If, like me, you enjoy spending time listening to podcasts, reading magazines, and catching up with websites all about photography, you may well think the holy grail of camera lenses are the ones with the widest apertures.
Pro's often mention the 28-70 and 70-200 f/2.8 zoom lenses, or the 50mm 1.4 and 85mm 1.2 prime lenses. These large maximum aperture pieces of glass offer the ability to shoot photographs with silky blurred back grounds by way of offering a small depth of field.
The thing that doesn't get mentioned is the challenge that these lenses also offer, which mainly is how easy it is to take out of focus shots. Having just picked up a 50mm f/1.8 I thought it might be helpful to go over some of these difficulties, offer some solutions, and help us get the most out of these great pieces of glass.
I'll start off with a quick explanation of Depth of Field. It's the area of a photo that is sharp around the point at which you have focussed. A small depth of field means that only your subject is sharp, and the background and foreground are soft. Think of a typical Wedding Photo of a Bride, shot from above, with only her eyes sharp and the rest of the image soft.
With a large depth of field most of the photo will be in focus, this time imagine a landscape shot where it's nice and sharp from the front to the back.
To achieve a small depth of field you can get closer to your subject, and / or use a larger aperture (smaller f number). You can also help to blur the background by moving your subject further away from it.
One of the more effective ways to achieve a narrow depth of field is to use a longer focal length lens, or the telephoto end of your zoom, and fill the frame with your subject. Your depth of field will be small because long lenses compress a scene (because the camera is further away), making the DoF appear smaller, and the out of focus foreground and back-ground will be magnified.
To get a large depth of field, move away from your subject and / or use a tighter aperture (larger f number).
So far, so good, so what's the problem with shooting at wide apertures to get that narrow DoF look so loved by portrait and still-life photographers? The problem is that the DoF can be so narrow that it is difficult to get you want in focus, but fore-warned is fore-armed, so let's try and understand what's happening.
By using guides such as the DOFMaster we can put in some figures and find out some facts, and they may well surprise you.
Lets start off with the nifty fifty. If we were shooting a head-shot, about 1 meter away, at f/1.8 to get a really small DoF, we'd actually only have 3cm of depth of field to play about with, about 1.5 cm in front and behind of our subject, and that would probably be their eyes.
Think about that for a moment. You get in close and try to focus on the eyes. You've therefore only got 3 cm of sharpness to play with. How long is the subjects nose? Do they have any hair hanging down in front? How much are you and your subject naturally rocking backwards and forwards with your breathing? What about if you're trying to capture subjects that are moving, or it's windy?
No wonder shooting at wide apertures can be tricky. Before we tighten up that aperture we can also try a couple of tricks. Practice rolling your finger across the shutter rather than pressing it to reduce the chance of you moving the camera. Next up have your camera in continuous or drive mode, and shoot plenty of shots each time, this should guarantee you at least one sharp image.
We may have an aversion to it, but what happens if we do tighten up that aperture? If we stay the same distance away, 1m, but tighten up to f/2.8, the Dof extends to 4cm. Not much change there then. However, if we go all the way down to f/8 (what?) the Dof stretches to... 12cm. That's much better, giving us a fighting chance to get what we want in focus - 6cm in front of their eyes, and about 6cm behind, with everything else soft. Good stuff!
Let's now back up a bit and get a head and upper body portrait, say about 3 metres away with a 50mm lens. With an f stop of 1.8 our DoF is 24cm, with 2.8 it's 38cm and at f/8 the DoF is over a meter. With all of these figures getting our subject sharp shouldn't be a problem, in fact it could be that we want a narrower depth of field. We could zoom out to a longer focal length, say 85mm, or slap on an 85mm prime. If we get further away so that our subject has the same framing, at the same f stop the DoF will be the same as with the 50mm, but it will actually look shorter because of the compression effect you get when using longer lenses.
Also bear in mind that longer lenses also tend to be more flattering due to that compression effect. 85-135mm are traditionally considered nice portrait focal lengths, but there are no rules so try going longer or shorter to taste.
Perhaps with these two examples we can understand how subject distance and aperture effect our Depth of Field, and that by using a longer focal length we can make our DoF appear smaller, although if you measured it it would be the same as with a shorter lens.
Returning to the original problem of getting too many out of focus shots when shooting with large aperture lenses to achieve that soft and blown out background look, here's some things we can do:
1) When really close, at wide apertures, your Dof will be tiny. Use continuous shooting mode to improve your chances of getting a sharp shot.
2) When really close, tighter apertures that we might normally avoid with portraits still give a small Depth of Field, so will again give a better chance at sharp photographs.
3) As we back away from our subject, in order to keep a narrow looking DoF we can use a longer focal length to compress the scene, and give the appearance of a larger aperture.
As with everything photographic, practice is the key. Try different focal lengths on your zooms, or different primes, at various apertures and subject distances, but play around with the three ideas above and hopefully you'll get some sharper portraits.
Cheers, Rob.
Friday, April 19, 2013 at 9:11PM 
