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Old Jul 22, 2009, 12:48 PM   #1
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Default Does the camera take IS into account in calculations?

Canon makes a number of IS lenses, which Canon claims can give you 4 stops of compensation. Do the EOS dSLR's take IS into consideration when calculating exposure? For example, if I am taking a shot in low light conditions without flash, and the camera would usually give a shake warning because the shutter speed needed may cause blur. Has the camera already factored in the 4-stop compensation from IS when it calculated the shutter speed? Or does it treat the shot as if there is no IS?

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Old Jul 22, 2009, 12:57 PM   #2
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I can't speak to the "shake warning" - I wasn't even aware the DSLRs had it (and I've owned 3 canon dslrs). For calculating exposure though - IS doesn't play a part in any exposure calculations. So if the camera calculated a shutter speed of 1/30 it would have done so with or without IS in the lens.
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Old Jul 22, 2009, 1:50 PM   #3
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Please try it and let us know. ;-)

With most cameras I've used with something like Auto ISO or Auto Exposure, where the camera is controlling more of the variables, the camera assumes you need a minimum shutter speed of 1/35mm equivalent focal length (the focal length you'd need to get the same angle of view on a 35mm camera), when adjusting exposure so that shutter speeds meet that goal.

Ditto for shake warnings. For example, if using a 100mm lens on a camera with an APS-C size sensor, I'd expect shutter speeds of around 1/160 second or faster to be the camera's goal, with it giving you a shake warning if shutter speeds are slower for any reason.

But, I don't know how Canon models behave with an IS lens. Again, try it and let us know. ;-)
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Old Jul 22, 2009, 2:13 PM   #4
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I've never seen a shake warning.

Are you sure the DSLRs have them?
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Old Jul 22, 2009, 2:27 PM   #5
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You're probably right. I just looked at the photos of the Viewfinders from Canon and Nikon models and don't see one. With my KM Maxxum 5D and Sony A700, I get a graphical representation of how camera shake may impact blur via a bar graph in the viewfinder, which appears to take shutter speed, focal length, and how steady the camera is into consideration. But, other dSLR brands probably don't give you the same type of viewfinder indication.

The Auto Exposure Algorithms should still take focal length into consideration when determining the minimum shutter speed target (using wider apertures and/or higher ISO speeds to keep shutter speeds faster in lower light). You'd have to try it to see if those algorithms take the use of an IS lens into consideration or not.
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Old Jul 23, 2009, 1:59 AM   #6
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I think it may be a camera-based vs lens-based issue.

Canon and Nikon have IS in the lenses, so you can see when you look through the viewfinder how much shake you have.

For camera-based systems the viewfinder is more shaky so it's harder to judge, hence the usefulness of the shake indicator.

I have noticed that the P mode on my 5DMkII seems to assume that you are using a lens with IS. Presumably because the 24-105 IS is the lens most commonly used with it.

So where with the 5D in P mode it would only go down to 1/50s with a 50mm lens, on the 5DMkII it happily goes down to 1/40s and even below. This is very irritating because I used to shoot P mode a lot and I am now getting soft pictures from camera shake that I never used to before. So I have had to stop using P mode with non-IS lenses.
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Old Jul 23, 2009, 5:52 AM   #7
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From what I can see, the algorithms on my Sony A700 try to keep the shutter speed at 1/35mm equivalent focal length or faster, even with stabilization turned on.

I rarely use Programmed Auto, but even with Aperture Priority using Auto ISO, it does the same thing. For example, if I use Auto ISO and a 28mm f/2, the camera increases ISO speed as needed to maintain 1/50 second.
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Old Jul 23, 2009, 6:06 AM   #8
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I just checked it again with stabilization turned off, and it does the same thing. The more I stop down the aperture, the higher it sets the ISO speed, keeping the shutter speed at 1/50 second with a 28mm f/2.
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Old Jul 23, 2009, 8:18 AM   #9
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Just to make sure it's not using something faster at shorter focal lengths only, I mounted a 70-210mm f/2.8-4 on my A700. At 210mm it's maintaining 1/320 second by increasing ISO speed as I stop down the aperture when using Auto ISO, with or without stabilization turned on.

So, in my Sony A700's case, the Exposure Algorithms don't appear to care if you're using stabilization when computing a target shutter speed. It would be interesting to compare camera behavior with a greater variety of cameras to see if some models do take stabilization into consideration for Auto Exposure purposes (i.e., use faster shutter speeds without it and slower shutter speeds with it).

I've used other dSLR models with stabilized lenses before. But, I didn't pay any attention to how the algorithms were working in that area (i.e., target shutter speeds if using any Auto modes).

For most dSLR owners, I suspect it wouldn't make much difference (since they're more likely to be concerned with shutter speeds needed to prevent motion blur versus blur from camera shake, and/or setting ISO speed manually). But, for some subject types, Auto ISO can come in handy in changing lighting so that you're not using a higher ISO speed than necessary to reduce blur from camera shake.
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Old Jul 23, 2009, 2:38 PM   #10
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To be clear the camera algorithms on the 5DMkII don't actually check whether you're using a stabilised lens or not. They have changed them from the 5D so that they are perhaps 1/2 stop more forgiving in terms of the shutter speed they will choose. (i.e. they will choose a slower shutter speed, so I find that on non-stabilised lenses I am now getting camera shake in P mode, where I never used to on the 5D).
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