Reverse mount adapters

[max_ebb]

I'm using macro tubes with a pentax 67 with 90mm and 165mm lenses. I'm curious to know what advantages, if any, a reverse mount adapter would give me with those lenses.

On a side note, for anybody that does macro work with a Pentax 67, what is the best lens to use with the macro tubes to get the most depth of field at about 1:2 (1/2 actual size)?

Thanks

 

[Dan Fromm]

Lenses made for normal use don't have to be reversed at magnifications below 1:1. Above 1:1 they should be reversed. If you're not going to shoot above 1:1, reversing will cost you image quality and ease of use.

All lenses. i.e., all designs and all focal lengths, give the same DoF at 1:2 and the same aperture. The only way to get more DoF is to stop down, i.e., use a smaller aperture, larger f/number. And the gains are limited by diffraction, stopping down too far can actually reduce DoF.

 

[max_ebb]

Lenses made for normal use don't have to be reversed at magnifications below 1:1. Above 1:1 they should be reversed. If you're not going to shoot above 1:1, reversing will cost you image quality and ease of use.

Ok, for above 1:1, what is the advantage of reversing the lens? How much difference does if make? How much above 1:1 before it starts making a difference? Does it work the same with longer lenses (like a 165mm)? The macro tube set I'm using now will get me to a little over 1:1, and I'm either going to add another tube to the set, or get a bellows.

All lenses. i.e., all designs and all focal lengths, give the same DoF at 1:2 and the same aperture. The only way to get more DoF is to stop down, i.e., use a smaller aperture, larger f/number. And the gains are limited by diffraction, stopping down too far can actually reduce DoF.

No, that's not true. If I use the 165mm lens with a shorter extension tube, I get substantially more DOF than I do with with the 90mm lens (with a longer extension tube) at the same magnification (ie: 1:2), due to the increased distance from the subject.

 

[Dan Fromm]

The advantage of reversing the lens when shooting above 1:1? Well, a lens can be optimized for only one magnification. Most taking lenses are optimized for 1:20 or smaller. By reversing a lens when shooting it above 1:1 we take better advantage of its optimization than if we use it above 1:1 facing normally. Clearer?

The only lenses that don't need to be reversed when used above 1:1 are truly symmetrical. There aren't many, the Apo Ronar line is an example. So are dialyte type Apo Nikkors and Apo Saphirs. Some Apo Nikkors are tessar types.

As a practical matter, the gains above 1:1 from reversing the lens depend on the lens. In general, fast lenses do worse facing normally above 1:1 than slow ones. Really asymmetrical lenses, e.g., tessar types and retrofocus lenses and telephoto lenses, do much worse than more or less symmetrical ones, e.g., plasmat types and slow double Gauss types. I don't have what you do, so can't do the right experiment for you. But you have your lenses and can ask them. Ask them!

Also, as a practical matter there are a few lenses that somehow manage the impossible. For example, I have a 100/6.3 Neupolar that shoots very well facing normally from around 1:8 to 8:1. It is a reversed tessar, i.e., a tessar with the cemented doublet facing the subject. And when I can focus it, it is spectacular at distance but doesn't have much coverage.

You are mistaken re DoF. I just did the calculation. At 1:2 and f/x and the same CoC a 90 and a 165 have the same DoF. The 165 has more working distance, that's all. Try the experiment. I once did it with the same subject, illumination, and emulsion. At f/8, KM transparencies of the same subject shot at 1:4 with flash and a 55/2.8 MicroNikkor AIS, 105/2.8 MicroNikkor AIS, and Questar 700 were indistinguishable. Equally sharp, same DoF, i.e., not much but the same in all three. The Questar 700 is a 700/8 (nominal, actually T/11) mirror lens, highest magnification 1:4.