Just for information would someone please explain how an aperature is determined?
For example, I have a cook 25mm c mount for my super 8 which has a maximum aperature of .95
A canon 200/1.8 L, etc etc.
With large format 4.5 is generally considered very fast (or good for focusing at least).
How are these numbers derived?
George
The diameter of the lens opening divided by the lens focal length.
Each stop is twice or half the light, depending on which way you go.
Each f/stop divides the area of the circle by 2.
Area = PI * (R squared).
If you want to double the area, you only need to open the diaphragm by the square root of 2, namely 1.4... not 2.
So you get 1.4, 2, 2.8, 4, 5.6, 8, 11, 16.
As has already been noted, the relative aperture or f-number is the ratio of the focal length to the diameter of the lens opening. (It is actually a bit more complicated since a lens has substantial extension, and the aperture may not be just where you think it might be. But you can assume the lens is confined to a plane and the aperture is a hole in that plane for all practical purposes.)
The relative aperture is used in determining exposure and also in calculating depth of field.
Large forma lenses have largest relative apertures anywhere from f/4.5 to f/9 or more. Generally, the wider apertures (smaller f-numbers) appear in wide angle lenses and the smaller apertures (larger f- numbers) in longer lenses. Longer lenses are easier to focus at the same relative aperture than shorter lenses would be. For example, I have a 75 mm f/4.5 lens and a 300 mm f/8.9 lens.
You would almost never use a large format lens wide open. It is just for focusing.
Leonard is right, however it is fun to occasionally shoot LF wide open for special effects. These were made with a Symmar f5.6/150mm on 4x5".Originally Posted by Leonard Evens
Slow Motion
By the waterside
I think the crux of this question is how can you have an aperture of .95. i.e. an aperture which has a diameter of greater than the focal length of the lens?
Someone care to answer that?
Canon did it with their rangefinders, as a 50mm f/0.95
Some details here:
http://www.tet.uni-hannover.de/~kapa/Photo.html
http://www.canon.com/camera-museum/c.../s_50_095.html
Marc
robc, the basic answer is big glass. Besides the Canon, there have been several special purpose lenses with apertures greater than f1.
I want to quibble on one point. It's not the actual aperture that is used in figuring the stop size, it is the apparent aperture, the size the hole seems to be when looking at the front of the lens. The front cell has a magnification factor which has to be taken into account. This has some relevance to robs's question, also.
Besides exmples like those provided by Patrick Roseen, large apertures are sometimes used in portraiture so as to provide shallow depth of field or deal with fidgety kids. Many of the old brass cannons provided selective focus to an extreme.
So using my 200/1.8 this means that the diameter of the aperature (apparent or actual) is 111.11mm's?
and the cooke 25/.95=26.32mm's?
yes, at least close enough for the purpose of the explanation. The iris diameter values will be slightly different because the figures you give are for the entrance pupil which is the magnified iris diameter.
if you set the aperture at .95 and measure the diameter accurately you should be able to find the difference between iris diameter and 26.32mm from where you could calculate the entrance pupil magnifcation if you really wanted to and providing you could measure accurately.
Last edited by robc; 7-Sep-2006 at 15:41.
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