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10-05-2012, 09:31 PM   #1
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Exposure values between LV and PDAF mode different?

Hi guys,

I was out this morning shooting and for the first time I realized that the exposure readings on LV mode and vs PDAF mode is quite different....I am not sure why.

I tried out using my K-7 and on LV mode, it consistently underexposed about 1 stop...say a shot I took (using the DA15) has an exposure of 1/8sec, F4, ISO100 EV0, centre weighted metering on LV mode, the same scene taken on PDAF mode is 1/20sec, F4, EV0, ISO100. Metering modes and other parameters remain the same.

The same scene using the same lens on my K-01 is 1/6sec, F4, EV0. ISO100, centre weighted.

I tried out my wife's K-30 and again I find that metering exposures on LV and PDAF mode is also different. Using her 18-135, the same scene on LV measures 1/6sec, F4, ISO100, EV0 and on PDAF, the exposures become 1/30sec, F4, ISO100, EV0. All on centre weighted.

Could anyone please tell me what am I missing here?

Thanks

10-06-2012, 04:19 AM   #2
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It's just that two different exposure meters is used in LV and OVF mode. In LV mode the main sensor is used for exposure metering, and with OVF a sensor inside prisma housing is used for exposure metering. These two exposure meters are not matched to give same exposure.
10-06-2012, 04:30 AM   #3
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QuoteOriginally posted by Fogel70 Quote
It's just that two different exposure meters is used in LV and OVF mode. In LV mode the main sensor is used for exposure metering, and with OVF a sensor inside prisma housing is used for exposure metering. These two exposure meters are not matched to give same exposure.
To add to this, in the OVF mode image will expose slightly higher because some light leaks in through the viewfinder (as the manual says).... try use the viewfinder cover and see what you get.
10-06-2012, 04:38 AM   #4
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Thanks guys...I am aware of the light leak via the viewfinder and I have tried out the steps covering the view finder with my hanky...the settings remain the same...

I can accept that the OVF and LV may not give the *Exact* exposure setting....but at 1.5 to 2 stops....isnt this kind of serious?

I also googled abit....looks like nikon D4 is experiencing the same issue somewhat.
Metering Discrepancies - PDAF vs CDAF: Nikon FX SLR Talk Forum: Digital Photography Review

Looks like a difference of 2 stops between CDAF and PDAF metering?

10-06-2012, 07:19 AM   #5
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I've just tried a few test shots with my K-30 and I am getting exactly the same exposures using the OVF and live view.

I made the test shots using the 18-135WR, getting 1/15" f5.6 at ISO 800 both in live view as well as OVF. Metering is multi segment.

I repeated the same scene with the K-r, using the same lens and getting almost exactly the same result, The only difference is that with OVF the shuuter speed is sometimes 1/15" and sometimes 1/20", so I suppose it is exactly in the middle and just goes to the nearest. With Live view it is always 1/15". So it looks like with the K-r there is evidence of some difference between OVF and LV but it is less than a third of a stop.

I then tried the K-30 with my Tamron 10-24. At 10mm I got some difference 0.2s with OVF and 0.3 seconds in LV. But at 24 mm I got 1/6" both in OVF as well as live view. (The difference in exposure between focal lengths is entirely expected due to the different scene composition)

I do seem to recall some cases when I got significantly different results between OVF and LV which makes me wonder why now that I do a controlled test I am not seeing this difference. Could it depend on the lens perhaps?

Could someone who is getting the very different OVF/LV results try with one of these lenses and see if they get same results as me ? (that is identical OVF and LV exposures?

Unfortunately these are the only two lenses I have with me now but when I get home I'll try the test with some other lenses to see if I get different results.
10-06-2012, 04:48 PM   #6
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I did another quick test with my K-30 this morning before i rush to church....

K-30 with 18-135. If the scene is evenly lit, I get not too difference between OVF and LV exposures. In this case I get:

LV: 1/400sec F5.6 ISO100 EV0
OVF: 1/500 sec F5.6 ISO100 EV0

difference is less than a stop.

But if the scene contains a mixture of bright and dark subjects (say indoor), the difference in exposure is easily 1 stop or so.
10-07-2012, 03:16 PM   #7
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OVF exposure depends on the focusing screen, or rather on its brightness at whatever the fully open f-stop of your lens is (variable f zooms make it even more complicated). I have long suspected that Pentax cheat and that there is a bit of software tweaking done for their lenses with the stock focusing screen.

The LV exposure should be the more accurate, for OVF simply tweak the EV offset for a given lens/focusing screen combination to match. With the Canon EE-S screen fitted I find that I need about +2/3 to +1 stops offset for most lenses.
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