Diffraction Limit Discussion Continuation

Started Feb 21, 2014 | Discussions thread
bobn2 Forum Pro • Posts: 57,704
Re: Cutting to the chase.

Jonny Boyd wrote:

bobn2 wrote:

Therefore the relative drop in resolution is great for a higher resolution sensor than a lower resolution one.

You do like wasting your own time. I don't think anyone has disputed that would be the case.

You seemed to be highly sceptical of everything I've previously posted.

I've been very sceptical of the methodology of your 'proof','demonstration', whatever it was. What I've said is that it doesn't prove anything, not least the thing you think that it proves, which is that the f-number of the peak moves with pixel size. That doesn't mean that some of the random assumptions that you built in don't accord with reality.

Therefore the absolute drop is also bigger.

Also probably true.

Then I don't understand why you kept disagreeing with me.

I only disagreed with the bits where you were clearly wrong, or your line of reasoning made no sense or your method for demonstrating the 'truth' was nonsense. I have a strong aversion to bogusly quantitative demonstrations, because they fool a lot of people into thinking they're based on reality. McHugh's 'diffraction limit calculator' falls into exactly the same category.

The final implication of this is that as sensor size decreases, the drop in resolution when stepping down constantly decreases.

Pixel count, you probably mean. Best not to complicate things with different sensor sizes at this point.

I meant pixel count, yes.

Given that human eyes have finite ability to distinguish resolution,

That statement is very ill formed, so much so as to be meaningless. What do we mean by 'resolution' and how does one 'distinguish' it? Best to stick with measurable things rather than make unevidenced and undoubtedly simplistic assumptions about human perception. What, for instance, is the role of acuity in all this? Is MTF50 a good model for perceived resolution?

How exactly is it 'ill-informed'?

I said and meant 'ill formed', not 'ill informed'.

It's a general statement about the ability of the human eye to resolve detail, saying that it has limits.

Now you say 'detail', then you said 'resolution'. Which do you mean? Again, when you say 'finite' you are suggesting something quantitative - that 'resolution' or 'detail' could be measured, and there is some result which says that below some limit of this metric, the eyes cannot resolve. So, which do you mean, how is it measured and where is the perceptual evidence o back up what you say?

I'm trying to establish what the limits are, merely that they exist.

You haven't even done that. Clearly the human visual system has its limits, but so far you have given no information about their nature. I raised the word 'acutance' in a previous post. Is this important? Does the perception of 'detail' or 'resolution' depend on light levels? Does it depend on the type of image? Human perception is a complex thing, and the existence of a simple limit, where you could say that at some point something is invisible based just on an MTF50 measurement, is not a given.

It would be ill-informed to suggest that the human eye has no limit on its ability to perceive detail or differences in resolution.

No-one said 'ill informed'. But it is indeed 'ill informed' to assume that human perception has very simple limits that can be predicted with a metric like MTF50, especially when there can be such variety in viewing conditions, which have not been stated.

there will come a point where the resolution at an aperture lower than the peak aperture will be indistinguishable from the resolution at peak aperture to the human eye.

However, I would hazard a guess that is probably true.

Why guess? I did the maths for you. I've outlined the logic. Why guess?

The maths was done based on fictitious figures, the logic was fallacious - therefore your exercised shows nothing. That doesn't mean that some of the results you were purporting to show don't stack up in the real world.

And if you agree, why did you keep trying to tell me I was wrong?

I told you that your experiment was bogus beginning to end, see above about its relationship to real world results.

So what? When the image is degraded so much that you can't distinguish between one f-number and another, are we really interested at all?

Without looking at actual examples that's an entirely speculative question.

Take it as a hypothetical. Does your running the numbers show you just how low a pixel count a camera would need to take diffraction out of the picture? Is that a pixel count most serious photographers would be content with?

I was merely establishing that the principle that at some point resolution becomes too low to distinguish between the peak aperture and another aperture.

You just had to say it - that isn't controversial.

Remember, the decision we're trying to inform here is the trade between DOF and resolution. Just thinking about that trade presupposes a concern for image quality which certainly won't be satisfied by a system so un-diffraction-limited that diffraction has ceased to be a player in resolution at any f-number (and interestingly, this 'diffraction limited' you're talking about has become precisely the reverse of what it means in an astronomical or microscope context).Let's just be clear, as camera with those properties is a dreadful camera, one no serious photographer would want.

I've established a principle,

Not at all. The 'principle' you established - that you can reduce pixel count to the point where diffraction is the least of your worries - was pretty obvious to everyone. But that wasn't the 'priciple' you purported to be establishing, which was that there would be a 'plateau' which would effectively move the peak resolution. You never established that.

I haven't run numbers to say whether such a camera would always be dreadful. To a extent that's a subjective statement.

To an extent Let's go back to your own numbers. We seem to get a pretty flat line when the 'sensor resolution' is 30. 30 what, you don't say, but we can make an estimate, because the lens resolution for the lens at f/22 is 60 of whatever they are. f/22 in practice is usually diffraction limited and seems to give an MTF50 of 30 lp/mm on lenstip's methodology (http://www.lenstip.com/392.4-Lens_review-Olympus_M.Zuiko_Digital_12-40_mm_f_2.8_ED_PRO_Image_resolution.html). So '60' means 30 lp/mm, which means that '30' means '15'. So we're talking about 15 lp/mm or 30 pixels/mm, or 200k pixels on a mFT sensor. Do you think that is a resolution anyone here is seriously going to aim for?

That's exactly what I've said with words, illustrated with numbers and charts and have proved with numbers.

You've 'proved' nothing with numbers - working fictitious numbers cannot prove anything. Working fictitious anything can't prove anything.

The numbers illustrated the exact mathematics I did above. If the mathematics you seem to now agree with are correct,

I don't agree that the 'mathematics' are 'correct'. As i said, you used a sometimes useful approximate formula, ran some unrealistic numbers with it, and came out with a bogusly quantitative result which did no more than show what was obvious before you did it. You wasted your time and brought nothing new to the discussion. The fact that I agree that the obvious was all along obvious in no way validates your 'mathematics'.

then the numbers are examples of what resolution looks like for certain combinations of sensor and lens. If you took a sensors with some of those resolution, and a lens with that resolution, you would get those results, or something close enough since the formula is only a rule of thumb.

200kP is your measure of quality? Why, exactly, do you have an interest in mFT equipment?

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