Strange settings when using auto ISO

[gwinegarden]

I was just looking at some photos that I took recently and found some of the picture very noisy.  When I looked at the metadata, the settings were a bit strange.

I was using a t5i with the Canon 55-250 IS STM lens.  The camera was set to Apeture priority @ f11 and Auto ISO.  An example of the settings, on the noisy photos, was 1/400 with ISO 6400!!  This seems odd.  I can see why it might pick that ISO at 1/400 but why choose that speed?  The others had similar settings.

On other photos, all seemed more correct e.g. 1/100 @ ISO 320 & 1/160 @400.

To me, the issue is that the camera is selecting too high a shutter speed, at certain times.  I do not know if Auti ISO has anything to do with it.

Any thoughts would be, greatly, appreciated.

[Waddizzle]

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@gwinegarden wrote:

I was just looking at some photos that I took recently and found some of the picture very noisy.  When I looked at the metadata, the settings were a bit strange.

I was using a t5i with the Canon 55-250 IS STM lens.  The camera was set to Apeture priority @ f11 and Auto ISO.  An example of the settings, on the noisy photos, was 1/400 with ISO 6400!!  This seems odd.  I can see why it might pick that ISO at 1/400 but why choose that speed?  The others had similar settings.

On other photos, all seemed more correct e.g. 1/100 @ ISO 320 & 1/160 @400.

To me, the issue is that the camera is selecting too high a shutter speed, at certain times.  I do not know if Auti ISO has anything to do with it.

Any thoughts would be, greatly, appreciated.

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You can go into the menus and limit the range of ISO settings the that the camera can use in ISO Auto mode.  

When I am using Av mode, I prefer to dial in an ISO value.  I do not want the camera to control more than one leg of the exposure triangle, because undesirable things can happen, just as you have experienced.

You need to be careful of when you lock in focus in One Shot mode because it also locks exposure.  This can be an issue when you focus and recompose, or when you use spot focus, which only meters with the center AF point in your camera.

[gwinegarden]

Thank you.  I realise that I can limit the ISO (I had set it to 6400) but this does ot explain what I believe s the issue which is why it chose 1/400.  

As I indicated, I am not sure if this is a metering issue or is caused by the Auto ISO being on (or both).  The camera was checked out a year ago, by Canon.

I will be avoiding Auto ISO from now on.

[Waddizzle]

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@gwinegarden wrote:

Thank you.  I realise that I can limit the ISO (I had set it to 6400) but this does ot explain what I believe s the issue which is why it chose 1/400.  

As I indicated, I am not sure if this is a metering issue or is caused by the Auto ISO being on (or both).  The camera was checked out a year ago, by Canon.

I will be avoiding Auto ISO from now on.

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The problem is less using Auto ISO, and more of an issue in allowing the camera to select two legs of the Exposure Triangle.  I doubt if your camera is malfunctioning.  May I suggest that you try using Manual mode with ISO Auto.

Also, I almost never shoot at f/11, not even for landscape shots.  I take advantage of the hyperfocal distances of lenses.  The only time I will use f/8 is with a Sigma 150-600mm "C" lens, which has an aperture range of f/5-6.3.  

[Waddizzle]

As a couple of regular members are fond of saying, "The camera will do exactly what you ask it to do."

[gwinegarden]

Well, I, certainly, did not ask it to choose 1/400 at f11.

[Waddizzle]

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@gwinegarden wrote:
Well, I, certainly, did not ask it to choose 1/400 at f11.

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What are you trying to photograph?

ISO is not exactly "exposure", as it is supposed to correspond to the type of film yo would use.  Exposure comes form aperture and shutter speed.  In digital camera, ISO is basically just a gain setting, albeit a slightly complicated gain setting.

[gwinegarden]

In this case, I was shooting an icy stream.

[Waddizzle]

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@gwinegarden wrote:
In this case, I was shooting an icy stream.

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Have you ever used a Depth Of Field chart or table.  Check out dofmaster dot com.  They have a DoF field table.

Depending upon your focal length and distance to the subject, you could probably use a wider aperture setting, and avoid high ISO issues altogether.  My advice is to not let the camera control more than one leg of the Exposure Triangle.

If you want to use Av mode, then dial in an ISO value.  Do not use ISO Auto in Av mode, which means the camera controls both ISO and Tv.  I only use ISO Auto in Manual mode, which means I control both Av and Tv.

[kvbarkley]

It might be trying to maintain a shutterspeed based on the 1/f rule, does the shutterspeed/iso depend on the focal length?

1.6 * 250 is 400.