Originally posted by ramshot PDL, isn't pressing the AF button to auto focus, and pressing the shutter release half way to auto focus, performing the same function?
Jeff
In a general way - yes - the half press or the AF button engage the AF subsystem, but it is how and when it is used that separate the concept.
Here is my put. Ever since I have been into photography, the act of taking a picture is basically the same. I focus on the thing that I want in focus, use what ever black magic I have to screw up the exposure - then I frame the image and shoot. During this period of time the exposure meter was always linked to the shutter button - so I had to push the shutter button to activate the light meter. Focusing was done by choosing the subject and using the split screen prism and ground glass mat screen to determine what I wanted to be the focus point. The object used to set the exposure was not necessarily the same object as the focus.
Now enter the autoeverything SLR (my first and most frustrating one was a Pentax SF-1). With autofocus the object in the center of the focus screen was put into focus when I pushed the button as was the exposure. Back in those days, at least for the Tokina 28-70mm I got with the camera, to manually focus you had to turn off AF or you would chew up the AF motor - no clutch on the lens. This meant that I how had the camera making two decisions for me right out of the box and I take that to mean that the camera is making choices that are mine.
Now comes the DSLR and the autofocus default settings are still there - the camera wants to make those decisions for me again. But - I go to a week long photo workshop and the three pros who are instructors - to a person - said 'de-couple AF from the shutter - take back control'.
Here is a scenario. I decide to take an image of some flowers but I do not want the one in the center to be in focus - I want the one in the back to be in focus. I pick up the camera and pick the first flower I can see push the AF button and look at the composition - I decide that that composition is cr*p but realize that the exposure should be based on this "thing" - so I push the shutter button half way and hit AE-L. Now I want a flower in the back to be in focus, I point the center AF (I have it set to the center point) and push the AF button - but wait - a better composition jumps out - I move the AF point hit AF again and now the image has the point of interest. Now I go about composing the shot - moving the in focus image to the point in the viewfinder that makes the image work.
To do this with a coupled AF/shutter would have mean that I would have -- release the shutter to get the AF to engage, blowing the AE-L and causing the camera to hunt - again and again until I got it the way I wanted.
So Yes - half press and AF button do the same thing - in that they turn on the AF circuits and focus. But, with the AF de-coupled it is up to me (you know the photographer) to decide what is in focus. So No they are not the same - the way I have it set up - I am more comfortable and have greater control over the function of the camera. I spent many years experimenting with DOF and using the focus ring to set the upper and lower limits to use DOF to the maximum. Letting the camera choose what is in focus basically gives you some engineers best guess - take control - it is more interesting.
AF is a way to assist the photographer to get the shot they want; moving the function from the shutter button gives me more control. Your mileage will vary.
The Elitist - formerly known as PDL
PS - the image from above. Shot with my *ist Ds - OK button used for AF. Exposure off of foreground flower, focus on the right hand flower. Kit lens, 55mm, f/5.6 (shallow DOF - on purpose) 1/250, ISO 200.