Why are my mirror lens photos still soft, and does the focus calibration ring need adjustment?
Asked 5/15/2015
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2 answers
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I’m using an older manual-focus mirror lens (500mm f/8, sometimes with a 3x teleconverter for 1500mm) and I can’t get truly sharp images. Even when the viewfinder focus indicator confirms focus, and even when I use live view, the results still look fuzzy. I’ve also tested at fast shutter speeds to reduce camera shake, but the images remain soft.
There is a ring near the distance scale that loosens with a small screw, and I suspect it may be for focus calibration or back-focus adjustment. I’ve tried moving it through its range before, but didn’t notice much improvement.
Do mirror lenses just tend to look soft, or is this lens likely miscalibrated? What is the correct way to test whether that ring needs adjustment?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
11y ago
2 Answers
5
No, you don't need to calibrate it, you need to calibrate you expectations: I could be wrong, but (judging from the cap and the use of green for the metres) that looks a lot like the old Vivitar 500mm F8 from the stone age (Pentax mount?), maybe one of the cheapest and worst mirror lens ever sold, unable to be sharp even if shooting at a knife. So yes, your 500mm shots are more or less what one could expect from a mirror lens; the 1500mm shots, on the other hand, are a really an interesting mistery, as your 500mm is...well, a 500mm: how can you shot at 1500mm with a 500mm lens? Are you using a 3x teleconverter? If so...that is all the quality you can expect.
In both cases, if you just want a confirmation of its "qualities" and to just check the focus ring (but you'll need a lot of space for this) you can download from everywhere a front/back focus test image -I usually use this http://2.bp.blogspot.com/-1G5zs4s9ctQ/UwMLB2ZhJdI/AAAAAAAAAbY/f9fAF6zDQVk/s1600/Test+Focus+2.jpg - and do a bit of testing. It's easy, cheap and instructive.
Originally by user38944. Source · Licensed CC BY-SA 4.0
user38944
10y ago
0
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You probably don’t need routine “calibration” so much as a reality check on the lens and setup.
Older mirror lenses—especially inexpensive 500mm f/8 designs—are often not very sharp by modern standards. If you’re also using a 3x teleconverter to reach 1500mm, a major drop in image quality is expected.
That ring is likely a back-focus/infinity adjustment. To test it properly, use a sturdy tripod or other solid support and focus on a point source at infinity, such as a star. Set the lens to infinity and take a shot, then manually find the sharpest focus and compare where the focus ring ends up. If there’s a consistent offset, adjust the ring slightly and repeat until the best focus matches the infinity mark.
If the 500mm images are only modestly soft, that may simply be normal performance for this type of lens. The 1500mm results with a teleconverter are likely about as good as that combination can deliver.
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