how do extension tubes work?

Does anyone know exactly how extension tubes work?

 

What I know:

 

they shift the focus range of the lens with which they're used. The lens will

focus closer but it won't focus at infinity.

 

they slightly magnify the image - based on a single experiment I performed

 

what I've heard and assume to be correct:

 

they're best used with the lens' focus point set at infinity and final

focusing performed by moving the camera lens assembly. This supposedly gives

the best performance.

 

Diffraction is greatly exaggerated at small apertures because the effective

aperture is much smaller.

 

I use tubes:

 

with my 200mm macro lens to get a little closer than I can otherwise

 

with my big glass to focus on objects (mostly birds) that are within the

minimum focusing range

 

in combination with my 100mm and 200mm macro lens and teleconverters to get

some high magnification.

 

My questions:

 

what exactly happens optically when I put a tube between my lens and my

camera?

 

How do tubes work the way they do?

 

Is there something I can do to optimize their performance?

 

thanks for any replies (even the smart alecky ones)

 

Tom Reese

All they do is add distance between the lens and film. A toilet paper roll would make a perfectly functional extension tube, except for meter coupling, structural rigidity, occasional light leaks, and the like.

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In lenses with no moving elements, focusing is accomplished by simply moving the entire lens forward and back. Moving the lens away from the film focuses closer. An extension tube just extends the range of motion that the helicoid can achieve, so it focuses closer (but it prevents the helicoid from moving the lens all the way back to focus at infinity).

Tubes are just tubes, hollow tubes. You can "optimize performance" by understanding the exposure compensation needed for different focal lengths when using tubes. Also, you should find a booklet explaining exactly how much magnification you are dealing with, etc.

Extension tubes work by moving the lens further from the film than the focusing mechanism alone can handle. The basic optical formula is as follows:

 

1/f = 1/f' + 1/f"

 

Where f = the nominal focal length, f' = the distance from the film plane to the rear node, and f" = the distance from the front node to the subject.

 

f' = f + e

 

Where e = the total extension (including the focusing helix and extension tubes).

Did you ever notice that if you focus on something, say 20ft away, with a 50mm (or shorter) lens, that everything beyond your subject is in focus as well? Well, it's a very similar thing that is going on on the other end of the lens, i.e. at the sensor/film plane, in that everything at or beyond the film or sensor focusing plane remains in focus (to some limit).

 

So if you now apply the concept of the so-called "digital crop factor" you can see that the film frame or sensor is filled with less of the entire contents at the focal plane proper. And that gives the pseudo-effect of increasing the magnification.

 

That's about as simply as I can explain it at this time. Hopefully you catch my drift and this helps you understand it a bit better.

 

C Painter

If you have a modern camera and lens, you can get extension tubes that are called "Automatic". There are various levels of the "automation" out there, starting with the Auto Aperture, and going through the mechanical Auto Focus (or in-lens auto focus motor), Vibration Reduction (VR or IS). If all the electrical signals, (mechanical links), are transfered from the lens through the tubes to the camera, then the camera should be able to auto focus, meter compensate for the macro proximity, provide auto aperture, and possibly provide distance information from the lens to the camera, as well as Vibration Reduction. - Not that you need all of that. If you understand all the macro photography aspects, you can possibly do all of that manually, without any automation, with any tube and metering system.

Here's my shot at an analogy....

 

Hold a flashlight vertically, pointed downward over a table top and you'll see a bright concentrated circle of light on the table top. That's the equivalent of a lens being mounted to the camera at it's optimal distance to the sensor/film (ie. nothing between the rear element and the sensor/film).

 

Now slowly start raising the flashlight away from the table, and you'll see the light pattern on the table start to become larger, more diffuse, and slightly dimmer. That would be the equivalent of adding extension tubes to the lens. Since the size of the film/sensor stayed the same, but the image circle got bigger, you have an effective increase in magnification as the edges of the image circle aren't projected onto the film/sensor anymore, but the center covers proportionately more of the film/sensor area.

I asked the question because I performed an experiment that yielded some unexpected results. I measured the field of view with my 200mm macro lens at a distance of around 18" and got 3 1/2". I mounted a 25mm tube and refocused the lens at the same distance and found that the field of view was 3 1/4". I expected more magnification than that and I started wondering exactly what goes on when I use tubes.

 

Thank you for the replies everyone.

 

Tom Reese

Which 200 mm macro lens are you using? Where are you measuring the 18" from? Some macro lenses focus internally and the focal length of the lens alters - therefore the lens has one effective focal length when you use the internal focussing and another when you use extension tubes for the same focussing distance. It's not so much what the extension tube does, but what your lens does when you focus it.

 

There may also be some movement of the lens optics within the lens barrel as you move the focussing ring, so that you may be measuring the 18" from two different places in the optical system. I think that this is the more likely explanation, given the way you have described the change in field of view.

 

Best,

Helen

That's because extension tubes don't increase the focal length (nodal distance) of the lens. That's what teleconverters are for.

Oops, in my haste I misread the last question - I was answering 'Why is there a small difference between the field of view with tube and without tube?" not "Why isn't there more magnification with the tube at approx the same subject distance?". It's exactly as Steve explains, of course.

 

Best, Helen

To get more magnification with extension tubes you have to get closer to the subject. The extension tube allows you to do that because it increases the distance between the lens and the image plane (the rear nodal point of the lens and the image plane).

 

Best,

Helen

Helen wrote "To get more magnification with extension tubes you have to get closer to the subject. The extension tube allows you to do that because it increases the distance between the lens and the image plane (the rear nodal point of the lens and the image plane)."

 

Small quibble. Another way of saying much the same thing.

 

With a unit-focusing lens, getting more magnification requires more extension. We normally use extension tubes to get more extension than the lens' own focusing mount can give. And, increasing magnfication reduces "front of lens" to subject distance.

 

Tom, there's not much to optimize.

 

Cheers,

 

Dan

Steven, thanks after thinking about this a little, I did finally figure out that all the tubes do is shift the focusing range and provide no magnification.

 

Dan, the optimization question was in regard to whether the lenses focusing position affects image quality. I know it affects focusing distance and magnification but is there a real world difference in image quality when the lens is focused at infinity vs. the closest possible distance?

 

Thanks for all the replies everyone.

Thanks to Dan for clarifying my sloppy writing.

 

Tom,

 

If you have a lens that has floating elements for close-up correction (eg Nikon's CRC) then it is usually best to have those in the close-focussing position for close-ups. So if you add an extension tube you will not get optimum performance with the lens itself set at infinity.

 

If, on the other hand, all the focussing helix on the lens barrel does is to move all the elements en bloc it doesn't matter where the helix is set.

 

Best, Helen

Helen, another quibble. There are lenses with elements that move during focusing and then there are lenses with elements that move during focusing.

 

According to Brian Caldwell, the 55/2.8 MicroNikkor AIS' floating element moves to reduce coma at, IIRC, far distances and the lens' focal length doesn't change with focused distance. Since coma is an edge aberration and the edges are off the film at 1:2, it is safe to use the lens at 1:2 on an extension tube.

 

The 105/2.8 MicroNikkor AIS' focal length gets shorter as the lens is focused closer. I've shot mine at 1:2 on its mount, i.e., at full extension and minimum focal length, and on its PN-11, i.e., at minimum extension and maximum focal length. At the same aperture there's no noticeable difference in the results BUT there's less working distance with the lens at full extension. Its variable focal length has one practical implication that I've had to work around; it defeats the arithmetic behind flash holders like Spiratone's MacroDapter. Helen, if you're curious about that arithmetic, I have a short essay about it and a handy spreadsheet that does the calculations.

 

The 200/4 MicroNikkor AIS loses focal length as it is focused closer. I've never tried mine off its own mount, so can't report whether image quality given magnification depends on how the magnification is obtained.

 

Cheers,

 

Dan

Now I'm puzzled. You appear to be saying that there is no point in having CRC for macro (or micro as Nikon say) lenses because they perform just as well at close and far settings, or am I misunderstanding you? I have to admit that until I read your post I hadn't questioned the received advice that I passed on here, but now I will. I'll try it with my 60 mm AF and a 200 mm AF as soon as I get the opportunity.

 

Thanks,

Helen

Helen, I must have been unclear. If I'm not mistaken -- always possible -- for the 55/2.8 MicroNikkor AIS the point of having the floating element is to improve performance at distance. I may still have the user's manual that came with mine, will check it tonight if it is in the manuals drawer.

 

No one has ever explained to me why the 105/2.8 MicroNikkor was designed with moving elements -- my 105/4 was unit focusing with no moving elements and I'm still not sure what I gained by replacing it with a 105/2.8.

 

My 55/2.8 and 105/2.8 shoot as well on tube at 1:2 as on their own mounts.

 

I believe -- correct me if I'm mistaken -- that your 60/2.8 and 200/4 AF MicroNikkors are rather different designs than the older 55/2.8 and 200/4 MF MicroNikkors. The AFs go to 1:1 on their own mounts so there's no incentive to use either on an extension tube since they're better reversed above 1:1.

 

Digression. There's a dogma to the effect that lenses made for use at normal distances are better reversed above 1:1. This is true for most, not for all. EKCo used to recommend using the 50/4.5 and 75/4.5 Enlarging Ektars above 1:1 facing normally. These two are heliar types, might be symmetrical except for diaphragm placement; I've never dismantled 'em to find out. WOCo didn't, AFAIK, say anything about using the 4"/5.6 Enlarging Pro Raptar as a taking lens, let alone above 1:1, but in the range 1:1 - 4:1 mine does much better than well enough facing normally. Again, I haven't dismantled and measured it, but I suspect it may be more nearly symmetrical than the typical 6/4 Plasmat enlarging lens.

 

Cheers,

 

Dan

Helen, as threatened I rooted around in the manuals drawer and found the one for my 55/2.8 MicroNikkor. It says this about the lens:

 

"It has a modified Gauss-type optical design of six elements in five groups and incorporates a floating element system in the rear lens group for improved performance. In fact, this lens produces excellent image quality at high magnification rations as well as normal shooting distances, because definition and distortion-correction remain virtually unchanged over the entire focusing range."

 

There's no mention of CRC, as used in the 24/2.8 Nikkor, anywhere in the manual and no additional discussion of the floating element. I believe -- could be mistaken -- that my ideas on what the 55/2.8's floating element does came from Brian Caldwell, most probably from a post on Usenet.

 

On another topic, you might want to look at the question that started this thread:

 

http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=00HPk7&tag=200607250207 Not quite a straight cinematography question, but you may be better qualified than most of us to answer it well.

 

Cheers,

 

Dan