[slackercruster]

What is the benefit of mounting a lens backward?

[Philoslothical]

Check RioRico's article, specifically the sections on reversal and reverse stacking: https://www.pentaxforums.com/forums/pentax-lens-articles/152336-cheap-macro-b...lose-work.html

In a nutshell, it lets you get close to your subject. If you reverse a short prime on the end of a longer one, you can get some impressive magnification, too.

[RioRico]

Thanks for the plug, Phil! Reversing a single lens, usually with a cheap mount-reversal adapter, does two things:

1) You can|must work rather close to a subject. With a reversed prime, that working distance is the lens' flange-focal distance aka register, which for PK and M42 lenses is about 45.5mm / under 2in. With a reversed zoom, the distance varies. I can reverse my lousy A35-80/4-5.6 and focus from about 3.5cm to beyond infinity.

2) You gain edge-to-edge flatfield sharpness. This can be important if you want to shoot macros of flat subjects like postage stamps, microcircuitry, butterfly wings, etc. Even that lousy A35-80 zoom produces good images when reversed.

Going back to point (1): We often reverse lenses to shoot macros, but it's not the lens reversal that produces magnification. We get large images because we're working close, yes. But magnification requires extension, such as with tubes, bellows, focusing helicoids, etc. A reversed lens that has a deep inset provides its own built-in extension there. For a little extension, knock the glass out of cheap or damaged filters of the same thread diameter as the lens. With a lens of 55mm or shorter, just a little extension can make a noticeable difference.

Another correlation with point (1): A non-reversed lens can't focus closer than its focal length, which is also where magnification is greatest. That focus distance is from the lens' optical center, not its front. With short lenses (under 28mm) on extension, the close-focus point may be INSIDE the lens. That's hard to work with, eh? But a reversed prime has the register as its working distance. Thus we reverse short lenses so that they CAN be used for macros.

Phil also mentioned reverse-stacking. I go into that in the article, but here's a brief:

3) If we reverse a shorter prime (the SECONDARY) onto a longer camera-mounted prime (the PRIMARY), magnification is the ratio of the focal lengths. So if I put a 105mm prime on the camera and reverse-stack a 35mm onto it, magnification= 105/35 = 3:1. With longer primaries and shorter secondaries, we can reach amazing magnifications. Working distance is still the register. There are some tricks and requirements with reverse-stacking; see the article.

Hope this helps!