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09-10-2008, 05:01 AM   #1
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K20D Upper ISO Limitation in TAv Mode

Has anyone else noticed this?

I had my K20D set on TAv mode in low light (standard room lighting at night), EV steps set to 1/3 with ISO set to auto range between 100 and 3200, and a Sigma 18-50 f2.8 DC lens. When the aperture is set to f2.8, the max ISO achievable is 2500, not 3200. Manifests itself by a flashing 2500 in the LCD screen and won't lift to 3200.

It is as though the upper ISO limit has been set to 2500. Problem goes away when aperture is reduced to f4 or higher number, or if EV steps set to 1/2. This behaviour is consistent and repeatable.

Although I would probably steer away from taking photographs at these settings, it seems like an unexpected limitation when the options are either getting the shot or not. there is really no point in having a f2.8 lens if the camera won't use it.

09-10-2008, 07:49 AM   #2
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You haven't mentioned the shutter speed you set. What happens when you increase the shutter without changing f or EV ? Does the ISO change ?

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Nish
09-10-2008, 05:18 PM   #3
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I seem to recall a glitch in the camera whereby if you increase or decrease the aperture, the ISO goes up or down accordingly all the way through whatever range is set. But changing the shutter speed , it does not! I had forgotten about this til now. I wonder if the latest firmware fixed it? Are you on the latest?
09-10-2008, 05:25 PM   #4
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Oops. Just checked - that wa in P mode. Problem discussed here.

09-11-2008, 03:40 AM   #5
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Hi bnishanth. To explain it further, in TAv the ISO is supposed to change as speed or aperture changes, but it is the upper ISO limit that appears to be artificially set at a reduced level of 2500 instead of the 3200 it is actually set at. The issue as I found it last night solely related to f2.8 and appeared to be independent of other variables such as shutter speed.

However, I have just now done a bit further exploration with another f2.8 lens (Sigma 20-40 f2.8 DG EX) and discovered that the problem only occurs at short focal lengths. For longer focal lengths in the zoom range, the max ISO happily lifts to 3200, but not at short focal lengths. From 28mm and higher, the upper ISO limit raises to 3200, but below that is set on 2500.

There is a bit of a story with the firmware load. I recently had to have a warranty repair and I have just discovered that the camera was returned to me with firmware 1.00, not the 1.01 that I had loaded before it went in for repair. I have now loaded the latest firmware (1.01) and the problem remains unchanged.
09-11-2008, 05:00 AM   #6
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You haven't answered the question: what is the shutter speed at that 2500 ISO ? If you've already reached 1/4000s, the camera obviously can't go any higher...
09-12-2008, 12:17 AM   #7
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More on TAv Upper ISO limit

Maybe I am not explaining it properly. In TAv mode, for a set (fixed) aperture, the higher shutter speed should force the camera to go to a higher ISO until it reaches the upper ISO limit set by the user. When the upper ISO limit is reached, this is indicated by a flashing ISO number in the LCD display. Essentially, this is saying that the photo will be underexposed because the ISO cannot increase any more. The issue is that the camera is telling me that it cannot go above 2500 (which is indicated by a flashing 2500, not a steady 2500) even thought the ISO upper limit has been set at 3200. So it is telling me that it is underexposing, but won't let me go to the ISO limit of 3200 that is actully set. It really doesn't matter if the shutter speed is 4000th or any other value because in TAv mode, the ISO should go all the way to the set limit before it flashes, not a third of a stop short.

The bit that is really strange is that it appears to be a function of focal length and this is just weird, because I can't imagine how this is part of the camera's calculations for exposure.

Last edited by AdrianM; 09-12-2008 at 12:50 AM. Reason: incomplete
09-14-2008, 04:17 AM   #8
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Hey, does anyone out there own a K20D with a short focal length 2.8 lens and want to give it a try? I'm trying to work out if my camera is unique in not performing to specification.

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