Aperture question, re: sharpness

Started Feb 26, 2014 | Questions thread
Guy Parsons
Guy Parsons Forum Pro • Posts: 30,327
Keep well away from f/22

a13 wrote:

Just a couple of musings that I'm posing to myself..

1. Is the absolute sharpest a lens can and will ever be, when it is stopped all the way down to its smallest aperture?

Most M4/3 lenses are at their absolute worst at f/22 - due to aperture diffraction effects. Best quality appears usually somewhere in the range wide open to f/8 for most lenses.

Follow-on from above,

2. If so, is that because it's using the very centre of the lens when it's stopped all the way down like this? Meaning that 1) will be true for each and every lens, without exception (well, except maybe ones designed to be sharper on the edges than in the centre...)?

Wide open most lens design and build compromises come into play and may have an effect on the image, stopped right down diffraction causes a very obvious drop-off in quality, so at some point in between the two there is a good sweet spot or good spread of apertures. Some M4/3 lenses are so good that they really are at their best at maximum widest open aperture.

In theory the highest resolution possible for a perfect lens always happens at maximum open aperture, but real life intervenes and it usually is some smaller stop where it is best overall. Though some M4/3 lenses do approach perfection....

Leading question in relation to above,

3. If a FF lens is stopped down to f/22, what is the f-stop on an m43 that would match the same exact PHYSICAL opening diameter of the FF lens @ f/22?

If a 50mm lens at f/22 then 50/22=2.72mm, same for any format be it pocket camera or 8 x 10 inch plate camera. Same at the other end so 50mm lens at f/2 = 25mm aperture size. Divide the actual focal length of the lens by the aperture to get the aperture diameter.

For a nice interactive display of lens performance, look at the lenses at http://www.slrgear.com/reviews/showcat.php/cat/7 (that's for the Oly lenses) and choose ones marked "(Tested)" and click on the Blur Index graph and play with focal length and aperture to see how the quality changes. The lower and flatter the graph the better. Also check out some full frame primes and zooms from Canon/Nikon and see how they compare for quality.

After you have checked a few of those you can see why f/4 may be more preferable to f/22.

Regards.... Guy

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