To infinity and beyond!

[shutter_blitz]

I've tried many-a-times to understand infinity as related to photography but always ended up feeling more confused than before. Time and again I pushed it to the back of mind.
I've googled it several times and found all the information confusing.

Here's is what I think I know so for:


Infinity is a focal point in the image beyond which everything is in focus, regardless of aperture size.
This point can also be quantified as 100 * focal length.Right?Wrong?

My questions:

If infinity is indeed measured as 100*FL, what is the associated unit of measurement? mm, metres?
Semantically infinity should mean something that can't measured, so why call this focal point that can be measured (1000 * FL) infinity? It is counter-intuitive.
If everything beyond infinity is in focus, this must not apply to wide apertures where everything in the background, and thus presumably beyond infinity, is out of focus or blurred. Right? Wrong?

[rc13k]

It's called infinity because everything past that point is in focus and keeps going forever and ever. Even on a wide aperture if you set the focus to infinity everything past a certain point will be in focus but that focal point is a lot further away. Focal length also changes this point. You can use a DOF calculator to check where these are, but I don't have the link right now. Maybe another user can help.

[FlyingPhotog]

http://www.dofmaster.com/

Pick a body, lens, f/stop and distance...

[iamaelephant]

Optical infinity is the distance an object is from the lens that the objects light rays can be approximated as parallel. I've never heard the 100*FL rule but it depends on the object size also.

[shutter_blitz]

I tried downloading the DOF software but didn't work for some reason. It freezes everytime I click on the application.
I tried some of the DOF calculators online but that didn't give me any insight into the point of infinity. I appreciate the two replies, but I am more confused than before.
I have attached a random test image I took with my 70-200 for some objective analysis.
The exif in the image doesn't list the focusing range that was set on the lens and I don't remember what it was set to, it probably must have been 1.4m - infinity. How would I calculate the point of infinity in this image?
http://farm4.static.flickr.com/3150/2541284969_d95de378ee_b.jpg

[rc13k]

This pic doesn't have infinity. It has a narrow DOF. Basically infinity means everything at the back part is in focus but to achieve this you have to focus on an object that's further away or using a wider focal length. Using a smaller aperture will also help.

[kniteshade]

If infinity is indeed measured as 100*FL, what is the associated unit of measurement? mm, metres?It would be whatever units FL is in. So a 50mm lens would be 100x50mm = 5m
Just means if you stack 100 lenses end-to-end its where the infinity point (or the point of infinite focus) would be



If everything beyond infinity is in focus, this must not apply to wide apertures where everything in the background, and thus presumably beyond infinity, is out of focus or blurred. Right? Wrong?

I believe the 'infinity' focusing only works if the camera's focus ring is set to infinity. But if your rule of 100*FL is correct, then yeah a 10mm lens should perfectly focus everything beyond 1m (which makes me believe the 100*FL rule must be somewhat incorrect)