F Stop

Wikipedia has a decent, clear explanation of f-stops, and what the f/# actually means.

If you're asking what stop points are, they are: f1.0, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 45, 64 and 90. Anything other than those numbers means it is in a 1/3 or 1/2 stop increment.

There is no "standard" f-stop as such. Each stop represents the same amount of light reaching the film/sensor plane, regardless of the lens, regardless of the camera. In other words, f/11 on any lens mounted on my Canon EOS 20D admits the same amount of light to the sensor plane. Same for my Hasselblad, Oly OM-1, Toyo 4X5 field camera, etc. When you have a 2.8 lens, that simply means that f/2.8 is the widest aperture, and f/32 is the smallest aperture. You choose f-stops based on the amount of light available, the depth of field you want, the kind of scene, etc.

An easy way to remember the standard f stop numbers is that doubling (or halving) the f number represents a two stop change. For example, starting at f 1.0, f 2, f4, f8, f16, etc are two stops apart. similarly, starting at f 1.4 (one stop less than f 1.0) the progression is f 2.8, f 5.6, f 11, f22, etc, is a two stop change. It helps me.

Each change by a "whole" f-stop represents a halving or doubling of the amount of light coming in, requiring a corresponding doubling or halving of shutter speed, if you want to maintain the same exposure.

The standard shutter speeds are set up similarly to the f-stops: each one is (roughly) half (or double) it's neighbour. For example, if you initially were using f8 and 1/250 sec. exposure, and you change your f-stop to 5.6 (doubling the amount of light admitted, you would need to decrease your shutter speed to be 1/500 sec. to maintain the same exposure.

when someone says to "under/over expose by x stops" they are referring to how they should adjust the aperture to compensate for the meter being tricked. For instance, if i am shooting a plane flying, i over expose (according to the meter) by atleast 1 stop. My lens is a 400 4.5, and i often shoot wide open so if my meter tells me to shoot at 1/500, then i'll use 1/250 etc.

Dan, sorry, but you are sooo inconsistent and this may be confusing the poor newbee:

First you talk about stops of aperture, then in the same sentence you compensate with stops of shutter speed while trying to explain yourself:

" .. they are referring to how they should adjust the aperture to compensate..." and a second later "...if my meter tells me to shoot at 1/500, then i'll use 1/250 etc..." So: either one or the other like this:

Dear John: one stop difference in exposure refers to double or half the amount of light hitting the sensor or film.

This can be achieved by closing or opening the lens up one stop, or by exposing the film for half the time or double the time. Lens apertures doubling or the aperture stop sequence traditionally goes by the numbers engraved on the lens from 1/1, 1/1.4, 1/2, 1/2.8, 1/4, 1/5.6, 1/8, 1/11, 1/16, 1/22, 1/32 etc. (1/1 being wide open and letting much light in; 1/32 being teeny weeny and shutting most light out) This fraction measures the proportional opening diameter of the iris inside the lens and since the area for light passage is proportional to the square of its linear dimension, these number do their silly steps. f/1.4 lets 4 times (2 stops more) light in than f/2.8 for example, as does f/8 compared to f/16.

With shutter speeds it is more simple: 1/60 sec exposure obviously allows twice the amount of light to reach sensor or film as 1/125 sec does, etc .

All in all proper exposure depends on getting the right equivalent aperture and speed settings:

An exposure using f/5.6 and 1/500 sec will give the same darkening of the film, sensor, i.e. the same exposure as (less light through the lens) f/8 and (longer exposure to compensate) 1/250 sec [1 stop compensation in both aperture and shutter speed]; similarly for 3 stop equivalent compensation from f/5.6 and 1/500 sec: f/16 and 1/60 sec will give you another equivalent exposure.

The photog chooses the aperture and speed combination according to his desire for sharpness and depth of field and dictated by the physical limits of his lens. You may not own an f/1 lens, I don't; they are very rare and expensive. So are f/1.4 lenses, but less so; f/2 lenses are more common and f/2.8 are everywhere. But such a f/2.8 lens maximally lets only 1/8 of the light in compared to that exclusive f/1 Noct lens ... and hence at its fastest setting it requires an 8 times longer exposure time or more sensitive film and sensor setting.

The actual number works out to be the approximate square root of the number 1 doubled. So if you take the series 1,2,4,8,16,32,64,128,256,512,1024 (each of which is the previous number doubled), and take the square root of each number you get the approximate values of 1, 1.4,2,2.8,4,5.6,8,11,16,22, and 32. I was taught that the square root values were used to reduce the size of the numbers so they would fit on the lens easily.

THANK YOU Frank! I've also been a bit confused with f stops since I'm just an amateur in photography and have had no schooling. I was wondering the difference with the f stop to the shutter speed and you explained it well.