Extension Tubes v. CULs - macro math questions

[slejhamer]

Is there a handy way to directly compare the effective magnification of extension tubes and close-up lenses? I'm also interested in how extension tubes and CULs affect working distance, which I believe might also be a function of actual lens focal length.

How do I use these number to compare working distances provided by extension tubes? And how do these numbers relate to magnification?

I know that a +1 diopter (CUL) will focus at 1m when the lens is set to focus at infinity, a +2 focuses at 500mm, etc. The formula I've seen is:

focal length in mm = 1 / diopter adjustment

but it seems that focal length is actually referring to working distance (focal distance from the front of the lens to the subject) rather than focal length of a lens (distance from film plane to front of lens.) However, extension tubes seem to be categorized by the latter terminology, so a 50mm extension tube physically increases lens focal length by that amount. Am I confusing issues? Are these in any way related? Whew ...

TIA,

[slejhamer]

And a follow up: how do I calculate how much light is lost with tubes? My biggest concern with using these with slower lenses is that AF seems to go away above a certain effective aperture.

[Scottes]

http://www.bobatkins.com/photography/eosfaq/closeup.htm
http://xoomer.virgilio.it/ripolini/Close_up.htm

Just to clarify - because I'm a little confused by some of your statements - a +1 diopter will let you focus 1 meter from the film plane. I don't think I've ever seen info that mentions the working distance, which I would define as the distance from the front of the lens to the subject. You can only calculate this on a per-lens basis since a +1 will focus at the same distance (from film plane) regardless of whether the 300mm lens is 3 inches long or 10 inches long. The 3 inch lens will give you 7 more inches of working distance though.

I *don't think* that light loss from tubes is a constant. A 36mm tube on a 50mm lens will cause much more light loss than if it were mounted on a 400mm lens. Again, this is just a gut feeling and I have no proof of this. But if you think about the length of the tube in relation to the length of the lens it seems to make sense.

However, I can say that a 36mm tube on my 100-400 L caused no *discernible* light loss when considering AF or looking through the viewfinder. It did have an effect on actual light loss and shutter speed, which I will guess is in the range of 2/3 - 1 stop light loss on that setup. But AF didn't care, and my eye adjusted for it easily, so I used the word discernible.

I've done a number of tests with tubes and 500D - search here for my name and "tubes" or "500D" - and have some to the conclusion that it's not worth testing, just go for it. That is to say, after 3 or 4 solid nights of measuring and comparing I realized that it was FAR too much work to conclusively measure and test more than a couple setups. I have a 70-200 L and a 100-400 L along with a 500D, and 12, 20, and 36mm tubes. The possible permutations is astounding.


I will say that overall that I like tubes. You have a greater variation in working distance - with a 500D you're stuck at one distance plus or minus 2 inches. With tubes I can go anywhere from 2 inches working distance out to 36 feet.

Tubes don't give the possible magnification of a 500D unless you go nuts, like 68mm of tubes on a 70-200 @ 70mm. Given that this yields 2" working distance it's not very useful.

Tubes are a pain in the neck when you're trying to switch them around in the field. A 500D is a pain when you have a hood and need to take it off and put it back in the case. I can shoot butterflies all day without taking off my 20mm tube, but if the butterfly is 22" away I have to take the 500D off my lens.

Tubes have no optics, though I don't believe that the 500D gives an optical loss on a digital. But this also means that tubes are easy to clean, can be thrown in your pocket, and don't require as much care as the 500D.

Tubes are light and go between the lens and camera. The 500D is not nearly as light (at least the 77mm one) and goes on the end of the lens. This throws balance off, and will be noticed after chasing butterflies for 3 hours.

Tubes work with any lens.

[slejhamer]

Yes, you are right, film plane to subject.

Those links are exactly what I was looking for, esp. the Bob Atkins table.

This is very helpful Scottes. Thanks!

8)