6x7 Arca, minimum lens aperture question?

I have a question regarding minimum lens aperture on my Arca 6x9, IBm currently using a 6x7 film back. I have seen numerous comments regarding lens diffractio n while using an aperture smaller than the so-called ideal for the format. I am using two different Schneider lenses on this camera, a 58mm Super-Angulon XL and a 150mm APO-Symmar. I have read that the optimum aperture for 6x7 is f11 and f 11Bf16 for 6x9. I have conducted preliminary film tests with this camera and e xposed several scenes at f11, f16, and f22 each. Upon examination on the light table I have found that my 6x7 black and white negatives exposed at f22 seem to have the best overall image quality and depth of field, not that either the f11 or f16 is bad mind you. It might just be that the increased depth of field at f 22 appeals to me. How will lens diffraction make its presence know? I might al ready be experiencing it, and just not know it. But if I am it seems a minor pr ice to pay for the added depth of field. Thanks.

Bob P.

" I have read that the optimum aperture for 6x7 is f11 and f11 to f16 for 6x9..."

Wouldn't that be more dependent on lens design ather than format?

To clarify my statement about reading recommended best apertures for a given format. This information is given in the ( Rodenstock brochure "Lenses for Large Format Cameras" dated 6/95 # 9543 5121012- 9302845 ) in the form of a table based on film format across a range of focal lengths. I will attempt to quote the table for both 6x7 and 6x9 formats:

(6x7 format / focal length lenses 35,50,60,75,100,180, and 280 best aperture f11-16) (6x9 format / focal length lenses 45,55,65,105,135,210, and 360 best aperture f16)

No mention of lens construction is made, FYI this same table states f16-22 as being the best aperture range for 4x5 format. I also recall seeing similar information from Schneider but I cannot locate it at this time, it too was film format driven. I too would think that lens design plays a key role in performance for a given format, but I can only q1uete what IBve read. However as stated in my earlier post IBve found that my f22 to negatives look very good on the light table. Hence my question on diffraction at smaller apertures.

Bob P.

Robert, I use both 6x7 and 6x9 and find that, for me, the optimum aperture for these formats is f22. I NEVER go larger. I have read a great deal concerning lens diffraction at small apertures but IMHO how many "normal" photographers would recognise it if it jumped out and bit us on the nose!! By "normal" I mean photographers who spend their time "in the field / studio" photographing subjects other than lens test charts!! I enjoy landscape work and the increased depth of field with a small aperture is welcome. I DO notice a difference if I use f11, not so much at f16. But my lenses appear to me to perform best at f22, so I carry on using this aperture. The proof is in the picture after all!! Regards Paul

One of the things about comparing smaller formats to larger formats is that the resolution requirements for a smaller format are higher since the magnification factor for any given size enlargement will be higher (4x enlargement of a 6x7cm = 8x10, whereas you only need a 2x enlargement of a 4x5 negative to get an 8x10 print).

Let's assume that there is a theorectical and real world difference between the resolution for a specific lens at B/11 and B/16 or B/22, but if you need B/16 or B/22 or even B/32 to get the image you want, would that difference stop you from making the image the way you need to make it? Ideally you should use the largest diameter aperture and combination of movements that provides the depth of field an image requires

Interesting: I was typing in "option + f" to get a cursive f when i wanted to type f/16 etc. But how did "option + f" become a capital "B"? I am using a Mac

The question of optimum aperture vs. sufficient DOF is always a judgement each photographer must make for him or herself under actual shooting conditions. You do raise one potential issue though and that is the fact that you are shooting roll film. What roll film backs are you using? Perhaps that answer is obvious (I'm not that familiar with Arca products), but I mention it because I've had some less than glorious results with after market roll film backs on my 4x5. Where the problem cropped up is in film plane alignment. I know my gg was right on the money, as I proved that with stringent film tests at wide open apertures. Not so with the roll film back. It was way off! the interesting thing is that the ANSI spec for 2-1/4" x 3-1/4" film holders is identical to that of 4x5! this makes no sense inasmuch as the tolerance (variance from the nominal spec) should get tighter (smaller) as the film size decreases. This, for reasons mentioned in a previous post. You magnify a smaller negative more and so it needs to be sharper to achieve like results with a similar size print from a larger negative. What I'm heading toward here, is that if your roll film holders are not holding the film at the right depth, you may be relying on DOF to make up for inaccurate film plane/ground glass coincidence! This might be giving you the impression that your lenses are working better at f22 than at f11. I would suggest doing a film test with a suitable step target at wide open aperture. Hope this helps in some way.

Bob the only time I noticed diffraction was using a 180mm Fujinon AS lens stopped down to f90 and that was on 6x9cm. (Horseman roll back). The image was carefully focused on the gg but subsequent enlargments (16"x12") showed that nowhere on the image was critically sharp. The lens however was critically sharp at other f stops.

In response to the last post, I believe diffration effects can be demonstrated better with certain subjects than with others and that with anything affecting apparent sharpness, there is an element of subjectivity involved when observed by the human eye as opposed to making measurements with suitable test targets, film and measuring instruments. I'm not suggesting that if the image looks good to us, it should be rejected because it was made at f-this or f-that. All I wish to point out is that the laws of physics can't be easily repealed. As you reduce the aperture of a diaphragm, the rays of light passing through it are forced to make a "tighter turn". In every book I've read on the subject, that means more diffraction.

With roll film holders one must be aware of two factors concerning emulsion location.

First, there is the matter of design distance from seating surface. ANSI specifications for 4x5 and 2 1/4 x 3 1/4 sheet film holders are the same, and would place the emulsion 0.190" away, assuming film thickness of 0.007". However, each manufacturer of small view cameras and roll holders seems to use its own value for this distance. Arca roll holders are Horseman roll holders. I don't know what ground glass distance Arca has settled on, but Horseman's holders for 6x9 cameras (and the ground glass on my Horseman VH) place 120 emulsion at exactly 0.185" from the seating surface. Any other ground glass distance will result in a discrepancy with those holders.

Second, for any frames that have spent some time on a reverse-curl feed roller - - and the threshold time can be *very* short depending on ambient temperature and humidity - - there will be a bulge toward the lens when film is advanced to place that "set" section into the gate. I have measured displacements of 0.035" at peak; thinner film ramps up gradually, while thicker stock exhibits a more localized bulge. Even if the ground glass and first frame emulsion locations match, this effect will wreak havoc with any critical focus work exposed on subsequent frames, particularly when shorter focal lengths and larger apertures are used. In such situations I have taken to exposing only the odd numbered frames on a roll, advancing quickly past the others. That way no section of exposed film ever spent time on a reverse-curl feed roller.

I suggest you perform some tests to rule out these potential problems before spending too much time worrying about diffraction.