Trouble with exposure using aperture priority

[ChadS]

Folks, does my camera need to go in for repair? Here's the symptom:

Shooting aperture priority I have several times run into a situation where the exposure is set horribly slow. This morning was the most recent example. I just shot a couple of outdoor shots in full sunshine using my T2i + 70-200L f/4 IS set to f/5.6 and ISO-100 +0 EB. The first three shots all set the exposure to 1/200. I changed to f/4 and the exposures jumped to 1/1000. This is the same light, same settings (evaluative metering), same everything just a few seconds apart.

Clearly the f/5.6 @ 1/200 are overexposed, but my question is what would make the camera do that. Given that 1/200 is my max sync speed is it possible that it was trying to sync with the flash? The flash was down but is there any way to tell in the raw file if that was the case?

I've seen the problem more often with my 50mm f/1.4 as it's more likely to want a fast shutter at wider apertures. I'll have to go back and see if it was always catching at 1/200.

Thanks.

[Sean Setters]

Were these tripod shots? Or did you possibly focus and recompose? Any variation in framing when the exposure values are calculated can greatly affect the exposure.

[neuroanatomist]

A long shot, but when it's doing that, what happens when you try to pop up the flash? Thinking maybe the microswitch in the hotshoe, based on the 1/200 s sync speed (although I think that limit only applies if the flash is powered on).

[ChadS]

A long shot, but when it's doing that, what happens when you try to pop up the flash? Thinking maybe the microswitch in the hotshoe, based on the 1/200 s sync speed (although I think that limit only applies if the flash is powered on).

@neuro that's what I was thinking. Is there any way to tell in the raw file if the camera was sending a flash with this shot? If it's a stuck microswitch it could have come unstuck just from me turning the select wheel to change apertures.

[ChadS]

Sean,

Handheld and I'm certain I did a focus and re-compose on every shot. There's not a lot of DoF at f/4 or f/5.6 even. However, I find it curious that the three shots at f/5.6 are all blown out by 1.5 stops but the shots are f/4 are all dead-on. I'll upload low-res versions. You can see there's not much variation in the lighting to cause that much of a shift - not with evaluative metering at least.

These are not back-to-back shots but are the closest in terms of framing and background so their exposures should have been the closest. I walked around following these birds so the shots are over a few minutes. All the f/4 shots are well-exposed, none of the three f/5.6 shots are.


f/5.6


f/4

[neuroanatomist]

There are several relevant entries in the EXIF data (viewed using exiftool on a Mac):

Flash : On, Fired
Canon Flash Mode : External flash
Flash Activity : 146
Flash Bits : E-TTL, External

[HDNitehawk]

I would look at the exif data, I think you will find the flash didn't fire on the f/4 sample. Flash on this kind of subject can be tricky. Also another variable with this kind of shot is what type of metering you are using and where it read. You can be off just a few inches one way or the other and the metering will not be correct. The picture has quit a few light to dark spots that could affect your shot. If you can create this in other circumstances it may be something else.

[neuroanatomist]

I would look at the exif data, I think you will find the flash didn't fire on the f/4 sample. Flash on this kind of subject can be tricky.

I think the flash wasn't supposed to fire at all - it's a T2i with the popup flash down. To me, the scenes in the attached shots look similar enough that they shouldn't meter 1.5-stops differently.

[HDNitehawk]

I think the flash wasn't supposed to fire at all - it's a T2i with the popup flash down. To me, the scenes in the attached shots look similar enough that they shouldn't meter 1.5-stops differently.

So the camera decided it wanted to fire the flash, fired it but didn't pop up? Yet the picture is over exposed not under exposed. I always hated the pop up flash on the 50d and 7d, they always wanted to pop up when you didn't want them. I would update the firmware, check all the settings in the camera and possibly return the settings to default and see if you can still replicate the fault.

[ChadS]

@Neuro's suggestion I pulled all the EXIF data with exiftool (handy utility that). There are many difference between shots, obviously, but two discrepancies that I cannot explain (just looking at two photos, let's assume that pattern holds)

In the overexposed shot I have:
AE Setting: AE Lock
Raw Measured RGGB: 0 0 0 0

For the properly-exposed shot I have
Raw Measured RGGB: 142716 283278 285415 128586
and I have no entry for AE setting.

I have no idea why the Raw Measured RGGB data is all null for the overexposed shot but the AE lock does tell me what the problem with that exposure is. However (and I just tested this and it work properly) one must hold down the AE lock button between shto carry the exposure through multiple shots. While I might have accidentally pressed this button I'm certain I did not hold it down for those three shots. I'm guessing that microswitch is sticky.

At least I know what to do to get out of that problem now. Thanks for the suggestions folks.