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03-29-2011, 07:07 PM   #1
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Auto ISO in Av Mode

I use a K-7 and lately I've been experimenting with different shooting modes and settings within them that I never bothered to screw around with. For this reason I've only just recently discovered it's even possible to put Av into Auto ISO with the Green Button.

So my question is pretty straightforward: when you toggle over to auto ISO, what kind of minimum shutter speed is the camera trying to maintain? Does it simply set the the ISO high enough so that a stationary subject can be captured crisply at a given FL? Or will it choose settings sufficient for a target that is moving at a reasonable clip?

I guess what I'm trying to figure out is: is there really any reason to use Av mode with Auto ISO activated over just switching over to TAv and choosing both an appropriate aperture and shutter speed for your given aims?

03-29-2011, 07:22 PM   #2
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I've always kept Auto ISO turned off in Av and Tv modes. I set my dials so that the ISO is on the front dial. In My experience, with auto ISO, it will always Max out the ISO (to the top of the predetermined range) and then set the shutter speed accordingly.

03-29-2011, 08:03 PM   #3
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Av doesn't just max ISO in my experience (on a K-x body).

As it gets darker, Av will favour the lowest ISO it can until the shutter speed gets down to what it guesses is a risky level for camera shake given the focal length. It will then maintain that anti-camera shake shutter speed and start pumping up ISO.

It takes no consideration of subject movement as obviously it has no clue what the scene is.

That's why I've mentioned a couple of time elsewhere now, that taking pics of people (esp. kids) with wide primes in Av mode is fraught as the shutter speeds get too slow (at least on a K-x).

I believe the K-5 can be tuned so that it maintains a higher shutter speed by pumping up the ISO earlier. Not sure about the K-7.
03-29-2011, 09:01 PM   #4
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QuoteOriginally posted by twitch Quote
Av doesn't just max ISO in my experience (on a K-x body).

As it gets darker, Av will favour the lowest ISO it can until the shutter speed gets down to what it guesses is a risky level for camera shake given the focal length. It will then maintain that anti-camera shake shutter speed and start pumping up ISO.

It takes no consideration of subject movement as obviously it has no clue what the scene is.

That's why I've mentioned a couple of time elsewhere now, that taking pics of people (esp. kids) with wide primes in Av mode is fraught as the shutter speeds get too slow (at least on a K-x).

I believe the K-5 can be tuned so that it maintains a higher shutter speed by pumping up the ISO earlier. Not sure about the K-7.

Yeah this is kind of what I was wondering, that is: is the goal of Auto ISO in Av to keep shutter speed fast enough that SR is effective for a stationary scene, which would still allow blur of subjects in motion (an effect that would be exaggerated by widening FL as you stated).

I suppose there are plenty of situations I've been in where I have not been aware of shifting ambient light, or when I move into shadows, and thus don't change ISO to something more appropriate. In such cases I think Auto ISO in Av mode would be useful. But it sounds like for anything moving where I want to control DOF TAv is the way to go.

03-29-2011, 09:04 PM   #5
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You have a K-7 so why not just use TAv? It's a killer feature.
03-29-2011, 09:13 PM   #6
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QuoteOriginally posted by twitch Quote
You have a K-7 so why not just use TAv? It's a killer feature.
Yeah, I'm going to start playing with it. It's just a shift from the way I've approached managing shooting parameters since I got my camera. Jumping in at a different point of the good old rock-paper-scissors dynamic of aperture, shutter speed and sensitivity.
03-29-2011, 11:19 PM   #7
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Hi V5p,

This is probably hard to understand, but it's my process.

I shoot birds, and this means quickly going from one lighting situation to another, sometimes extremely different. I've found that I need quick changes to Av, Ev Comp, and ISO, and Av priority and Auto ISO give me this with the K-7 and K-5.

I shoot ultra teles, mostly with TCs, so max apertures are relatively slow (f4.8-7.7). Before the K-5, I didn't have a wide range of apertures that I could shoot because of this and my requirements for fine feather detail (which is destroyed by noise and NR).

Here's an often encountered scenario:

Bird with some white or yellow feathers lands in bright sunlight. I need relatively low ISO, high shutter speed, and small aperture + some - Ev comp to expose reasonably to keep the white/yellow from clipping. DOF at under 10 feet is very shallow so small apertures are important

Next bird lands on a branch in shadow, backlit by a bright blue sky with some branches in the relatively near background. I need larger aperture, and + EV comp to expose the silhouetted bird so I can get some detail and color in the shadows, and I might need higher ISO since I'm limited to how wide an aperture I can choose, plus I want reasonable DOF to get as much of the bird as possible, but not too deep as I want to isolate the bird from the background branches. I am shooting a non-cooperating subject, so I still need relatively high shutter speeds for a good keeper ratio.

Next bird has white or yellow, lands on a similar branch, but in direct sunlight. I need to go to - EV comp and lower ISO.

Next bird lands in the shadow of the tree trunk. I need high ISO to keep shutter speeds reasonable just to control camera shake. I'll compromise DOF just to get the shot.

In AV priority, I can program the e-dials so I have the rear dial control Av, the front dial control Ev comp, and I can have Auto ISO highlighted on the Info screen, so a press of the Info button gives me direct control over the high limit for Auto ISO with the rear e-dial. I have the Program line set to prefer higher shutter speeds, and as JeffJS noted, the camera usually defaults to the highest ISO in this case, so in effect, I have direct control over ISO, while maintaining Auto ISO's ability to lower the ISO automatically for subsequent shots if needed. I want as much detail as possible, so normally set a relatively low ISO for the top of the range unless it's pretty dim at brightest.

I tried using the ISO button and fixed ISO settings and Auto ISO -- ISO button to raise the ISO, but that also gives you a fixed ISO, and you need the ISO button + green button to return to Auto ISO, and these were too fiddly for me. This is the only mode that allows me to control Av, Ev comp, and ISO this conveniently.

The bottom line is that Av priority/Auto ISO gives me the best setup to control the the most important shooting parameters for me quickly and instinctively for any situation. I can do all of this stuff without looking at the camera and confirm with a glance on the top LCD before I raise the camera to my eye. I'm pretty sure that few others would find this setup useful, but it's the best way for me to achieve what I want.

Scott
03-30-2011, 04:51 AM   #8
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QuoteOriginally posted by v5planet Quote
I use a K-7 and lately I've been experimenting with different shooting modes and settings within them that I never bothered to screw around with. For this reason I've only just recently discovered it's even possible to put Av into Auto ISO with the Green Button.

So my question is pretty straightforward: when you toggle over to auto ISO, what kind of minimum shutter speed is the camera trying to maintain? Does it simply set the the ISO high enough so that a stationary subject can be captured crisply at a given FL? Or will it choose settings sufficient for a target that is moving at a reasonable clip?

I guess what I'm trying to figure out is: is there really any reason to use Av mode with Auto ISO activated over just switching over to TAv and choosing both an appropriate aperture and shutter speed for your given aims?
My experience with auto ISO mode is that the camera will typically move ISO up to maintain the minimum shutter seed for the focal length. The result can be somewhat unpredictable, if you put the range of ISO too great. For that reason, many people who shoot in Auto ISO mode, have it screwed down to a very limited range. They would prefer after that to change shutter spoeed, as opposed to increasing ISO further

Also,m in flash mode, the camera will adjust ISO first, when auto ISO is used, before it attempts to use any flash. so what happens is that in Av mode with auto ISO on, in flash, it attempts fill flash only. it never considers the thought you want the flash to be the primary light.

03-30-2011, 09:46 AM   #9
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QuoteOriginally posted by Lowell Goudge Quote

Also,m in flash mode, the camera will adjust ISO first, when auto ISO is used, before it attempts to use any flash. so what happens is that in Av mode with auto ISO on, in flash, it attempts fill flash only. it never considers the thought you want the flash to be the primary light.

Very good point about the flash. I always use a fixed ISO for flash.

Scott
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