Photography News | Issue 41 | absolutephoto.com Technique 59 Camera School PART 5 Here we lift the lid on all things camera related, showing how to get better results from your CSC or DSLR and providing all the info you don’t find in the manual. So, stick with us and you’ll soon be wielding your camera like a pro. This month, how your chosen focal length and where you focus affect depth-of-field...
Words & pictures by Kingsley Singleton
Depth-of-field – that part of the image which is considered acceptably sharp – is controlled by the aperture setting of the lens, and broadly speaking, large apertures (f/2, f/2.8 etc) will produce a shallower depth-of-field than small apertures (f/16, f/22 etc). However, other factors also contribute to depth-of-field. Focusing distance The first contributor is the point in the frame at which you focus. The closer you focus to the camera, the shallower the depth-of-field in the image will be at the same aperture. For instance, shooting with a full-frame DSLR and using a standard 50mm lens set to an aperture of f/4, if you focus at 1m, the depth-of-fieldmight only be around 10cm, but focus at four metres, and the depth-of-field will increase to something around 1.5m. To see this in action you can take test shots with your camera in aperture-priority mode, focusing at different points in the frame to see how the depth-of-field changes, or download a depth- of-field calculator app for your mobile device. The effect of the focus point on depth-of- field is so great that a picture focused closely using a small aperture can actually have a smaller depth-of-field than a picture focused further away at a large aperture. Try this with a macro lens, which by its design will focus very close to the camera, and you’ll see that even small apertures, like f/16, will produce only a shallow depth-of-field, with just a tiny part of the subject kept in sharp focus. Focal length Focal length also has a pronounced effect on the depth-of-field created. For example, you’ll find that using an aperture of f/2.8 at a focal length of 24mm will generate a larger depth- of-field than f/2.8 at a 70mm focal length if you shoot from the same camera position.
Focus at 50cm
Above Knowing how to maximise the effects of aperture on depth-of-field you can adapt your technique to subjects where you want lots of sharpness, or just a little.
Of course, this is how photographers usually work – if you need a wider view, you fit a wide-angle or if you want the subject to appear closer a telephoto lens is used. Without altering shooting position using a wider lens gives more depth-of-field. But, if you decide to move closer with a wide-angle or retreat a few metres back with a telephoto so the subject’s size is the same in the viewfinder, the amount of depth-of-field is the same if the same aperture is used. Again, you can test this yourself using a typical standard zoom lens. Frame up your subject in at the longest focal length, and in
aperture-priority mode, set the maximum aperture allowed by the lens (the lowest f/ number). Now focus on your subject and take a shot. Next, stay put and zoom out to the widest focal length and repeat the shot with the same aperture. You’ll find a distinct difference in how much of the subject and the rest of the scene appears to be in focus. For your next shot, leaving the lens at the same settings, take a few steps closer and frame up a shot so the subject is the same size in the viewfinder as the first picture. If you now compare this shot with your first one, you will see that the depth-of-field in both is almost identical. Putting it all together From these basic principles you can see how the aperture settings, the focal length and the point in the frame at which you focus can all be adapted to make the most of different subjects. For instance, if you’re shooting a landscape and want it sharp from front to back, you’d not only set a small aperture, but also use a wider focal length and try not to focus too close to the camera’s position. Or, if you want a portrait with a shallow depth-of-field, you’d use a wide aperture, a longer focal length and focus comparatively close to the camera.
Focus at 1m
24mmat f/3.5
70mmat f/3.5
Full image, f/3.5
Above The closer that you set the focus to the camera, the more restricted the depth- of-field will be, even when identical aperture settings are used. So if you want lots of blur in the background, focus near to the camera, and if you want more sharpness, focus further off.
NEXTMONTH We’ll take a look at the secondary effects of shutter speed and how that part of the exposure triangle affects moving subjects.
Above Shoot at wide and telephoto focal lengths using an identical aperture and keeping the subject the same size in the viewfinder you’ll find depth-of-field is similar.
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