focusing on infinity

[Elaine]

OK...this sounds like a dumb question in my own head, but could someone please explain what focusing on infinity really means? I think I get the basic idea, but I'm not sure how to force it to happen or even when the best time to use it is. I imagine myself trying to do this and just getting a blur. It talks about this in the fireworks article someone posted, and I thought, "Oh yeah, OK, I can do that." But now I'm not so sure. Could someone clear me up, please?!

[ziggy53]

OK...this sounds like a dumb question in my own head, but could someone please explain what focusing on infinity really means? I think I get the basic idea, but I'm not sure how to force it to happen or even when the best time to use it is. I imagine myself trying to do this and just getting a blur. It talks about this in the fireworks article someone posted, and I thought, "Oh yeah, OK, I can do that." But now I'm not so sure. Could someone clear me up, please?!

You need a camera which allows the technique, some P&S do not.

Any dSLR, or SLR for that matter, should use lenses which can allow autofocus to be turned off. It's usually a switch on the lens itself, and it allows you to manually focus the lens.

The infinity mark usually looks like a number "8" lying down, and it represents the longest distance setting for any lens. The reason that is important for fireworks is that the fireworks display itself may not be a good candidate for autofocus, but it is almost always something at a considerable distance. If you set the lens for infinity and then use a relatively small aperture, f5.6 to f8 seem to work well for ISO 100, then you should get the display in focus.

[Elaine]

OK...it was as basic and simple as I thought! LOL! (I'm definitely not using my P&S and I am aware of how to turn on manual focus and what the infinity sign looks like. :D ) I guess I just hadn't experimented with it to actually "trust" that it would focus correctly. I think I need to take a few days and shoot ALL manual!

Now another question...at what distance does a wider aperture (say 5.6) cease to affect DOF, or does it cease to affect it? Maybe we just don't notice it because a particular scene is so far away? I feel like I've read the answer to this somewhere, but I can't remember where. Looking at my different lenses, the distance scales are all different (of course). My 28-75mm only goes up to 7 ft before the infinity sign. My 70-210mm goes to 30 ft before the infinity sign. How does this info combine with info from a DOF chart? Bryan Peterson mentions the whole DOF via aperture combined with distance scale usage in "Understanding Exposure" and that's the one part I wasn't sure how to actually use myself...combining both sets of info successfuly. If it would be simpler to send me to a link, I'd appreciate it! :D

[ziggy53]

DOF is always apparent, it just varies with focal length and aperture (mostly.)

This is a subject we cover pretty frequently:

http://www.dgrin.com/showthread.php?t=7426
http://www.dgrin.com/showthread.php?t=3377
http://www.dgrin.com/showthread.php?t=59691
http://www.dgrin.com/showthread.php?t=6653
http://www.dgrin.com/showthread.php?t=2795
http://www.dgrin.com/showthread.php?t=37613

[Elaine]

Thanks much, Ziggy! I've read through most of the links and found some helpful info. Now I think I'd better get outside and try some experiments!