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03-31-2010, 07:01 AM   #1
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Linking ISO steps with EV steps

There is a custom setting that allows ISO steps "to coordinate it with the EV steps". I understand what this means, but am not sure how it works. If I am using AV mode, with ISO on auto, does this mean that if I adjust the EV to + 2/3, ISO will automatically adjust by the same increment? Is this preferable to using the default sensitivity adjustment at increments of 1 EV? Or since I have ISO set to auto, it makes no difference?

03-31-2010, 07:29 AM   #2
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No, it will not automatically increment the ISO when you select 2/3 exposure compensation. Somewhere in your menu is a setting that defines if your EV compensation goes in steps of 1/2 or 1/3 EV.

What the 'coordination' does is to allow you to increase iso in smaller steps when selecting an ISO manually. If you set EV to be incremented/decremented in steps of 1/3, you can also change your iso in steps 1/3 (instead of steps of 1EV).

Normal ISO steps are 100/200/400/800 etc (1 EV). When selecting 1/2 step EV and you have this 'coordination' set, you can choose your ISO 100/140/200/280/400 etc.

I am not sure how it affects automatic ISO as I did not pay attention to that.
03-31-2010, 08:05 AM   #3
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I tried that setting on my K-x, and indeed in auto-iso mode, it will vary the ISO in smaller increments, instead of going from 100/200/400, it will go 100, 120, 150,200, etc. What I wonder is if this actually is a finer grain control for sensor sensitivity, or just changes some mathematical algorithm used by the cpu and the sensor is still at 100/200/400.
03-31-2010, 08:07 AM   #4
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Oh, I see! I have to be manually selecting the iso setting to see the difference in incremental settings. Thanks for clarifying this!

03-31-2010, 08:14 AM   #5
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QuoteOriginally posted by kiwibird Quote
Oh, I see! I have to be manually selecting the iso setting to see the difference in incremental settings. Thanks for clarifying this!
Yeah. Like I shoot in full manual, so I now have a wider range to choose from.
04-01-2010, 12:16 AM   #6
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QuoteOriginally posted by justDIY Quote
I tried that setting on my K-x, and indeed in auto-iso mode, it will vary the ISO in smaller increments, instead of going from 100/200/400, it will go 100, 120, 150,200, etc. What I wonder is if this actually is a finer grain control for sensor sensitivity, or just changes some mathematical algorithm used by the cpu and the sensor is still at 100/200/400.
Sensor does not have variable sensitivity. It has it's base sensitivity (usually ISO 100, some sensors have ISO 200) and then signal from it is amplified according to selected ISO.
04-01-2010, 12:56 AM   #7
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QuoteOriginally posted by Edvinas Quote
Sensor does not have variable sensitivity. It has it's base sensitivity (usually ISO 100, some sensors have ISO 200) and then signal from it is amplified according to selected ISO.
Yes, but answering his question is not quite that simple. For example, on the K100D, there is a range of 200-1600 which is amplified in the analog domain, at the stage before the sensor ADC -- then there is a 3200 mode which is "fake 3200", i.e. 1600 pushed a stop digitally in firmware. In the same manner, many cameras utilize "digital pushing" to achieve the speeds in between the full stops. So for instance, again on the K100D, ISO 640 is going to be pushed ISO 400 (a full stop of analog amplification from ISO 200 to 400, then a half stop of digital amplification from 400 to 640)

This means that ISO 640 is potentially noisier than ISO 800. Again, I'm sure this isn't true for all cameras, but for some. Not sure how all the Pentax models stack up.
04-01-2010, 01:50 AM   #8
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Erik, thanks for useful information.

04-01-2010, 03:06 AM   #9
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QuoteOriginally posted by Erik Quote
Yes, but answering his question is not quite that simple. For example, on the K100D, there is a range of 200-1600 which is amplified in the analog domain, at the stage before the sensor ADC -- then there is a 3200 mode which is "fake 3200", i.e. 1600 pushed a stop digitally in firmware. In the same manner, many cameras utilize "digital pushing" to achieve the speeds in between the full stops. So for instance, again on the K100D, ISO 640 is going to be pushed ISO 400 (a full stop of analog amplification from ISO 200 to 400, then a half stop of digital amplification from 400 to 640)

This means that ISO 640 is potentially noisier than ISO 800. Again, I'm sure this isn't true for all cameras, but for some. Not sure how all the Pentax models stack up.
Interesting.. Can anyone confirm that the above would be true (or not) for latest Pentax models like K-7/x ? Thx.
04-01-2010, 11:53 AM   #10
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QuoteOriginally posted by Erik Quote
Yes, but answering his question is not quite that simple. For example, on the K100D, there is a range of 200-1600 which is amplified in the analog domain, at the stage before the sensor ADC -- then there is a 3200 mode which is "fake 3200", i.e. 1600 pushed a stop digitally in firmware. In the same manner, many cameras utilize "digital pushing" to achieve the speeds in between the full stops. So for instance, again on the K100D, ISO 640 is going to be pushed ISO 400 (a full stop of analog amplification from ISO 200 to 400, then a half stop of digital amplification from 400 to 640)

This means that ISO 640 is potentially noisier than ISO 800. Again, I'm sure this isn't true for all cameras, but for some. Not sure how all the Pentax models stack up.
See some recent discussions on dpr about ETTR, in which these same issues come up. GordonBGood shows pretty conclusively that this sort of thing doesn't matter in practice - there is no way you're going to be able to tell the difference between ISO 640 achieved by analog amplification directly versus ISO 640 achieved by digitally pushing ISO 400 output. That is, while digital pushing leaves "holes" visible in a RAW histogram, they're not visible in the image becaue we can't discriminate that finely, and they're gone by the time we convert to JPEG anyhow (whether that's done in camera or in PP).
12-30-2010, 10:17 AM   #11
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I discovered this ISO sensitivity / EV deal as I was working through the manual for my K-x, had some questions, and discovered this thread. I trust it is not bad form to revive the thread...
Here's kind of the bottom line question:
If I leave the default setting, I can take pic X at 1/80 at 3200ISO.
If I change the sensitivity, the exact same pic is taken at 1/60 at 2500ISO.

I certainly can't discern a clear difference in the pics, but that may just be this instance.
I realize the tradeoff is between faster speed (w/ less likelihood of hand shake) and higher ISO (w/ greater likelihood of noise?).
Or is the above discussion basically saying the difference between 2500 and 3200 is simply mathematical and that there really is no difference in noise, and in that case, go with the default and 3200 and take advantage of the faster shutter speed?
Thanks
12-30-2010, 10:22 AM   #12
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Mgvh:

I shoot full manual anyway, but this is why I always tell people to use manual ISO.

It's a way to get better control of what the camera is doing.
12-30-2010, 10:34 AM   #13
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Yes, I was using Program mode which I have found helpful as I am learning how to use my K-x. But, even if I was to work full manual, is it really better to use, for example, 2500ISO instead of 3200ISO if hand shake will not be an issue?

Last edited by mgvh; 12-30-2010 at 11:15 AM.
12-30-2010, 10:53 AM   #14
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You always, always, ALWAYS want to use the lowest ISO you can if your shutter speed and aperture work for the particular shot. Unless you want to use high ISO noise for effect, which is rarely. (You can always add noise in post processing in the computer.)

For example, if you're shooting outdoors in bright sun, nothing fast moving, there's zero reason to shoot faster than 200. I always shoot 100 myself.

Your ISO setting is the one setting which dramatically affects the image quality. Various apertures will affect depth of field, and shutter speeds affect what shutter speeds affect.

But unnecessarily high ISOs will screw you up.

Last edited by Ira; 12-30-2010 at 11:00 AM.
12-30-2010, 10:59 AM   #15
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And stay away from P mode. It teaches you nothing.
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