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02-11-2009, 08:39 PM   #1
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limits of light metering...?

Hi there!
I have just discovered strange thing with k20d. When the light conditions are really poor, light metering just stop working and treats everything, below some level of light, at the same way. For example: if i put the cap on the lens and cover the viewfinder so any light couldn't get into light sensors, camera at manual mode f4 (with the lens f4), iso1600, makes me set some shutter speed according to EV bar at the 1/5s position. Is this normal? Is this a limit af light meter accuracy? I dont know. When i put f1.8 lens at the same parameters camera makes me use 1,3s shutter speed... Idont really know what to think about it.... Maybe some of you could make the same experiment: put the cap on the lens set some iso set aperture and try to scroll what shutter camera makes you use... Please...

02-11-2009, 08:52 PM   #2
Ole
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The lightmeter can meter down to EV 0, which is 2 sec, f/1.4 at ISO 100. If there is less lighgt than EV 0 you're probably going to get some arbitrary result.
02-11-2009, 09:35 PM   #3
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QuoteOriginally posted by filorp Quote
Hi there!
I have just discovered strange thing with k20d. When the light conditions are really poor, light metering just stop working and treats everything, below some level of light, at the same way. For example: if i put the cap on the lens and cover the viewfinder so any light couldn't get into light sensors, camera at manual mode f4 (with the lens f4), iso1600, makes me set some shutter speed according to EV bar at the 1/5s position. Is this normal? Is this a limit af light meter accuracy? I dont know. When i put f1.8 lens at the same parameters camera makes me use 1,3s shutter speed... Idont really know what to think about it.... Maybe some of you could make the same experiment: put the cap on the lens set some iso set aperture and try to scroll what shutter camera makes you use... Please...
Why bother? You are in total darkness - not "poor light".
02-11-2009, 11:32 PM   #4
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YIKES

You beat me to this post! I have been doing some exposure series related to the Katzeye focus screen and noticed the same thing. To address the comments above:
  • Yes, the K10D and K20D do meter to EV(100) 0. This is the amount of light to allow 1 second exposure at f/1 at ISO 100.
  • With full-open aperture exposure, this sensitivity is more than adequate
  • With stop-down metering, however, it is fairly easy to present this amount of light to the meter.
I will reiterate the last point. Consider the following when using stop-down metering:
  • Exposure of 1/30s at f/2
  • Same amount of light at f/22
The above light level is not unreasonably low, but is below the meter sensitivity at narrower apertures when attempting green button metering.

What I found was that the series would look something like this with the meter response flattening to underexposure at about EV(100) 1.5:

f/2 1/30s EV 5 (at the meter)
f/4 1/15s EV 4 "
f/5.6 1/8s EV 3 "
f/8 1/5s EV 2 "
f/11 1/3s EV 1 "
f/22 1/3s EV 0 "

(The above series is not actual data)

I suspect that much of the observed issues with stop-down metering at narrower apertures are the result of exceeding the range of the meter.

Steve


Last edited by stevebrot; 02-11-2009 at 11:41 PM.
02-12-2009, 01:00 AM   #5
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Out of Metering Range

QuoteOriginally posted by Ole Quote
The lightmeter can meter down to EV 0, which is 2 sec, f/1.4 at ISO 100. If there is less lighgt than EV 0 you're probably going to get some arbitrary result.
And it is 0EV (or anything else as specified) only if a F1.4 lens is mounted. With a slower lens, the "darkest" limit will be raised.
02-12-2009, 06:00 AM   #6
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Thanx guys - you are sociaty i can always count on - so such behavior is absolutely normal you think? Can i start to take a normal pictures instead of pictures of lens cap at last? This is another point to our wish list to Pentax... improve the light measure!

Last edited by filorp; 02-12-2009 at 06:23 AM.
02-13-2009, 11:21 AM   #7
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I discovered this, too, when I decided to take some very low-light shots. I ended up just kind of estimating the exposures. Of course, wide open + long exposure + ISO 6400 = a very...unique look.
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