Genuine focal length of Nikkor lenses?

[Ian Rance]

I was tinkering around yesterday 'honing' my holiday kit, and I was sighting

through my lenses imagining that I was using them on my trip, when I noted

something this:

 

My prime 35mm lens 'sees' quite a lot less than the 35mm end of my 35-70mm

zoom. At closer ranges the zoom almost looks like 28mm. I then compared

another zoom lens (24-85mm at 35mm) and that was different again!

 

Now, either the prime lens is NOT really 35mm as stated on it, or my 35-70mm

zoom is actually a 30-70mm (which is fime by me).

 

Has anybody done any checking to see what the real focal length of the nikon

lenses are (apart from contemporary magazine reviews)? Is there a website

somewhere?

 

On this subject, I do recall hearing that Nikon 28mm lenses were not this at

all (more like 30mm).

 

If it is not possible to belive what Nikon states on each lens I would be most

interested in finding out the real focal range of lenses so I can plan any

future purchaces (not worth buying a 28mm lens if it is identical to my 35mm

zoom for instance).

 

Thanks for any pointers.

 

Ian, UK

[rob_sato]

Hi Ian. There is more reason then this. First , there is something known as manufacturing tolerances. Basically in anything there are two ratings, nominal or what it is officially labelled as and the actual. Good example would be computer memory. I have a so called half a gig memory stick. Half a gig is also 512 megs of ram nominally, but in reality, my pc only reads 488 megs.

The higher quality companies like Zeiss, Leica, Nikon, Canon, Etc. tend to have tighter tolerances. Something like 5 or 10 percent on either side of the marked focal length is considered acceptable.

Then some lenses just sound better for marketing. There is a no longer made generic 18-28mm ultrawide zoom made by Samyang that has been sold as both a 17-28mm and a 18-28mm but they are one and the same lens! Now wouldn't we all rather have a 17mm over an 18?

 

Also, it is partially legacy. The Nikkor 28mm f/1.4 is actually a 30mm f/1.4. We'd all rather get a 28mm then a 30mm right? Well, the slightly longer focal length makes it easier to design then a true 28mm while still making it close enough to be labelled as such. (Of course, the Sigma 30mm f/1.4 is another story, but this digital only lens can't even be used on full frame cameras, film or digital).

 

In todays lenses , particularly supertelephotos, optical designers use a little trick to allow unbelievably close focusing at less then infinity distances. This has the side effect of reducing the focal length of the lens as it changes focusing distance.

There is a very popular 100mm f/3.5 generic autofocus macro lens out there that is actually only about 90mm or so when focused to its minimum focusing distance. This doesn't count the manufacturing tolerance factor mentioned earlier. Superzooms in particular use this trick, that is why so many users of these lenses complain at feeling somewhat gypped when they compare it against a non superzoom lens ending at 300mm at less then infinity or a lens which can't focus as close.

 

Combine manufacturing tolerances and the close focusing/focal length reducing trick and no wonder your lens doesn't seem to be its marked focal length!

Hope this helped.

[umut_arslan]

http://old.photodo.com/nav/prodindex.html

 

All lenses have +- 1-2 mm difference, thats normal.

[Ian Rance]

Thanks Rob and Umut.

 

The Photodo site is spot-on, and just what I was looking for. Thanks for the link.

 

Concerning the variation, it would seem that my 35mm lens is in fact 36mm and my zoom 35-70mm is correct (but wrong at the long end). It would have been good if Nikon had been honest and written the truth on the lenses. 35mm should mean 35mm NOT 36mm (and 1mm at the wide end is a noticeable amount).

 

Anyway, enough with the griping and back to enjoying getting my kit sorted.

 

Regards, Ian

[ronald_moravec1]

My NIKON 18/70 on a D200 at 18 matches my Leica 28R lens pretty close, viewfinders may differ in what they show so it is hard to judge without actually making a pic.

 

Someone alluded to focal length changing when focusing up close. I noticed this the other day when I set the 18/70 to 65 and then compared it to a true 65mm fixed focal length Leica mounted on the same camera . I had to move in to about 2/3 the distance with the zoom to maintain the same picture area. Also when doing portraits, the 70 should be equal to 105mm, yet I was having to get too close, closer than I should have with a 105.

 

I guess changing focal length accounts for the discrepancy.

[work-page]

Even though the lenses' actual focal length may be a tad different from the named value, the 1mm or so that this concerns isn't the most significant factor in what you see.

 

The 35mm uses helicoid extension to focus closer. It's 35mm all the way from infinity to close-up.

 

The 35-70/2.8 does not depend on helicoid extension, but moves the location of the lens groups with respect to each other. Focussing closer is achieved by shortening the focal length. Great when it comes to the 35mm setting, less so at 70mm as it reduces magnification at the long end.

[Ian Rance]

Thank you Peter - that explains why the lens was becoming VERY wide when I used it really close.

 

Photo.net really is a good place to get balanced and informed answers.

 

Ian

[rob_sato]

Oops. Meant to say superzooms, not supertelephotos. Sorry.

[oceanphysics]

It's certainly not a matter of "manufacturing tolerances." That term implies that different lenses coming off the line would have noticeably different focal lengths, which is certainly not the case.

 

It's marketing and has already been mentioned, the fact that some lenses are only at their nominal focal length when focused at infinity.

[Ed_Ingold]

Most zoom lenses, including the Nikkor 35-70/2.8, focus using an internal mechanism which moves selected elements rather than racking the whole lens in and out. In the process, internal focusing shortens the focal length as you focus closer, giving a wider field of view. The nominal focal length applies only when the lens is focused at infinity. Actual sample-to-sample variations of focal length are insignificant, on the order of tenths or hundredths of a millimeter or less.

 

Also the focal lengths may vary by a millimeter or two from the nominal focal length, depending on the specific design. Even small differences can have significant effects on FOV at short focal lengths. For example, an Hasselblad 80/2.8 Planar has a true focal length of 81.2 mm. Hasselblad is one of the few companies that is so precise in their specifications.

[Ian Rance]

"For example, an Hasselblad 80/2.8 Planar has a true focal length of 81.2 mm. Hasselblad is one of the few companies that is so precise in their specifications."

 

Precice - and honest!

[Michael R Freeman]

<i>"It would have been good if Nikon had been honest and written the truth on the lenses."</I> -- It has nothing to do with honesty, but rather the inherent optical properties of certain lens designs. :-)

<P>

Stated focal lengths for lenses are always the "truthful" (within manufacturing and "marketing speak" tolerances) focal length at infinity focus.

<P>

At any other distance, the effective focal length can and often does change, depending on the optical design. This is particularly true of lens designs that feature internal focus (IF), rear focus (RF), close range correction (CRC) or focus via the front group. Then there is the deliberate focal length shortening "trick" that Rob mentions. Nikon uses this bit of optical magic with all of their AF Micro-Nikkor lenses to allow focusing to 1:1 without extension tubes (the AF 105/2.8D is only about 60mm at 1:1).

<P>

In all of these designs, one group of elements changes position relative to another group(s) during focusing. When that happens, the focal length of the lens changes (if you think about it, that's how zooms change their focal length). I have never used the AF 35~70mm f/2.8, but I suspect it focuses via the front group, while the rear group remains fixed. The AF 28/1.4D has both CRC and rear focus (RF). As such, it's focal length will vary with focus.

<P>

Many prime lenses (including IIRC all of the 35mm Nikkors except the 35mm f/1.4) focus solely by extension, i.e. the entire set of optical groups move together as a unit when focusing. Thus, they show much less (insignificant) focal length shortening as they are focused to closer distances.

[briany]

Rob - "manufacturing tolerances" for computer memory?! That would be quite a semiconductor manufacturing facility. Look on the box and it will say that 1MB = 1,000,000 bytes. Your computer uses the traditional definition: 1 MB = 2^10 Kilobytes (1024KB). 1 KB = 2^10 bytes (1024 bytes). So 512,000,000 bytes / 1024 / 1024 = 488MB

[alex_lofquist]

Technically, all focal lengths are measured with the lens focussed at infinity. (In actuality the testing may be done at 51 focal lengths for convenience and then calculated for infinity.) All IF lenses focus by changing focal length rather than by barrel extension.

 

In going back over Pop Photo, and Modern Photo tests it was not uncommon to see discrepancies in FL as much as to 10%.

[dan_long]

Ian:

 

You have received a lot of good information regarding the variability of field of view of various lenses with the same "marked" focal length. What I have gleaned over the years regarding this same subject is that this issue is not so much about honesty or accuracy as it is design. Please keep in mind that your basic premise and initial assumption was regarding what field of view you saw in the viewfinder as that corresponds to lenses of the same "marked" focal lenghth. If you also examine other optics such as telescopes and binoculars, you will find a similar inconsistancy, but with those devices it has nothing to do with "labeled" focal length. The actual focal length of an optic is a mathematical measurement of how far in front of a given plane, or focal distance, the image produced by the optic reverses within itself. On the other hand, the perceived angle of view, may be very different from one 35mm focal length to another. This would be dependant not only on the actual focal length, but also the internal design, baffling and how the image is compressed within the optic. It may also be a simple function of the tube it is mounted in. If you examine enough stuff, you will eventually come to the conclusion that all 35's are not equal, either within a given manufacturer, or across various manufacturers. Angle of view is not directly related to focal length, only partially. This is most evident when you examine the wide angle products each company produces. You may see a lens of say 15mm focal length that is rectilinear in nature, and covers an area of view that is more narrow than say, a 16mm fisheye from the same company. The design of the lens other than its focal length contributes to this.

 

regards,

Dan