Designations (mm, f/, etc.)

[Sicily1918]

OK, here're a few questions... trivia, if you will :D



When your lens has a 1:4 - 5.6 designation (or whatever numbers), what is that a measurement of? What's the ratio all about?
On the f/stop subject -- just how did we arrive at these numbers?
The mm rating on a lens -- what does it mean when it's 50mm? What length are we measuring here?
P.S. -- I know how they relate to photography, I just want to know what they're measurements of. For example, the Hertz ratings in CPUs (500MHz, 1GHz, 3.2GHz, etc.) measures how many cycles per second the CPU can handle, RPMs measure how many times something rotates in one minute, and so on.

Thanks -- next time maybe we'll have existential questions :mrgreen:

[sid]

OK, here're a few questions... trivia, if you will :D

When your lens has a 1:4 - 5.6 designation (or whatever numbers), what is that a measurement of? What's the ratio all about?
On the f/stop subject -- just how did we arrive at these numbers?
The mm rating on a lens -- what does it mean when it's 50mm? What length are we measuring here?
1: Thats the aperture of the lens.
2: Look here for a looong explanation http://www.uscoles.com/fstop.htm Thats the best I can do.
3: The mm measures the focal length of the lens

[Sicily1918]

[/list]1: Thats the aperture of the lens.
2: Look here for a looong explanation http://www.uscoles.com/fstop.htm Thats the best I can do. 3: The mm measures the focal length of the lens
Nonononono.... What's the number for? If I have an aperture of 2.8, it's 2.8 what?
Cool, I'll read that... thanks! :)
OK, so it can focus out to 50mm? That doesn't work... once again, what is it a measurement of? Is it that every element in the picture is 50mm in real life? Is it like in NASA probes, where the resolution equals some distance (like 3km/pixel)?

[Jim_T]

This stuff is all on the web and easily found with Google :)

The focal length deals with the physical length of the lens.. It's measured in millimeters... 50mm is a touch under 2 inches. Modern compound lenses behave like they're a certain length, but they can be made shorter due to the way they're constructed.

You've probably noticed when you fully extend your 100-400mm lens, it's somewhat longer than your 50mm lens. This is because it has a longer focal length.

Here's a site explaining focal length..

http://www.photoxels.com/tutorial_focal_length.html

[Sicily1918]

This stuff is all on the web and easily found with Google :)

The focal length deals with the physical length of the lens.. It's measured in millimeters... 50mm is a touch under 2 inches. Modern compound lenses behave like they're a certain length, but they can be made shorter due to the way they're constructed.Ah! So, basically, it's a holdover from a long time ago when my 400mm really would be that long (from first to last lens, I assume).

Very cool... thanks for the input! :)

[Bodog]

The f stop is a ratio of the diameter of the aperture opening to the focal length of a given lens. So an f stop of 2 on a 50mm lens would equal an aperture opening of 1/2 X 50, or 25 mm. f4 would be 1/4 X 50, etc. Smaller (numerical) f stop, bigger opening, larger f stop, smaller opening... :D

[kb244]

http://www.photo.net/photo/tutorial/

There ya go.

[jrobert]

Nonononono.... What's the number for? If I have an aperture of 2.8, it's 2.8 what?
Cool, I'll read that... thanks! :)
OK, so it can focus out to 50mm? That doesn't work... once again, what is it a measurement of? Is it that every element in the picture is 50mm in real life? Is it like in NASA probes, where the resolution equals some distance (like 3km/pixel)?

F/2.8 means the diameter of the lens' aperture is equal to the focal-length of the lens divided by 2.8 (hence the 'f' in 'f/2.8'). For a 50mm lens, that would be 50/2.8 = 17.85 millimeters. But since the ratio of the aperture's diameter to the lens' focal length determines how much light the lens delivers to the film, the actual size of the aperture usually isn't of any interest.

The focal length is not the overall length of the lens, but the distance behind the lens (actually, behind the rear-most element, I believe) where the image of a subject infinitely far in front the lens, would be in focus. This is where the film or sensor needs to be placed, and is the distance that you change when you 'focus' the lens on a subject nearer than 'infinity' (most of the subjects we take pictures of, unless you're an astronomer).

Hope that helps a little.

-jeff-