Know your f-stops

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To many photo novices, the f-stop is a mystery. We’re help to help…

If you’re looking for a new camera or fancy upgrading to a DSLR – and you have little experience of cameras – you are going to get a lens full of jargon spilling into your consciousness.

There is, however, an important piece of jargon that you simply have to know. Not just because you don’t want to look dumb in the camera shop, but because it really matters. It underpins creative photography, so getting to grips with it as soon as possible can only benefit your images.

I refer to the f-stop (which is short for Focal-Stop).

We are not going to discuss every nuance of the f-stop here (you can find more in our future posts), but we’re going to debunk the jargon for the novice.

First things first

To understand the f-stop, we need to understand some other, foundation terms first:


The exposure is the amount of light that hits the camera’s sensor (or film). “Under exposed” means the image will be too dark because it does not get enough light, and “over exposed” means the image will be too light as there was too much light.

ISO rating

The ISO rating is the measure of how much light the camera’s sensor (or film in the day when film was the main medium) needs to make an image with the correct “exposure” (the lightness/darkness). In the days of film, we have “slower” (low ISO) film, and “faster” (high ISO) film. The “faster” the film, the lower amount of light was need to produce a correctly exposed image. Typically, ISO 100 would be used on bright days (and give fine resolution images) and ISO 400 when things were less bright (but giving grainier images).

In the digital world (where we have an electronic sensor instead of film), you would think this would be irrelevant, but in fact it isn’t. The ISO rating can be changed on the camera, but higher ISO ratings tend to produce lower quality images. Published tests on your specific camera will show what ISO seeing will produce the best quality images, but generally speaking, keep the ISO at a low number; the more light that hits the sensor will generally make the image quality better so set the camera to receive more light. ISO 100, 200 and 400 are typical for fine quality, whereas ISO 1,600 and ISO 3,200 will tend to produce images of lower quality.

If you are thinking that ISO 100 needs twice the light of ISO 200, you are correct. Every doubling of the ISO number means a halving of the amount of light is needed to produce a correctly exposed image. ISO 200 is 2x faster than ISO 100.

The aperture

This is the size of the hole in the lens that lets in the light. There is usually an arrangement of small, thin metal plates that change their position to make the hole smaller or larger. The aperture plays a key role in photography and it is this variable that is expressed in terms of the the “f-stop”.

The focal length

This is a measure of how “strongly” the lens bends the light to a point. Inside a lens, the incoming light is focused to a point, and the focal length is the distance between where that point is and the sensor. In a traditional 35mm format, a lens with a focal length of 50mm is the magnification roughly equivalent to how the human eye sees the world. The “longer” the lens (i.e. with a bigger focal length) will bring objects that are further away to appear “closer” in the image. Think about wild life photos as an example.

Shutter Speed

The shutter is a mechanical device – usually in the camera directly in front of the sensor (or film) that moves aside to expose the image to the sensor. The speed of the shutter can vary but it usually works pretty quick (i.e. the shutter will usually open for times shorter than 1 second).

Putting these all together means the following:
For any ISO setting, the shutter must expose the image to the sensor for a certain time period that will vary depending upon the size of the aperture.

What is the f-stop (focal-stop)?

The f-stop is a comparison between the focal length and the size of the aperture. In “technical terms” it is the focal length of the lens divided by the size (the area) of the aperture of the lens. You will see standard f-stops marked on your lens such as f2, f4.5, f8 etc

For example, a lens with a focal length of 50mm with an aperture area of 25mm will have a f-stop of f2 (50/25)
If the aperture is made smaller to 6.25mm, then the aperture will be f8 (i.e. 50/6.25)

In fact this calculation produces an f-stop numbering scheme that is the opposite to what it seems. i.e. you might image that an f22 has a bigger aperture that f8, but in fact f8 is a bigger aperture that f22. The bigger the f-stop the smaller the aperture.

It also means that lenses with longer focal lengths will usually have to be physically much bigger for the same f-stop. e.g. a 200mm lens will need an aperture area of 100mm for a F2 f-stop – for a 50mm lens the aperture size would be only 25mm.

So what’s the point of this?

Now that, hopefully, we have explored what the f-stop is, let’s discuss why it matters, and why the f-stop is the critical factor to determining how your photos turn out. This is what you need to know:

To attain that exposure, you need to open the shutter for a time period dependent upon the aperture size (the f-stop). The smaller the f-stop (i.e. the bigger the aperture) then you need to open the shutter for a smaller amount of time.

So, if you want to “sharply freeze” some action in a sport photo, you will need to use a fast shutter speed (otherwise the image will be blurred), so in this case you will need a bigger aperture (smaller f-stop). But to grab close-up action far away you will generally need a lens with a “long” focal length (say 500mm to 1000mm), so to attain say an f-stop of f2 (to freeze the action), you will need a lens with a very big aperture. And I mean big – you will have seen the enormous (and expensive) lenses used by professional photographers at football matches. They need those lenses to get close up but still have a small f-stop – getting close to the action AND freezing the action.

If you want to blur some movement, you set the aperture smaller (higher f-stop like F16) which means the shutter opens for longer (thus blurring the image).

It is also important to note that, no matter how rigidly you think you are holding the camera, your body naturally trembles and so if the shutter speed is too slow the image will be blurred. It is therefore a good idea to use a solid support (like a tripod, roof of a car, a wall or whatever is convenient) to keep the camera rock-steady. When you must hand-hold a camera, then you should use a faster shutter speed to prevent blur (the rule of thumb being the slowest shutter speed to use = 1 divided by the focal length of the lens. So, if you have a 50mm lens, then keep the shutter faster than 1/50th second (i.e. 1/125th, 1/250th etc). Do this by adjusting the f-stop.

If you make the aperture smaller, you get a greater distance that is in focus. This is called “depth of field”. So, if you want to isolate one specific part of the image (by making the rest of the image out of focus) then you need to make the aperture bigger (smaller f-stop). If you want to make a greater range of the picture in focus, then make the aperture smaller (bigger f-stop) and use a slower shutter speed.

You could also do this by using a lens with a smaller focal length – but that will create a bigger image with the object of the image being smaller in the frame

Applying these rules as a novice

In summary, here are the rules for the novice. I totally understand if you need to read these a few times and yes, you may print them:

  • Smaller f-stops = bigger apertures (i.e. f16 is a smaller aperture than f8 – opposite to what you might think)
  • To blur an image, use a bigger f-stop and slower shutter speed
  • To freeze an image, use smaller f-stops and faster shutter speed
  • To have more of the frame in clear focus, use bigger f-stop and slower shutter speed
  • To isolate a part of the image by making the rest blurred, user smaller f-stops and a faster shutter speed

All modern DSLRs (digital SLR) have a vast array of automation to help you achieve good images. But many people – even armed with that array of technology – switch their camera to manual control. Personally, I feel “manual” gives me total creative control. But other will disagree, saying the composition and framing are equally important so why not let the camera take care of some settings.

In summary

You can see that you might find it tricky to achieve the desired image, and so the art of taking the photo could be described as a “creative compromise”. As your photo skills develop, you will see that interchangeable lenses (i.e. with different focal lengths) are important to attain the right balance of f-stop and shutter speed (and magnification and framing – but that’s for another article).

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