What is the real value of lens sharpness tests?

Some articles and comments I read around the web suggest that sharpness tests and crops of shots designed to show how sharp lenses are, are valuable.

Others suggest they are just pixel-peeping, or that somehow, 100% crops don’t present a valuable picture of what is going on.

I take a different view. The fact that each lens manufacturer publishes mtf charts showing the sharpness capabilities of their lenses shows they at least think there is value in knowing how sharp a lens is.

But there is another reason for testing a lens.

Read enough articles and you will build a picture that tells you that the variation between samples in consumer-grade lenses is greater than with more expensive lenses. No surprise there.

And then there’s the matter of how long that consumer grade lens will stay sharp, assuming it was sharp when new. Parts wear, lenses get knocked against things. What is to say that lens will still be sharp six months after you bought it?

Photodo published their independant test results for years and it was the first port of call for many people looking for a new lens. The Photodo site that is published now is somewhat different than under its old incarnation because it now publishes user reviews. But it still maintains the old database.

There are other sites that rely on user reviews, such as photozone but, no site can answer the question whether a particular sample is a dud, or if it was not a dud when it was bought, have the insides worn away or, what damage was done when it was accidently knocked against that door jamb yesterday?

And when I go out shooting, I like to know that the lens is known to be sharp and that my sample is sharp. And I want to repeat that test at some point so I have something to compare my results with. And I don’t want to make the tests so long and complicated that I forget what I intended to prove in the first place. So if I shoot at f8 and at a particular focal length a lot of the time, a good test shot like that should tell me what I want to know. If there is a problem, I will see it.

How to test
Tripods vary. Some tripods cause more shake than they prevent. Flimsy tripods, long shutter speeds, and jerky shutter releases all add up to blurred photos and results that have nothing to do with the showing the sharpness of the lens.

So I like to use a support that is not going to move. Brick walls are a good bet. Better than tables or other pieces of furniture (though heavy wooden furniture can be good). Put your ear against a wooden table and you will feel the vibration running through it. I can hear the vibration from the whizzing hard drive on this computer on this table now.

After a steady support, the next thing is an aperture that is small enough to ensure that any missed focus or focus errors are not going to show up in the shot. And as I often shoot at f8, that is an aperture that will work for the test and provide usable results.

I took these shots at f8, which on dx sized sensor, gives a reasonable depth of field. The subject to camera distance was approximately two feet (60cm) and the depth of field was therefore about 11 inches (28cm), which is enough to cope with any focus errors.

The next thing is to help the camera to expose correctly, which in this case meant two things. One was to take a shot that was predominantly darker rather than lighter. Too much of a light area in the frame means the camera will underexpose, and then you have to correct for that in processing the image.

The second thing was to cover the viewfinder window with my finger (near but not touching) so that no stray light came in and altered the exposure reading. Normally, the photographer’s eye would cover the viewfinder and prevent this problem, but I had the camera propped on a low wall.

Nikon provide a little plastic cover for the viewfinder, but it is sitting somewhere in my bag and I have never used it.

The Results
So, these are the shots from the Nikon 18-55mm kit lens that came with the D40. The exif data tells me the shot was taken at ISO 200, f8 and 1/4 second.

The crops are 475×475 pixels. I shot the images in RAW and converted them in Camera RAW 4.2 in Photoshop CS3. The sharpening parameters I applied were Amount 69, Radius 1.0 Detail 25 Masking 0


cones crop 1

cones crop 2

cones crop 3

Camera Raw 4.2 became available a few days ago and can be uploaded from the Adobe site, and a good place to start the search to find the exact download page is HERE

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