Mirror Woes

Mirror woes

I recently bought a used 500mm f/8 Sigma Mirror Ultra telephoto for a song. I know ,you get what you pay for. But for $65 it was worth the gamble. The lens looks like new. The problem is the image I see in the view finder is very sharp. But the film doesn’t show that at all. I shot a series using tree branches about 500 yards away on an angle to reduce the problem of depth of field, in other words something should have been in focus. The lens was mounted on my heaviest tripod which I use for my 4x5, the Olympus OM attached to the lens. With mirror up the was shutter tripped by cable release and exposure were 1/1000 and 1/500 Oddly enough the 1/1000 was the worst with the typical out of focus donut highlights, Obviously something moved. What mystifies me is I am not getting what I see. If the lens were of coke bottle quality I would expect to see that in the viewfinder .Just the opposite the image in the viewfinder is as sharp as my prime lenses which are pretty sharp Anybody have suggestions or experience with these long lenses?

I quit using my 500mm f/8 Nikkor mirror lens in favor of other brand refracting lenses. Being able to stop down after focusing was an important consideration. Focusing is critical in mirror lenses. SLR focusing screens may not be optimized for f/8. What appears sharp on any SLR screen may be quite unsharp in an enlargement. There is the possibility of some mechanical problem. My ancient Celestron C8 telescope is afflicted by gradual focus shift that affects photography much more than visual observation. At least your Sigma was much cheaper than my Celestron.

Your Sigma Mirror Ultra is probably a higher quality lens that the Nikkor 500mm f/8 mirror lens I bought over 20 years ago. When I first used my Nikkor 500 on a Nikon F2, I could get nice sharp photos. The photos were not as contrasty as those shot with a normal lens, but they were sharp in 7x enlargements. Recently I used the exact same lens on a Nikon F5, mounted on a tripod, and the photos were definitely not sharp in 4x enlargements. So maybe it is the difference in the focus screens (the F2 was a split image, and the F5 is matte), or a mechanical problem with the focus screen alignment in the F5 (although no other lens on the F5 gives unsharp results), or maybe it's just my aging eyes? In any event, I don't like mirror lenses because, and as Jim said, being able to stop down after focusing is a very important consideration.

Have you used that focusing screen for similar length lenses before with good results? The OM has interchangeable screens, perhaps that screen is not suited for that length lens?

mirror lense

That is a very good point. I had the micoprism screen in during the tests, I'll try it again with the split image. Thanks

olympus finder screens

I checked out my Olympus manual and sure enough they recommend the number 4 full matte for long telephotos Fortunately KEH has one and I ordered it.In a few days I'l see if it helps.