DPReviewTV: What is diffraction in photography?

Canon Rumors Guy

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It’s Monday, so it’s time to head to class and today the always educational Don Komarechka shows us what’s behind the dreaded “diffraction” when you’re stopped down significantly with your lens.
Why do the images lose sharpness? Don shows you a couple of simple experiments to help you understand what is actually a pretty complex phenomenon.
Check out the video above and give Don a follow on Twitter or Instagram

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dilbert

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This will make for interesting reviews of the upcoming high-megapixel Canon RF that is aimed at a branch of photorgraph that typically uses narrow apertures for increased depth of field: landscape photography.
 

privatebydesign

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This will make for interesting reviews of the upcoming high-megapixel Canon RF that is aimed at a branch of photorgraph that typically uses narrow apertures for increased depth of field: landscape photography.
No it won't. I have two really big issues with the video, first, he is demonstrating a visual diffraction between f17 and f96, at which point f17 looks pretty darn good. Besides, I don't know any Canon lenses that stop down past f32.

Second he implies that more pixels make diffraction more apparent and that simply isn't true, more magnification makes diffraction more apparent. If you have a 30mp image and retake that image with the same settings with your new 90mp camera the diffraction in both is the same.
 
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SteveC

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Second he implies that more pixels make diffraction more apparent and that simply isn't true, more magnification makes diffraction more apparent. If you have a 30mp image and retake that image with the same settings with your new 90mp camera the diffraction in both is the same.

Before this turns into a gigantic talking-past-each-other fest, I'll suggest that you are assuming that the images are viewed at the same final size...rather than being 100 percent cropped or something like that.

But we have quite a number of pixel peepers here.

I believe that at 100 percent crop you will see a difference because you are (effectively) blowing up the image, making the diffraction more apparent. It's the same absolute size, but won't be the same displayed size under those circumstances.
 

privatebydesign

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Before this turns into a gigantic talking-past-each-other fest, I'll suggest that you are assuming that the images are viewed at the same final size...rather than being 100 percent cropped or something like that.

But we have quite a number of pixel peepers here.

I believe that at 100 percent crop you will see a difference because you are (effectively) blowing up the image, making the diffraction more apparent. It's the same absolute size, but won't be the same displayed size under those circumstances.
Of course, but by the same token if you made a larger print of the smaller resolution file you would see more diffraction in the lower resolution file. If you don't 'compare' things at the same size, speed, magnification etc etc then that is not a comparison. It is like saying my car goes faster than yours because mine goes 150km/h yours only does 100mph, it simply isn't true.
 

dilbert

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Second he implies that more pixels make diffraction more apparent and that simply isn't true, more magnification makes diffraction more apparent. If you have a 30mp image and retake that image with the same settings with your new 90mp camera the diffraction in both is the same.

Yes, the diffraction is the same on both sensors as it is defined by the lens aperture. What changes is the ability for the diffraction to be recorded by the sensor.

A web page with only old cameras is here:


... using the 7D, which is the closest to the R5 (4.3um pixel), setting F/22 shows a very large Airy disc. Compare that to F/1.4. Then there's the interferance pattern (not shown by that web page.)

The ability to record fine detail drops as the aperture narrows.
 

usern4cr

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Could somebody please "illuminate" me on this particular issue?
Look down the barrel of the RF 800mm f11 lens (which has NO adjustable iris blades as it is always "wide open").
It looks like (to me) the light bundle is around 5/8" or so wide, visually.
Now I don't know for a fact how narrow the smallest light bundle is at the circular fixed iris, or pupil. Does anyone know the size of it?
If the fixed pupil is in the range of 5/8" or so, then how could light be appreciably affected by diffraction since that's such a big hole?
 
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Several problems with this demonstration. First and foremost, light does NOT bend. And discussion of light that justifies some light property by saying that light "bends" is automatically incorrect. Period. Second, using liquid wave tables or sound waves to describe what the action of electromagnetic radiation is like is not really representative.
 
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privatebydesign

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Could somebody please "illuminate" me on this particular issue?
Look down the barrel of the RF 800mm f11 lens (which has NO adjustable iris blades as it is always "wide open").
It looks like (to me) the light bundle is around 5/8" or so wide, visually.
Now I don't know for a fact how narrow the smallest light bundle is at the circular fixed iris, or pupil. Does anyone know the size of it?
If the fixed pupil is in the range of 5/8" or so, then how could light be appreciably affected by diffraction since that's such a big hole?
The ‘fixed pupil’, apparent aperture opening, is 800/11 in mm, rounded out that means it is 73mm or 2.87 inches.

Looking down a lens and trying to estimate anything is an exercise in futility as the lenses you are looking through change the very view of what it is you think you are seeing.

As for how light is affected by such a ‘large hole’, well that is just the nature of waves and edges. But as I pointed out previously, the example shown was as extreme as it is possible to get even in a specialized situation and really can’t be replicated in non macro real world situations. I think few would argue the example at f2.8 was not sharp yet that equates to a non macro real world f17, more closed down than you 800 f11!
 

privatebydesign

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Several problems with this demonstration. First and foremost, light does NOT bend. And discussion of light that justifies some light property by saying that light "bends" is automatically incorrect. Period. Second, using liquid wave tables or sound waves to describe the action of electromagnetic radiation like like is not really representative.
Gravity bends light.
 
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AlanF

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Could somebody please "illuminate" me on this particular issue?
Look down the barrel of the RF 800mm f11 lens (which has NO adjustable iris blades as it is always "wide open").
It looks like (to me) the light bundle is around 5/8" or so wide, visually.
Now I don't know for a fact how narrow the smallest light bundle is at the circular fixed iris, or pupil. Does anyone know the size of it?
If the fixed pupil is in the range of 5/8" or so, then how could light be appreciably affected by diffraction since that's such a big hole?
I don’t know if this helps but I did post a thread on diffraction.
 
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Gravity bends light.

The force of gravity on light is so weak that the only observation of light bending is when it travels past a star or black hole in space. There is nowhere on earth that anyone or any instrumentation will ever see light bend. Also, the Einsteinian prediction of the bending of light is actually a confirmation of the curvature of space predicted by Einstein. You should read about Einstein and Eddington and the extent that Eddington, Dyson, and Crommelin had to go to in 1919 to see the effect of light bending around our sun. So, light bending inside your camera is something you will never see.
 

Joules

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There is nowhere on earth that anyone or any instrumentation will ever see light bend.
What exactly do you mean when you write of light bending? Are you saying that light behaves like a particle, traveling in a straight line from its source until it strikes a surface?

The common explanation of how light is not (just) a particle and diffraction works conceptually is the double slit experiment. And you can easily find tons of material on it, often with pictures that show the difference between 'particle' (no bending) and 'wave' (bending in the sense that there does not need to be a straight line of uninterrupted medium between the source of the light and the surface it hits) like this:

What you would expect from a particle:
double_balls.png


What the experiment actually produces:
double_waves.png


Source

So, are you saying the lower picture is misleading, or is the effect it shows not what you refer to with the term bending? In the former case, how is the diffraction pattern explained? Do you have any good material for an alternate explanation?

Edit: Hadn't watched the video previously, but the laser beam producing a disk instead of a point is not a double slit, but it shows the same kind of 'bending'.
 
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Del Paso

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To me, the only question which really matters is:
Will an f.16 picture taken with a 90MP camera be VISIBLY better than the same picture taken at f.16 with a 30 MP camera?
DLA of the 90 MP camera should be around 4,5, if I'm not mistaken.
Any thoughts would be welcome!
 

privatebydesign

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The force of gravity on light is so weak that the only observation of light bending is when it travels past a star or black hole in space. There is nowhere on earth that anyone or any instrumentation will ever see light bend. Also, the Einsteinian prediction of the bending of light is actually a confirmation of the curvature of space predicted by Einstein. You should read about Einstein and Eddington and the extent that Eddington, Dyson, and Crommelin had to go to in 1919 to see the effect of light bending around our sun. So, light bending inside your camera is something you will never see.
You said, and I quote “And discussion of light that justifies some light property by saying that light "bends" is automatically incorrect. Period.” That is inaccurate, period.

But that was simply referring to life in general, now you limit that statement to “ So, light bending inside your camera is something you will never see.”, of course this again is false. What effect is there on light when it goes from one medium to another? Say, air to a piece of glass, or two pieces of glass with different refractory properties? The kind of stuff you’d find inside a lens..... It bends.

A3BE46C6-3FA7-47F7-A3CA-FBF608CA2FFE.jpeg
 
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dilbert

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To me, the only question which really matters is:
Will an f.16 picture taken with a 90MP camera be VISIBLY better than the same picture taken at f.16 with a 30 MP camera?

How will you be looking at the image?

90MP reduced to HD screen size (1920x1080) is going to be much the same as 30MP reduced to HD.

90MP will give you more room to crop to get that 1920x1080 but few crop.

90MP will give you bigger prints at the same DPI, but few print.

If you zoom in for some 1:1 pixel peeping, then what you find will depend on the interpolation algorithm use by the raw image conversion that tries to work out what color a given pixel should be based on known sensor pixel size, whether it is red/green/blue being processed, focal length, neighbors, etc.
 

Joules

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To me, the only question which really matters is:
Will an f.16 picture taken with a 90MP camera be VISIBLY better than the same picture taken at f.16 with a 30 MP camera?
DLA of the 90 MP camera should be around 4,5, if I'm not mistaken.
Any thoughts would be welcome!
They will look identical when you view them at full size. Once you start zooming in (the same amount on both images), the lower MP one will start to look worse, as it will show pixels and debayering artifact or moire more easily.

The sensor resolution does not affect how apparent diffraction is. It just is one of the two limits to the detail you can capture - the other is the diffraction, which is a property of the lens (+ settings).

The 'concern' about using a high MP sensor and a narrow aperture is simply that you aren't getting the full amount of detail the sensor on its own is capable of. But in comparison to a lower MP sensor, that is absolutely not a disadvantage, as that sensor does not have the capability to sample as much detail in the first place.
 

privatebydesign

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To me, the only question which really matters is:
Will an f.16 picture taken with a 90MP camera be VISIBLY better than the same picture taken at f.16 with a 30 MP camera?
DLA of the 90 MP camera should be around 4,5, if I'm not mistaken.
Any thoughts would be welcome!
Yes, it will.
 
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