Lens sample variation - A closer look
By
Dave Etchells
(Sunday, January 31, 2010 - 21:12 EST)
Today we have something a bit different for you over on SLRgear: Rather than testing and reviewing a single lens, we've partnered with LensRentals.com to test *5* different samples of the Canon EF 50mm f/1.4 USM.
Sample variation is a significant issue for users, reviewers, and manufacturers. As a user, the issue is whether a positive review and glowing comments by other users mean that you'd have a similar good experience with a lens should you purchase a copy. On the flip side, do a few negative reports about a lens' performance mean you should avoid it, or are they merely flukes, with the majority of samples actually performing well?
There's also the question of the extent to which manufacturers may tweak either lens designs or their manufacturing process over time, and the effect that has on lens quality. In the case of the Canon EF 50mm f/1.4 USM, this seems to have happened, as our earlier tests of this lens showed very different behavior at f/1.4 than we found in these five more current samples.
The topic of sample variation came up in a conversation I had with Roger Cicala last year, the principal of LensRentals.com. With over 2,500 copies of over 350 lenses in stock as of the time writing, Roger is in a unique position to know about lens variability. Other than checking center focus accuracy with a LensAlign unit, though, he's not been able to quantify the issue at all. We'd both been thinking about sample variation for some time at that point, so the solution seemed obvious: given LensRentals' deep inventory, we could take advantage of the regular ebb and flow of their business to siphon off small groups of popular lenses to run through our testing process, to see just what sorts of variation might be found between samples.
Our lens testing process is very resource intensive, so it will never be practical to test multiple samples of more than a handful of lenses, and at that, we can only realistically test 5 or so samples of a lens at any given time: We just don't have the time or resources to do more. The time and work involved also means that we'll likely focus (sorry for the pun) our efforts in this area on prime lenses: zooms are proportionately more difficult, as we need to run the whole test process multiple times on each lens; once for each focal length we characterize. Even restricting our attention to primes, we expect to only be able to manage a few such tests per year.
Despite the limited extent of these tests, we expect the results to be both highly interesting and useful to readers. LensRentals.com rolls over their inventory as an ongoing process, so at any given time, they have samples of lenses purchased over about a year's interval. Selecting samples for us to test from across a year's worth of purchases should give us a good look at the state of current production for each lens design, spanning a pretty wide range of serial numbers.
Check out our first lens sample variation test, the Canon EF 50mm f/1.4 USM.
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