Reversing lens to lens

[lartamax]

I have a canon 6D with a canon 100mm macro. I want to increase my magnification and was thinking of reversing my 35mm 1.4 sigma art onto my macro lens.What magnification would I get and do Only use the macro for focusing. Finally I read in an article you may not be able to shoot at a variety of apertures unless you have manual aperture control on the lens.
Other equipment would include both a 1.4 and 2x convertor. Canon 25mm extension tube and a full set of kenko tubes. So anything really getting me 2:1 or better.
Thanks in advance
John

[rjlittlefield]

John, you're in the fortunate position of having lots of options, all of which will work well.

If you reverse your 35 mm on front of your 100 mm, with both lenses focused at infinity, then you will get magnification of 100/35 ~= 2.86:1 . From there, focusing the 100 mm closer will probably get you more magnification, but whether that's true, and by how much, depends on details of the lenses that I don't know. You would have to experiment to be sure. Yes, changing the aperture will be an issue. Ideally you would stop the front lens in order to get best edge performance, but stopping the rear lens will also work just as well at image center. See http://www.photomacrography.net/forum/v ... php?t=8336 ("FAQ: Stopping down a lens combo") for more discussion.

Alternatively, if you add a 2X teleconverter behind the 100 mm macro, then with the 100 mm focused at 1:1, the combination will give 2:1 magnification. In this case you'll get the same working distance as the 100 mm alone at 1:1. Using a teleconverter has the advantage that infinity focus remains possible, so this combo works continuously from infinity to 2:1. For more discussion of this approach, see http://www.photomacrography.net/forum/v ... hp?t=21751 ("What does a teleconverter do and why might you use one?") for more discussion of this approach.

Or, if you add around 75 mm of tubes behind the 100 mm macro, then when the 100 mm is focused at 1:1, the extension will bring you to about 2:1. (The actual focal length of the "100 mm" lens gets shorter as its focus ring is turned. At 1:1, its actual focal length is more like 75 mm, and whatever the exact value is, that's the amount of added extension that would take it to 2:1.)

Some of these approaches also work well when combined. Putting a 1.4X teleconverter on the camera, behind 75 mm of extension tubes, will bring you to 2.8:1, when the 100 mm is focused at 1:1. Or using the 2X teleconverter would bring you to 4:1.

It's impossible to say which of these approaches would give you the highest image quality.

It's important to remember that there will be some optimum aperture for each approach, and the optimum may be different for each approach. The optimum for you and the exact lenses you have may also be different from what's optimum for someone else, even if they have exactly the same model lenses.

Careful experimentation is your best friend in sorting out the issues of image quality.

--Rik

[pulsar123]

I wouldn't put the reversed 35mm in front - it will be hard (you loose aperture control), plus this is not the way to use expensive lenses, this is more like for 50mm f1.7 MF lenses for 40$ : you expose to the elements the (rear) part of the lens which is not designed to be exposed.

Kenko tubes will likely be the best option overall - no extra glass involved (meaning potentially better quality and transmission), full control of the AF and aperture, and significant flexibility (by changing the number of tubes).

[rjlittlefield]

tubes will likely be the best option overall - no extra glass involved (meaning potentially better quality and transmission)

I always twitch at the implication that less glass means higher quality. It doesn't. When you add tubes, you drag a lens away from the focus arrangement for which it was designed. That adds aberrations, which sometimes can be quite significant. In contrast, adding glass that is properly matched to the application does not have this problem, which if you're careful (or lucky) can result in excellent quality.

I would actually expect the reversed 35mm approach to give the best image quality of all the options discussed here, when operating at 3.8X. That's because a) the reversed lens approach ends up using each half of the combo in exactly the focus arrangement for which it was designed, and b) the 35mm f/1.4 Sigma Art has excellent sharpness even at comparatively wide apertures like f/4 (test report HERE), while the 100 mm macro probably reaches its peak at a somewhat narrower aperture such as f/5.6 or even f/8. At 3.8X, with the front lens operating at f/4, the combo as a whole will be running at effective f/15, still not into diffraction territory with the 6D sensor. In contrast, the 100 mm macro, set on f/5.6 and extended+teleconverted to give 4X, will be running around f/30, which will give quite a bit more diffraction blur in addition to whatever aberrations result from the extension.

That's not to say that I'd actually recommend using the reversed 35mm. I agree with pulsar123 that the other issues he mentions are very important and could well make one of the other options more attractive overall. But if you really want to work as high as 3.8X, then I would definitely include the reversed 35 mm in the list of setups to be evaluated.

--Rik

[lartamax]

Thanks for the replies. Certainly a lot of options to consider and a lot more reading to do. Looking forward to it whichever set up I go with and hopefully some image stacking as well.

John.

[pulsar123]

tubes will likely be the best option overall - no extra glass involved (meaning potentially better quality and transmission)

I always twitch at the implication that less glass means higher quality. It doesn't. When you add tubes, you drag a lens away from the focus arrangement for which it was designed. That adds aberrations, which sometimes can be quite significant. In contrast, adding glass that is properly matched to the application does not have this problem, which if you're careful (or lucky) can result in excellent quality.

--Rik

Yes, I am perfectly aware of these nuances, hence my "potentially"

I suspect the practical aspects (like not being able to control aperture) would be the most important factors here (regarding using the reversed 35mm lens).

[rjlittlefield]

tubes will likely be the best option overall - no extra glass involved (meaning potentially better quality and transmission)

I always twitch at the implication that less glass means higher quality. It doesn't. When you add tubes, you drag a lens away from the focus arrangement for which it was designed. That adds aberrations, which sometimes can be quite significant. In contrast, adding glass that is properly matched to the application does not have this problem, which if you're careful (or lucky) can result in excellent quality.

--Rik

Yes, I am perfectly aware of these nuances, hence my "potentially"

Understood, and no offense intended.

The reason I twitch is that while the person who writes those words may be perfectly aware of the nuances, the person who reads them probably is not, and thus is very likely to see just the bits about "no extra glass ... better quality".

That gap is heavily exploited by people who sell extension tubes. Kenko, for example, advertises that their tubes

convert almost any lens into a macro lens at a fraction of the cost while maintaining its original optical quality.

Fotodiox is even more forceful. From their ad at Amazon:

Unlike the add-on close-up lens, it will not alter the optical quality.

I'm not sure whether that stuff is written in ignorance, or optimism, or puffery, or just plain "say anything to make the sale".

But in any case, ads like those are the reason why I twitch.

--Rik