Program for comparing sharpness of similar images

[Jonathan Wienke]

With such scenes AF shouldn't change targets between shots, but even if it does one can always go manual.

The likelihood of AF not locking on the indended subject is actually pretty good, given that BSS is intended for use when light is poor and motion blur is likely to be a problem. In such conditions, the AF system in a DSLR like the Canon 1D-MkII (which by itself costs more than some digicams) can have difficulty locking on to the correct subject and maintaining focus properly. So it's a bit naive to think that the digicam's AF is going to reliably lock focus right where you want it locked, every frame in a series. By the time you realize that the AF isn't always locking on the intended subject, you've already lost shots if BSS is engaged.

[feppe]

The likelihood of AF not locking on the indended subject is actually pretty good, given that BSS is intended for use when light is poor and motion blur is likely to be a problem. In such conditions, the AF system in a DSLR like the Canon 1D-MkII (which by itself costs more than some digicams) can have difficulty locking on to the correct subject and maintaining focus properly. So it's a bit naive to think that the digicam's AF is going to reliably lock focus right where you want it locked, every frame in a series. By the time you realize that the AF isn't always locking on the intended subject, you've already lost shots if BSS is engaged.
[a href=\"index.php?act=findpost&pid=88430\"][{POST_SNAPBACK}][/a]

If the photog presses the shutter while the AF is locked on the wrong subject, it is again an operator mistake. And if one is concerned about losing focus between shots, just lock it. Problem solved.

[larsrc]

The likelihood of AF not locking on the indended subject is actually pretty good, given that BSS is intended for use when light is poor and motion blur is likely to be a problem. In such conditions, the AF system in a DSLR like the Canon 1D-MkII (which by itself costs more than some digicams) can have difficulty locking on to the correct subject and maintaining focus properly. So it's a bit naive to think that the digicam's AF is going to reliably lock focus right where you want it locked, every frame in a series. By the time you realize that the AF isn't always locking on the intended subject, you've already lost shots if BSS is engaged.
[a href=\"index.php?act=findpost&pid=88430\"][{POST_SNAPBACK}][/a]

If you're holding down the shutter to take images as fast as the camera can handle it and use one-shot AF, the focus doesn't change between frames.

Like any other tool, it can be used or misused.  There are many situations where I wouldn't use, say, a polarizer, but that doesn't make me throw it out.

I did a simple test using the compare() function from the ImageMagick suite, which can give some sort of number for frequencies, but it didn't do what I wanted either.

[Jonathan Wienke]

If you're holding down the shutter to take images as fast as the camera can handle it and use one-shot AF, the focus doesn't change between frames.

That's not usually true. Most cameras will still attempt to refocus between frames even when shooting in continuous mode. All Canon DSLRs in their default configuration will do this. You can change this behavior, but it requires assigning AF activation to the * button via custom function settings. So that's a dangerous assumption to make without confirming it with the camera manual and actual testing.

[jani]

That's not usually true. Most cameras will still attempt to refocus between frames even when shooting in continuous mode. All Canon DSLRs in their default configuration will do this. You can change this behavior, but it requires assigning AF activation to the * button via custom function settings.

No, setting the AF focus control to "one shot" is sufficient, at least if we can believe the manual for the 1D MkII and assume that the 20D and 1D MkII behave similarly.

And as far as I've been able to determine, they do behave similarly; I've used both in situations where I'd expect the camera to refocus between frames if I used AI servo mode, but both cameras did not refocus between frames in one shot mode.

Note that this is while keeping the shutter button fully depressed for continuous shooting.

AI servo may, however, be the default configuration; I've forgotten.