom Hogan's review of the 16-85/3.5-5.6 DX VR Nikkor:
"Quick question, which gives you more range: the 18-200mm or the 16-85mm? The answer might surprise you a bit. The 16-85mm has a horizontal angle of view range of 16 to 73 degrees, the 18-200mm has an angle of view range of 7 to 66 degrees. However, because the 18-200mm changes focal length so much at the long end when focused close, for many situations its angle of view is only 10 to 66 degrees, which is not looking a lot better than the 16-85mm."​
This is the type of question some folks will have when considering a purchase, and it's the type of question that isn't readily available from the manufacturer's specifications.

31. ### kelly_flanigan|1

Here what I would fine usefull are these 3 variables
Plot/Graph distance from the front of the lens to subject (stamp/bug/heart valve)
*versus* zoom (mm focal length setting)
Showing magnification too.
Thus if one wanted to shoot a snake/heart valve/postage stamp from a distance of X (front of lens to subject) they could see if they could get the magnification required.

32. ### bebu_lamar

I don't have the lens in question so I don't know. It's quite expensive and I don't need it. Although the lens doesn't focus by simply extension that is simply moving the whole lens away from the focal plane but rather only a number of the elements move, we however, can force focus the lens by setting it to infinity on the focusing ring and then adding enough extension to focus the lens at a subject of 2.5m from the focal plane. I believe what Bob said, and I think it's true, that when focusing this way the lens would give a narrower angle of view and higher magnification then if you focus the len at a subject of 2.5m from the focal plane using the focusing ring. If so the lens focal length is indeed shorten as you focusing it at distance closer than infinity.

33. ### bjørn rørslett

It is of course possible to add extension to (virtually) any lens. Then, you treat the lens as being unit focusing. You will lose any optical correction of the design for the actual distance setting. If the lens has short focal length, you might end up with the plane of focus inside the lens assembly. If the lens has a long focal length, you need a massive amount of extension. In either case, you are confined to shoot a perfectly static subject and may need to move the entire lens (+tripod) to a new location in order to bring the subject into focus (unless you are using a bellows unit for extension). The need to relocate the setup is certain, however, if you're treating a zoom lens like it was unit focusing by adding extension, then zoom the lens. For each scenario, you'll experience a bigger drop in effective aperture than would occur if you just focused the lens in its normal fashion.
All the above should indicate why the comparison you suggest, is an academic exercise and moot for practical photography. Besides, if the exercise indeed had been academic, a peer reviewer would have a field day of pointing out the wrong use of optical concepts, for example, focal length. Moreover, since virtually all modern lenses don't use unit focusing, for obvious reasons, what's the point anyway? Your "reference 400mm" will behave in a similar way.
To return to a real world situation, an idea would be to publish a graph showing the magnification as a function of total distance (i.e., distance subject to film plane). Nikon in fact did exactly that with their Macro-Nikkor lenses. Since magnification is non-dimensional and doesn't depend on the film format, you could use the graph to compute the field of view for any format the lens was intended for, by simply multiplying magnification with the format diagonal. For a zoom lens, the graph would have to be 3D and thus likely less practical. Most practical - as always - is just looking in the viewfinder of the camera.

34. ### ilkka_nissila

I'll admit I'm not feeling up to par this week, fighting off bronchitis, and might be missing some relevant post, but so far I haven't seen anything to support the assertion that there's any nefarious hidden agenda in Bob's question.

Well, I didn't mean to suggest that he has a hidden agenda, but I don't think he has an unbiased point of view either. His comment "Just look at the new Nikon 70-200 which gives up to 30% less magnification at close distances than the old 70-200. You may not care. Others certainly do." suggests that this lens is a prime example of why he thinks the article is needed. Yet what some people have reported (using the D3X) is that the newer lens is sufficiently sharp at f/2.8 at near distances that it actually more than makes up for the loss in magnification as long as you use a body with small enough pixels. So it's not a good example at all unless you're going to be measuring how well the lenses in question reproduce detail to show which optical design actually reproduces the subject better at close focus distances. Now, doing such experiments is no doubt labour-intensive - all the more important that one person carries them out so that they are made in a consistent manner. With regards to the 70-200 Mk II, it is my user's view that the new lens works great at close distances and I believe it's the first autofocus Nikon f/2.8 telezoom which actually gives really sharp images at 200mm, at f/2.8, where at least I use such a lens quite often. These factors (along with the price and features) should be what people look at when they choose a fast telezoom for their Nikon DSLR, and not the simple lens equivalent focal lengths which have caused so much stir in people who haven't bothered actually to find out what the lens can do in real life. This is one of those cases where an excellent product has been lambasted by the online mob. It has led me to believe that it is actually a small minority of the people who write in online forums that actually use the gear they talk about enough to actually find out how it works in practice. It's apparently so much easier to ask people do do measurements of EFLs and take pictures of tiny pores inside a lens and talk about that rather than make images. (I can imagine all the coke bottles and pizza boxes near the terminal along with a mannequin head and rulers.)
That Bob is asking for others to measure and calculate the corresponding focal length in the simple lens model to match the framing of the real lens is problematic. 1) An experiment is easy to make incorrectly, and 2) without doubt someone will use the angle of view vs. focal length at infinity formula rather than the simple lens formula, leading to results that cannot be compared with each other. By using easy to understand quantities such as magnification vs. distance, there is no possibility for error in the interpretation of the experimental data (apart from actual measurement error), and the results will be understood correctly by everyone reading the article. Magnification will also be meaningful to people other than those who actually use long unit-focusing lenses every day. Yesterday I calculated the simple lens EFL for my 300mm prime at closest focus - it was about 240mm ... great. With regards to the 70-200 Mk II, there are numerous conflicting numbers on the EFL at minimum focus floating around dpreview.com, leading me to doubt if it would be even possible to outsource this kind of data collection and expect consistent results. A lower risk of error is with magnification as a function of distance, and those who have the mindset that they think in terms of imagined unit-focusing lenses, can convert the results to their favorite EFL quantity, and as long as this is done in the same way for every data point then things are fine.
If you want to teach, you need to use scientifically sound, easy to understand, quantities that are meaningful to practical photography in as far as possible.

35. ### kelly_flanigan|1

Ilkka; to measure this actual focal length requires at best simple algebra. To run a controlled test requires folks to use a ruler or steel tape. None of this is rocket science; it really is about a 7th or 8th grade issue; but the average photographer really has only an 8th grade education; and often worse cannot do algebra anymore; and often cannot fathom a test procedure. That is why a lot of stuff on the web has errors.

36. ### oskar_ojala

I would say that since this has been a hot topic on the net, an article trying to somehow quantify this and explain it in layman's terms is in place. Whether or not Bob's article fulfills those goals is a bit hard to debate when the actual article hasn't been completed yet ;-)
Of course, one should strive for using technically accurate terminology without trying to confuse the reader too much.