Question about 7D

I was taking pics on the weekend with my 7D. I had it on high speed continuous TV priority. I noticed it often would change the aperture in a succession of shots when the conditions were the same. I realise if I'd had it all set on manual this wouldn't happen but I'm not so experienced I can judge the right settings. I was hoping the camera would get it right. I dont know if I am doing someting wrong or I have a problem with the camera.
Not sure if I am explaining this properly. Here's an example

File Name IMG_2825.CR2
Camera Model Canon EOS 7D
Firmware Firmware Version 1.1.0
Shooting Date/Time 18/09/2010 11:49:31 AM

Shooting Mode Shutter-Priority AE
Tv( Shutter Speed ) 1/1600
Av( Aperture Value ) 4.0
Metering Mode Spot Metering
Exposure Compensation 0
ISO Speed 200
Auto ISO Speed OFF
Lens EF70-200mm f/2.8L IS USM
Focal Length 200.0 mm
Image Size 5184x3456
Image Quality RAW
Flash Off
FE lock OFF
White Balance Mode Daylight
AF Mode AI Servo AF
AF area select mode Automatic selection
Picture Style Standard
Sharpness 5
Contrast 1
Saturation 2
Color tone 0
Color Space sRGB
Long exposure noise reduction 0:Off
High ISO speed noise reduction 0:Standard
Highlight tone priority 0isable
Auto Lighting Optimizer 0:Standard
Peripheral illumination correction Enable
File Size 22955 KB
Dust Delete Data No
Drive Mode High-speed continuous shooting
Live View Shooting OFF
Date/Time(UTC)

HTTP response: NOT FOUND | MIME changed to 'image/gif' | Redirected to error image by FLICKR

HTTP response: NOT FOUND | MIME changed to 'image/gif' | Redirected to error image by FLICKR

Subscribing.

What exposure mode did you have it in... Center weighted, evaluate ect? Unless everything was exactly the same in each shot (exactly where the camera was pointing as well) the exposure setting could easily change. How drastically they could change could depend on which exposure mode you were in.

The camera's metering I would guess. The scene has changed slightly between the two shots so the metering may have decided it wanted a slightly wider aperture for that shot. This is one of the reasons many will shoot in Manual.

Others may disagree but, that seems perfectly normal and reasonable to me. You've set the camera to shutter priority mode which means it will adjust aperture and/or ISO to acheive proper exposure. Additionally, I'm assuming you left the camera set for evaluative metering. Given the fact that lighting conditions may very well change from one frame to the next, it stands to reason the scene will meter differently from one frame to the next leading the camera to choose a different aperture to maintain a correct exposure. Having said all this, I can't say I'm any better at shooting that situation than you. I personally would have shot it in either aperture priority or manual because those were the only mode supported by the cameras I grew up with and it's what I'm used to.

thanks guys, I sort of thought it was probably the case, just a very slight change in lighting and conditions. This is the first opportunity I've had to use the camera like this so it's all a learning experience for me.

You took away the Exif before posting the images (that's always a bad idea when asking for help), so we can't see which metering you did use?
Evaluative?

apersson850 wrote in post #10947459
You took away the Exif before posting the images (that's always a bad idea when asking for help), so we can't see which metering you did use?
Evaluative?

I didn't do it deliberately, actually I didn't do it at all, I dont know why it doesn't show.

I've managed to copy the exif, not sure why there's a smiley stuck in there though.

Ah! I was wondering why everybody was trying to guess the metering. Obviously the exif was added just a short time ago.

Spot metering will take a reading from a very small part of the scene (2.3%). Here you're shooting black and white subjects. If the spot happens to be on a predominantly black area then you'll get overexposure, if white you'll get underexposure.

This is obviously a grey day with very constant light - perfect for manual exposure. Go into Tv mode, set your desired shutter speed, and point the camera at the grass. Half-press the shutter button and check the aperture it suggests. Then dial that aperture and your shutter speed in manual mode and you should be ready to go, with consistent exposure. Have a chimp at the first couple of shots and increase/decrease aperture if required.

thanks!
yes, I went back and added the exif.

Yep, a combination of Tv and spot metering on a black & white moving subject - An accident waiting to happen!

As already pointed out, on a day like that (looks like flat even light) Manual exposure works a treat. Just be aware of the light changing though, check the histogram occasionally and adjust as required.

hollis_f wrote in post #10947802
Go into Tv mode, set your desired shutter speed, and point the camera at the grass. Half-press the shutter button and check the aperture it suggests. Then dial that aperture and your shutter speed in manual mode and you should be ready to go, with consistent exposure.

Seriously? When I want manual mode, I set it to M to begin with, not Tv.
Set to M, select desired shutter speed, aim at something representative (grass is usually good), select an aperture that zeroes the meter. If impossible, adjust ISO and/or shutter speed until possible.
This is the way you want to work later anyway, when you now and then re-meter the grass to see if your fixed exposure still is OK.

Auto lighting optimizer is also enabled in the images. Sometimes it plays a little with the images as well. Here the main reason is the different apertures, of course.

apersson850 wrote in post #10947838
Seriously? When I want manual mode, I set it to M to begin with, not Tv.

I was trying to describe a method that was as automatic as possible. Yes, I would do it the way you suggested myself.

All right. There's always the question about teaching something that works, the re-teaching properly, or gaining the proper technique from the beginning. But I understand your purpose, so I'll not debate it.

I see a lot of people who claim to use spot meter with the various auto-exposure modes (Av, Tv, P). This thread is a good example of how that workflow will create large variations in exposure.

IMO, spot metering is not for fast burst shooting of action because then you are just telling the camera to expose whatever happens to be under the spot to a medium tone. This can vary all over the map.

Best use of spot meter is to individually meter various known tones in a scene (like grass) to come up with a correct exposure.

As was mentioned, in constant lighting such as in the example shots, there is pretty much no reason to ever use anything other than M mode.

If the clouds had been patchy and coming and going, then Tv with either evaluative or CWA would work. You'll still get some exposure variation that way, but way less than with spot.