Hi Jim,
I wonder if you are perhaps struggling with the explanations because we are all so familiar with the concepts, taking shortcuts .. Apologies if I am now teaching granny to suck eggs.
So my understanding is that when you use AV, you set the 'f' factor but the shutter is automatically set? Right or wrong?
Yes
Each different mode is aiming at the same thing - a "perfectly" exposed shot. Exposure means exposing the sensor to the light aiming to get the right brightness of image, a fundamental principle with the terminology going right back to the beginnings of the art. That perfect exposure is a balance between ISO (sensitivity of the camera); aperture ("f stop" - amount of light allowed through the lens); and shutter speed (amount of time that light falls onto the sensor). Adjusting one forces an adjustment to at least one other in the opposite direction to achieve the same perfect exposure. Adjusting any of these has other effects which you may or may not desire.
The different modes on the camera do not alter any of that. What they do is change the parts of the process that the camera controls :
In the basic modes on your camera the camera controls aperture, shutter speed and ISO.
In P mode you can change ISO, the camera controls aperture and ISO.
In AV, as you said you control ISO and aperture (f stop), but the camera controls shutter speed.
In TV you control ISO and shutter speed, but the camera controls aperture.
In manual mode, you control all three.
Now it gets complicated. In deciding what to use it is important to think in terms of many situations in photography.
1. Situations where lighting is predictable - where you can predict what is going to be required and set accordingly.
2. Situations where you cannot predict lighting.
3. Situations where you can predict that the camera will be fooled to a degree, but you don't know what the base will be.
... and so on.
Bird photography has massive numbers of special situations. For instance - white (or black) birds; shooting birds against the sky. For all these special situations people develop different approaches. Some of these are alluded to in other posts.
In general for birds you do want the highest shutter speed possible to reduce blur. Where you don't know exactly how much light a shot will require, many people usually allow the camera to set that shutter speed at the time of shooting to allow it to react instantly to adapt shutter speed to the maximum possible by using AV. Prior to shooting they will set ISO and aperture appropriately (usually to maximum acceptable values for their requirements). That way they know aperture and ISO will be acceptable, shutter speed is pot luck - but the best possible in the situation.
Maximum acceptable values ? Most people find aperture is best stopped down one or two stops from the maximum for their lens. There are two reasons for this. Almost all lenses are not at their sharpest at maximum aperture (wide open). Secondly depth of field is increased by stopping down - the distance at which focus is perfect becomes deeper. Wide open a bird may not even be in sharp focus from front to back.
As ISO is increased the image tends to become grainier - less sharp. Manufacturers have been working very hard on this. On my camera (30d) I find anything under ISO 800 pretty good, so usually shoot birds at 640. Newer and more expensive cameras can achieve similar quality at even higher ISOs.
I used the AV setting and had great photos...F8, ISO 400 and the shutter automatically set....
But I noticed that any birds on the ground moving around when I was taking the photo were blurred. The ones sitting on a fence etc turned out fine
Apart from the movement, I'd guess the intensity of lighting, overall, was better for birds sitting on the fence, so the shutter speed selected would be higher. It might help you to use centre spot metering only (if available on your camera) so that the exposure is optimized for the bird as far as possible.
Remember your eyes are far more adaptable than a camera. What you see fine, the camera may find too bright or too dark for it's ideal operation. Watch the settings the camera is selecting in the viewfinder as you shoot. You may be surprised and learn a lot about when you are likely to get good shots and when the lighting is too extreme. Remember that rule of thumb about shutter speed being 1/focal length or better for sharp shots ? That really is a minimum for birds - even for set ups with some kind of image stabilization, because that doesn't stop the subject moving !
but movement wasn't taken into account in AV. TV takes into account movement right?
Not really. That is a different issue. It is shutter speed that minimizes effect of movement. The question is how to arrive at the best possible shutter speed ?
Quite the learning process...but fun! jim
Not half
Mike.
