Diffraction Limit Discussion Continuation

Started Feb 21, 2014 | Discussions thread
Jonny Boyd Regular Member • Posts: 105
Re: Diffraction Limit Discussion Continuation

Great Bustard wrote:

Jonny Boyd wrote:

It tells you plenty. You have made a universal claim that diffraction always causes peak sharpness at the same aperture, regardless of pixel count. While I agree that that is mathematically correct...

Not merely "mathematically correct", but supported by all the lens (system) tests.

My point was that this isn't always experienced in practice if the difference cannot be perceived. I'm not disputing that the peak is there, I'm just disputing whether it always makes a visible difference in an image, so potentially, as far as the eye can tell, there can be a plateau in resolution instead of a peak. It's like looking at a piece of ground and saying 'it looks flat,' then someone comes along with a laser and tells you where the highest point is. There might actually be a peak, but for practical purposes, there may as well not be.

I'm not making any claims about which lens/camera combinations this well apply to, I'm just saying that potentially this can happen.

...I also believe that a drop in resolution will in some cases only become noticable at a smaller aperture for a low res sensor than a large one.

When the drop in resolution becomes apparent depends on many factors, not the least of which is how large you display the photo.

Obviously. I'm dealing with a situation where you print at the same size and view from the same distance so that we can simply determine whether pixel count makes a difference to when diffraction limits resolution, when all else is equal. Hasn't that been the stated assumption in plenty of people's posts: 'all else being equal'?

In any case, we also all agree that the sensor with more pixels will have higher resolution, all else equal.

I don't think anyone has ever suggested otherwise so I don't know why it keeps bring brought up.

Thus, since a given lens peaks at a particular aperture regardless of the pixel count of the sensor, and a sensor with more pixels will always resolve more than a sensor with fewer pixels (all else equal), then in what sense does "diffraction limit" have any meaning,

In the post you've just replied to, I'm discussing the issue of where diffraction begins to visibly limit resolution. Mathematically (used as an antonym of 'visibly') diffraction always limits resolution at the same aperture for a lens, regardless of sensor pixel count. But the difference in resolution between apertures may be so small as to not be visible, meaning that diffraction doesn't visibly limit resolution until lower apertures than the mathematical peak.

or than how Bob characterized it:


The 'limit' is just a bogus idea. McHugh has taken a well defined optical term - a 'diffraction limited' system is one so good that diffraction is the only limit on its performance - turned it inside out and made it into something senseless.

Sometimes Bob talks nonsense. Or disagrees with someone without justifying why.

For instance he insists that neither the lens nor the sensor limit the resolution of the final image, implying that using a better sensor always gets a better image. The reality is that the lens and sensor both put limits on the resolution of an image. Increasing the resolution of one allows you to get progressively closer to the limit of the other, but never to exceed it.

That's all I'm claiming.

In what way are sensors with more pixels any more "diffraction limited" than sensors with fewer pixels? That when viewing at 100% on a computer monitor you can see the resolution fall faster from the peak aperture, even though the peak aperture is the same, regardless of the pixel count, and the sensor with the higher pixel count has greater resolution?

That's effectively what I've said many, many times. And it's not just at 100% on the computer monitor. It's reality.

If I can find one instance where numbers demonstrate it, then I am correct.

"A number multiplied by itself is always the same number. For example, 1x1 = 1." So because I found a single instance where numbers demonstrate the claim, does that make the claim correct?

You've got things backwards. You're making the universal claim. My claim is that there is an exception to the universal claim. If I find an exception, then I'm right.

If I said "A number multiplied by itself is always the same number' and you said 'no, there are exceptions,' then you'd only need one example of an exception to be proved correct and disprove the universal claim.

The universal claim I'm disproving is that peak visible resolution always occurs at the same aperture.

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