I just received an EL-Nikkor 80mm f/5.6 . Unfortunately there is a small fleck of dirt in one of the elements. The seller has a return policy if I don't like it.

What is the best way to test the lens to see if it affects the image?

Thanks Roger

-- Roger Bird (rpbird@home.com), November 23, 2001

Answers

I can safely say that if it's small piece of dirt (< 0.25 mm dia) it will have no observable effect on the imaging quality. Any contribution it will have will be proportional to square of the ratio of the diameters of the dirt to the element it's sitting on. ie. if the element is 5 mm the dirt may reduce light transmission by (0.25/5) ^2 = -0.25%. Any diffraction or reflection effects will be similarly reduced in effect.

The dirt will have a bigger effect on the resale value. If this is a problem then you would be better off returning it. BTW, there's a good chance that if you turn the lens on it's side and lightly tap it, you could dislodge the particle and it could stick to the mounting ring instead of the element.

Good luck & cheers,

Duane

-- Duane K (dkucheran@creo.com), November 23, 2001.

If you're handy- grip the very small lip where the lens cap goes and unscrew it (hold the lower mount section, not the aperture ring). Use a piece of rubber if it's too slippery. The front section of the lens will come off, exposing the aperture area in the middle. Use a ear syringe or canned air (carefully) and blow off the offending speck of stuff. Screw the front back on and tighten securely. You aren't removing any individual elements and there's no risk of messing up alignment or decentering, just don't drop anything or take it apart any further. I'm holding one in my hand and just did this sitting at the computer, but proceed at your own risk! Or leave it alone- it won't affect anything unless it's huge.

-- Conrad Hoffman (choffman@rpa.net), November 23, 2001.