Diffraction Limit Discussion Continuation

Started Feb 21, 2014 | Discussions thread
Anders W Forum Pro • Posts: 21,468
Re: Diffraction Limit Discussion Continuation

Jonny Boyd wrote:

). ) Anders W wrote:

Jonny Boyd wrote:

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.

Yes you did. Do you want me to look up the specific posts where you denied it or are you going to acknowledge it voluntarily?

There was a post (Re: Nope.) where I said that peak resolution for a lens affected only by diffraction, would be wide open. Other optical effects would limit that. The lens itself will have an inherent peak aperture and I made that point that as you increase the aperture and the resolution of the lens drops, cameras become less and less able to take advantage.

The way I described was over simplified because I forgot to take into account the fact that you do still gain advantage from having a sensor resolution higher than the lens resolution. If you had three sensors with resolution s_1 > s_2 > s_3 then when the lens is stopped down to reduce the resolution to about 3 * s_2, then you get no meaningful advantage from using s_1 instead of s_2, though you're still better using them than s_2. Stop down so that the lens resolution is about 3 * s_1 and there's no meaningful difference between the resolution of the sensors. That's due to diffraction, so that's the practical limit imposed by diffraction if you're trying to decide whether it's worth using a higher resolution camera or not. That seemed to me to be the practical point of asking where diffraction causes a limitation.

As I said, in that particular post I forgot to account for higher resolution sensors being useful up to a point and left out the factor of 3, but otherwise the broad point I was making was still correct.

In this post (Re: So, what are the m4/3 diffraction limits?) I said that for a sensor with much loser resolution than the resolution of the lens, then there would be a plateau of sharpness, rather than a peak. I've modelled that and put on a chart which demonstrates exactly that. I'm guessing the disagreement here is over the definition of peaks and plateaus, so I probably should have clarified that I wasn't talking about a perfectly flat sharpness, but rather a peak is so spread out, with minimal drop across multiple f-stops, certainly much flatter than the curve for higher res images, that it's virtually indistinguishable from flat. If system resolution only drops 1% between peak aperture and f/22, then that's a plateau compared to a system where resolution drops 40% between peak aperture and f/22.

In retrospect I should have taken more time to spell out exactly what I meant by plateau.

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.

No you didn't say that. You said the peak would occur at different apertures depending on sensor resolution (just as Cambridge in Colour). Do you want me to look up the specific posts for you?

I havent read everything on the CiC site, but the bit I agreed with, which I thought was in dispute, was when diffraction begins to degrade detail, which is a different discussion to the question of when it limits you so much that you may as well use a lower resolution camera. CiC is right that diffraction becomes noticeable at wider apertures for higher resolution sensors and therefore degrades their detail sooner. As I've repeatedly said in many posts, that's not the same as saying that those higher resolution cameras don't still have a detail advantage.

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.

Look! A number of us took time to teach you (I don't apologize for the expression) what things are actually like.

Your first reply to me basically ignored the length post I'd written to try and work out where disagreement originated, and instead of engaging with any of the points, you basically said 'Here's a formula that shows why you're wrong. Go work out why.'

What I actually said was:

"If you work out the implications of the formula, you'll see that Bob is right. Let me know if you have any questions/problems."


Maybe you didn't mean it that way,

No, I certainly didn't mean it that way. I simply took you on your words. You said you had a degree in physics so I assumed you were familiar with formulas and in a position to work out the implications for yourself (as eventually, but not initially, turned out to be the case). Math is there for a reason and for those who are reasonably familiar with the language of mathematics, it is a far more precise and less ambiguous way to get the message across in cases like the present than verbal expressions.

but it came across as rather arrogant with the lack of engagement and disinclination to explain things.

See the offer to help out with any questions/problems in my initial response quoted above. Moreover, I immediately responded to your request for further information in the form a point-by-point response to the argument you had outlined as well as a first description of the implications. I did so in spite of the fact that your initial reply was anything but gentle. See here:


We were rewarded by all sorts of insults.

Really? Where? I suggested that your first reply was unhelpful for reasons that I've outlined again here, and elsewhere suggested that you didn't understand diffraction and disagreed with the laws of physics (Re: So, what are the m4/3 diffraction limits?). In retrospect that was probably a misunderstanding over the word limit. I was talking about diffraction limiting the amount of detail that could be resolved while you were saying that higher res sensors would always improve detail. I thought you meant that you could get more detail than what diffraction effects limit you to i.e. the system resolution could exceed the diffraction-imposed lens resolution, but presumably you actually meant that you could always get closer and closer to that limit? Similarly you thought I was saying that higher resolution sensors stop adding detail at a point, when I meant that there's a level of detail that no sensor can exceed and the gains you get, at a certain point, are so negligible that you can ignore them.

In so far as I may have misunderstood you or failed to express myself clearly, I apologise.

OK. You did misunderstand and you did fail to express yourself clearly but the apology is accepted.

Now we are somehow made up as the guilty party just because you not man enough to stand up and say you made a mistake.

I've stated quite clearly several times now that I regard the problems in the previous thread as largely being down to misunderstanding and had no interest i pointing the finger at anyone, preferring to make a fresh start. You're the one who thought it necessary to point the finger and assign blame, so I find your comments here rather ironic.

Not recognizing that you are wrong when you are implicitly puts the blame equally on both sides alike (and I certainly don't think that's appropriate in the present case). This in turn makes it more difficult to accomplish what you say you wanted: a "fresh start".

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.

For pretty obvious reasons.

Yes, the reason I stated: I would rather move forward with the discussion and add to the forum, rather than dig up the past and waste people's time. I'm sure other comemnters would rather read about photography than our personal disagreements, so can we get back the actual discussion?

No problem with that.

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