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

Subject: Re: [OM] Ugh! It's that pesky diffraction stuff...
From: "Jez Cunningham" <jez@xxxxxxxxxxxxxx>
Date: Fri, 21 Nov 2008 15:12:55 +0100
So if we really should keep to f/5.6 or f/8 we should all be out buying ND
filters for those bright days...

On Fri, Nov 21, 2008 at 2:55 PM, Chuck Norcutt <
chucknorcutt@xxxxxxxxxxxxxxxx> wrote:

> 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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