Diffraction Limit Discussion Continuation

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

Anders W wrote:

Jonny Boyd wrote:

there,s nothing there that I hadn't already said to you in other ways.

There most certainly is: The recognition that the point along the aperture range where peak image resolution occurs is independent of sensor resolution.

I never denied that. I said that at low resolutions it's more of a plateau that a peak, so you effectively get the same resolution at smaller apertures.

Substantively, I have only two comments: That peak sharpness will occur at exactly rather than approximately the same aperture and that "my/our" side is hardly the one to blame for any conceptual or terminological misunderstandings.

Anders, I avoided assigning blame to anyone and put it down to misunderstanding.

Yes I saw that. So I pointed out what was missing.

Don't be in ass in response.

I am not being an ass. You decidedly are by calling me one for absolutely no good reason.

You felt it necessary to assign blame and point fingers when I had hoped the conversation could have a fresh start.

If you substantively agree then you'll also agree that when the sensor resolution drops below a certain level, the aperture size won't perceptibly reduce the system resolution in the same way as a system with a sensor resolution similar to the lens resolution. And once you get to a high enough sensor resolution, diffraction overwhelmingly determines system resolution so that the whole system is effectively diffraction limited from wide-open.

What I substantively agree with is the following:

When the sensor resolution is much lower than lens resolution, variations in lens resolution will have but a small impact on image resolution (but still always such that increased lens resolution leads to increased image resolution). When it is the other way around (lens resolution much lower than sensor resolution), variations in lens resolution will have a much stronger impact on image resolution.

I tried to put that to you earlier, but you seemed dismissive of the idea. I'm unclear about whether that is still the case..

Where was I dismissive about the idea as I spelled it out above? Please provide specific references (the post/posts you have in mind and the passage/passages in those posts).

I'm not interested in dissecting the previous discussion. I'd rather go forwards than backwards. What I said before and think is most relevant to the present discussion, is that lower resolution sensors have more of a plateau than a peak, so you get the same resolution effectively at lower apertures as you would at the theoretical peak resolution aperture. Therefore a low res sensor isn't diffraction limited at the same aperture as a high res camera.

Let me illustrate what I mean. Here's a hypothetical lens attached to a number of different hypothetical sensors covering a large range of resolutions. The units for resolution are the same throughout and the table below calculates the systems resolution for each aperture for each sensor.

When we plot the data on a chart, it looks like this:

The highest resolution sensors are virtually indistinguishable from the lens resolution. Below is a table showing what percentage of lens resolution is reached at different apertures by different sensors.

With the low res sensors, resolution is pretty flat across the range. The table below shows the percentage of sensor resolution used at each aperture by each sensor.

If we looked at the numbers with a high enough precision, we would indeed find peak resolution at the same aperture for every lens. In practice though, there is no noticeable difference in resolution at any aperture for the lowest resolution sensors. With high res sensors, you'll gain sharpness by using the peak aperture, but as sensor resolution decreases, you get less and less advantage from the peak aperture and suffer less of a penalty for stopping down.

Here's one last table that shows the % of peak resolution that you get at each aperture:

If we said that a resolution difference of 0.1% was the limit of perception, then for all practical purposes, peak resolution is indeed a plateau that stretches over several apertures for smaller sensor sizes.

Mathematically of course, there is always a peak aperture independent of the sensor. And in practice, the relationship between lens and sensor resolution for a system may be such that no combination of lens and sensor produces a plateau.

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