What's an effing F stop?
Let's recap what elements a photographer can use to control exposure. Exposure is made up of three elements:
- Shutter speed - how long the film is exposed to light
- Aperture - how much light is allowed through the lens
- ISO/ASA - how sensitive the film is to light.
So why is it called an F-stop (or F-number)? 'F' actually stands for fraction. The number that follows the 'f' represents the relationship of the effective focal length of the lens to its aperture. For example, F4 really means 1/4 i.e. the diameter of the aperture at f4 will be a quarter of the effective focal length of the lens. In the 35mm format, a standard 50mm lens set to f4 would have an aperture diameter of 12.5mm (50mm X 1/4).
What about the sequence of numbers written on a lens? Typically the progression is a subset of the following:
1 - 1.4 - 2.0 - 2.8 - 4 - 5.6 - 8 - 11 - 16 - 22 - 32 - 45 - 64
In this sequence the number preceding a value represents approximately double the area of the value. The number following represents approximately half the area of the value. In real terms this means that f2.8 allows twice as much light to pass through to the film as f4; f5.6 provides half the light of f4.
Clear as mud right?
What do you really need to know? You don't need to memorize any equations or do any complex mental arithmetic to understand f-stops. You only need to know that the larger f-number allows twice as much light as its predecessor and half as much light as its precursor. When you are advised to stop down a stop you are being told to half the light getting to the film. When you're told to open up a stop you will allow twice as much light to hit the film plane. This doubling and halving of values is consistent with the other exposure measurements, shutter and film speeds, which also double and half in their settings (1/250th of a second is twice 1/125th, 800 ASA film is twice as fast (1 stop faster) as 400 ASA).
What effect does the aperture have on the image?
As well as controlling the amount of light hitting the film plane different apertures have different characteristics on the resulting image. The wider open the lens (the smaller the f-number) the shallower the depth of field. Conversely, the smaller the aperture (the larger the f-number) the greater the depth of field. Why is this? Go back to your high-school physics books and look at how a pin hole camera works. What happens if you increase the size of the hole? The smaller the pin hole the sharper (and darker) the resulting image is. The same is true of lenses containing glass and this effect becomes more pronounced the closer the subject is to the camera.
Controlling exposure is a constant compromise. If I want as much depth of field in my resulting image as possible but I'm in a low light setting I would typically set the shutter as slow as could for the support I am using; rarely less than 1/60th of a second if I am hand holding the camera but slower if I am using a monopod and slower still with a tripod. I would then set my f-stop to balance the shutter speed and hopefully I would have the depth of field I needed.
When the amount of light isn't a problem and I want the maximum depth of field I can achieve I would stop down the lens to its smallest aperture (it's largest f-number). However, there are times when this isn't the most desirable setting. When I want to isolate a subject, for example, a person from a busy background, a swallower depth of field and hence smaller f-number would be preferable.
There are other considerations when choosing your f-stop and these have to do with how your lens performs at particular apertures Typically a lens does not perform at it's best at the extremes of the f-stops available. Wide open an image is typically softer and lacking a little contrast. Stopped fully down and lens can be too hard. Often a lens performs at its best 2 or 3 stops bellow its smallest aperture but you're results may vary depending upon your specific lens and camera and it is worth taking a little time to test your own glass to find the optimum settings for your lenses
What about half and third stops?
Sometimes f-stops are known as click-stops. This is because lenses often incorporate a spring mechanism that 'clicks' as you turn the aperture setting of a lens. This allows the photographer to more accurately select a specific f-stop. Prior to this most lenses had what is a called a 'continuously variable aperture'. This means that the aperture smoothly moved from one stop to another with no 'clicks' to indicate specific settings. This makes it difficult to adjust a lens without looking at the aperture ring but it does allow more control. If your light meter indicates a little more than a particular f-stop with a lens without click-stops you can set the aperture to a f-stop plus a little bit. To allow similar control some click stop lenses have half stop or third stop settings. As the aperture in modern cameras has become under the control of the camera itself and an aperture ring on the lens itself is becoming a thing of the past, fraction of stops are commonly seen and are available in the viewfinders and settings of cameras.
