Rodenstock 135mm APO Sironar-S at f/64

[JPlomley]

This forum has been terrific for all sorts of information and advice and I was hoping for some feedback about the following situation. I have purchased the titled lens as my first LF optic and have been blown away with it resolution, contrast, and accurate color rendition. There is also a "glow" associated with this lens that is difficult to describe unless viewing the chromes on a color corrected light table. Currently, I am viewing all my chromes with a Schneider 3x 6x7 loupe which I find makes lens performance easy to gauge. I recently took an image at about 1/4 life-size that required a focus spread of 10mm after optimizing the plane of focus. Information from this forum suggested an aperture of f/64. The results were less than impressive-very soft edges on all the ferns and venation was poorly resolved. Up to now, this lens has been a stellar performer for landscape use even stopped down to f/45. I'm just guessing here, but am I seeing a diffraction effect that is exasberrated by the fact that I am at 1/4-life size (i.e. my effective aperture is greater than f/64), or in general, should LF lenses just never be stopped down to their minimum aperture?

[Dan Fromm]

Its a question of how big you want to enlarge the negative. Diffraction limits a perfect lens' resolution to approximately 1500/(effective f/#) lp/mm. The generally accepted rule of thumb is that a print in which 8 lp/mm is resolved looks sharp when viewed at normal viewing disance. Normal viewing distance usually means 10".

So do the arithmetic. f/64 ==> at best 23 lp/mm. 23/8 = 3. The most you can enlarge a negative shot at f/64 is 3x if you want a print that will pass critical scrutiny. And so on. Note that real lenses, which are imperfect, won't do quite as well.

Since you're examining y'r chromes at 3x, perhaps the plane of best focus wasn't where you intended. Or perhaps you're not looking a subjects in the plane of best focus.

BTW, at 1:4 effect aperture with f/64 set and pupillary magnification = 1 (probably not exactly true for an Apo Sironar-S) is f/(64*1.25) = f/80. It is smaller, not larger, than f/64.

[JPlomley]

Cheers Dan. When I said greater than f/64 I meant greater numerically, not greater in terms of aperture opening. Apologies for the confusion. Looks like I should have shot the image at f/45 as opposed to f/64 considering your formula for effective aperture.

In terms of focus plane, it was straightforward, and the fact that all elements in the image had an equivalent "softness" to them suggests the plane was optimal, otherwise I would have expected some sort of gradation in resolution. But now you have me thinking about another possibility. Perhaps I jarred the rear standard slightly when inserting the film holder. I did do an initial Polaroid test to make sure I had sufficient DOF, and as I recall I did not re-loupe the GG before inserting my RVP-100. I guess another possibility is that since the exposure was ~ 10 sec, perhaps the film was not quite seated in the holder and there was some movement of the film during the course of the exposure.:eek:

[Bob Salomon]

Rodenstock states that the lens is diffraction limited at f22 and that is the best working aperture. The Apo Sironar S series is designed for reproduction ratios of 1:10. That would ensure that the best results are at image ratios from 1:5 to infinity. For closer work the Apo Macro Sironar lenses would be best.

[Dan Fromm]

Hmm. With a 10 second exposure, motion blur due to vibrations caused by the shutter's opening shouldn't be a bad problem. Is it possible that the subjects were blown around by the wind? I ask because at times wind can be a real problem.

Cheers,

Dan

[JPlomley]

Dan:

My wife and I both work at this together. She's watching the subject while I'm checking the stop watch. She said there was no motion of the ferns whatsoever. It was an extremely still day. Concerning shutter vibration, I would expect it's contribution to a 10 sec exposure is negligible, especially since the lens is mounted in a Copal 0 shutter.

Bob:

Agreed about the f/22 diffraction limit. However, the chromes are blistering at f/32 and still unbelievable even at f/45 for landscape work. This lens is truly an outstanding optic. However, as you mentioned, it seems I may be pushing the design of the optic in trying to utilize it for 1/4 life size. I'll do a few tests and use Dan's calculation for effective aperture and see what is actually acceptable for real world field work. I have seen images by David Muench and Jack Dykinga using non-macro medium telephoto lenses for macro work with acceptable results (Schneider APO Symmar's) so I guess I was hoping I could make due for now with the 135mm APO Sironar-S. As an aside, does the 120mm APO Sironar macro lens also contain the ED glass found in the S-series?

[Bob Salomon]

Linos makes no reference to the glass used in the Apo Macro series or the Apo Macro Digital series.

They use the best glass and techniques for the lenses application.

Yes your images at f45 may be spectacular. But that doesn't mean that you are not into diffraction.