Good grief, let's stay on earth ! Flare from reflection on the edges of the iris is a red herring - there is no surface to speak of and the direction of rays is not prone to a reflection either. If not, we could also speak (with much more reason but the same lack of realism) about flare from reflections on lens blackened edges or - why not, staying in the fishing industry - about light reflected of dust particles present in the space around the aperture...Originally Posted by Emmanuel BIGLER
Thanks Emmanuel, I appreciate your informed, enlightening contributions. I was not aware of the use of the Fourier Transform to analyze diffraction at an arbitrary edge or aperture. But I was thinking about instructors of undergraduate physics and photography classes, not engineers trying to solve real world problems when I suggested that the diffraction at a simple sharp edge is easier to model. A Fourier Transform will get you results, but it may be a little too much of black box to be of great teaching value at an introductory level.
The science is interesting.......... but then so are the real-world applications in our craft of the information that science can provide us, regarding this issue of diffraction.
I understand fully that many posters are focussed on gaining a deeper insight into how exactly it is that diffraction occurs, and it is in that spirit that I am following the discussion here. Yet there does seem to be such an enormous discrepancy between the automatic assumptions that any small aperture (below f22 according to some, below f32 according to others, and not until f64 according to others) will result in degraded sharpness apparent on film, and the post from Kirk Fry quoted above.
Since the apertures that most of us commonly shoot at lie in this same range f22 to (gasp) f64, it seems to me it would be useful to be able to define for a given focal length exactly what is the aperture at which diffraction will be even detectable to the microscopic gaze, and indeed to the viewer/buyer of the resulting image.
For myself, I'm not really clear at this point if there is a definite consensus about whether focal length alters the aperture at which diffraction will be detectable. Some of the evidence seems to indicate that, but others seem to be disagreeing, yes? no? I'm not wishing to create acrimony or dissension on this, but "confused about diffraction?" certainly seems to invite clarification on issues such as this.
The difference even between f32 and f45 will be a hugely significant step in the series of trade-offs and balances that make up the movements used and the choice of exposure length and aperture, prior to releasing the shutter. (These apertures were chosen with 8x10 in mind, but the equivalents for your own format are equally applicable of course.) That may determine whether to re-compose to leave out elements that cannot be brought into focus, and so on. Or alternatively, to accept a different rendition of water, foliage etc.
In the end, the whole process rests upon a set of choices or trade-offs. Where something like film speed is concerned, there is a certainty that's broadly definable as, say, within half to one stop of manufacturer's rating. With film choice, a certain grain structure must be accepted (give or take small improvements from processing technique, developer choice etc). Where DoF is concerned, there is a fairly clearly visible change from unsharp to sharp, when a good loupe is used. With a certain choice of shutter speed, movement or blur can be predicted or eliminated.
In the case of diffraction from small apertures however, there seems to be much less certainty than with these other elements of the process. If we cannot confidently say, yes, at f/so-and-so this lens will inevitably be reduced from the sharpness to be had at one stop wider an aperture, then I am left feeling that the certainty that "ought" to be available has not yet been arrived at.
If we were talking about a 28mm lens on a 35mm camera, I would likely never use f16 unless I had no alternative. (It may be that f11 is actually an issue, maybe f8, but I haven't experienced diffraction per se at f8. Who knows?)
But this is LF, so what are the equivalent apertures for a 65mm lens, a 90, a 135, a 180, a 210, a 250, a 300, a 360, a 480, a 600? And let's assume for the sake of argument, that we're talking about a 6x enlargement, say.
Maybe in this way, we can move away from sensing a dark and possibly unwarranted cloud of assumed diffraction hanging over quite innocent and usable apertures. Or maybe we won't, but the discussion is sure to be thought-provoking.
For those who still have an enlarger, there is a way to explore these issues. Just put a grain focuser under the enlarger, and look at different apertures through it. The effects of diffraction will be plain to see at the smallest apertures, as will the effects of lens faults at the widest apertures. Even at f/45, I never saw any loss of resolution from my ancient enlarging lens enough to mask the visible grain structure of the film.
One might do the same thing with a very fine ground glass and a very strong magnifier--maybe 20x. One would need a strongly lit subject to make viewing possible, but it's not something we'd need to do for every image, of course. It's just a way to calibrate our experience. I would make the comparison not only with subjects at the plane of sharp focus, but with subjects not focused but (hopefully) within the depth of field, to see where depth of field stops and diffraction begins. For most practical subjects, I suspect that diffraction will turn out to be a pretty minor effect compared to depth of field.
I have a cheapie Radio Shack 30x microscope. I guess I'll have to try my own experiment to see if it really works. Looking through a 30x microscope to a 4x5 ground glass would be like looking closely at a 10x12-foot print made from that film. If it works, I'll report back.
Of course, if I can't see the effects of diffraction in a 10x loupe on the ground glass, then I won't see them on a 40x50" print (from 4x5) looked at without aid.
If we are designing systems (or if we just like thinking about it in those terms), we need the science. If we just want to make good usage decisions, the empirical approach may be easier than trying to understand the science.
Rick "realizing that most practitioners are probably not comfortable with the mathematics of the frequency domain" Denney
In terms of the practical application I thought that there had been a pretty strong consensus. The size of your negative and your desired print gives the resolution you need in the negative, typically by something like 6 or 8 lppmm X the magnification in the print; and some rule of thumb like 1400/F-No, 1600/F-No or 1800/F-No tells you the best resolution that a particular aperture can deliver. If you want to be pretty conservative you might choose to use 8 lppmm for the print resolution and 1400/F-No as safe choices. For my typical 3-4X magnification prints (from 4x5 or 5x7), that allows me to stop down to f/45. To make an occasional 5X print (assuming I still intended to view it from as close as my bifocals will allow), I should probably respect a limit of f/32.
On the other hand, If I wanted to make a 12X print from a 35mm negative then I have keep the aperture faster than f/16, and I have to hope my lenses and film can resolve 96 lppmm under the prevailing conditions.
Finally, even if the performance of your enlarging set up matches that of your taking set up, you will get something worse through the two processes in series, so you should probably stay inside of the aperture limits calculated above by one or even two whole stops.
Greetings all, I come in late to this discussion.
The 'f' stop on a lens, any lens , eg. f8 the diameter of the hole is one eighth the focal length of the lens. Therefor
a 50mm for 35mm camera has approx 6mm aperature @ f8.
a 150 mm lens for 4X5 @ f8 has approx. 18.5mm hole.
a 300mm lens for 8X10 @ f8 has approx. 37.5mm hole.
a 450mm lens for 'any film format' @ f8 has approx. 56mm hole.
Compact digital lenses 8mm or less VERY SMALL HOLE.
It is my belief that the size of the hole , NOT the number of the 'f' stop is what determines ' defraction '.
A large format lens with an f2 aperature would have a 150mm hole if it were a 300mm focal length , an expensive and very heavy piece of glass and a very large shutter if it were to be in the lens .
cheers Barrie B.
Only if the diaphragm is in front of the lens. In any other cases the f stop isn't a real diameter but the diameter of the lightbeam that can pass through the lens. If the diaphragm is placed between the lens the diameter of the incident beam is greater as the 'hole'.
The f-stop cannot measured directly by measuring the 'hole'. With most lenses the 'hole' diameter is more or less 30 per cent smaller as the f-stop, f/focal-lenght.
Cheers
Peter
I once thought diffraction was governed by objective truth that didn't require belief, only agreement, and then I read the inner workings of this thread. I'm still sure there's objective truth in there, but it seems to be known by a smaller group than I once thought.
In any case, the f-number includes both the absolute diaphragm diameter and the focal length. f/5.6 is indeed bigger on a longer lens, but then that longer lens also magnifies the image more. That increase magnification enlarges the smaller diffraction effect. So, it's an approximate wash, apparently, leaving us with just the f-number.
Rick "glad the practical math ends up being simple" Denney
It is my belief that the size of the hole , NOT the number of the 'f' stop is what determines ' diffraction '.
Hello from the other side of the Earth (France)
Depends on which quantity you measure for the effects of diffraction
- if you consider the angular resolution limit in object space, then you are right, the absolute aperture diameter is the relevant quantity ; the angular resolution limit for an aperture "a" is lambda /a (in radians).
For the human eye if we take lambda = 0.5 to 0.6 microns (maximum of sensitivity of the retina to the solar light) and an iris diameter of 2 mm (during day time), we get an angular resolution between 0.25 and 0.3 milliradians ; one second of arc is about 0.29 milliradian, this is consistent with experimental tests of visual acuity ; at least for jet fighter pilots, my visual acuiy is probably closer to 2 seconds of arc, this is what I use for computing my home-made Depth of Field tables ;-)
- if you consider the linear details (in millimeters or in microns) recorded in the image plane, hence the f-number is the relevant quantity, not the absolute aperture diameter, and as mentioned above, the formula is extremely simple, the absolute cut-off frequency is equal to N. lambda. N = f/a
Both approaches are of course identical, depending on your application, since the connection between the angular resolution in the object space and linear resolution in the image plane for distant objects is simply :
(linear resolution in the focal plane) = (focal length) x (angular resolution in object space)
BTW this is the best definition for the focal length of any thick compound lens system, the focal length transforms a certain feature of angular size "alpha" in radians into a linear detail in the focal plane of size "f . alpha" in millimeters or microns (of course, lenses engraved in inches do the same, but in mils on output ;-) )
This definition allows to escape endless discussion about the position of principal or nodal planes of a thick asymmetrical system ;-)
Emmanuel, shouldn't there be a 'tan' in there too?
i.e. D = f*tan(alpha) for rectilinear lenses.
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