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09-27-2013, 03:20 PM   #1
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Best focusing technique with Tamron 500mm f8.0 SP

Hello all,
I have posted a few notes on my experiments with the Tamron 500mm f8.0 SP mirror lens and I have noticed that I have trouble focusing accurately that lens. Moon shots turned out..not too bad, other shots have been hard to pin point correctly ( I am using a K5 fitted with a katzeyes focusing screen that has given good results so far...). I have used some form of focusing bracketing with no real success. I read elsewhere that this is a common problem with mirror lenses and I have also used the "black hat" technique to eliminate camera shake - exposing using Bulb - B - to some advantage in order to minimize camera shake on a tripod - great stuff actually.
But now... the same friend who lent me the Tamron has lent me a (original) Pentax rear converter K T6-2X which gives me more pains to focus properly! See images below...
Does someone have a hidden magic bullet?
Image 1 is a window 280 feet away,
Image 2 is the screen in the main door at 25 feet,
Images 3 and 4 are at 28 feet

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09-27-2013, 03:29 PM - 1 Like   #2
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The Tamron mirror lens while somewhat better than some other mirrors like the Vivitar, is not that sharp. However, to get focus for moving objects I set camera to AFS and then Catch-in-focus works with that lens. Hold the shutter down and it will not fire until it is in focus.

Dale
09-27-2013, 03:35 PM   #3
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Have you tried using live view? (might show up any slight forward/back focussing issues with your replacement focus screen)
09-27-2013, 04:07 PM   #4
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Use live view with a tripod, turn SR off, and shoot with a remote or timer to eliminate shake. With live view you should be able to achieve critical focus (i.e. the best possible) -- use magnifying function if you need to. The results will show you how sharp the lens is. Like any mirror lens, it is not going to be razor sharp, but the Tamrons are pretty good. Some contrast boosting and sharpening in post and the results should be quite acceptable. For tests you can also focus on things at an angle so there will always be something in sharpest focus even if you are "missing" the point you want to focus on for whatever reason.

09-27-2013, 06:00 PM   #5
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Original Poster
Thanks for all the pointers, guys. I guess you have to be extra careful and take several frames with small changes in focus. I always use mirror lock up and cable release and will also that "black hat" trick in bulb for moon and deep space shots. I'll post what I get tonight or tomorrow for the last quarter with the 2x converter.
Thanks again.
09-27-2013, 06:17 PM   #6
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QuoteOriginally posted by DDoram Quote
The Tamron mirror lens while somewhat better than some other mirrors like the Vivitar, is not that sharp. However, to get focus for moving objects I set camera to AFS and then Catch-in-focus works with that lens. Hold the shutter down and it will not fire until it is in focus.

Dale
That's what I do with my cheap Opteka 500/8 mirror and all of my long manual focus lenses for that matter. I also need at least a monopod or preferably a tripod to keep it steady.
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