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05-31-2011, 12:42 AM   #1
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I don't understand why stop down metering doesn't work well at higher f stops.

I mean the silly camera does through the lens metering when stopped down. Why then when we snap it, is the exposure off. When we snap it, it should stop down to the same amount no?

05-31-2011, 12:55 AM   #2
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What lenses and camera are you using?
Under what conditions are you shooting?
What metering mode are you using?
Can you please post an example of an image that reflects your perceived problem?
There are just far too many variables to consider before suggesting metering issues on your camera.
05-31-2011, 12:58 AM   #3
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You will need to tell us: What mode are you photographing in? With a purely manual lens? With an A-lens? With a screw mount (M42) lens?

Assuming it is a manual lens, you should shoot in M-mode and you will need to measure the light using the green button. Your camera's crippled KAF2 mount does not allow for proper light metering unless you use the green button.

Note: Some claim that you may use Av mode and stop-down metering with screw mount lenses, making the system work as you describe. However, as the manual says itself, this may not give reliable results - and it surely does not do so with my K200D.

So, the safe approach is: Take your pictures in full manual (M) mode and use that green button.
05-31-2011, 01:19 AM   #4
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I am testing manual aperture lenses. e.g. Pentax-M, Pentax K, and Sears. I am using manual mode and pressing green button for stop down metering. It meters fine wide open, but is totally unpredictable as you adjust the f stop up, especially after f8 on upwards to f22 +

05-31-2011, 01:25 AM   #5
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QuoteOriginally posted by geekette Quote
I am testing manual aperture lenses. e.g. Pentax-M, Pentax K, and Sears. I am using manual mode and pressing green button for stop down metering. It meters fine wide open, but is totally unpredictable as you adjust the f stop up, especially after f8 on upwards to f22 +
What is the Green button set to in Manual Mode? Anything flashing in the viewfinder when you stop down via the Green button. Can you hear it actually stop down?
05-31-2011, 01:29 AM   #6
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It's stopping down and it works fine up to a certain point. Once it gets to f8 or so anything past that always returns the same shutter speed.
05-31-2011, 01:34 AM   #7
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QuoteOriginally posted by geekette Quote
It's stopping down and it works fine up to a certain point. Once it gets to f8 or so anything past that always returns the same shutter speed.
- and you are sure that you have set the camera to work in "M" mode?

If so, I am afraid that your camera is not "silly" but rather faulty!
05-31-2011, 01:36 AM   #8
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Yes I am working in manual mode and it stops down with the +/- button on the K-X for manual lenses.. I have the green button assigned to DOF preview.

Anyways, I found out reading another thread it is a problem with stop down metering in LOW LIGHT conditions.. My conditions atm are very low light.

05-31-2011, 01:37 AM   #9
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Yes it is stopping down.. the aperture blades close up, it gets dim.. yuo can hear it.. same as DOF preview.. etc... yeah I am doing it right.. it's just that the pentax stop down metering sucks ><
05-31-2011, 01:40 AM   #10
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.......aaah, that is a very common constaint/limitation: As soon as you get very low light and exposure times around 0.5 - 1 second, you cannot rely on the automatic metering any more.

You will either have to do some trial and error (you will soon get the "feel" of what is needed) or buy a more sophisticated light meter.
05-31-2011, 01:42 AM   #11
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Or you can just meter wide open and double the shutter time in your head for each stop you go up So say at f2.8 it is 1 second shutter speed, at 4 it is 2, 5.6 is 4, 8 is 8, 11 is 16, 16 is 32 and 22 is 64 seconds
05-31-2011, 01:49 AM   #12
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QuoteOriginally posted by geekette Quote
It's stopping down and it works fine up to a certain point. Once it gets to f8 or so anything past that always returns the same shutter speed.
Is anything flashing in the viewfinder or in the LCD?
05-31-2011, 01:49 AM   #13
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QuoteOriginally posted by geekette Quote
Or you can just meter wide open and double the shutter time in your head for each stop you go up So say at f2.8 it is 1 second shutter speed, at 4 it is 2, 5.6 is 4, 8 is 8, 11 is 16, 16 is 32 and 22 is 64 seconds
Yep, or you could also adjust the ISO or do a combination of both.
05-31-2011, 02:25 AM   #14
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I wish there was a way to have the Pentax K-X show the lightcandles or whatever it gets when it meters wide open. That way I can just write an iphone program where you enter in the lightcandle amt and then spin the f stop, iso selectors and it gives you the resulting shutter speed.

I understand why it is accurate with auto aperture lenses. Because it meters wide open and knows what it is going to set the aperture to and does teh appropriate calculations. It doesn't know what aperture the manual lens is set to. Would be nice to have an option in manual mode where you set the aperture in the camera of what the lens is set to.. that way you'd have perfect metering results.
05-31-2011, 03:38 AM   #15
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In fact, there is a huge limitation with Pentax cameras concerning stop-down metering : their focus screens!

Metering is done through the focus screen (the cell actually meters the image formed on the screen), and thus is highly dependent upon this screen's capabilities.

Unfortunately, modern screens are not "linear", meaning that they are designed to boost perceived brightness with the usual aperture range found in lenses today (they increase brightness through a fresnel lens located on the bottom of the screen, usually for apertures between f/2-4).

As a side effect, our screens do not react in a linear manner anymore (closing one stop does not induce a one-stop change in metering), and they don't depict true DoF neither.

And there is a limit, with really small apertures (around f/16-22 IIRC), where the screen will "dark out", and the metering cell will not see changes anymore.

K10/K20 had very bad screens in this regards.
Only screen I've found so far with a linear response is the Canon EE-S screen...

Last edited by dlacouture; 05-31-2011 at 04:31 AM.
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