# Extension Tubes?

Discussion in 'Canon EOS' started by jorge_ituarte|3, Sep 11, 2003.

1. ### jorge_ituarte|3

Never really been into serious macro photography. So, I don't know a
darn thing about extension tubes. Educate me guys, what size does
what? In other words lets say I have a lens that gives me .25x at
min focusing distance of 4' what size tube would I use to get close
enough to get one to one, one to two ect....

I know a lens specifically for macro photography is best but I don't
do enough macro work and don't want another lens lying around.

2. ### mark u

Look at the following for all the answers you're likely to need:-

http://bobatkins.photo.net/info/faq30/closeup.htm

3. ### jorge_ituarte|3

In regards to the chart. I don't understand what is meant by a magnification of .32 Is that .32x. On lens with a magnification of .25x that would give me a magnification of .08. I'm sure I'm not reading that chart correctly and sound like an idiot but please clarify.

4. ### jorge_ituarte|3

Also if someone could give me a more general explanation of the relation between extension tube size in mm and decrease in min focusing distance.

5. ### minh_thai

You want a detail explanation? You got it <p>

At min focusing distance w/o ext tube we have:<br>
1/f=1/d_min_1+1/d_film_1 (1)<p>

At min focusing distance w/ ext tube of thickness d_tube:<br>
1/f=1/d_min_2+1/(d_film_1+d_tube) (2)<p>

Solving (1) and (2) gives you d_min_2 as a function of f, d_min_1 and d_tube:<br>
d_min_2 = f(f*d_min_1 - f*d_tube + d_min_1*d_tube)/(f^2 - f*d_tube + d_min_1*d_tube)<p>

6. ### minh_thai

Magnification ratio is M=D_film/D_min.<p>

I do a little calculation and graph the result as follows. M

7. ### minh_thai

FYI, this calculation is for my EF 50/1.8 with min focusing distance of .45m.

8. ### bill_goldman

Jorge;

You're probably totally confused about lens extension and magnification. I'll try to clarify a couple of things.

First of all, consider a simple lens with linear focusing. That means that all elements move forward and backward to focus. When racked all the way back, the lens is focused at infinity; when racked out all the way, it is focused at its minimum subject distance. This becomes more complex with inner and rear focusing systems.

At any focused distance the lens has a subject magnification relative to the size of the image on film, in this case the 24X36mm frame. So each lens has a range of magnification; at its minimum extension, when focused at infinity, this is determined by its focal length. When focused at its maximum extension, this depends on its focused distance. In order for a lens to capture a subject at 1.00 (1:1) magnification, it would have to have a total extension equal to its focal length (e.g., 50mm for a 50mm lens). This can include the extension of the lens itself plus an extension tube(s).

Therefor, when you see a range of magnifications for a lens with a given extension tube (e.g., a 25mm tube), this range is the magnification with the lens at its infinity setting and at its minimum focus setting, now altered by effectively being racked out 25mm. You do not multiply magnification numbers together.

Those are the basics. Hope this helps.

9. ### bill_goldman

I forgot to mention one thing, if it was not obvious. At greater magnifications, as the lens to film plane increases, camera to subject distance decreases. That's why many people prefer longer focal lengths for macro work as it increases the working distance.

10. ### bill_goldman

I was thinking about what I said relative to magnification. I should have added that a subject captured at 1.00X (1:1) will be its actual size on the slide or negative. Obviously, if an entire object is larger than the slide or negative, only a portion can be captured at 1.00X. Of course you don't need this much magnification if you're going to photograph flowers or other moderate sized objects.

11. ### jorge_ituarte|3

So in a nutshell. A lens which at its closet focus distance intrinsically has a macro capability of .25 and the extension tube EF12 acordoring to the chart with provide a magnification factor of .32 on said lens. Would I add .25 + .32 to get a .57 macro. Or is it just as the chart says .32 with the tube? How many mm is extension tube EF12? If I had an extension tube on this lens that was 2x the mm deep of EF12 would the magnification of my lens then .64 at half the min focusing distance of the EF12?

12. ### jorge_ituarte|3

OK Bill I think one of my two synapses went off. If I have a 300mm lens that intrinsically has a macro capability of .25 at min focusing distance of 4" it must have internally an extension of 75mm therefore to get a 1:1 from this lens I would need an extension tube of 225mm and would have a min focusing distance of 1". Am I right?

13. ### jorge_ituarte|3

But that wouldn't make any sense considering the tubes I have seen for sale are 25mm 36mm ect.

14. ### bill_goldman

Jorge;

Yes, it does make sense that you would need a lot more extension than normal extension tubes can supply to achieve a 1:1 result. The Canon EF12 is 12mm and the EF25 is 25mm. They are designed to help you focus a little closer with your lenses than you can normally. Canon does not even recommend that their two extension tubes be stacked, but only used individually. If you're looking for 1:1 capability, you need a real macro lens.

In the past, camera manufacturers offered a bellows attachment to allow a large range of magnification but they were not intended for use with 300mm lenses, but rather lenses in the range of 35mm to 100mm focal length. Novoflex offers a bellows for the EOS system but it is very expensive. Canon now offers the MP-E f/2.8 macro lens which provides magnifications of 1X to 5X. It has a huge built-in extension capability and is eminently suitable for insect photography.

15. ### jorge_ituarte|3

Yea, I guess your right looking at the table for the 100-300 f5.6L - EF12 is .32 and EF25 is .39... Well I learned something new. Thanks a lot Bill!