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02-19-2013, 10:55 AM - 2 Likes   #1
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A comparison of reversed wide-angle primes

As part of my education in extreme macro techniques I just did some testing of various Pentax wide angle lenses mounted in reverse.

Lenses tested:
A24/2.8
M28/2.8 (first version)
K28/3.5
K30/2.8
M35/2
K35/3.5

About the test method: I used a bellows to set each lens at 3:1 magnification or as close as I could get. (For the A24/2.8 the bellows at minimum extension was too much, so I used the #2 manual tube, 19mm, which got very close to 3:1.) I used flash to factor out camera/subject shake as much as possible, and I focus-bracketed each lens/aperture setting using a micro rail. In post I adjusted exposure for each shot, then applied the same sharpening settings to all shots. I made subjective comparisons to find the best aperture for each lens, then compared those shots against each other. I only compared center sharpness.

This was all rather tedious, but at 3:1 magnification the effective f-number is about 4 times greater than the nominal (i.e., aperture ring setting) value, and it becomes a stark trade-off between inherent lens sharpness and diffraction. For this kind of work it is very important to find the optimum aperture for a given lens at a given magnification, because performance degrades quickly as you move away from that aperture. I was getting decidedly mixed results before, so the testing was well worth the tedium.

Here are the lenses in order from best to worst, with the best aperture shown in parentheses:

1. K28/3.5 (4.5)
2. A24/2.8 (4.8)
3. tie between M28/2.8 (5.6) and M35/2 (4.8)
5. K30/2.8 (5.6)
6. K35/3.5 (6.7)

The K28/3.5 is the clear winner, although all of the top 4 do at least reasonably well. The K30/2.8 and K35/3.5 did poorly and are not suitable for this magnification. This rather proves the point that performance in the normal direction is not a good predictor of performance reversed, because those latter two lenses are very sharp when used normally.

The tie between the M28/2.8 and M35/2 was interesting in that here you see the trade-off between diffraction and lens sharpness quite clearly. The M28/2.8 must make a big jump in sharpness going from f/4.8 to f/5.6. In contrast, the M35/2 gives very similar results at all apertures from f/4 to f/6.7, meaning a steady increase in lens sharpness, offset about equally by diffraction.

Wide angle lenses are interesting for reversal because you get high magnifications without much extension, allowing a compact field rig. I expect that the best 50s will do better at 3:1, but need a lot of extension to get there. So another reason I wanted to test these was to see if going past 3:1 is worthwhile. The K28/3.5 is clearly going to win here, because of its slightly wider optimum aperture, and you want the widest aperture possible at high magnifications. (It is a thousand pities the DA35 limited doesn't have an aperture ring.) I'd venture a guess that 4:1 would be acceptable, but not outstanding, on the K28/3.5, and that the rest of these lenses aren't really suitable for it.

At some point I'll try a similar test with 50mm lenses.

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aperture, diffraction, k-mount, k28/3.5, lens, lenses, m28/2.8, m35/2, magnification, pentax lens, sharpness, slr lens, test
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