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05-26-2012, 08:20 PM   #1
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Issues with Reversing Lens for Macro

I was watching someone reverse their lens for a macro shot. Now, most lenses have a good coating on the normal end, but what coating or protection exists for the glass on end that inserts into the body? Eg. Is lens reversal a poor idea?

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05-26-2012, 08:30 PM   #2
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I'm no expert in optics or macro photography, but I've done a bit. Lens reversal allows closer focusing, kind of a poor man's macro. And the film side of the optic is designed for a flatter field than the subject side of non-macro lens. Yes, it puts the antiglare coating on the wrong side, but in macro work, you're not likely to have a strong light source in frame with the subject.
So - no, it's not generally a poor idea for its purpose - inexpensive (relatively) macro photography.
05-26-2012, 08:35 PM   #3
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I was not aware that coating is universally different on the front vs. the rear element. Of my SMC Takumars, at least in appearance, only the 35/3.5 appeared to have a visibly different coating on the front and rear element. But I'm only judging by appearance, which could be misleading.

Are you talking about anti-reflection coating, or some sort of physical damage protection? Some of the newer lenses have a harder coating on at least some of the elements; I'm not sure if that's different on different elements.

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05-26-2012, 09:25 PM   #4
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I can think of three types of lens reversal:

1) Reversing an enlarging lens: This swaps subject and image fields without changing focal length or working distance. The image field is usually flatter than the subject field, and we seem to get sharper results with such reversal.

2) Reversing a camera prime lens: This also swaps image and subject fields, but reduces the working distance to the lens register aka flange focal distance. For Pentax lenses, that is about 45.5mm. Reversal does not increase magnification, just shortens the working distance. For more magnification, add some extension.

3) Reversing a camera zoom lens: The field swap again, but the effect on working distance varies. From my experience with reversed 35-80mm and 28-90mm zooms, close-focus working distance ranges from about the register to several times the register, while far-focus at the long zoom end may be beyond infinity. A true macro-zoom!

With any of these reversals, aiming into a light tends to produce flare. Enlarger lenses typically aren't coated AFAIK and so may be most susceptible. But I avoid shooting directly into bright light and I haven't noticed glare with any reversal, not when shooting close. When I use a reversed zoom for non-macro shooting, I put a section of macro tube on the end to act as a lens hood. So far, so good. We'd expect that newer digital-format zooms will have better coatings on rear elements. Unfortunately, these lack aperture rings, and so are more difficult to use reversed. Oh bother.

05-26-2012, 09:30 PM   #5
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Assume for the moment no filter is used, and rearmost external surface of the lens was not coated, since reversed it has no glass in front of it--it does not need any coating.

Actually (I am almost positive) all the the external lens surfaces receive multi-coating so (otherwise one would get flare from internal reflections off film or sensor) so it also protects for reflections from front-to-filter and then back again.
05-27-2012, 12:52 AM   #6
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Interesting observation, but I don't think it's really a problem =). First of all, even of the lens is reversed, maybe those coatings still have some positive effect (ie does it matter where they are, I don't know). But secondly, when I've reversed lenses for macro work I've actually never found there to be a particular issue with this, be it stacked (which hides the worst defects) or unstacked. Good luck if you have a go at it!

-Johan
05-28-2012, 08:18 PM   #7
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One aspect not yet mentioned is the physical damage the reversed lens is susceptible too. That rear element is not protected by a filter or lens hood and depending on lens design, could be protruding. Since these reversed combinations also generally give very good subject/film magnification ratios you will probably be putting it very close to your subject and there's a real danger of bumping into it if you're not careful.
05-28-2012, 08:43 PM   #8
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QuoteOriginally posted by HGMonaro Quote
One aspect not yet mentioned is the physical damage the reversed lens is susceptible too. That rear element is not protected by a filter or lens hood and depending on lens design, could be protruding. Since these reversed combinations also generally give very good subject/film magnification ratios you will probably be putting it very close to your subject and there's a real danger of bumping into it if you're not careful.
The working distance for a reversed Pentax-compatible prime is about 45mm. Working distance for a reversed zoom varies, but isn't closer than the minimum focal length. And nervous folks can put a section of macro tube on the reversed lens to act as a hood and fender.

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