# Lense speed?

Discussion in 'Photography' started by Jasuhn, Aug 19, 2003.

1. ### JasuhnGuest

How do you know how fast a lense is?

JASON

Jasuhn, Aug 19, 2003

2. ### Jan PhilipsGuest

That means the maximum aperture.

Jan Philips, Aug 19, 2003

3. ### Rod O'ConnorGuest

Actually, a 1.4 is 4 times faster than a 2.8, don't forget f 2....

Rod O'Connor, Aug 19, 2003
4. ### J CGuest

Check your math and I think you'll find that the f/1.4 is FOUR times
as fast as the f/2.8. You cannot compare the f/stop number. You need
to actually calculate the AREA of the aperture as follows:

50 mm / 1.4 = 35.714 mm = diameter of the aperture
and the area of this opening is calculated with the formula for
calculating the area of a circle (pi x radius squared)
So the maximum area of the aperture in a 50 mm 1.4 lens is
3.14 x 17.857 x 17.857 = 1001 square mm

and for the f/2.8 lens the calcuation is
50 mm / 2.8 = 17.857 mm = diameter of the aperture
so the area of this aperture is
3.14 x 8.928 x 8.928 = 250 square mm

1000 is four times 250 so the 1.4 f/stop lens lets in FOUR TIMES as
much light as the 2.8 lens, not twice as much. A lens with a max
aperture of f/2 would let in twice as much light as one with a max of
f/2.8.

By comparison, we know that at f/11 the shutter lets in twice as much
light as it does at f/16. That is because a 50 mm lens at f/11 has an
opening of 16.22 square mm, whereas at f/16 the opening is 7.66 square
mm (and at f/22 the area is 4.05 square mm). Whereas your math above
would lead one to believe that f/11 let in twice as much light as f/22
and we know that is not true

HOWEVER to be precise the above calculations do have rounding errors.
AND it should be noted that as the aperature gets smaller it is less
of a circle and more of a polygon so the above calculations using the
area of a circle are slightly flawed.

-- JC

J C, Aug 19, 2003
5. ### J CGuest

NOTE that in this post I did not proceed to further elaborate that the
diameter is only part of the comparison, ... because I assumed that
people would have a basic grasp of geometry and therefore realize that
the diameter had an impact on the area of the aperture. However, as I
see in another post this is not the case and needed to be further
explained.

-- JC

J C, Aug 19, 2003
6. ### Dan UnekenGuest

I would warn against using f 22.
Stopping down the lens a few stops will eliminate or reduce many lens
errors (aberrations) .
But using very small apertures such as f 22 does not increase general
sharpness in a picture, it increases depth of field (bringing nearer
to be "in focus" those areas further away and closer to the camera
that have not been focussed on), but not general sharpness. On the
contrary, using the smallest aperture on a lens will almost certainly
reduce sharpness due to diffraction of the light around the diaphragm
edges. Every lens has its optimum sharpness somewhere in the middle
aperture values, which may safely be estimated at around f 8 on 35 mm
camera lenses.

Dan Uneken, Aug 22, 2003