I'm sure this is a dumb question! Focusing past the infinty mark?

52Caddy

I read the advice about shooting the moon using the infinity mark for focus, but since you can turn the ring past that mark, what is the purpose?
Thanks,
Eric

gtc

infinity varies
actual infinity varies with heat/cold changing the focal length of the lens-the barrel expands/contracts. sometimes you may hit the infinity mark and things are not in focus and you need to move it a little more forward or back to achive correct focus

52Caddy wrote:

I read the advice about shooting the moon using the infinity mark for focus, but since you can turn the ring past that mark, what is the purpose?
Thanks,
Eric

gtc

however
i could however be wrong...temperature may not be the only factor at work here-someone else may like to chip in here?

gtc wrote:

actual infinity varies with heat/cold changing the focal length of the lens-the barrel expands/contracts. sometimes you may hit the infinity mark and things are not in focus and you need to move it a little more forward or back to achive correct focus

ScottMcLeod

gtc wrote:

i could however be wrong...temperature may not be the only factor at work here-someone else may like to chip in here?

I can.

How can a lens autofocus to infinity without being able to go beyond it?

The answer is, it can't. To my knowledge the reason for being able to focus to infinity is to allow for the camera to determine autofocus at extremely far distances.

But then again, I could however be wrong as well...

David_S85

I googled your question
look halfway down on this page to the subject of Focusing:
http://www.kenrockwell.com/nikon/1735.htm

Safari

David_S85 wrote:

look halfway down on this page to the subject of Focusing:
http://www.kenrockwell.com/nikon/1735.htm

On first reading, that didn't add any clarity for me. But on further thought I came up with the following, which may or may not be correct and may or may not help:
It just does not make logical sense (to me) to focus "beyond infinity" - how can you focus on something that is further than infinity (indeed, how can anything BE further than infinity?) However, the process of focusing, either manual or auto, is a classic simple "feedback loop" - you move in one direction so long as you get "positive feedback" and reverse direction when the feedback becomes negative. Here, we move lens elements in one direction so long as that improves the focus, then reverse the direction when focus ceases to improve and begins to degrade. This requires that the focus ring be able to move slightly past the infinity mark - but does not mean that the lens is actually focusing at a point further than infinity.
I'm not even sure this relates to the original question, but for what it's worth.......

gtc

give and take
its a little bit of "give" in the focussing helicoid to allow focussing to infinity when the lens has expanded in hot temperatures and thus changed the tolerances and therefore the focal length-eg a 200mm prime becomes a 200.025 mm on a hot day.

if the give wasnt there ,and it was hot ,one would not be able to focus to infinity

the reverse would apply in very cold temperatures-you would be able to focus to infinity before you reached the infinity mark-therefore the infinity mark is a point at where infinity should be at a given temperauture.

Safari wrote:

On first reading, that didn't add any clarity for me. But on further thought I came up with the following, which may or may not be correct and may or may not help:
It just does not make logical sense (to me) to focus "beyond infinity" - how can you focus on something that is further than infinity (indeed, how can anything BE further than infinity?) However, the process of focusing, either manual or auto, is a classic simple "feedback loop" - you move in one direction so long as you get "positive feedback" and reverse direction when the feedback becomes negative. Here, we move lens elements in one direction so long as that improves the focus, then reverse the direction when focus ceases to improve and begins to degrade. This requires that the focus ring be able to move slightly past the infinity mark - but does not mean that the lens is actually focusing at a point further than infinity.
I'm not even sure this relates to the original question, but for what it's worth.......