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Old Mar 7, 2006, 1:43 PM   #1
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The more I read and learn about buying SLR lenses - the more I see it needed to test the lens before purchase. Is the situation really that bad when buying lenses that you will see that big of a difference in sharpness at center and corners between two identical lenses that you must test the lens ?

If so - How would you go about doing this in-store before purchase ?

I know enough to know that you must shoot a pic of something at say f2.8 28mm 50mm and 70 mm assuming I'm looking at a tamron 28-70 2.8.

then do same at f4 , f5.6 ect

Put image on SD card - then take home to evaluate edges and center sharpness.

then what ?

What do they mean 100% crop ?

Should you use the cameras auto-focus or should you manually focus when taking the test shots ?

I kinda got the idea - but not totally.

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Old Mar 7, 2006, 2:02 PM   #2
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You're unlikely to see a difference in two identical lenses from the same manufacturer but there is a difference between similar lenses by the same manufacturer or different manufacturers.

For example Sigma make two 70-300 zooms. The specs are almost identical but one is labelled APO and is slightly more expensive. The tests I have read say that this is a far superior lens to the non APO version and gets better reviews than the Nikon 70-300 zoom.

If you look at a 6Mp picture on your monitor and you view the entire picture your software will be scaling the picture down to about 0.7Mp if your screen is the standard 1024x768 pixels. To look at the actual pixels either view it at actual pixel size which means you only see a small part at a time on your screen or crop out an area of say 500x500 pixels that you can actually view at full res on the screen. The second option is often done for posting on the web as it creates a small file for downloadingatfull resolution, tjis is a 100% crop.

I tend to look for magazine reviews of lenses I'm interested in as they have the gear to set up objective tests.

Having narrowed down the choice I'd try to take pictures in the way I plan to use the lens and see what the results are like. No pointhaving a lens that's great atat f2.8 if you always shoot at f8.
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Old Mar 9, 2006, 10:49 AM   #3
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hi ...
there is no need to take such a pain. You can sit back at home and browse through 1000's of picture taken using different lenses at,

just click on the lens icon you want to evaluate and you will see 1000 of pictures

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Old Mar 9, 2006, 11:06 AM   #4
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But isn't it true that there can be a great deal of variance between different copies of the same lens ?

(i.e.)- your exact same lens looks sharp at this aper/shut/focal length - but my lens isn't.

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Old Mar 9, 2006, 12:06 PM   #5
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there will be some variance in the performance. But I guess at least all high-priced lenses are well tested for quality assurance such that they end up with only little variance but not significant.
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Old Mar 9, 2006, 4:45 PM   #6
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sportster64, I understand your frustration, I went through the same thing, until I finally decided I'll just get them and test them myself. I was actually hesitant to buy it online, but I can't find any decent photo store near me. Plus, I found a really good deal!

I just bought a D50 with three lenses, and I'm planning on testing them when I get them. I'll do a little newspaper, chart and brickwall tests, but mainly I'll do real world tests.

The lens I bought are cheapies (Nikkor 18-55, 70-300, 50 f/1.8 ), so I don't expect much. The testing is more for understanding the lens' limitations so I know when to use specific settings. I know I won't be expecting the 70-300 to produce sharp images beyond 200mm.

I have questions of my own for everyone:

From what I've read, for zoom lenses, the best image a lens can produce for a certain focal length is two stops down from it's fastest aperture setting. Anyone know if this rule of thumb fairly accurate?

Are there tests I should do that would determine right away if the lens is a lemon or not? I know about the back and front focusing issue is one, but are there others?

sporster64, here's something that may frustrate you more:
I also read that for some lens (depending on its internals), the image you get from a lens on a specific focal length and aperture setting can vary base on the actual glass alignment inside! For example, say on the 70-300, if you start off at focal length 300mm and then adjust to 150mm, and take a picture, the lens alignment inside may not be identical as the one if you had start from 70mm and then moved up to 150mm. Not only that, but if the front lens rotate upon focusing, it is also possible that the soft spot of the lens would move round. So it may be soft on the left on one shot and then soft on the right on the next shot!

This is the reason I finally decided to buy the lens. I'll go crazy thinking about all these permutations. The mag tests gives you some general background on the lens' capability, but there will be variance from lens to lens. And I assume chhetri is right, the variance will be less on more expensive lens. But in the end, the lens is just a tool and as long as you don't get a DOA lemon lens, you just have to make do with what you get and understand its capabilities and limitations.

good luck!

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