@mike21 wrote:Thanks for all the replies. I usually set shutter speed at 1/250 for static birds and 1/1500 for birds in flight, with an aperture of f8, which is optimal resolution for my 55 - 250 mm lens, plus auto ISO. Av would be no good because the speed would invariably be wrong. I will try Tv and have the option of exposure compensation, although the aperture will then probably be f5.6, which is the widest at 250mm. I use spot metering and spot focus with AI servo, and the centre group of focus points for birds in flight. It is interesting that some higher end models have exposure compensation. If the image appears over exposed I can get small incremental changes by increasing shutter speed slightly, but generally when there is a problem it is under exposure. when it meands adjusting the ISO.
That’s what I thought. Stop using spot metering for action photography. Use the default setting, evaluative metering. If you are using AI Servo, then enable all focus points in the camera.
The AF system will always try to lock focus on the nearest subject. More times than not, the nearest subject will be your BIF.
Perfect pictures do not come out of the camera. They often can benefit from a little post processing in the digital darkroom. Save your photographs as RAW file format, and edit them in DPP. I used Adobe Lightroom to brighten the area under the wings.
@mike21 wrote:Its usually one particular water bird out of many, about 100 or more yards away - hence spot focus, also most birds in flight are against the sky where I assume evaluative would result in under-exposure of the bird itself. I use the excellent Afinity Photo for post production - cost equivalent to several months Photoshop subscription and its mine.
How big are these birds? A distance of 100+ yards is pretty far, and a tall task for 55-250mm lens, which is roughly equivalent to a 100-400mm lens on a full frame body. The only good thing about that distance is that all of the birds would be in focus, because they would all fall within the estimated Depth of Field.
http://www.dofmaster.com/dofjs.html
Take a look at the calculator at the above link. It is old, so use the Canon EOS 7D as your camera. The 7D is the default, which is half the reason I like this site.
Look at the estimated range of the DoF. It is nearly 140 feet deep. I do not think you need to worry about spot focus. Again, I suggest that you use Evaluative Metering, and AI Servo with all AF points enabled. I would only use Spot Metering [and Focus] with a MUCH closer subject, one that was not moving, and the camera mounted on a tripod.
Your photo software should be able to make subtle adjustments in light and dark areas, assuming that you shoot as RAW.
"...about 100 or more yards away..."
That is way, way too far away. It is too far away for even a 600mm lens. Unless you are shooting a pterodactyl. 
