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05-14-2014, 09:44 PM   #1
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Focus confirmation only sometimes with a 500mm mirror lens

Hi,

I have bought a 500m 6.3 mirror lens on eBay (relatively) cheap and in general I am quite pleased with the results. What I can't figure out is how to reliably use focus confirmation when pointing at the moon. I have set the camera to back button focus and sometimes when I press the AF button the red square shows and after adjusting focus I get the confirmation beep. But often the square doesn't show when pressing the button and hence no focus confirmation. I can't figure out why it sometimes works and sometimes doesn't work especially since I don't change settings between shots.

Does anybody know what the problem could be?

05-14-2014, 09:54 PM   #2
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QuoteOriginally posted by maxxxx Quote
Hi,

I have bought a 500m 6.3 mirror lens on eBay (relatively) cheap and in general I am quite pleased with the results. What I can't figure out is how to reliably use focus confirmation when pointing at the moon. I have set the camera to back button focus and sometimes when I press the AF button the red square shows and after adjusting focus I get the confirmation beep. But often the square doesn't show when pressing the button and hence no focus confirmation. I can't figure out why it sometimes works and sometimes doesn't work especially since I don't change settings between shots.

Does anybody know what the problem could be?
When you half-press the shutter, the AF system will beep once it thinks the subject is in focus. It should work the same way with the AF button. If it only works intermittently, it could be because the lens is too slow or there just isn't enough contrast.

But in any case, with a MF lens that long you pretty much need to focus manually when shooting the moon. I recommend using live view to ensure the focus is spot-on. Nailing the focus is key to getting good moon shots

Adam
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05-14-2014, 10:05 PM   #3
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A constant aperature of f/6.3 is on the hairy edge of the camera's ability to detect focus. Add to that the low contrast of the long & cheap lenses, and you're lucky to have it work even some of the time. I guess that's pretty much what Adam said, isn't it?
05-15-2014, 07:44 AM   #4
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I'm using Tamron Adaptall 500mm f/8 mirror lenses and I have found that you can't really rely on the elctronics. Even though I often get a beep confirmation by half pressing the shutter release the result if relying on it is almost never good enough. If you like me aren't all that fond of live view (which of cource is the safest way to achieve correct focus), you actually have to be better than the electronics and learn how to focus manually the old way.

You could also exchange the focusing screen for one that is more suited for manual focusing. There are a number of threads in this forum that could guide you. There might also be necessary to adjust the screen with shims in order for it to be accurate. This is also described in or referensed in these threads.
If you chose to buy a new focusing screen I must warn you about one cheap dual 45 deg split image screen on eBay that is completely useless with lenses slower than f/3.5.

05-30-2014, 03:17 AM   #5
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QuoteOriginally posted by maxxxx Quote
Does anybody know what the problem could be?
Phase detect focus rarely works with mirror lenses, presumably due to their optical layout. The only answer is liveview or a better focusing screen, such as the cut-down Canon Ee-S.
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