Question About Extension Tubes

[bob williams]

Well, I have done some research on this topic, but have found no real answers---so here is the question: Are any of you familiar with a chart, graph or calculation that you can estimate minimum focus distance based on the size of an extension tube and lens? For example, I want to shoot humming birds with my 500mm F4L which has an MFD of 15 ft. What would the MFD be if I used it with a 24mm extension tube? or how much of a reduction in MFD could I expect with a 24 mm Extension tube? I know I can simply go out and do the trial and error thing----But I am looking for some formula, graph or chart that would get me in the ballpark based on the lens/extension tube combination that I am using. If you have any Ideas or refrences please share.


Thanks


Bob

[Fast Glass]

Additional magnification=(extension tube length)/(lens focal length)


Based on this rough rule, the extension tube mounted between the camera body and your 500mm will result in a .05x magnification gain (25mm divided by 500mm).


Hope this helps,


John.

[bob williams]

Additional magnification=(extension tube length)/(lens focal length)


Based on this rough rule, the extension tube mounted between the camera body and your 500mm will result in a .05x magnification gain (25mm divided by 500mm).


Hope this helps,


John.
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Thanks John, I think your getting me pretty close----But how do you apply this formula to MFD? Would this .05 increase in magnification be the same as a .05 decrease in MFD?

[Jon Ruyle]

ut how do you apply this formula to MFD?

The formulas you need are


1) 1/f = 1/a + 1/b


where a = subject distance, b = distance between the lens and the ccd, and f is the focal length of the lens. and


2) magnification = b / a


From these two formulas it is easy to derive the formula John gave you, and also easy to figure out your new mfd .


Your mfd comes from the fact that travel in your lens only allows it to get so far from the ccd (that is, b can only get so big), thus a can only get so small. Adding a 24mm extension tube essentially adds 24 to your maximum value of b, thus allowing a smaller minimum value for a, thus increasing magnification.


Example: a 100mm macro lens has a maximum magnification of 1. This means that when focusing as close as possible, b / a = 1, so b = a. Thus formula one gives


1 / 100 = 1 / a + 1 / a, so 1/100 = 2/a or a = 200. The mimimum focus distance is 200mm, and when at this mfd, the lens is 200mm from the ccd.


Now imagine adding a 100mm extension tube. Then the furthest we can get the lens from the ccd is b = 300, (we used to be able to get 200mm from the ccd, but we've added 100mm) and we can compute the new a using formula 1), as follows:


1 / 100 = 1 / a + 1 / 300


This gives a = 150mm for our new mfd. (And, of course, our new magnification is 300 / 150 = 2, just as predicted by John's formula).


Having said all of this, it the formula does not really work for some lenses in real life, for two reasons. The first is that focal lengths of some lenses change as they get close to the subject (so the approximation gets worse as you get closer). I guess for your 500mm, the formula will be pretty close, though.


The other problem is that a is the distance of *something* to the subject, but it might not be the front element of the lens. Thus what you really have to do is compute mfd with and without the extension tube, and subtract the difference from your old mfd to get your new mfd.


I hope this helps...

[Fast Glass]

MFD is measured from the image plane too the point of focus, not from the lens element.


John.

[Jon Ruyle]

Do you mean the image plane to the ccd? (ie, a + b above) Are you sure?


In that case, mfd = a + b above, so according to the lensmakers equation, I get mfd = f[(m+1)^2]/m, where f is the focal length and m is the max magnification. This formula seems to overestimate the mdf reported in lens specs.


If mfd is the distance between the "ideal optic" and the focal plane (a above) I think the formula would be mfd = f(m+1)/m, which agrees much more closely with specs in Bryan's reviews for lenses like the 500 f/4.


Possibly I've made a mistake in my algebra, but I don't think so


Anyway, it doesn't really matter what mfd is. Bob- if you want the distance from the front element to the subject, use the second formula. If you want distance from the subject to the ccd, use the first.


Just know that real lenses differ from ideal, so neither formula will be exact.