Extension tubes vs Canon 100mm macro

[SUSOM]

A while back, I was playing with my kenko tube set on my 50mm f/1.2, and 70-200 f/2.8, and I found it impossible to get much DoF. I.e. I could get a bee's eye in focus, but the body would blur, and this seemed to be regardless of where I set the Av.....?

Would a dedicated macro lens be different somehow? If so, in what ways?

[Naturalist]

There is a WORLD of difference between extension tubes and a dedicated macro lens. I shot with extension tubes back in the 80's and 5 years ago I got my first macro lens (EF-S 60) and WOW!!!

[Earguy]

Yes, depth of field with extension tubes is razor-thin. Because the lens is further from the sensor, there is light fall-off too...so you tend to want to open your aperture (even less depth of field) and use slower shutter speed (motion blur).

And the longer the extension tube is, the less depth of field you have, and the more light fall-off you have.

So, yeah, a good macro lens has several advantages over extension tubes.


As an example, here's a shot of a wasp I took with the shortest extension tube. Note that the true sharp focus is on the thorax, and the eyes and abdomen are a little soft. Distance is probably 3/16".

View image in gallery

[scoundrel1728]

The depth of field of a 100mm macro will be comparable to a 100mm fixed focal length lens on extension tubes, assuming that the macro is designed so that its focal length does not change with its focus setting. Some macro lenses do shorten their effective focal length when focused at close distances; some do not. It depends on the design of the individual macro lens. Lenses that do shorten their focal lengths at close distances (high magnifications) may have slightly greater depth of field - I haven't done the math yet - but depth of filed will still be hard to come by. A dedicated macro lens may be easier to use and give better image quality, but whether these advantages outweigh the increased cost is a question only you can answer.

If your subject is static, i.e., doesn't move, and you need depth of field greater than is otherwise possible, you can try the technique known as focus stacking. This amounts to taking a series of images at different distances or focus settings and combining the sharp parts of each into a single image. Ideally, this would be done with a telecentric lens, but a lens with its no-parallax point far enough behind the lens will usually work. This technique will produce ghost images and mismatches between component images to the extent that the image magnification or no-parallax point shifts between images. Of these two, correcting for image magnification changes is generally easier than correcting parallax errors.

[Gautam]

For the same magnification and aperture DoF of 100mm + ET & 100mm macro would be the same for all practical purposes. The light fall-off would also be similar. With 100mm macro lens when the focus is moved from infinity to 1x, there is a loss of 2 stops of light. Only way to get deeper DoF is to use a small aperture. Unless your subject is static you will need to use a flash so that the aperture can be set to 11 or smaller.

[zemlin]

The trick to getting more DOF with macro work is to bring your own light so you can stop way down. Small sensor cameras also have a DOF edge because of the shorter focal length for the same FOV.

[Epicuros]

Talking of macros and rings (or tubes), I have been using my Canon 100mm Macro lens together with three Kenko tubes, believing that I was getting higher magnification. This proved to be false, as I gave a try to my 50mm/f1.4 Canon lens with the same tubes and it turned out that I got far greater magnification than with the 100mm macro lens.

[Gautam]

The magnification you get with a certain length of extension depends on the focal length of the lens. The general formula is

m' = extension/FL, m" = the inherent maximum magnification of the lens
the total magnifcation (m) is m' + m"

So with shorter lenses you would get more magnification. However, a bit surprised that you got more magnification with 50mm lens. Did you set the macro lens for maximum magnification ?

[scoundrel1728]

Let f be the focal length of the lens,
let i be the distance from the lens' (image) no-parallax point to the image, i.e., its total image distance,
let s be the distance from the lens' (subject) no-parallax point to the subject, i.e., the subject distance, and
let x be the length of the extension tubes. For a full set of Kenko extension tubes, x will be 68 mm.

If the lens is of classical design, it will obey the thin lens equation
1/f = 1/s + 1/i
and the lens magnification will be
i/s

The 50 mm lens focused at infinity, together with the three Kenko tubes for a total extension of 68 mm, will hava an image distance of x + f, or 118 mm. This gives a reciprocal object distance 1/s of 1/f - 1/i, or 1/50 - 1/(68 + 50) mm^-1. The resulting magnification will be i/s or (x + f) / f - 1 or (68 + 50) / 50 - 1 or 1.36:1, slightly greater if the lens is focused to its minimum focusing distance.

Assuming that the 100 mm macro is focused to 1:1 and is of a "classic" design, the subject distance and its image distance will both be twice the focal length, or 200 mm. However, with 68 mm of extension, the image distance becomes 268 mm. The focal length is still 100 mm, so the reciprocal object distance 1/s becomes 1/f - 1/i, or 1/100 - 1/268 mm^-1. The magnification for this system therefore becomes 268 / 100 - 1, or 1.68:1. You should therefore get slightly greater magnification with your 100 mm macro when it is focused to its closest distance, but there is no guarantee that your macro obeys the thin-lens equation as it is focused. In other words, it is entirely possible that the focus mechanism also acts as a zoom mechanism, changing the lens' "infinity" focal length when the lens is focused close. There are some practical reasons for designing a lens this way.

On the other hand, if you left your macro lens at infinity focus when ahooting with your extension tubes, I can understand why the shorter lens gave you more magnification with the same amount of extension tubes. In this case, the 100 mm lens would give a magnification of (68 + 100) / 100 - 1, or 0.68:1.