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[OM] Ugh! It's that pesky diffraction stuff...

Subject: [OM] Ugh! It's that pesky diffraction stuff...
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>
Date: Fri, 21 Nov 2008 08:55:48 -0500
In a really long and very technical article from Luminous Landscape 
titled "Do Sensors “Outresolve” Lenses?" the cut-to-the-chase answer in 
in many cases is... yes.  The reason is the diffraction limits of lenses 
at various apertures.  You can read the entire, nerdy article here
<http://www.luminous-landscape.com/tutorials/resolution.shtml> or just 
skip to the conclusions which can be found in table 3 near the end.

The table lists different sized sensors from four thirds to medium 
format and, under each format, three different wave lengths of light 
ranging from red to blue with green-yellow light in the center column at 
0.55 microns wave length.  Down the left side of the table are various 
apertures.  The larger the aperture the less the diffraction (and 
ignoring other pesky optical problems) the greater the resolving power. 
  The data in the table are the maximum number of megapixels that can be 
resolved with that sized sensor, wave length and aperture... and this is 
key... using a perfect, diffraction limited lens.  Since none of us own 
any of those the implication is that our real world results will be 
somewhat less than the theoretical maximum.

Since green-yellow at 0.55 microns is where we see best that's a good 
color to choose.  The implications then are as follows.

If, like me, you own a 12.7 MP Canon 5D you can scan down the table for 
35mm sized sensors under 0.55 microns and see that the maximum 
theoretical resolution at f/11 is 16 MP.  Since that's for a perfect 
lens, in all probability my real world lenses won't do that well but 
maybe I'm still OK at 12.7 MP at f/11.  But going to f/16 is a no-no for 
best resolution since the theoretical limit drops to 7 MP at f/16.

If you own a camera with an APS-C size sensor (most of the Canon, Nikon, 
Pentax, Minolta/Sony DSLR market) at f/11 the theoretical limit is 7 MP. 
  You have to limit the aperture to f/8 in order to reach a theoretical 
limit of 13 MP which will maybe cover those 10-12 PM sensors with real 
world lenses.  Even a Canon 20D's 8 MP sensor isn't fully resolved at f/11.

And finally we have the four thirds sensor where the pixels are getting 
even smaller.  The Olympus E-1 at 5 MP is OK up to f/8 but for newer 
cameras like the 10 MP E-3 you should be limiting lens aperture to f/5.6 
for maximum resolution... assuming your lens performs well at f/5.6.

Remember the Olympus exec's question about using OM lenses on the newer 
four thirds cameras:  "Why you want to use old lens on new camera?"  If 
he had explained about resolution limits and that many 35mm lenses 
perform best at f/8 or f/11 we, perhaps, might have understood better.

Of course, resolution is hardly the only characteristic of a good lens 
but when you've got that camera mounted on a tripod and shooting some 
landscape or architectural subject for maximum detail remember this 
advice on how to set (limit) the aperture.

Chuck Norcutt
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