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Author Topic: Diffraction Limit of Film?  (Read 4598 times)

Bevan.Burns

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Diffraction Limit of Film?
« on: November 17, 2007, 11:45:44 AM »

Hi all, wondering if someone could enlighten me. I read so much these days about the diffraction limits of various digital cameras, and was wondering how diffraction effects film. I assume it does in the same way. Is there different limits for different film stocks? Different formats? What's the deal?
Thanks
Bevan
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Morgan_Moore

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Diffraction Limit of Film?
« Reply #1 on: November 17, 2007, 12:40:37 PM »

Quote
Hi all, wondering if someone could enlighten me. I read so much these days about the diffraction limits of various digital cameras, and was wondering how diffraction effects film. I assume it does in the same way. Is there different limits for different film stocks? Different formats? What's the deal?
Thanks
Bevan
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I may be wrong but as far as I am aware diffraction occurs when the various ratios of light wavelenght, aperture and pixel size come in to line

The random distribution and size varience of film 'pixels' means it doesnt happen

I may be wrong

S
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AJSJones

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Diffraction Limit of Film?
« Reply #2 on: November 17, 2007, 01:39:07 PM »

This page explains nicely why diffraction is important in photography in general.  Whether it's regular arrays of defined sensor elements or random distributions of grains of various sizes in a film emulsion, the effect on the incoming image is the same, but how it affects what is recorded will be a bit different.  Also pixel peeping is far more prevalent than "grain peeping" ever was, so it's more "noticeable" these days.  Still, going to f/22 on a 35mm film camera and modern digital FF sensor will both show substantial losses due to diffraction, when a traditional sized print is created.  With higher res sensors it is reasonable to make bigger prints than before and geometric enlargement is greater so both DoF and diffraction effects will be altered - generally narrower DoF and greater visibility of losses due to diffraction
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grepmat

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Diffraction Limit of Film?
« Reply #3 on: November 17, 2007, 03:19:04 PM »

Diffraction occurs because light bends around an edge. That's it. It has essentially nothing to do with film, or digital sensors, or even lenses (*re*fraction is how lenses work).

Diffraction impacts photography because our lenses are of finite size and furthermore we stop down our lenses. When the aperture blades form a smaller circle, the diffraction of light at the edges of the aperture causes more and more blurring of the image. Stop down too much, and blurring due to diffraction becomes dominant.
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John Sheehy

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Diffraction Limit of Film?
« Reply #4 on: November 17, 2007, 07:51:03 PM »

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Diffraction occurs because light bends around an edge. That's it. It has essentially nothing to do with film, or digital sensors, or even lenses (*re*fraction is how lenses work).

Diffraction impacts photography because our lenses are of finite size and furthermore we stop down our lenses. When the aperture blades form a smaller circle, the diffraction of light at the edges of the aperture causes more and more blurring of the image. Stop down too much, and blurring due to diffraction becomes dominant.
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Another way to look at it is that as you stop down, the diffracted light reduces at a lower rate than the non-diffracted light.  When you halve the aperture, you approximately quarter the amount of light that doesn't hit any aperture edges, but only halve the amount of light that expeeriences diffraction, since the circumference of the blade edges has only halved.

So, there is actually less diffracted light at smaller apertures, but it is a larger percentage of total light.
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grepmat

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Diffraction Limit of Film?
« Reply #5 on: November 17, 2007, 09:06:42 PM »

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Another way to look at it is that as you stop down, the diffracted light reduces at a lower rate than the non-diffracted light.  When you halve the aperture, you approximately quarter the amount of light that doesn't hit any aperture edges, but only halve the amount of light that expeeriences diffraction, since the circumference of the blade edges has only halved.

So, there is actually less diffracted light at smaller apertures, but it is a larger percentage of total light.
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Good point that I neglected to say. Thanks.
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panoak

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Diffraction Limit of Film?
« Reply #6 on: November 17, 2007, 11:01:09 PM »

You remind me of how we used to discuss film limits along the way to the sweet spot.  General film wisdom landed in saying that a lens resolves optimally within 2 stops of the middle of the aperture range.  This actually arrives at a 4 stop range, with the "universal optimum" for 35mm being F/8.  That still basically applies, as this relates to lens performance more that the medium in use.  Your criteria will vary according to circumstance, as you'll prefer a smaller aperture for macro, and a larger one for portraiture.  The old rule of thumb still holds true.  If the aperture range of your lens is F/2.8~F/32, the sweet spot of the lens is very near the middle.  Stay within F/4~F/16 as the working range for that lens, and you'll be sure to be "safe".  Use the extremes only when you must, and expect to pay a little for it.
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ejmartin

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Diffraction Limit of Film?
« Reply #7 on: November 19, 2007, 03:34:30 PM »

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Another way to look at it is that as you stop down, the diffracted light reduces at a lower rate than the non-diffracted light.  When you halve the aperture, you approximately quarter the amount of light that doesn't hit any aperture edges, but only halve the amount of light that expeeriences diffraction, since the circumference of the blade edges has only halved.

So, there is actually less diffracted light at smaller apertures, but it is a larger percentage of total light.
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This statement is rather misleading.  ALL light passing through an aperture is diffracted; the diffraction pattern is due to the superposition of light coming from all across the diaphragm to a given point on the sensor.  Diffraction is a wave phenomenon; it is not possible to localize it to a particular part of the aperture.
« Last Edit: November 19, 2007, 04:23:06 PM by ejmartin »
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emil

ejmartin

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Diffraction Limit of Film?
« Reply #8 on: November 19, 2007, 03:41:25 PM »

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Hi all, wondering if someone could enlighten me. I read so much these days about the diffraction limits of various digital cameras, and was wondering how diffraction effects film. I assume it does in the same way. Is there different limits for different film stocks? Different formats? What's the deal?
Thanks
Bevan
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Yes it works the same way.  The diffraction pattern is caused by the wave nature of light, as the light passes through the aperture.  The size of the diffraction spot is proportional to the f-stop.  What recording medium you place at the focal plane to record that diffraction pattern, whether it be film or digital sensor, is up to you -- but it has no effect on the diffraction pattern itself, which formed at the point the light passed through the aperture, and before it reached the focal plane.
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