I responded to a question the other night asking for some guidance on Depth of Field and if possible an article. I thought about this for a while and it’s quite a complex subject involving ideas such as circle of confusion and hyperfocal focusing. Personally I don’t like complexity as it tends not to be that practical in the real world. Here then is my Lightweight guide to depth of field with a Micro 43 camera.
The first thing to realise is that Depth of Field is something completely different to lens sharpness. Sharpness is about how well defined edges appear in your image. Lots of things can contribute to an image being sharp or not. Camera shake will detract from sharpness as will vibrations. Unfortunately the aperture also contributes to sharpness which is possibly why people sometimes become confused.
When a lens is wide open at its maximum aperture e.g. f/1.8 it is unlikely that it will produce its sharpest results. Whilst high quality lenses will perform well when wide open, most lenses achieve their best results when stopped down around 2 stops from wide open. My experience with Micro 43 lenses is that they usually perform well when wide open and achieve excellent results when stopped down by around 1 stop from the maximum.
Stop a lens down to the other extreme and you will see the effects of diffraction creep in. This is where the light entering the lens diffracts on the blades of the aperture. This causes it to spread and the image becomes softer. Different lenses will start to suffer from diffraction at different apertures so I can’t give you any guidance other than to say test your lenses.
Now for depth of field. The first thing to realise is that there is only one point of true focus in an image. The further you move from this point the more the focus deteriorates. Near to the point of focus you probably don’t notice this but further away the image starts to appear blurred and out of focus. The area that appears in focus to the eye is the zone of acceptable focus; remember it’s not actually in focus, only the point of focus is in true focus.
The acceptable zone of focus extends beyond the point of focus and also in front of the point of focus. This is said to be the depth of field. How far this zone extends is determined by a number of factors which include:
- The size of the sensor – the smaller the sensor the greater the depth of field at a given aperture. Micro 43 is therefore good if you want lots of depth of field. Generally speaking you can’t do anything about sensor size unless you change camera.
- The distance of the point of focus from the camera – the nearer the point of focus to the sensor then the less the depth of field.
- The aperture – a smaller aperture will produce a greater depth of field than a larger aperture on the same lens assuming the other factors are constant. Take care however as you could make the aperture so small that the lens suffers from diffraction. There is therefore a balancing act between depth of field and optimum aperture.
- The focal length of the lens – Now all you science types don’t all cry out saying there is no difference it’s just down to compression and magnification (ignore this comment if you don’t know what I am talking about). Remember, this is the simple approach to depth of field. The wider the focal length of the lens then the greater the apparent depth of field that can be achieved, all other things being equal. Putting this in simple terms, take a picture with a 14mm lens and take the same picture with the same aperture and focus point using a 45mm lens and the 14mm will appear to have a greater depth of field.
The final key piece of information is that the depth of field extends roughly twice as far beyond the camera as in front of it.
So, how to use this assuming you want to create a large depth of field from the foreground to the distance:
- By the time you are about 50m away from the camera, you will effectively have reached infinity focus on your lens. Remember, this is a practical real world simplified guide.
- Select a lens that will allow you to create the composition you want.
- Estimate how far the nearest point to you (in the frame) is.
- Estimate how far 1/3 of the distance is from this point to 50m and identify something around that point in the frame. This is where you should pick your point of focus. Don’t leave this up to the camera to decide.
- Select an aperture that gives you the depth of field you need. Until you are adept at judging this you may need to take a few shots and check them at 100% magnification on the back of your camera.
- When you check shots the foreground is more important to judge than the distance. The foreground will show up areas that are out of focus much more than the distant hills.
- If you find your image is slightly out of focus in the foreground but the hills are fine you need to move the point of focus nearer to you.
- If the hills are blurred but the distance is fine you might need to move the point of focus away from your OR use a smaller aperture. Try both.
- Try to keep the aperture within the range for optimum sharpness.
All this might sound like a lot to remember but after a while it becomes second nature. I tend to shoot landscapes with a 14mm lens set to f/8.0. I know when the focus point is well selected this will give me good depth of field on most of my compositions. Selecting the focus point becomes automatic for me based on years of experience gained by taking a picture and checking the results.
Hope this helps.