What are F numbers?

So, now that we've discussed one corner of the exposure triangle, it's time move on and discuss another. For this blog I'm going to tell you F numbers or aperture and what effects they have on your photography.

The aperture of you camera is the hole in the lens, through which light enters and reaches the sensor or film. In all but the cheapest of cameras, this aperture is variable in size. In some cameras there are a limited number of options, for example, there may only be three settings; called close up, portrait and landscape. In more expensive cameras there is a much wider range of options, which can be selected automatically by your camera or manually selected by you.

This aperture not only controls the amount of light entering the camera over any given time but also controls the 'depth of field' in your image. I will give more detail on this soon.

The size of your aperture is indicated by the F numbers and they work in reverse order. In other words, larger F numbers indicate smaller apertures and smaller F numbers indicate larger apertures. This may seem odd but the F numbers are in ratio to the 'depth of field' of the image, which is the inverse of the aperture size.

The chart above shows the relative size of different apertures and the F numbers than indicate them, in the top line. The scale in the lower line indicates the size of the depth of field, assuming that you are focusing on the same point with each aperture. It is not to scale, as the depth of field also depends on the lens being used. In most cases it is at infinity when using F22 and would therefore, be impossible to show to scale, as you would expect.

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So what is Depth of Field and how do you use it, I hear you ask? (Must get something done about those voices in my head).

The 'depth of field' is the area of the image that appears sharp. When you focus on a subject at, for example, 3 meters, everything in the image that is 3 meters from the sensor is sharp. This forms an arc in the image. Nothing else is sharp, the items at 2.99 m and 3.01 meters are out of focus. However, they are so slightly out of focus that we can't tell. 

As we move further from the 3 meter point, objects get more and more out of focus, until eventually we reach a point where they are so out of focus we can see it. Depth of field is a measure of the distance from that point in front of your subject to the similar point behind your subject. This will vary, depending on what aperture you use.

In the image on the left I needed a wide 'depth of field'. I needed both the boat, in the foreground and the mountains, in the background, looking sharp. 

Checking the chart above, you will see that larger F numbers give wider depth of field. This image was taken at F 28, not in the chart. At this setting, the depth of field extends from the camera's minimum focus point to infinity. The minimum focus point on most cameras is at about 1.4 meters but depending on the lens you are using, this may be different, as it's the lens that controls it.

When using large F numbers, the way to get the widest depth of field is to focus on a point about 15 to 16 meters from the camera. Don't focus on a point much further away, as you will have objects near the camera out of focus. The depth of field extends from the point of focus, not the camera, so it's possible to have an obviously out of focus area close to your camera. You can, however, use this fact to create a sense of depth in an image. I will discuss this in a later blog.

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In this image of my wife, you will notice that the curtains  and even her far shoulder are out of focus. Also, if you look closely, you will see that some of her near shoulder is out of focus.

This is the result of using a small F number, F 6.3 in this case. The resulting depth of field is very narrow, barely enough to include her head. This ensures that all of the attention is on the subject and not something in front or behind her.

As you would expect, a narrow depth of field is ideal for portraits or any image where there is a lot of distracting detail. It is vital that, when using a small F number, you focus on the most important item in the image. In the case of portraits, that is the eyes. When working with a narrow depth of field it's important to ensure that your subject is sharp and draws all of the attention.

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Now you have seen how the two extremes of F numbers effect your image and what they suit best, go out and experiment. People will tell you that the rule is - "use a small F number for portraits, use a large F number for landscapes" but there are no rules in photography, just guidelines. The best way to learn is to use the guidelines until you are confident with them and then break the rules and have fun.

In the image on the left, I used an aperture of F 9. Why? I hear you ask (those voices again).  Well, you have to balance the three points in the exposure triangle, namely - ISO, Aperture and Shutter Speed. I had chosen an ISO rating of 640, a compromise between speed and grain (see ISO blog, for details). To get a shutter speed that would allow me avoid camera shake, I need a wider aperture or smaller F number(the same thing, as you know by now). At F 9 the waves would all appear in focus and I was able to use a shutter speed of 1/250 second.

You will need to experiment with your camera and lens, as the depth of field will vary depending on what lens you use and how far out your are focusing. Stick with the "rules" until you're comfortable with what you're doing, then go crazy.

The size of the aperture will always be the same. The effect is changed by the lens and the position of the glass within it. 

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