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Thread started 22 Dec 2012 (Saturday) 04:23
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explain something to me re: diffraction

 
Maverique
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Dec 22, 2012 04:23 |  #1

Ok, so I was reading DPReview's review of the Sigma 35mm and this bit stuck out to me as odd:

Diffraction starts to degrade the image at smaller apertures, but F16 is still eminently usable if you need the extra depth of field.

Wait, what? Isn't diffraction a factor of the camera's sensor/film and not of the lens? Are they stating that it's so sharp that the loss of detail to diffraction is mitigated by the lens' "raw power"? I don't quite get it. Someone halp?


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hollis_f
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Dec 22, 2012 04:36 |  #2

The amount of diffraction is the same at any aperture, regardless of how sharp the lens is or the resolution of the sensor.


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Dec 22, 2012 04:40 |  #3

Maverique wrote in post #15395923 (external link)
Ok, so I was reading DPReview's review of the Sigma 35mm and this bit stuck out to me as odd:

Wait, what? Isn't diffraction a factor of the camera's sensor/film and not of the lens? Are they stating that it's so sharp that the loss of detail to diffraction is mitigated by the lens' "raw power"? I don't quite get it. Someone halp?

Pretty much. Both sensor/film and lens have an affect on diffraction. Some lenses can start to degrade by f/8, and some f/16.. most around f/11.


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Dec 22, 2012 05:46 |  #4

Maverique wrote in post #15395923 (external link)
Wait, what? Isn't diffraction a factor of the camera's sensor/film and not of the lens?

Other way around. Diffraction is due to the lens and aperture. The sensor/medium simply affects how much of it is captured in the final image.


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SiaoP
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Dec 22, 2012 06:45 |  #5

Yup, more diffraction occurs when the hole the light can flow through (the aperture hole) is made smaller. The sensor is there to collect the light and diffracted light, the bigger the sensor the more it can record. Generally all lenses have pretty bad diffraction after f/16. I usually don't like going that small.

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Madweasel
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Dec 22, 2012 08:18 |  #6

SiaoP, your diagram is misleading, because the edge diffraction you get at the aperture does not split wavelengths - you're thinking of a diffraction grating, which splits colours through interference between many parallel edges.

Frank is strictly right (not surprising for a retired [mass] spectroscopist) in that the total amount of diffraction is unchanged at any aperture, but as the aperture closes, so less "straight-through" light is admitted and the diffracted light, whose effect on the image is to smear details, forms a larger proportion of the image-forming light.

The association with sensor resolution is simply that a higher resolution is able to show the image-smearing effects of diffraction at a wider aperture than lower-resolution sensors can. As a proportion of the entire image, the effect is unchanged.

In my opinion, based on real-life experience, the whole diffraction limitation is overplayed. Yes, if you pixel-peep a comparison of two identical shots, one at f/11 and one at f/22, you will indeed see a difference, but you wouldn't look at the f/22 picture on its own and declare it "soft". I've taken many shots at f/22 or even f/32 and they've been fine. At the same time, if I want ultimate sharpness, then yes I might be drawn towards f/8 or f/11. The best thing is to try it yourself and see what you're happy with in practice.


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EL_PIC
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Dec 22, 2012 08:34 |  #7
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Maverique wrote in post #15395923 (external link)
... Someone halp?

You need help not halp.
Stop reading and take pictures.
If you take a series of sunset pictures at every f stop from wide open
to closed you will see some effect at great mag but prob not throw
out any images due to diffraction.
You may prefer some at certain f stop due to sun star bust rays.


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Dec 22, 2012 09:00 |  #8

In my opinion, based on real-life experience, the whole diffraction limitation is overplayed. Yes, if you pixel-peep a comparison of two identical shots, one at f/11 and one at f/22, you will indeed see a difference, but you wouldn't look at the f/22 picture on its own and declare it "soft". I've taken many shots at f/22 or even f/32 and they've been fine. At the same time, if I want ultimate sharpness, then yes I might be drawn towards f/8 or f/11. The best thing is to try it yourself and see what you're happy with in practice.

That is the truth of it. There is 'way too much angst over diffraction these days, primarily because digital makes pixel-peeping too easy when it almost never matters when the picture will be displayed.

Frank is right about the mechanics. Not even sensor size matters--f/32 on a lens in front of an 8x10 sheet of film produces exactly as much diffraction as f/32 in front of a 24x36mm sensor.

This is a matter of gradation, and like every other choice in photography, always a matter of compromising one factor for another. Once you realize that diffraction is an observable factor at every aperture with several factors that may mitigate it, you can settle down and start to think in terms of what factors are important to your image and how they balance.

So if I carefully compare the finest detail at the focus plane of an image shot at f/4 with the same image and sensor shot at f/22, I may see that the f/22 image is very slightly degraded. But if I'm shooting a group of people that needs the depth of field of f/22 to get everyone in focus, it would be stupid to shoot at f/4--yes, that one person would be slightly sharper, but everyone else would be totally blurry.

And depending on the actual final display, there might not be any discernible diffraction difference at all, even on comparison. Uncropped images shown on a digital monitor or on prints even up to 30x or 40x probably won't show differences even on head-to-head comparison--the difference simply isn't resolved in the display medium.




  
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Dec 22, 2012 09:25 |  #9

An f/16 lens setting for 4/3 format has the same amount of diffraction as an f/16 lens setting for 8x10" film format. The difference in the ability for the large format lens to be used at smaller apertures without VISIBLE amount of distortion is simply caused by the fact that the tiny 4/3 format has to be magnified so much more (15.6x) to make its 13mm tall frame fill an 8x10" print compared to the 8x10 large format (1x), so diffraction is made 15.6x more visible due to that magnification.


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Maverique
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Dec 22, 2012 10:29 |  #10

EL_PIC wrote in post #15396318 (external link)
You need help not halp.
Stop reading and take pictures.
If you take a series of sunset pictures at every f stop from wide open
to closed you will see some effect at great mag but prob not throw
out any images due to diffraction.
You may prefer some at certain f stop due to sun star bust rays.

Meh I was just curious. I don't use those apertures, ever, but do like to understand these things. Again, simple curiosity ;)


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Dec 22, 2012 15:46 |  #11

hollis_f wrote in post #15395938 (external link)
The amount of diffraction is the same at any aperture ...

This doesn't seem right to me, and appears to conflict with the other responses and my own, limited, and probably faulty understanding, that is, that diffraction becomes more of an issue at smaller apertures.

Is it that the diffraction is actually the same at all apertures, but just more evident at smaller ones?




  
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hollis_f
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Dec 22, 2012 16:09 |  #12

xarqi wrote in post #15397406 (external link)
This doesn't seem right to me, and appears to conflict with the other responses and my own, limited, and probably faulty understanding, that is, that diffraction becomes more of an issue at smaller apertures.

Sorry, I was not at all clear, was I?

The amount of diffraction at any given aperture will be the same, regardless of sensor type and resolution or lens sharpness. So the amount of diffraction with any lens at, say, f16, will always be the same. As others have pointed out, this diffraction my nor be visible if the sensor doesn't have the resolution to detect it, or if the lens is not at all sharp.


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Dec 22, 2012 16:27 |  #13
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Maverique wrote in post #15396579 (external link)
Meh I was just curious. I don't use those apertures, ever, but do like to understand these things. Again, simple curiosity ;)

Curiosity is a good thing. But you should not take this too literal.
As I previously stated ...
you will not throw out pics based on Diffraction or even Refraction.
No one has mentioned this but ... smaller sensors with pixels below say 5 microns will behave quite differently than larger ones {S/N Ratio}.

For the real Curious into detail ..
Diffraction also limits the detail and contrast in an image. It is the loss of contrast and detail that limits the Apparent Image Quality. As the pixel size becomes smaller, lenses must be used at lower f/ratios and those lenses must deliver better performance in order to increase Apparent Image Quality. Few lenses are diffraction limited at f/8 over their entire field of view, so this is an optimistic upper limit to image quality.

Diffraction affects image detail by reducing contrast. The technical term for the contrast reduction is called the Modulation Transfer Function (MTF) and describes the contrast the camera delivers as a function of the spacing of lines (called the spatial frequency), or fine detail. Here the spatial frequency is expressed in terms of pixel spacing. As the f/stop increases, the diffraction spot becomes larger, and fine detail in the image becomes reduced in contrast. The red, green and blue lines show the diffraction effects for red, green and blue wavelengths of light for f/ratios 1, 2, 4, and 8. When MTF reach 0, there is no detail in the image at that scale.


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explain something to me re: diffraction
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