*D 300 F/11 diffraction too limited?*

Started May 16, 2008 | Discussions thread
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bobn2
bobn2 Forum Pro • Posts: 48,878
Re: *D 300 F/11 diffraction too limited?*

Dear Hide,

Diffraction causes the image projected on the sensor to get blurrier as you close down the aperture. The visible effect depends on how much you magnify the image to view it. How much blur is acceptable to you depends on your style and standards, but you'll get sharper results if you magnify less, and you'll tend to magnify less using a camera with a big (FF or MF) sensor. The 'diffraction limit' quoted by Thom Hogan and others is a very misleading figure, what it actually means is the point at which the diffraction in the lens is limiting the sharpness of the image rather than the pixel density of the camera. The resolution at this limit is the most resolution that camera will ever have, high pixel density cameras are diffraction limited at lower apertures simply because they can capture more detail, so the lens diffraction limits them earlier.

To help people understand the practical effects, I worked out the following table, which shows the maximum resolution (in dots per inch) available in an A4 print for different cameras. Sorry for the nasty formatting, I hope you can make it out.

Cam~ D40, 20D, 40D, D200, D300 ~ 1DII, 5D, D3, 1DIII, 1DsII, 1DsIII
f/2.8 ~ 260, 301, 344, 347, 378 ~ 299, 372, 376, 343, 432, 477
f/4 ~ 245, 266, 290, 297, 313 ~ 286, 355, 359, 314, 395, 428
f/5.6 ~ 219, 234, 253, 260, 272 ~ 257, 319, 322, 279, 351, 377
f/8 ~ 189, 199, 212, 218, 227 ~ 223, 277, 279, 239, 300, 319
f/11 ~ 161, 167, 177, 182, 188 ~ 191, 237, 240, 203, 255, 268
f/16 ~ 129, 132, 138, 143, 147 ~ 154, 192, 194, 162, 203, 212
f/22 ~ 105, 105, 109, 113, 116 ~ 125, 156, 157, 130, 163, 169

A few things about this table:

  • You can chose what your minimum acceptable resolution is, many would say 300 dpi, I would say something less than that.

  • If you want another print size, simply divide by the ratio of that size to A4 (linear), so for instance for A3, divide these by 1.414.

  • If your camera's not in the list, you need to choose one with the same sensor size/pixel count. So, for instance the D40x and D60 are the same as the D200.

  • It assumes perfect lenses, in which the definition is entirely diffraction limited. In practice real lenses aren't as sharp as they should be at wide apertures.

  • You'll find it doesn't agree with normal 'diffraction limit' calculations. That's because they ignore the effect of the anti-alias filter. This limits the resolution of any image projected on the sensor by 'spreading out' each point of the image. It does this even to images which are already limited by diffraction (it's not really a 'low pass filter', more a 'muffler', so its effect needs to be added in.

Details about my calculations are here: http://forums.dpreview.com/forums/read.asp?forum=1021&message=27453566

I hope that helps.

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Bob

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