Understanding F stop

Understanding F Stop


As a beginner photographer, you might have heard of such terms as f-stop or f-number and wondered what they actually mean.



The ‘f’ stands for focal length. The number following it is a fraction of the focal length. So to calculate the size of your aperture at a certain f-stop you have to divide the focal length by the fraction. For example, if you are shooting with a 200mm lens at f/4 the diameter of the aperture is 50mm.

Here are a couple of f-stop settings examples:

  • A 50mm lens with the aperture of f/2 = a lens opening 25mm wide (50mm/2).
  • A 50mm lens, with the aperture of f/8 = a lens opening 6.25mm wide (50mm/2.8).

This is what the aperture scale looks like (not to scale):

How the F-stops Affect Your Image?

The most important thing to know about these f-stop numbers is that, from each number to the next, the aperture decreases to half its size. If you are changing from f/2 to f/2.8, you are halving the exposure. In doing so, you’re halving the open area of the aperture in the lens. By this, allowing 50% less light through the lens (1 f-stop). This is because the f-stop numbers come from an equation used to work out the size of the aperture from the focal length of the lens.

Let’s say you are photographing portraits of someone at the beach during sunset. As the sun keeps going down, you will have less and less light which you need to compensate somehow. One of the possible solutions is to open up your aperture, letting in a bigger amount of light through the lens onto your camera’s sensor. Jumping up an f-stop will brighten up your image but also cause a change in the depth of field.

If you are shooting with a wider aperture such as f/2.8 you will notice that the area of focus is much smaller than if you were using a bigger f-stop, for example, f/5.6 or f/8. So to put it this way: The wider the aperture is, the shallower the depth of field.

You can get very creative with adjusting your f-stops for a different depth of field but you also need to be aware of how to do it properly. A wide aperture can easily make your picture to be blurry in undesirable areas. Especially in portraits where if you use a very small f-stop such as f/1.8 your subject’s nose could be out of focus while their eyes are still sharp. So, in this case, the area of focus can be less than 10mm, which is a very shallow depth of field.

ISO Stops

Let’s start with the easiest to understand: ISO. One-stop up from ISO 100 is 200. And one-stop up from ISO 200 is 400.

The intervals aren’t equal but, instead, the ISO doubles between stops. Easy enough to understand, so I’ll leave it at that.

Be careful though, the bigger you set the ISO the more noise your image will have! Why? Because what raising the ISO does is that it makes the sensor of your camera more sensitive to light. The camera does this with an increased electric charge and the noise on the shot is the by-product of this adjustment.

Although you should not be afraid of stepping up your ISO. This can help you to be more flexible with adjusting shutter speed and aperture to achieve different ‘effects’ in your for your photo. For example, if don’t have that much light and your aperture is also as wide open as possible, adding some ISO can help you to expose your shot correctly.

Shutter Speed Stops


The majority of the time when you use your digital camera, you’re shooting at a fraction of a second. If you shoot at speeds of 1 second or longer, the same principle as above applies. You double the time from 1 second to 2, then from 2 seconds to 4. Simple.

When shooting at a fraction of a second, such as 1/200, to double this number, halve the denominator (the number on the bottom of the fraction, in this case, 200).

If you’re new to photography, don’t worry; this will soon become second nature.

1/100 is twice the length of 1/200 so that’s one-stop and the exposure is doubled. 1/50 is twice the length as 1/100 and so on.

Cityscape at night

Let me make this simpler for you:

You’re shooting at f/2.8, at 1/100 of a second, with an ISO of 200 but you want a very shallow depth of field. You know that widening your aperture opening to f/2.0 will produce a shallower depth of field.

Perfect! But it will also triple the amount of light that’s entering your camera lens. You have jumped up two f-stops with your aperture and made the exposure brighter.

You need to counter this with a change in shutter speed or ISO. To do this, you can halve the ISO to 100 or double the shutter speed from 1/100 to 1/200 of a second.

Blurry field with a castle in the background

So you see, this is quite important to know.

To briefly summarize, increasing the exposure by one-stop will double the exposure and decreasing the exposure by a one-stop will halve it.

But Wait, There’s More!

You will have noticed with your aperture, shutter speed, and ISO that there are more intervals than just doubling and halving exposures. For example, between f/1.4 and f/2, you will also find another f-stop, f/1.8. These are third stops, which give you more control over your exposure.


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