Modern focusing screens in DSLRS are not actually designed to help
the AF system since they are unrelated to how the AF works and are
used only by the human user.
The normal DSLR is provided with a plain focus screen (without a
split prism) because that is less distracting for general work and
split prisms can affect light metering. But these plain screens are
lousy for manual focus, especially with small aperture lenses (f4,
f5.6) like those mostly used for birds.
Most up-market and many mid-market DSLRs allow you to replace the
focus screen with OEM or after-market screens. Here's a link to the
best-known manufacturer's replacement screen for my camera (Canon
30D). The only reason I haven't got one yet is because of the cost
and that I can't work out from the installation instructions how
tricky it is to install, or how much can go wrong, Also there is the
issue of "funny" performance around the centre of the screen where
the split image and microprisms are, but where many of my birds live
their entire lives (i.e. within the small centre circle). I'd like
to try before I buy. Other than those unknowns, I agree that manual
focus with the standard screens is all but impossible for critical
work, so split prism screens would be a great improvement. And help
prevent those hundreds of photos I've taken with perfectly focused
twigs and blurred bird.
http://www.katzeyeoptics.com/item--Katz-Eye-Focusing-Screen-for-the-Canon-30D--prod_30D.html
On this site can also be found lots of other good info on this topic.
Cheers
Julian
At 11:49 AM 12/12/2008, you wrote:
> In the old days of film and manual focus only cameras
> focussing screens were fantastic and had split prisms etc to
> ensure you were getting what you wanted in focus. Since the
> advent of auto focus teh focussing screens are designed to
> help the AF system not the user.
> This i think is the single biggest reason why manually
> focussing modern cameras is quite difficult.
I didn't know that. I assume the optical screens on DSLRs are much
better that the approx 100k pixel electronic viewfinder on my Canon
S3, but perhaps not so much better as I thought. Are there
particular brands and models that have better focusing screens than the rest?
Given the number of occasions that the bird is behind twigs or
leaves, I would have thought that the quality of manual focusing was
essential, yet it doesn't seem to be discussed that often.
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