009 - Aperture and Depth of Field

In previous posts I have written about the fact that aperture has an affect on the depth of field, but as usual that is only a part of the equation.  First of all let's just be clear about what I am meaning by depth of field. In layman's terms I am meaning the distance between the nearest point with good focus to the furthest point with good focus. Anything in front of the nearest point or behind the furthest point would become blurred and the further in front or behind would be more blurred. If you read more advanced descriptions it will talk about the circle of confusion and acceptable sharpness and other technical terms but basically it's what part of the image is sharp or blurry.

The other factors that also determine the depth of field are the focal length of the lens and focus distance.  I tried to cover focal length in my previous post so you could refer back to there if you need a refresher. I don't believe I've mentioned the focal distance before but approximately this is the distance from the camera to where the focus is at its sharpest. This is the point where when you are using autofocus that the camera has determined to be the best distance for a good photograph.  If you are taking a portrait photograph you would want the camera to focus on the face and more specifically on the eye(s) if they are visible. In a landscape photograph you would probably want the focus distance to be on the main subject in the landscape.  I will cover focus distance a bit more in a later post.

As there are three main factors which affect depth of field I think it might be best to remove one of those factors to make it a bit simpler to illustrate. So for the remainder of this post I will concentrate on aperture, focus distance and depth of field on my trusty 50mm fixed focal length lens. Another factor is the sensor size and my camera is a cropped sensor with a crop factor of 1.6 so any figures I report will be for this camera unless I state otherwise.

As I have mentioned previously, when you are taking a image the wider aperture will let in more light but will reduce the depth of field.  In a portrait photograph this may be the effect you want where the face is in sharp focus but the background is blurred so that it simplifies the image to concentrate on the main subject. Calculating the depth of field is complicated but there are plenty of calculators about which will work it out for you. I use a application on my iPod Touch (PhotoCalc) which is very easy to use and also has lots of other useful information.

I'll use a real world example to illustrate depth of field for a portrait. Let us say that the model is 5 feet (approx. 1.5 meters) from the camera and the aperture is set to f2.8 then the depth of field would be from 4 foot 10 inches to 5 foot 2 inches or 4 inches (100mm) in total.  Not much really, that's from the end of my nose to the front of my ear assuming the camera focuses at 5 feet. If the camera focuses on the end of my nose then my eyes could be a bit blurred with having such a big nose. If I set my camera to portrait picture style it was tending to set the aperture to f4 which gives me an extra 2 inches of depth of field which is a more realistic 6 inches (150mm) when using auto focus.

Now if you have the model a bit further away so you get more of them in the frame the figures are a bit different. Lets say the model is 8 feet (approx. 2.5 meters) from the camera then the aperture is set to f2.8 the depth of field becomes 10 inches (250mm) and at f4 increases to 14 inches (350mm). At the bigger distance you will get more of the model in focus and start getting the background in focus if it is near to the model.

If you are wanting the background in focus then you might want to stop the aperture down a bit further. In the automatic modes this might be where you change from portrait style to fully automatic to even landscape picture style. With the model still at 8 feet from the camera f5.6 will give 1 foot 8 inches (500mm) depth of field and f8 will get you 2 feet 5 inches (nearly 750mm).

If you have good eye sight and want to switch your lens to manual focus you could reduce the depth of field even more my opening up the aperture even more. This could enable you to make an image where only the eyes are in focus and the rest of the face becomes a little blurry (very artistic).  You would have to be in one of the creative modes rather than the automatic ones because the portrait picture style on my camera doesn't go to those lengths. Creative modes are for later posts when we start to find the automatic modes a bit too restrictive.

What about real world examples for landscape photography. Generally the aim here is to get as much of the scene in reasonable focus. Say we're in a woods and furthest distance you can see is about 65 feet (20 meters) and the nearest object of any size is 16 feet (5 meters) and there's plenty of light. Assuming the camera will focus on the nearest object then at f8 you would get about 10 feet (3 meters) in depth of field from about 12 feet to 22 feet or 4 foot in front of the subject to 6 feet behind. At f11the depth of field goes up to 15 feet and at f14 jumps up to 22 feet with about 6 feet in front 16 feet behind.

None of these are going to everything in focus and you may be running out of light if you close the aperture down any more. So what is the answer? As usual it's a compromise. The first thing to ask is how much you actually need in focus? Is there anything in front of the main subject you're really bothered about and how much detail do you need in the background?

 Let's say there is nothing in front of the main subject, so you could focus a bit further back into the scene to gain a bit more sharpness in the background. If you were to set the aperture back to f11 and then focus to 25 feet, the near limit for the depth of field would be about 15 foot and the far limit would be almost 70 foot. This would get your whole scene in reasonable focus although your main subject may not be as sharp as it could be. To move the focus to a different distance you could either focus manually or point the camera to an object further back, half press the shutter and then recompose.

Last example for this post. Another landscape scenario where you're on the top of a hill and you can see for miles. The nearest object of any size is 100 feet (30 meters) away and your aperture is set to f11. Now if the camera focuses on that nearest object then the near limit is about 30 feet but the far limit is out to infinity.  However there are parts of the scene which are closer that 30 feet which are going to be blurry and maybe you wouldn't mind being a bit sharper.

One option would be to use the hyperfocal distance which is simply the nearest point at which the far limit is infinity. On my calculator app. this is 39 feet 3 inches which gives a near limit around 20 foot. Maybe this is fine but perhaps you want just a bit more sharpness in the foreground. You could focus a bit closer but then you're going to lose the sharpness in the distance. It would probably be better to shut down the aperture to say f14 where the hyperfocal distance becomes around 30 feet with the near limit down to 15 foot. You might think about closing the aperture further but you can then hit problems with diffraction which will counteract the extra depth of field by making the whole image less sharp. All lenses are different but I would tend to make my limit f16.

There's a lot of numbers in this post and it's a bit long so I apologise for any confusion that combination may cause.  In summary for portraits you would probably want to have a wide aperture to reduce the depth of field and for landscapes you would tend to use a smaller aperture to increase the depth of field.  The focusing distance can make a big difference for both portraits and landscapes where generally the bigger the focus distance the greater the depth of field.

Next time let's add in the focal length dimension into the mix and see how that affects the depth of field.