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Lens potential comparison calculator
Posted By: beholder3, 05-13-2012, 11:47 AM

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Last edited by beholder3; 11-01-2012 at 05:11 AM. Reason: Updated for v2.5 of spreadsheet, direct acces to some more constants
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05-13-2012, 12:42 PM   #2
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Looks good! I haven't read it in depth yet, but my first reaction is: You should account for variant frame sizes. It seems the choices are FF or APS-C -- but is that nominal APS-C, with a frame diagonal of 30.1mm, or the actual frame size of something like my K20D, with a diagonal of 28.1mm? That 9% difference would impact the following calculations.
05-13-2012, 02:55 PM   #3
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Last edited by beholder3; 08-11-2013 at 07:33 AM. Reason: [deleted]
05-16-2012, 11:47 PM   #4
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Last edited by beholder3; 08-11-2013 at 07:33 AM. Reason: [deleted]
05-17-2012, 05:51 AM   #5
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This is really interesting--thanks and congrats!

QuoteOriginally posted by First Poster Quote
Language and wording might be more precise and scientific
While I understand the value of technically precise wording, there is also great value in using language the rest of us can understand. I hope, as you edit for precision, you will retain (or include) language which make this accessible to those of us with limited technical background.
05-17-2012, 06:41 AM   #6
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Last edited by beholder3; 08-11-2013 at 07:32 AM. Reason: [deleted]
05-17-2012, 09:33 AM   #7
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Beholder3,

Nice work! It looks complete & easy to use - but the number of choices may be daunting for novices - who often mistakenly believe they must know everything to do anything.

I didn't notice an explanation of what you mean by "Relative Blur" what is it? I see by looking in the results cell it is the Diameter of the blur circle/CircleOfConfusion? That seems to me to be a good measure!

(I've recently been working on the blur problem so am very interested in your work.)

I see that relative blur appears to be correct in hyperfocal conditions ( < 1), but under those conditions I got an error for "Total circle of confusion considering diffraction effects". The error was taking the sqrt of a negative number.

I don't understand what you mean by total CoC being related to the SQRT(difference between CoC and airy disk areas). Perhaps the negative sign is an error? Wouldn't the effective CoC be the greater of the two?

Dave in Iowa

Last edited by newarts; 05-17-2012 at 09:39 AM.
05-17-2012, 11:01 AM   #8
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Last edited by beholder3; 08-11-2013 at 07:32 AM. Reason: [deleted]
05-17-2012, 04:10 PM   #9
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QuoteOriginally posted by beholder3 Quote

In my cases the airy disk is always smaller than the maximum tolerable CoC. The formula is meant to calculate a resulting max tolerable CoC by subtracting the airy disk's area from the original CoCs area.

The whole diffraction effect thing also was part of a huge debate on a German forum. There was no real consensus how to calculate it. The current subtraction concept to me seemed to be the best fitting - some other guys calculated it slightly differently. Same with the constant of 2,44 ("2,44 is the diameter of the middle maximum of the bessel function until its first zero point"). After reading through lots of page on this I just took and used it. The guys on the forum at least didnt object to the formula I proposed (and which is now foudn in the spreadsheet).
If you want to delve deep into this stuff, I suggest you take a look at Depth of field and diffraction at the bottom of the page. You need to use some Bessel functions. At the latest that was when I got lost.
I'm pretty familiar with these topics.

I can't see that subtracting the Airy disk's area from the CoC has useful meaning. CoC is by definition the display area too small to resolve within (reduced in size by the ratio of image:display.) Therefore, referred back to the display, calculated CoC or Airy disk, whichever is greater should be used as the blur. Simply put, if the Airy disk is larger than the CoC, that's what will be shown on the display. Some function like MAX(Airy, CoC) might be useful.

CoC is a parameter derived from the characteristics of the viewer and display. It seems to me that all your calculations could be made without reference to CoC by starting with the diffraction disk diameter - ie independent of the viewing system.

Much of the previous work I've seen and done on out-of-focus point diameter has used the lens' absolute aperture as an important parameter because at infinity the image of the disk on the image plane is (subject magnification * aperture diameter)

I like your approach better. Best might be to derive it all without reference to CoC until the very end.

Dave in Iowa
05-18-2012, 12:38 AM   #10
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05-18-2012, 06:44 AM   #11
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QuoteOriginally posted by beholder3 Quote
In the classic way yes. And I prefer that. But today people are pixelpeepers and magnify pictures on screen to silly sizes. Then the assumptions on human eye resolving potential become meaningless. That's why the pixel version is included. The viewing distance assumtpion is not accepted by some.
I mostly use the display pixel size as the basis of the CoC. It is easy to explain and makes sense to novices. The sensor pixel pitch is not so useful but does represent a limit to display resolution. Clearly, the display defined CoC must be larger than the pixel pitch.

Dave
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