aperture question

[mrgas]

quick one here...

is the size of the aperture for a given F number always the same for every lens

or does the actual size of the hole vary for a given number from lens to lens

hope this makes sense

 

[andrewc]

the f stop is a ratio of focal length by diameter of lens.

the diameter of the aperture at f2.8 of a 50mm lens will be much smaller than that of a 300mm lens at f2.8

 

[Martyn...]

quick one here...

is the size of the aperture for a given F number always the same for every lens

or does the actual size of the hole vary for a given number from lens to lens

hope this makes sense

Its diameter differs as the f number is a ratio of the focal length.

So f2 is the focal length divided by 2, a 50mm lens would be 25mm dia, a 100mm lens would be 50mm dia

 

[mrgas]

thanks for your replies

obviously something i need to read up on

again thanks

 

[grumpybadger]

So f2 is the focal length divided by 2, a 50mm lens would be 25mm dia, a 100mm lens would be 50mm dia

Which is why, for the pedants amongst us, it is written f/2

 

[Martyn...]

Which is why, for the pedants amongst us, it is written f/2

I was just glad I got it the right way around

 

[HoppyUK]

Because they are a ratio, f/numbers are constant, regardless of camera or format, ie f/4 passes the same amount of light to the sensor whether it is a DSLR lens, long or short, or a compact, or a camera phone, whatever.

 

[classcams]

Apertures of lens is a complex subject. It is always best to talk in area of an aperture, because the light passing power is 4 times greater between say effective diameters of
2" and 4". Areas = 3.14 and 12.56

 

[HoppyUK]

Apertures of lens is a complex subject. It is always best to talk in area of an aperture, because the light passing power is 4 times greater between say effective diameters of
2" and 4". Areas = 3.14 and 12.56

I don't think it is complicated at all. It certainly doesn't need to be. People get confused by the relationship between f/numbers, but as soon as you realise that they are linked a factor of 1.4x (square route of 2) it all becomes clear.

The common range of f/stops is 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22. Each one has half or double the light passing power of the one either side even though the actual numbers are related by 1.4.

It was a goon that came up with such a daft numbering system, in an effort to put strict technical correctness ahead of basic common sense. Actually, it was a Mr Waterhouse wasn't it? He has a lot to answer for