diffraction limiting?

[Murray@uptowngallery]

How does one determine what is the smallest aperture/(largest f#) one can use without 'problems'?

Is it dependent upon factors like format and degree of enlargement like choosing a circle of confusion number for a given format?

I want to insert a stop between cells in a large barrel lens to allow practical exposure times with no shutter, and don't want to go so far as to impact the quality (at least not drastically).

Thanks

[Loose Gravel]

Not that simple. The smaller the aperature, the more diffraction. The trade is diffraction vs depth of focus. If you are using an older lens, then you trade resolution vs f/number, too. If the wind is blowing, you have to add in the blur due to movement vs all this other stuff. When all is said and done, you might just pick something about f/22 to f/45, depending on your format and enlargement ratio.

Try posting the question: "What's your favorite f/number?"

[gma]

I asked the same question a while back and there was not any definitive answer. Opinion was that there is no absolute diameter, but that it varies with the focal length, type of lens, shape and location of diaphragm, etc.,etc.
I suggest that you will start to lose image quality when the diameter of the diapragm gets to be less than about 3 or 4 mm. Since you are into experimenting, maybe you can make some test negatives using standard line charts with different diameter diaphragms and report the results in the experimental gallery.

[Loose Gravel]

Maybe I should rephrase...

It is hard to determine this from a scientific point unless you answer many questions, but from a practical point, set it for f/45 and take pictures. Diffraction does not all-of-a-sudden ruin your photograph. Diffraction is a gradual worsening of the image that slowly overtakes all the other 'problems'.

[clay]

Maybe I should rephrase...

It is hard to determine this from a scientific point unless you answer many questions, but from a practical point, set it for f/45 and take pictures. Diffraction does not all-of-a-sudden ruin your photograph. Diffraction is a gradual worsening of the image that slowly overtakes all the other 'problems'.

I shoot the 355 G-claron regularly at f/64 and f/90 for contact printed 12x20's and 7x17's. Frankly, if there is any diffraction problem, it is not evident on the prints. I agree with Loose G, just relax and go.

But if you are really interested, just remember that diffraction is related to the absolute physical aperture size, NOT the relative aperture (ie f/numbers) For example f/45 on a 300mm lens is the same size opening as f/22 on a 150mm lens, and f/11 on a 75mm lens, and in each case would have the same theoretical diffraction effects with all other factors (lens type, coating, etc) being held the same. That's why ULF people can get away with shooting at such small f stops. (That and contact printing! :^))

[Jorge Oliveira]

You guys with these funny little cameras...

Jorge O

[Murray@uptowngallery]

Great - I needed to hear 'real world answers'. I tend to be an 'armchair' photographer. I have so many projects going at once nothing gets finished so I rely on reading and planning, etc.

Thank you.

It will be some time before I get results but this gives me the perspective to inch forward with a couple ideas.

Murray

[gma]

Consider this to complicate the answer even more. The pinhole camera has no glass lens at all. Some "precision pinholes" are amazingly sharp (in a contact print at least). The pinhole relies solely on diffraction to make the image. Without the effects of diffraction there would be no image in that type of camera. Glass lenses work by refraction.

[sanking]

I shoot the 355 G-claron regularly at f/64 and f/90 for contact printed 12x20's and 7x17's. Frankly, if there is any diffraction problem, it is not evident on the prints. I agree with Loose G, just relax and go.

For contact printing diffraction should never be a problem.

Diffraction limited resolution varies according to color of light but can be approximated by the following formula.

R = 1500-1800/f-N, with R resolution in lppm, f = aperture of lens. The figure varies from 1500 - 1800 because of different resolution with color of light.

For example, a 355 G-Claron used at an aperture of f/90 would have a diffraction limited resolution of between 17 - 20 lppm, depending on color of light. This is considered more than adequate for contact printing since the human eye is said to have maximum resolution of about 15 lppm at a viewing disance of 10 inches.

For projection printing the limits are more stringent since the negative will most likely be enlarged. For example, a 2X magnification of a 5X7 negative made with a lens aperture of f/90 would result in an effective resolution of only 8-10 lppm on the print, less than the maximum resolution of *some* people at a viewing distance of 10 inches.

Sandy

[chrisg]

But if you are really interested, just remember that diffraction is related to the absolute physical aperture size, NOT the relative aperture (ie f/numbers)

Actually, that's not correct. F-number is the relevant figure. Oversimplifying a little here, but a diffraction limited lens can focus parallel rays of light down to a spot with diameter = 2.4 x (wavelength of light) x (f-number). For the sake of coming up with a useful number, say the wavelength of visible light = 500 nm (0.5 um), so the minimum spot size will be about 1.2 um x f-number. Suppose you wanted at least 50 line pairs per mm on your neg? That's 20 um per line pair or 10 um per line, so you could aperture down to about f/8 before diffraction would prevent you from achieving your desired resolution. That's a 'factor of two' kind of estimate. In practice, the resolution of most lenses is limited by something other than diffraction at f/8. (I believe silver particles on the order of 10 um to a few 10's of um, so 50 line pairs/mm is about as high a resolution as you'd get.)

I think the human eye can perceive 5-10 line pairs/mm. (I suspect that's under ideal conditions when you're standing extremely close to what you're looking at. Someone may correct me on that.) If you're exposing negatives for contact printing, then I might expect to perceive a diffraction effect somewhere in the range of f/45 to f/90 if I was looking real hard for it. If you expect people to view the print from at least a few feet off, you can probably go considerably slower than that.

Chris