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02-23-2009, 01:36 PM   #1
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optical path for exposure meter?

I'm trying to reconcile some apparently conflicting results about exposure measurements.

Can someone provide a link showing the optical paths appropriate for spot vs multi-zone exposure evaluation?

I recall seeing such info for older model pentax cameras implying that spot exposure measurements would not be affected by focusing screen.

Dave

02-23-2009, 02:49 PM   #2
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I can't provide you with a link, though I suspect a google search would turn something up but...
If you are using the spot meter, then anything in the center of the screen is going to have it's way with the meter. Split image aids are especially prone to affecting spot metering.
02-23-2009, 03:16 PM   #3
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QuoteOriginally posted by Wheatfield Quote
I can't provide you with a link, though I suspect a google search would turn something up but...
If you are using the spot meter, then anything in the center of the screen is going to have it's way with the meter. Split image aids are especially prone to affecting spot metering.
That's what I'm trying to get data on. Earlier Pentax cameras let some light thru the center of the mirror which was used for spot metering. Light bounced off the mirror was used for center-weighted and multi-segment metering.

Dave
02-23-2009, 03:18 PM   #4
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To my knowledge Wheatfield is correct. The complete exposure meter is in the viewfinder assembly above the screen. The spotmeter is just as affected by the screen as any other measuring method.(Indeed when switching to a split screen, iot ios affected much worse, as the central split screen covers nearly exactly its spot circle.) That was only different in the Pentax LX, because the light was directed down onto the lightmeter cell at the bottom of the mirror chamber through the semi-transparent main mirror and a small secondary one on its back. When the mirror was up, the LX would measure the light reflected from the film surface directly with that cell. All other Pentaxes, incl. the DSLRs have a more conventional lightmeter assembly...

Ben

02-23-2009, 04:00 PM   #5
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QuoteOriginally posted by newarts Quote
That's what I'm trying to get data on. Earlier Pentax cameras let some light thru the center of the mirror which was used for spot metering. Light bounced off the mirror was used for center-weighted and multi-segment metering.

Dave
I saw diagrams some months ago (can't remember where). All current Pentax dSLRs have the detector reading through the focus screen. The K10D and K20D have the detector located toward the front of the pentaprism while the other models (I think) have the detector adjacent to the viewfinder.

Steve
02-23-2009, 04:05 PM   #6
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My observations: Focusing screen has an effect on metering:

(1) When I installed an after market screen with split image, spot metering was so inaccurate it became unusable. Matrix and center-weighted are also effected, but not by much.

(2) My K10D, with the stock screen (LL-80 I think) and pre-PKa lenses, had inaccurate metering (-0.5 to +2.0 stops error). When I replaced the stock screen with an LL-60 (designed for the *ist series), the error became bearable, -0.5 to +1.0 stop.

I think there are 2 light sensors for the meter, sitting just above the pentaprism, one on each side of the pentaprism.

This is why it is necessary to cover the viewfinder (if your face is not doing that already, as when you have the camera on a tripod) when metering to prevent stray light from entering the viewfinder and fool the light sensors.

This is also why the preflash in P-TTL has to occur before the mirror flips up (put the flash in P-TTL mode and the camera in 2 sec delay to see this).

For whatever reason, OTF (Off-The-Film) metering in which the sensor is in the mirror box, measures the light refected by the film, is no longer used. Some say that it's difficult to measure the light reflecting off the digital sensor but I don't think so. The camera manufacturer can easily do this, tweaking the firmware if necessary. It must be easier to measure the light reflecting off the digital sensor than to measure light reflecting off the film surface (because there are so many different kinds of films out there).

Auto focusing, however, is done behind the mirror, before the light hitting the focusing screen.
02-23-2009, 07:37 PM   #7
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QuoteOriginally posted by SOldBear Quote
My observations: Focusing screen has an effect on metering:

(1) When I installed an after market screen with split image, spot metering was so inaccurate it became unusable. Matrix and center-weighted are also effected, but not by much.

(2) My K10D, with the stock screen (LL-80 I think) and pre-PKa lenses, had inaccurate metering (-0.5 to +2.0 stops error). When I replaced the stock screen with an LL-60 (designed for the *ist series), the error became bearable, -0.5 to +1.0 stop.
i think the -ve side of the metering error is a function of maximum apature my SMC-50mm F1.4 is worse
QuoteQuote:
I think there are 2 light sensors for the meter, sitting just above the pentaprism, one on each side of the pentaprism.
there are more than 2 in matrix metering, the K10D has 16 segment matrix metering.
QuoteQuote:

This is why it is necessary to cover the viewfinder (if your face is not doing that already, as when you have the camera on a tripod) when metering to prevent stray light from entering the viewfinder and fool the light sensors.
it has nothing to do with matrix metering, my ricoh XR2s which was bottom center weighted metering with one sensor had a built in blind to stop eyepeice illumination of the sensor. this has been an issue since the beginning of time because the prism is an optical device, light can go both ways.
QuoteQuote:

This is also why the preflash in P-TTL has to occur before the mirror flips up (put the flash in P-TTL mode and the camera in 2 sec delay to see this).
no, preflash does 3 separate things, it communicates data to remote flashes, it does wide open flash metering and it provides focus adjust allumination. the mirror is down because the lens stops down at the same time as the mirror goes up to reduce shutter lag.
QuoteQuote:

For whatever reason, OTF (Off-The-Film) metering in which the sensor is in the mirror box, measures the light refected by the film, is no longer used. Some say that it's difficult to measure the light reflecting off the digital sensor but I don't think so. The camera manufacturer can easily do this, tweaking the firmware if necessary. It must be easier to measure the light reflecting off the digital sensor than to measure light reflecting off the film surface (because there are so many different kinds of films out there).
it is all about A) providing a means of off camera flash, and B) Cost cutting. TTL flash needs a separate sensor and input A/D for the processor. the *istD has both, and in my opinion, any pro level camera should. Preflash is disruptive to some subjects, wild life etc, and this along with delay makes P-TTL almost useless for wildlife shots using flash (which I do use as a technique)
QuoteQuote:

Auto focusing, however, is done behind the mirror, before the light hitting the focusing screen.
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