Saturday, May 10, 2008

Back-Focus Issues on Chinese Focusing Screens

I ordered a Virtual Village focusing screen for my Canon EOS 350D, but it arrived after the Canon had been disabled; after I got a Pentax K10D, I used its stock focusing screen plus AF focus confirm for some time, but started to miss the split-image screen, so I ordered another Virtual Village screen, this time for Pentax.

Virtual Village makes relatively inexpensive split-image screens, compared to say the Haoda, focusingscreen.com, or Katz Eye screens. Inexpensive meaning around one-third to one-fourth the price.

However, the cheap Chinese screens are known for looking "dirty" and having focus issues.

Yesterday I tried using my K10D plus Pentax SMC-M 135mm f3.5 prime, compared to my office colleague's Canon EOS 400D plus 70-200mm f4.0 L IS zoom. Of course my prime cost $40 and the Canon zoom cost $1,000 plus, but I did not expect a huge disparity in the performance, after all a prime is a prime.

But there was a huge disparity: a photo taken with the Canon was visibly sharper even in small sizes. Since I've noticed that my split-image screen does not indicate in-focus condition at infinity with my 200mm prime, I figured that it might need some shims to correct for the back-focus condition.

I found some stickers, cut them thinly, and attached them to the upper edges of the screen (facing the pentaprism):



It does look messy, but the bits of sticker are actually not visible in the viewfinder.

I did some focus testing afterwards. Here's a focus test before adding the shim:



and after:



Could be just me, but the "in-focus" text does look a bit sharper with the shim in place, and the out-of-focus areas in front and behind the text are more symmetrical with the shim: without the shim, most of the in-focus areas are behind the "in-focus" text.

I also noted that the screen now correctly indicates in-focus condition at infinity with a 135mm and 200mm lens.

And now, to test the relative performance of my $12.95 Sears 70-210mm f4.0 zoom, against the highly-regarded Pentax SMC-M 135mm f3.5 prime. Here's the whole photo:



And 100% crops of the top-most microwave dish. With the zoom (wide-open):



with the prime (wide-open):



with the prime (at f/8):



The SMC-M prime doesn't give up a lot, wide-open, and it's a lot sharper than the zoom, which is to be expected.