200mm f/2.8L II oval aperture

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My new 200mm f/2.8L II EF USM lens has an oval aperture. Looking through the business end of this lens while holding down the DOF preview, I stop it down to 32 and I can see that the aperture is oavl shaped (slightly rounder than an egg). As I open the aperture, the oval becomes less and less pronounced until it appears round; the oval actually becomes indistinguishable around f18. I have shot two rolls of film with this lens and images are great, but I have yet to shoot anything smaller than f16.

My 50/1.4 and 100/2 have round apertures at f22 (as round as they can be with straight blades forming the 'circle'). Is the oval a problem? Should I be concerned about image quality at high f-stops?

-- Derrick Morin (dmorin@oasisol.com), July 26, 2001

Answers

It won't cause any optical problems. The only possible problem it could cause would be a small exposure error if the area of the aperture wasn't what the camera thought it should be. You can easily test that of course by shooting at f16, f22 and f32.

I wouldn't shoot at f32 unless you REALLY need the DOF since shapness will suffer significantly due to diffraction.

-- Bob Atkins (bobatkins@hotmail.com), July 26, 2001.


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