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11-28-2013, 06:34 AM   #1
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ISO variability issue

Hi
I have been a Pentax user since 1983 and currently use K10, K5 & K5iis

I need a fast shutter speed for much of what I do and use the Tv and TAv settings. Both the 5 cameras almost always significantly vary the automatic ISO number when the photographs could have been taken within seconds of each other from the same spot. eg today as a test 2 shots resulted in 800 and 3200 ISO.

This is frustrating me. Have I got a setting problem or is it inherent with the K5?

Ken

11-28-2013, 07:00 AM   #2
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Are you getting more or less similar looking pictures at those different ISO-values?

How is the metering set on your cameras: Spot? Center-weight? Multisegment?

I trust you do use the same metering settings n all three bodies.
11-28-2013, 07:13 AM   #3
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The images are materially differently exposed, as you would expect with such a variation in ISO. The images otherwise could be almost identical. I say almost as I shoot hand held.

I use spot or Centre selected focus with spot metering.

Ken
11-28-2013, 08:01 AM   #4
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Well, of course I cannot say anything for sure, but spot metering might be the culprit. I would try multisegment metering to see if I get more consistent ISO values in Tav-mode.

11-28-2013, 08:11 AM   #5
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How are you using spot metering, just letting it meter in the center of the frame or picking something to meter on and locking it there? If you are just letting it meter wherever center happens to be, then it is no surprise the exposure is all over. While it isn't 1 degree like a handheld spot meter, it is a very small area that it is metering from. Even if something varies in color it can throw you off.

You might want to switch to center weighted metering metering if you want to just let it meter somewhere around the center. This is more like the old manual film SLRs.

Can you post the images?
11-28-2013, 08:13 AM   #6
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Spot metering is the most likely culprit. Even a change of an inch or so in where the 'spot' meters could make a significant difference in the reading. As suggested already, try center weighted or multi as a test and see what happens. You could also try spot on a tripod to be sure there is no movement, assuming the subject is not moving. If on a tripod the metering is repeatable then you have your answer.
11-28-2013, 10:56 AM   #7
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Spot metering is definitely the problem. When shooting sports, I personally use manual exposure so that the image to image parameters remain the same.
11-28-2013, 03:49 PM   #8
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Here is a thought

Does your photo editor have the ability to perform a greyscale histogram on a selection? Corel PSP does and you could take the central 5% of the frame area and see if this histogram is at 120 for a centre average

If so, there is nothing wrong with the metering. It is just spot is not the best if the subject centre is changing from shot to shot

11-28-2013, 05:07 PM   #9
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Thank you all for your help. I will try with changed metering. I have uploaded the 2 test shots from yesterday.

I have a second problem that I have addressed but if my solution doesn't work I will be back. Thanks again.
Ken
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11-28-2013, 05:38 PM   #10
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If those were taken in spot then yes that is your problem. Dead center on the top one is on the grey bird and thus exposed not too far off. The second dead center was on the black bird and the camera will try to make black-->grey so it lightened the image to achieve that.

If you look at the second image the color of the black bird is now almost the same as that of the grey bird in the first image. Camera functioning as designed. If you want to use spot, then you have to pick the 'spot' you want to meter on, which should have a neutral brightness so darker items look darker and lighter items look lighter.
11-28-2013, 07:29 PM   #11
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Thanks for the advice. I take bird photos so don't choose the area to focus on, it is almost always centre; hence spot focus bias. I mistakenly thought that spot metering would be appropriate too.
11-28-2013, 08:45 PM   #12
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QuoteOriginally posted by Ken Glasson Quote
Thanks for the advice. I take bird photos so don't choose the area to focus on, it is almost always centre; hence spot focus bias. I mistakenly thought that spot metering would be appropriate too.
Spot metering is very, very good, but only if you are using manual exposure (my opinion). I use spot metering at times. If I am taking a picture of a splashing waterfall, I will spot meter the brightest water I want to keep detail in, in Manual exposure, then add two stops. If I am taking a moody picture in the depths of the forest, I meter the darkest log I want detail to be retained in, and subtract two stops. Works for me.
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