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03-17-2008, 05:30 AM   #1
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How to determine lens quality by looking at the pictures?

How do you guys determine if lens is sharp with no problems (good copy) when buying used? This is given that you have pictures taken with the lens. I know that I can check the picture for any soft spots, but is there anything else I should be looking for?

Thanks.

03-17-2008, 09:29 AM   #2
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QuoteOriginally posted by drabina Quote
How do you guys determine if lens is sharp with no problems (good copy) when buying used? This is given that you have pictures taken with the lens. I know that I can check the picture for any soft spots, but is there anything else I should be looking for?

Thanks.
Look for Purple Fringing and Chromatic Aberrations at high contrast edges. Ideally, you would test the lens by shooting toward a bright background, and then on your PC/Mac examine the photos carefully, particularly at the corners. I am lucky enough to have a perfect test are out my back door which faces South - pine trees.

Check the vignetting. If a zoom lens, at wide and long focal lengths.

Check the center/edge softness. I don't think there is a lens out there that resolves as well at the edge as at the center, so don't overdo this one.

If an autofocus lens, check for back/front focus by shooting something with a good bit of detail.
03-17-2008, 10:18 AM   #3
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Sluggish aperture blades can manifest in different ways depending on camera/mount design - e.g. one side of the frame may get more exposure than another (due to the lens not stopping down fast enough), or even a consistent underexposure (for the same reason).

You can also look at the photo to see if one edge or corner is less sharp than the others.
03-17-2008, 01:00 PM   #4
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QuoteOriginally posted by Nesster Quote
Sluggish aperture blades can manifest in different ways depending on camera/mount design - e.g. one side of the frame may get more exposure than another (due to the lens not stopping down fast enough), or even a consistent underexposure (for the same reason).

You can also look at the photo to see if one edge or corner is less sharp than the others.
Not trying to nitpick, as this is for my own knowledge, but wouldnt sluggish aperture cause overexposure? not underexposure? If it doesnt close down fast enough i think it would let in too much light, and over expose?

03-17-2008, 02:33 PM   #5
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You're right of course, I have very sluggish brain apertures

I just had this experience with a Fujinon - overexposure dependent on aperture chosen.
The other thing was caused by a Zuiko, where due to the way the OM meters off the film, I'd get a dark edge from where the lens finally shut down enough. My brother in law had returned a body because of that, it looks like a shutter problem, but I got it on two of my lenses, but not others where the apertures are snappy.
03-17-2008, 04:23 PM   #6
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QuoteOriginally posted by Nesster Quote
You're right of course, I have very sluggish brain apertures

I just had this experience with a Fujinon - overexposure dependent on aperture chosen.
The other thing was caused by a Zuiko, where due to the way the OM meters off the film, I'd get a dark edge from where the lens finally shut down enough. My brother in law had returned a body because of that, it looks like a shutter problem, but I got it on two of my lenses, but not others where the apertures are snappy.
M/K mount lenses do not meter correctly at all apertures, as I found when testing my M 400/5.6. Wide open, it metered for overexposed, from f/8 to f/16, it metered correctly, then switched back to overexposed. My solution is to either set the aperture and shutter speed using the sunny 16 rule (and variants) or to meter the subject at f/8.
03-17-2008, 05:22 PM   #7
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ACK! What is the sunny 16 rule??????
03-17-2008, 05:29 PM   #8
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Another test is to shoot at night at a subject with small light sources like Christmas lights.
This is to test for the aberration known as coma.
A lens that has poor coma correction will show the light sources to be non-circular, oval or comet like in appearance, particularly at the corners.

03-17-2008, 06:06 PM   #9
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QuoteOriginally posted by rmtagg Quote
ACK! What is the sunny 16 rule??????
google "sunny 16", first hit: http://en.wikipedia.org/wiki/Sunny_16_rule

It says: "The basic sunny 16 rule, applicable on a sunny day, is this: Set aperture to f/16 and shutter speed (reciprocal seconds) to ISO film speed."
03-17-2008, 09:48 PM   #10
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QuoteOriginally posted by falconeye Quote
google "sunny 16", first hit: Sunny 16 rule - Wikipedia, the free encyclopedia

It says: "The basic sunny 16 rule, applicable on a sunny day, is this: Set aperture to f/16 and shutter speed (reciprocal seconds) to ISO film speed."
Thank You!!!!!!!!!!!!!!!!
03-18-2008, 10:40 AM   #11
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QuoteOriginally posted by falconeye Quote
google "sunny 16", first hit: Sunny 16 rule - Wikipedia, the free encyclopedia

It says: "The basic sunny 16 rule, applicable on a sunny day, is this: Set aperture to f/16 and shutter speed (reciprocal seconds) to ISO film speed."
It also has a number of other exposures, which used to be found on little slips of paper in film boxes. It should be noted that these work for "average" subjects at average latitude. At the arctic circle, all bets are off.

Sunny day exposure, front lit: f/16 at 1/ISO (as in a full moon )
Sunny day exposure, side lit: f/8 at 1/ISO
Sunny day exposure, back lit: f/5.6 at 1/ISO
Partly cloudy, f/8 at 1/ISO
Cloudy bright, f/5.6 at 1/ISO
Cloudy dark, f/4 at 1/ISO and bracket like heck.
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