Exposure problems with new focusing screen


giofi

Link Posted 09/05/2010 - 19:02
I recently bought a new split image focusing screen for my K20D from Focusingscreen.com (K3 model). I find it very good and I am very happy with it.

Today I put a Pentax FA 100-300 on the camera and found out that it overexposes by some 3 stops with the new focusing screen. The same happens with the FA 28-105 zoom.

After some testing I found out that the focusing screen works fine with some lenses, but not with others. The only commonality I found is that it works fine with fast lenses (for instance the DA* 50-135, but I have not tried all lenses).

Do you have any ideas and recommendations? I have read many reviews about Focusingscreen.com and this particular focusing screen, and no one reported similar problems.

Thanks to anyone who can help....
Giorgio

Pentax Photo Gallery

Anvh

Link Posted 09/05/2010 - 19:13
It's a known problem with split-screen focusing screens.

Here this how every thing is placed inside of the camera.


#1 is the exposure meter
#4 is the focusing screen
#7 is the AF sensor.
I believe the rest is quite clear.

As you can see the exposure meter must meter though the focusing screen and that's causing the problem.
Stefan


K10D, K5
DA* 16-50, DA* 50-135, D-FA 100 Macro, DA 40 Ltd, DA 18-55
AF-540FGZ

giofi

Link Posted 09/05/2010 - 20:57
Thanks Stefan.
But is it normal that the problem exists only with certain lenses and not with others?
Giorgio

Pentax Photo Gallery

womble

Link Posted 09/05/2010 - 21:12
Were you using spot metering? I have only heard of problems with spot metering and split screens, not with specific lenses. Mine seems to neter fine with the katz-eye screen I bought from axl.

Best wishes, Kris.
Kris Lockyear
It is an illusion that photos are made with the camera… they are made with the eye, heart and head. Henri Cartier-Bresson
Lots of film bodies, a couple of digital ones, too many lenses (mainly older glass) and a Horseman LE 5x4.

My website

giofi

Link Posted 09/05/2010 - 21:29
Thanks Kris! Yes I was, now I tested with center weighted average and it seems to work fine. Problem solved.

Reading on KatzEye's site I also found the explanation of the difference between lenses:

Spot Metering Mode: Our testing showed spot metering mode to be within factory tolerance for all AF lenses having a maximum aperture of f2.0 or larger (lower f number). Lenses having a maximum aperture smaller than f2.0 (lager f number) will show a significant shift toward increased exposure, with the shift increasing for lenses with smaller maximum apertures. Spot metering mode is NOT RECOMMENDED when using a split prism focusing screen with lenses slower than f2.0.
Giorgio

Pentax Photo Gallery

womble

Link Posted 09/05/2010 - 21:31
Glad to be of service!

Having a split screen again has been bliss as far as I am concerned.

Best wishes, Kris.
Kris Lockyear
It is an illusion that photos are made with the camera… they are made with the eye, heart and head. Henri Cartier-Bresson
Lots of film bodies, a couple of digital ones, too many lenses (mainly older glass) and a Horseman LE 5x4.

My website

giofi

Link Posted 09/05/2010 - 21:56
Kris,
I fully agree with you.
Giorgio

Pentax Photo Gallery
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