What determines the F stop on your lens?

[Paul]

Okay I understand how stops work with Aperture & Shutter speed but what determines the F range on your lens and how do they (Nikon) calculate it?

[jonnydee]

Not entirely sure on this one, but I think the f-stop is a ratio of focal length to the size of the apeture. This is why the f-stop sequence of doubling reads like it does, (4, 5.6, 8, 11 etc). I think that the numbers are related to the diameter of the apeture, but give a doubling in area each time and hence a doubling in the light, or something... Hopefully I haven't confused the issue too much, perhaps someone more knowledgable than me can elaberate!

Jon

[Onyx]

F-stop is the ratio of the focal length of the lens over diameter of the hole letting light in. eg. 50mm prime, f/2 means diameter of the light hole is 25mm. But instead of saying you're shooting at 25mm aperture (which is more correct), to make it comparative with other focal lengths, some genius invented the use of ratios to describe aperture. So for example at 100mm, the same f/2 aperture will let the same ratio of light to hit the sensor or film as at 50mm, but requires a 50mm diameter opening instead of a 25mm one.

NB:
4^2 = 16
5.6^2 = 32 (approx)
8^2 = 64

When you square the numbers, you'll more clearly see the doubling and halving relationship between the seemingly arbitrary sequence of f-numbers: 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 64, 128, etc.

[Deano]

Onyx,

I found this explaination confusing when thinking of a 300mm 2.8 lens which would therefore have a 107mm aperture. I'm sure the hole in the front of my D70 isn't that big.

So I went googling and found "A Tedious Explanation of the f/stop" which answered this and a range of other questions.

Cheers
Dean