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02-23-2007, 09:12 AM   #1
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AUTO ISO: what triggers a change?

I'm trying to understand what the camera does if I set the ISO to AUTO. I've read the manual and also the info on the subject in Joe Farace's Magic Lantern book on the K100D. And I've experimented with the camera itself. Can't see what's going on.

I understand that AUTO ISO does nothing in full manual (M) or bulb (B) shooting mode, or if I'm in another mode and I've made an adjustment to the exposure with the Av/+- button.

So, let's assume then that the camera is in Tv or Av shooting mode. I've set the ISO to AUTO and I've also used the menu to restrict ISO to the range from 200 to 800. I set the aperture to, say, f/8 and I go to shoot my picture. Now what happens? At this point, the camera has been told to shoot at f/8 and it will do so - but in Av mode, the camera has also been asked to try to calculate a correct exposure, and it has two ways to do this, namely, by adjusting the ISO or by adjusting the shutter. How does it know which to prefer? What I've found so far is that it seems to prefer the shutter, even if it means slowing the shutter down to a point where I'm bound to get some blur in the picture.

Will

02-23-2007, 09:33 AM   #2
Ed in GA
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Will, that's a really good question and one for which I would like to know the answer as well.

I've found that I keep the ISO at 200 on my camera most of the time. Back in the film days, I rarely shot above ASA100 (except for sporting events) because I like the resolution of the photographs at that speed.

I'm curious to see what some of the more learned have to say when replying to your query.

Ed
02-23-2007, 12:03 PM   #3
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why not do a characterization og the AUTO ISO function.

Using a fixed light source and subject, do the following

1) Set ISO to auto, then set the Camera to program mode such that it follows one of the program curves, Perhaps Av mode

2) then change the apature from maximum to minimum in 1 f-stop increments

3) record the changes in shutter speed and ISO setting and see at what point it increases ISO in order to keep a minimum (assumed 1/focal length) shutter speed.

Do this with several light sources, and or focal lengths and let every one know.

without doing it myself, my bet is that the camera will first make adjustments within the program lines for trading shutter and F-Stop, and only when the camera falls outside of these program ranges will it increase the ISO,
02-23-2007, 12:10 PM   #4
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QuoteOriginally posted by Lowell Goudge Quote
why not do a characterization og the AUTO ISO function.
I already tried some quick tests and I could not get the ISO to budge from 200. Instead, the shutter speed just kept getting slower and slower.

And this strikes me as the kind of question that somebody ought to know the answer to. I mean, it's really kind of obvious, isn't it? There's an AUTO ISO setting. SO WHAT DOES IT DO? Answer: Automatically adjusts the ISO. Followup question: WHEN or WHY?

To be honest, I'm rather surprised that this question isn't answered in the operating manual. At least I can't find it in there.


QuoteQuote:
without doing it myself, my bet is that the camera will first make adjustments within the program lines for trading shutter and F-Stop, and only when the camera falls outside of these program ranges will it increase the ISO,
What do the words "within the program lines" mean?

Will

02-23-2007, 12:15 PM   #5
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I seem to have the opposite results. I also have a K100D, and when I have it in Auto ISO it seems to use the highest ISO possible. I had it set on a range from 200 to 800, and most of my shots came out at 800 ISO.
02-23-2007, 12:20 PM   #6
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and if u dial in any plus or minus ev it wont do any of the above bless it.. it will default to iso 200..

auto iso will use the lowest number it thinks acceptable within its safe parameters.. its has a vague idea of what safe parameters are and seems to work quite well apart from the ev thing..

it does seem to take focal lengh into account i think its more a shutter speed thing than anything else.. it in essence tries to keep it up to reasonable speeds..

trog

ps.. its possible more than the ev dial stop it doing its job.. i mostly use it in P mode..
02-23-2007, 12:31 PM   #7
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QuoteOriginally posted by xfraser Quote
I seem to have the opposite results. I also have a K100D, and when I have it in Auto ISO it seems to use the highest ISO possible. I had it set on a range from 200 to 800, and most of my shots came out at 800 ISO.
Interesting. I did a couple more quick tests, paying closer attention. I had ISO set to AUTO with the acceptable range defined as 200 to 800.

Shooting in aperture priority (Av) mode, every shot seems to come out ISO 800.

Shooting in shutter priority (Tv) mode, every shot seems to come out ISO 200.

Shooting in program (P) mode, there is some variation - some shots 200, some 800, but none 400.

That's the result of a dozen quick shots. If what I think I'm seeing really is the case - still not sure - then (a) I wonder why this isn't explained in the operating manual and (b) it might actually make sense.

Will

PS Added 2-23-07 4:50PM (hours after this post first appeared): See my post later in this thread, offering better test info.

Last edited by WMBP; 02-23-2007 at 03:52 PM. Reason: Draw attention to correction made in later post
02-23-2007, 12:53 PM   #8
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this is great!

I will try this with my K10D over the weekend and see what I get.

Normally I don't use many automatic features, especially white balance and ISO, as I don't trust them. Many times I don't even use auto exposure, but use a paved surface as an approximate equivelent to a 18% grey surface, to set the metering, and then keep that metering unitl the conditions change.

This is partially due to having a lot of K series lenses (especially long telephotos, that I don't want to replace instantly) and partially due to working this way all the time with film (From KX to Ricoh XR2s to PZ-1) before my *istD and K10D came along.

Program modes were generally used for snapshots only, but even then, I was careful about giving the camera too much control since no one has developed successfully a "mindread.exe" for the camera yet

02-23-2007, 12:59 PM   #9
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I mainly either shoot in AV or M, so my results are the same as yours as far as Av mode goes.

I'm with Lowell, I don't really like using Auto ISO, but it seems like I keep forgetting to adjust it before I start taking pictures and miss some opportunities, so I usually keep it in Auto.
02-23-2007, 01:17 PM   #10
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better tests, better results

Scratch my earlier post with test results. I've tried a little harder (again).

Just now, I WAS able to get the ISO to change from 200, to 400, to 800, all the time keeping the shooting mode set to Tv. It was 200 when I was pointing the camera at a window; 400 when I pointed the camera into a dark corner in the kitchen underneath a counter; and 800 when I went into the pantry without the light on.

Here are the settings. Tv mode, very slow shutter speed of 0.3s:

(window) f/22, ISO 200.

(dark corner) f/3.5, ISO 400

(very dark pantry) f/3.5, ISO 800

The hard boundary up against which the camera's exposure calculation seems to be bumping is the aperture. THAT sort of makes sense. But the limits of the aperture are, um, more usefully defined than the limits of the shutter.

Will
02-23-2007, 02:05 PM   #11
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QuoteOriginally posted by WMBP Quote
Scratch my earlier post with test results. I've tried a little harder (again).

Just now, I WAS able to get the ISO to change from 200, to 400, to 800, all the time keeping the shooting mode set to Tv. It was 200 when I was pointing the camera at a window; 400 when I pointed the camera into a dark corner in the kitchen underneath a counter; and 800 when I went into the pantry without the light on.

Here are the settings. Tv mode, very slow shutter speed of 0.3s:

(window) f/22, ISO 200.

(dark corner) f/3.5, ISO 400

(very dark pantry) f/3.5, ISO 800

The hard boundary up against which the camera's exposure calculation seems to be bumping is the aperture. THAT sort of makes sense. But the limits of the aperture are, um, more usefully defined than the limits of the shutter.

Will
I was just thinking the trying same thing, it almost seems to be related to maximum apature and ambient light. Next question, what happens if you put the flash on? someone has complained (in an earlier thread) that the flash always goes to maximum ISO when auto ISO is set? is this really true, or only when you are already in the dark?
02-23-2007, 02:06 PM   #12
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Just did the same tests, but this time, using Av mode, with aperture set to f/11. Once again, ISO was set to AUTO with a range of 200 to 800. This time, the shots of the dark corner and the very dark pantry both took the shutter to 4 seconds and in both cases, the ISO was bumped up to 800. Shot of the window (not bright, but much brighter than the pantry) was set to 1/60 at ISO 400. It LOOKS to me as if 4 seconds (not 1/4s, 4 whole seconds) may be the hard limit here, in other words, that the camera lowers the shutter speed without raising the ISO until it gets to 4 seconds.

At the moment, my VERY TENTATIVE conclusion is that AUTO ISO may be usable in shutter priority mode, but seems dangerous in aperture priority mode. Opening the shutter wide to f/3.5 or whatever is almost always safe, and the same can be said for stopping the shutter down all the way. It's usually safe, I suppose, to INCREASE the shutter speed to something very, very fast; but it's definitely not safe to slow the shutter down to 4 seconds. If there is a 4 second limit, then it seems to me that someone made a mistake. I would have set the limit at 1/30s or something like that.

Will
02-23-2007, 02:14 PM   #13
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"that the flash always goes to maximum ISO when auto ISO is set? is this really true, or only when you are already in the dark"

the k100 dosnt go to maxium iso when the flash is used .. the k10 does i think..to me its a bug just like the ev thing is a bug with the k100.. fix one bug let loose another..

trog
02-23-2007, 03:55 PM   #14
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QuoteOriginally posted by trog100 Quote
"that the flash always goes to maximum ISO when auto ISO is set? is this really true, or only when you are already in the dark"
Only in the dark. This only applies to K10D and not K100D.

QuoteQuote:
the k100 dosnt go to maxium iso when the flash is used .. the k10 does i think..to me its a bug just like the ev thing is a bug with the k100.. fix one bug let loose another..
We've had lengthy discussion here and in dpreview. I believe most posters in dpreview agreed with me that this was an intended behavior for the K10D and not a bug.
02-23-2007, 04:23 PM   #15
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i know we did.. not that it altered my opinion in the slightest.. the intended behaviour arguement was fully understood but on my part simply not agreed with.. he he

trog
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