Diffraction and using a small aperture

[8x10 user]

DOF versus diffraction is a trade off. Often the amount of DOF desired is an artistic choice. It is generally recommend that a photographer uses the largest aperture while maintaining focus of the area desired. There are a number of DOF calculators to help a photographer determine what the optimal aperture is for each shot. There is one built right into my Sinar camera. You focus at one end of the area that is to be in focus, you turn the gauge until it matches your viewing aperture and then you focus at the other end. The gauge will tell you the optimal aperture for the shot.

If everything else is perfect and you use the right lens/ film then the difference in sharpness between F/45 and F/22 is noticeable. F/45 will be more forgiving of focus mistakes, film sag, and various aberrations that may or may not be present in the lens design. Some lenses are sharper at F/32 then F/16. Most LF lenses are designed for working apertures of F/16 (ED glass) or F/22 (no ED glass)-F/45.

The diffraction limit of F/22 might be 2x that of F/45 however the quality per pixel will be higher from a F/45 chrome scanned at 1700 PPI versus a F/22 image scanned at 3400 PPI scan. Any short falling of the scanner and lens will be more noticeable with the higher resolution scan and since the film resolveablity versus grainalty of the film remains a constant there are diminishing returns when you increase resolution. So even if the theoretical limits of diffraction state that the limit of resolution is 4 times higher (by total number of pixels) at F/22 versus F/45 the actual level of obtainable "quality" is less then 4 times higher. A 1700 PPI scan on 8x10 is still ~206 MP at at F/45 the DOF should be higher then the DOF obtained from a 200MP MFD (Sinar, Hasselblad multishot) back because diffraction forces one to use a very large aperture with such a back.

Some use a larger aperture and focus stack to produce images of the maximum technical quality. I've even heard of this being done with 8x10.

Everyone keeps coming back to engineering and ppi/spi/dpi, lp/mm and everything else. I do not speak to anything but large format film and lenses, as I don't do digital capture, and haven't done the tests over there myself. FWIW, the issue for me isn't resolution but bit depth, or the ability of the capture medium to represent the largest number of tones.

I would say this. I think (not I know) that most large format photographers are expecting a certain amount of sharpness from their systems. Damn the numbers, and do a test for yourselves. Waste a whole sheet of film, take the shame shot at f22 and f45, do whatever else you do to it, print it in the darkroom or scan it and whatever... and see if you can tell the difference in your print.

My only point is that the difference is so small that most photographers can be happy about shooting with plenty of depth of field.

I have heard the adage "shoot 2 stops down from wide open" and I declare it a falsehood, or meaningless, or whatever you want to call it. I like depth of field and anyone else that also likes it, should be able to have plenty without degrading their image in any significant way. If its true at all, its splitting hairs...


Lenny

[Drew Wiley]

You've got to enlarge a 6x7 image over three times as much, so even a garden-variety 4x5 lens will record far more detail than any M7 lens. It's better to compare
apples to apples. I reprinting an exceptional 6x7 neg right now, and the problem is not about getting the same general look in a 16x20 print as with 4x5, but the
sheer headache of dealing with the fact that miniscule film flaws or bit of dust that would be almost imperceptible when enlarging from large format film mean a
lot more nitpickiness in prep and inevitable spotting when working with MF film. Per tonality, sure I'd be rather working with 8x10 film rather than either 4x5 or
6x7; but in this case, I barely had time to set the camera up on a tripod and get the shot. The light would have been totally gone if I had tried using the view camera (which I did have along). But given a lens like an Apo Rodagon N 105 in a very precise carrier, with a premium paper, the tonality itself is pretty darn acceptable.

[8x10 user]

My personal experiences with with Apo Sironar S and the 8x10 image format indicated that somewhere around 3000 PPI is possible under ideal circumstances. The chromes were evaluated with a 12x Schneider loupe, 2 different drum scanners, and an Eversmart Supreme.

[Jac@stafford.net]

My personal experiences with with Apo Sironar S and the 8x10 image format indicated that somewhere around 3000 PPI is possible under ideal circumstances.

I admit to innumeracy. I cannot wrap my head around the "PPI" metric. How big is a "P", and how does it relate to film resolution?

How about good, old "LP/mm"?
.

[Lenny Eiger]

Really?

The difference was huge. Undeniable.

As to Bob's question I did my best to match as many variables as I could. Same film, developer, exposure, etc. The cameras were all set up right next to each other and the shutter clicked within moments of each other - to make sure the light didn't vary. Everything was on a tripod, in my garage, no wind, etc.

There is one more thing I want to say here. I've been doing this as long as any of you. There are times when I was super technical, making graphs and everything else, and other times when I focused on the shooting and just did what I knew worked, adjusting slightly here and there, as needed. Most people that look at my work appreciate the excellence of it. I have the ability to be quite consistent, and I get the results I want. I am not stupid.

I get so tired, it seems any time we talk about technical issues, that some people feel the need to slip in a "well if you knew anything, you'd know that: whatever their point is."

All I am suggesting is that there are times when the numbers don't make sense. Maybe its the stochastic nature of film that yields a certain result, but there are plenty who will argue that "the numbers don't lie". They haven't taken that extra factor into consideration. As long as these conversations can stay mutually respectful I'm happy to participate. I have an opinion, and everyone is welcome to disagree. However, I'd like to have it toned down a bit.

Lenny

[jp]

I admit to innumeracy. I cannot wrap my head around the "PPI" metric. How big is a "P", and how does it relate to film resolution?

How about good, old "LP/mm"?
.

PPI is pretty simple, if it's 3000 ppi, then a P is 1/3000". It's comparable but a little more specific than dots per inch. If you scan film in, you base resizing resolution on pixels, as output detail is measured in PPI for displays or DPI for inkjet or some minimal fraction of DPI if you making a line screen. An image scanned or captured 6000 pixel wide and displayed or printed 20" wide is going to have 300 ppi of information maximum in the file, your monitor or printer will have to resample from that.

[8x10 user]

^ Exactly

DPI (dots per inch) and PPI (pixels per inch) are common terms in dot based printing (inkjet, screen, and printing presses). For example Epson's newest large format inkjet printer can output 360 pixels per inch using a 2880 dots per inch halftone pattern.

[BetterSense]

LP/mm at the film plane is also meaningless unless one specifies the degree of enlargement and viewing distance. If photography were a real field we would express resolution in minutes of arc or steradians.