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01-26-2009, 09:07 AM   #1
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New Viewer - Mirror lens problems

Hi,
I have a Samsung GX10, which I am sure you are aware is almost exactly the same as the Pentax K10D, but a lot cheaper, this is why I purchased it.
I recently bought a manual 500mm mirror lens with Pentax screw mount and an adaptor to K mount, but it is not fitting properly, when fitted on it's own the adaptor is a few thou below the camera face, which when it is on the lens it prevents the bayonet from attaching, a thin rubber washer between adaptor and lens helps but I am not over the moon about it.
The other thing is that the prints, however carefully I focus, are not sharp!
Focussing is very difficult anyway and I wonder if it is Mirror shake? I use a sturdy tripod, has anyone got suggestions how I can improve things please?
Gumpa:

01-26-2009, 09:19 AM   #2
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Post moved to correct forum to get better views. Welcome to the forums.

What Kmount are you using? An original pentax K to M42 adapter? If not it may be that a 3rd party adapter is not machined correctly.

Focus with mirrors can be a challenge. If you search the forums, many users have replaced the focus screen with a Katz Eye split screen to help with MF glass. A test would be to use a cable release with mirror lock up on a stationary subject in very good light. If those shots are sharp, then it's just practice. if not, then your mirrors could be out of alignment. But Mirror lenses will not be as sharp as the typical good refractive lens.
01-26-2009, 10:11 AM   #3
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QuoteOriginally posted by gumpa Quote
The other thing is that the prints, however carefully I focus, are not sharp!
Because they don't have aperture diaphragms, mirror lens generally use filters to control aperture (if you want to stop down from whatever fixed aperture the lens was built with you need to use a neutral density filter that will remove some of the light.)

Because of this, the filter is typically included in the optical formula of the lens. So if you don't want to stop the lens down, you need a "skylight" filter -- a filter consisting of a piece of clear optical glass -- in most mirror lenses. They won't be sharp without it. (Many won't be all that sharp with it, especially near the limits of the close focus. This is a limitation of the lens manufacturing, rather than mirror lenses as such.)

Generally, a split prism focus screen, turning focus confirmation on, and setting up Av mode to adjust only the shutter are good steps to getting better mirror lens pictures. Especially at relatively short distances, you're dealing with an extremely thin focal plane, too.
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