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11-03-2011, 01:53 AM   #16
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Hi!

I think that the formula cited below might be quite appropriate to calculate CoC if the photos are viewed only on computer displays.

CoC = (display.pixel.size) × (sensor.width / display.width)

For me the most complicated parameter is the display pixel size. I do not know the size of pixel for modern monitors. I presume that this parameter can be derived form such an indicator as pixels per inch (PPI) or pixel density. According to Wikipedia, PPI was about 85 for monitors capable of a maximum 1024×768 (or XGA) pixel resolution. Modern monitors are capable of higher resolutions, for example, 1280×1024. By assuming PPI is about 85, the pixel size is about 0.2886 mm. In some specifications of monitors I found out the parameter “size of point”. For example, the size of point is 0.294 mm for my work’s monitor. I presume that the size of point is the same as the size of pixel. So it comes out that the size of pixel is higher than 5 lp/mm or 0.2 mm that is used for standard calculations of CoC.
I tried to carry out another calculation of CoC based on pixel size and monitors size. I assumed the pixel size about 0.289 mm. I did not have information about the monitor’s with. So I used diagonal. The diagonal of the monitor was assumed 19” or 482.6 mm. The Pentax APS-C senor’s diagonal was assumed 28.3 mm according to my previous calculation. So:

CoC = 0.289 × 28.3 / 482.6 = 0.017 mm

The calculated CoC is lower than the standard one – 0.019 mm or 0.02 mm.

There is also an option to view a photo not in full screen but in so-called life size (100% zoom). That means the photo’s dimensions will be higher than the monitor’s dimensions. I am not sure how calculate CoC for such a situation. I presume that in this situation one monitor’s pixel corresponds to one photo’s pixel. If this assumption is right, then the enlargement will be equal to:

Enlargement = display pixel size / senor pixel size

If neglect changes in viewing distance, then the formula for CoC might the following:

CoC = display pixel size / enlargement

or after simplification:

CoC = sensor pixel size

My Pentax K-x has a sensor with 4288 × 2848 pixels. The sensor size should be 23.6 mm × 15.7 mm. So the sensor pixel size is about 0.0055 mm. So the CoC will be 0.0055 mm, and the enragement will be 52.47 (0.2886 / 0.0055). This CoC is much lower than standard one. However, it refers to quite extreme case – viewing photo in life size zoom (100% zoom).

I carried out some DOF calculations by using this “new CoC”. For focal length 18 mm and aperture F4.0 the hyperfocal distance is 14.75 instead of 4.07 m (if CoC 0.02 mm is used). The difference is very huge. I presume it will be even much larger for telephoto focal lengths.

I will highly appreciate comments about the “new CoC” formula. I am not fully sure that it is correct and that I have not omitted something significant.

11-03-2011, 03:28 AM   #17
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QuoteOriginally posted by Alberts Quote
Hi!

I think that the formula cited below might be quite appropriate to calculate CoC if the photos are viewed only on computer displays.

CoC = (display.pixel.size) × (sensor.width / display.width)...
Your conclusions are pretty much correct except that a computer monitor and sensor have different aspect ratios. Your calculated numbers will be more consistent when you take the proper aspect ratios into account. Typical display pixel size is around 0.25mm, 0.01".

Displaying at 100% crop maps each sensor pixel to a corresponding display pixel so in a sense is the maximum practical image enlargement.
11-03-2011, 04:14 AM   #18
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Diffraction limit & CoC

QuoteOriginally posted by newarts Quote
Your conclusions are pretty much correct except that a computer monitor and sensor have different aspect ratios. Your calculated numbers will be more consistent when you take the proper aspect ratios into account. Typical display pixel size is around 0.25mm, 0.01".

Displaying at 100% crop maps each sensor pixel to a corresponding display pixel so in a sense is the maximum practical image enlargement.
CoC plays an important role in identifying the f-stop beyond which diffraction softening is observable.

Diffraction effects become visible in a display when the diameter of the Airy Disk is greater than a display pixel.

Diameter.Airy.Disk = 2.44 (f-number) (wavelength)

For green light and a displayed image this becomes:

f-stop.diffraction = 3/4 [Display.pixel.width.micrometers (sensor.width/display.width)]

For example, with a display pixel of 0 .25mm and a display width of 10", diffraction will begin to show at an f-stop of:

f-stop.diffraction = 3/4 [250 (1"/10")] ~ f:18

If this image were enlarged by a factor of two (displayed at 20" on a 0.25mm pitch display), diffraction softening would become visible at about f:9.

Last edited by newarts; 11-03-2011 at 05:52 AM.
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