How good is YOUR lens?
Many publications, both in print and on the web, will test a product and give you a definitive rundown of its capabilities from that single test. Often, the results of these reports are at a variance to one another and some even go as far as total conflict! So are these differences in reviewers’ objectiveness and methods, or are there true differences in the products they are testing? Is quality control at fault or is it bias amongst journalists?
In order to produce a method of comparison, we constructed a test target to employ the ISO12233 slant edge test. We used Imatest software to calculate the uncorrected Spatial Frequency Response (SFR) at MTF50 in cycles per pixel. This measurement is considered the best measure of testing lenses on modern digital cameras.
The tests were all carried out on the same day at the same venue, on the same camera. Due to the need of shooting at a minimum of 25x the focal length of the lens, the tests were carried out outside in bright conditions with a little cloud. The camera, a Canon EOS 1D, was mounted on a Manfrotto 055 tripod with a Proball 468RC2 head. All the files were shot with the self timer and were captured in RAW format and processed with Rawshooter Essentials with no sharpening.
We chose a lens that has a good reputation and asked the manufacturer to supply one. We then sourced two more lenses of the same mark from ePHOTOzine members (many thanks to them). The lens of choice was the Canon 100-400mm L IS, you know the one, the big white zoom!
Please note that the figures achieved here are from a 4.2mp camera with a 1.3x crop factor. The same lens on a different camera is likely to produce different results. That is because, unlike in the old days when you could put the same type of film in different cameras, with digital every different camera has a different sensor (See Making Sense of Sensors)
As you can see from the graphs (higher numbers are better), there was considerable variation between the lenses, although they all proved to be better at the short end than at the long end and all peaked in their performance at around the same figures.
It does, however, show that the variation in equipment reviews is not down to bias by the reviewers, or not entirely anyway. The tolerance range in the manufacturing process is certainly a little larger than we would have expected from a top end lens, so it will be interesting in the future to try comparisons of the same kind on other marques. Watch this space.
Review by Ian Andrews
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