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06-06-2009, 10:42 AM   #1
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GX20 focussing screen

I started this thread after I couldn't find what I was looking for on another thread on Pentaxforums "Should K20D overexpose with M lenses?"

2. The GX20 I use overexposes by two stops with SMCP-M lenses. In the Pentax K10D or K20D, this can be cured by replacing the supplied focussing screen with an LL-60 screen from an older Pentax ist-D camera. This has a corresponding Samsung model, probably the GX1S.

3. I have already done some homework on the advice of fellow forum members on the other thread.

(a)There is nothing in the manual about interchangeable screens. Looked at the manual under Optional Accessories; has nothing listed apart from the flash, the camera case and some parts which are packed with the camera. It observes that "Dedicated accessories are available for this camera. Please contact a service centre for details regarding accessories."

(b) Samsung Romania referred me to their service centre, a private company. The service centre said that the K20D and GX20 were similar in most respects but had nothing to say about an interchangeable screen. The LL-60 is available locally.

4. The questions are (i) whether the GX20 which is very similar to the K20D has an interchangeable screen facility (ii) whether the LL-60 will fit the GX20

5. This would apply to those using Pentax K-mount manual focus lenses with GX10 or GX20 cameras.

06-19-2009, 01:47 AM   #2
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The KatzEye split screen for Pentax K20D fits perfectly in my GX-20. So I guess it's save to say they're identical.
07-04-2009, 12:44 AM   #3
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Most useful to know that a Katzeye screen intended for the K20D fits the GX20, even though the Katzeye is a third-party product. Did not have access to internet owing to travel, hence delay in acknowledgement. More views would be welcome.
07-04-2009, 02:34 AM   #4
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I have the LL60 fitted to my GX20, I needed the rule of 3rd lines. Do not use with manual lenses so couldn't comment on over exposure.

07-04-2009, 03:18 AM   #5
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I can't see how the focussing screen would affect the exposure. I examined my GX20 and it appears that the flip mirror is only partly silvered and there is a secondary mirror behind it redirecting light onto the metering sensors at the bottom of the camera. The light for metering shouldn't go anywhere neer the focussing screen. However if you don't have your eye to the viewfinder whilst metering, light entering the camera through the viewfinder goes back through the viewfinder optics and reaches the sensors. This can sometimes affect the metering.

Edit: Although this has affected metreing and caused exposure errors for me, the images are always under exposed and so this cannot be the cause of your problem.

Last edited by MattGunn; 07-05-2009 at 06:26 AM. Reason: brain fart
07-06-2009, 02:45 PM   #6
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I'm writing this base on Pentax DSLRs, not the Samsung counterparts. But I think they are VERY similar.

QuoteOriginally posted by MattGunn Quote
I can't see how the focussing screen would affect the exposure.
It's a well-known fact that Pentax K10D and K20D when used with old lenses (PK mount, M42 mount) produce wrong exposure.

The light meter is located in the light path, AFTER the focusing screen so it's natural that the focusing screen affects the exposure.

QuoteOriginally posted by MattGunn Quote
I examined my GX20 and it appears that the flip mirror is only partly silvered and there is a secondary mirror behind it redirecting light onto the metering sensors at the bottom of the camera.
They are sensors for autofocus, not for metering.

As a test, take a photo without the focusing screen. You'll see that the photo will be in focus.

QuoteOriginally posted by MattGunn Quote
However if you don't have your eye to the viewfinder whilst metering, light entering the camera through the viewfinder goes back through the viewfinder optics and reaches the sensors. This can sometimes affect the metering.
This tells you that the light sensor is somewhere near the focusing screen, not below the mirror.
07-08-2009, 12:19 PM   #7
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This certainly makes sense, now you put it like that. If the metering sensors are after the focus screen then they must be on a partially silvered surface of the pentaprism. If this is the case then they are exposed to scattered light from the focus screen, in which case there should be no spatial information in the light they receive. How therefore does the multi region aspect of the metering work?
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