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05-28-2011, 03:46 AM   #1
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K100DS ISO choice algorithm

I use a K100D Super, and usually have the ISO set on auto select from 200 to 800. (I avoid the higher ISO to reduce noise effects.) I have noticed the effect I am describing in both Auto and Av modes. (These are the main exposure modes I use.) I have noticed the effect with a variety of lenses of various ages: DA18-55, Super Tak 55/2, Tamron 28-300 (auto from mid 90's). I often shoot in dark situations, where speed - ISO and lens are useful - eg night shots of people etc.

Why does the auto ISO selection often choose 200 (slowest available) in the dark shots? This seems counter-intuitive!

Is there some way I can adjust how the algorithm works so that it will choose a faster ISO when confronted with a dark situation?

05-28-2011, 05:38 AM   #2
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I don't think the lighting in the room has as much to do with it as you might think. That is, the camera will always (as it should) choose the lowest ISO usable for a given meter reading. That is, if your shutter speed cannot compensate for your aperture setting, in terms of exposure, it will raise the ISO so a faster shutter speed can be used. If the shutter speed is going to be too slow to get a steady photo, you Should be getting a Flash warning (or the flash should pop up) or some other warning. The shutter speed may blink for instance. That's all a guess on my part, I don't have and never had the K100d.

Your manual states that ISO will not auto adjust if

EV Compensation used
B or M modes are used.

05-28-2011, 09:41 AM   #3
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You'll need to psot some images - with Exif intact - to demonstrate what you're seeing. Normally, the auto exposure modes with choose parameters to get a shutter speed of at least 1 / focal_length, or perhaps 1 / focal_length*1.5. If it can get that with ISO 200, why go higher? But if it's not actually getting a shutter speed that high, then indeed, that sounds like an issue.
05-29-2011, 02:04 AM   #4
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Do you use exposure compensation? When using auto iso on a K100 exposure compensation don't work and the camera revert to iso 200!


05-29-2011, 04:23 AM   #5
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Picture and EXIF data as requested

I post two images (in jpg) and the related EXIF data in TXT format. Both files were produced by the software supplied with K100DS in late 2008.

In the picture of the dog I was caught out with 1 sec exposure and ISO400 - but the auto setting would have allowed 800 - which still would have been a bad picture, but would not have raised my question.

The picture of the magazine was done to put on an eBay sale. The combination of the shake reduction and my hand made this acceptable, but I was still a little surprised at the choice of ISO200. I rate that picture as perfectly acceptable for the purpose, but not aesthetically pleasing.

Another interesting pint is that the EXIF data for the lens was different, but I actually used a Super Tak 55/2 for both images. The EXIF data is consistent with this for the image of the magazine, and I clearly remember using the same lens for the image of the dog - I was showing my recent acquisition to my sister, who owns the dog.

I apologize that the EXIF data is in TXT form, which makes it difficutl to read. It seemed the best form I could save compatible witht he posting rules.
Attached Images
Attached Files
File Type: txt imgp 1012.txt (929 Bytes, 120 views)
File Type: txt imgp 1016.txt (946 Bytes, 152 views)
05-29-2011, 08:04 AM   #6
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I don't think Auto ISO works with M42 type lenses (just like M lenses). I could be wrong about that however.

And I am wrong. At least with the K5 anyway. It does tend to pick the lowest one it sees as reasonable. Checked with a SuperTak 35 f2.0, no foil tape, just got the lens.

Last edited by JeffJS; 05-29-2011 at 08:54 AM.
05-30-2011, 05:17 AM   #7
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I have been using the m42 with Av and auto ISO selection, and most of the time it seems OK, except that I sometimes get caught out with exceptionally long shutter. I have NOT been using Ev compensation - I have been tricking the meter by shorting the 6 o'clock pin in the mount.

Even on the few times I have used Ev compensaiton with the m42 (with the pin shorted) I have had the effect I expected, so the deliberate over/under exposure seems to work.

I have been caught out by the effect even with auto lenses - such as the Tamron,although I do not think the DA18-55.

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