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

Elaine wrote:

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

distance scale questions?
OK...it was as basic and simple as I thought! ! (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. ) 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!

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!