How to fine tune your exposure with spot metering Metering is often something that leads to great confusion in newbie photographers (and often in more experienced ones, too). Basically, metering refers to the process where your camera measures the light and uses this measurement to produce an image that s well balanced in terms of light and dark. Photographers often talk about exposing for the sky or exposing for the ground but what does this actually mean? Whenever you take a shot, you re opening up (or exposing) your camera s sensor to light. This is what we mean by making an exposure. The way the light enters, how much of it gets in, and how long you give it to get in are determined mainly by two things: the aperture size and the shutter speed. So, if a photographer talks about exposing for the sky she means that she will use camera settings that make the sky look the way she wants it, and that this might be at the expense of other areas in the picture being correctly exposed. Imagine you re shooting a sunset with someone standing on a beach gazing into the sky. Because the sunset is the thing you re most interested in getting right, you expose for the sky/sunset. The result of this is that, because the sky is much brighter than the foreground, the person looking at the sunset is under-exposed and comes out as a silhouette, which doesn t matter in this instance as you re not bothered about recording any details of the person. This is what is meant by exposing for the sky. And of course it doesn t have to be the sky you can expose for anything. Why default settings don t always work Usually, your camera s default or factory setting will be for matrix (aka evaluative/ multizone/honeycomb/segment) metering, and set like this it will take light measurements evenly from all over your frame and then set the exposure using an average of the readings. Most of the time this is accurate and reliable, but in certain situations things start to go wrong. For instance, if you shoot into the light, or shoot against a very light or very dark background, you re likely to get an over- or under-exposed shot. You can use exposure compensation to rectify this, but it s not always easy to get the setting right, you only have two stops to play with in each direction, and it can take some trial and error. In the example above, of shooting the sunset, your camera will aim to get a good all-round exposure and you might end up with a slightly washed-out sunset and a slightly better exposed person. Technically this is the best all-round exposure, but artistically you want something entirely different. This is where spot metering can be very useful To take complete control, spot metering is the way to go. When you use this, your camera s metering system takes its reading from a tiny area of the frame instead of all over. The actual percentage of the total image area used varies from camera to camera, but it s often as small as 1.5%. check your camera manual if you want to find out the exact figure. By positioning the spot it uses over your subject, your camera will produce a good exposure for that small part of the image frame, giving you much greater control over the result. This isn t so crucial if you re shooting a large area like a sky, but if you wanted to photograph a person standing under a street lamp, and it s important that his face is correctly focussed, then you need your camera to take its readings from a very small area of the picture frame ie, his face. Most compacts and many entry-level DSLRs or CCSs use the centre of the screen for spot metering i.e. you place the centre point over your subject. However, more sophisticated cameras allow you to tie your choice of focus point to the metering point. What this means is that it will spot meter from 1
wherever your active focus point is and if this is somewhere other than the centre, then that s where it will meter from. If your camera has this option, then bear this in mind for later experiments. However, the easiest way to get started and for many there is no other choice is by using the central point to set the spot meter. For the moment, we re going to assume that this is how we ll be doing it - in a later section we ll talk about other options. Situations where spot metering is useful Where there is high contrast between your subject and the background eg, portrait of a person with a very light or dark background behind Shooting into the sun gives a very bright background which throws off your camera s usual metering abilities. A common example is shooting statues or stonework high up on a building. Even on an overcast day, the sky is so bright that you ll end up with a silhouette rather than a shot showing the details. Photographing the moon the moon is very bright against a dark background and if you use normal metering you won t get any detail in the moon itself. Image by Jason Jenkins, used under Creative Commons Licence (commercial). This beautifully detailed shot of the moon was achieved using spot metering. Very dark or light subjects eg, a white bird like a swan you re likely to lose detail in the feathers and end up with a well-exposed background but a white blob of a bird. What happens when you use spot metering? What happens is that your subject comes out well-exposed, while the exposure of the rest of the image is allowed to happen as it will. So, if you shoot a white swan for example, the swan will have lots of lovely feather detail and will come out the way you dream of, but the background will end up 2
under-exposed, sometimes looking almost black. This is usually fine, and can actually add to the effectiveness and drama of the shot. The opposite happens if you shoot a black coloured animal let s say a black dog. The dog itself will be correctly exposed, but because it s so much darker than the background, the background will overexpose and look almost white. This isn t usually as effective as the other way round, but it doesn t matter you will have captured the dog the way you want and in a portrait like this the background is usually unimportant. By using spot metering, the photographer has captured superb detail on the bird. The background has over-exposed and blown, but this doesn t matter and perhaps even adds something to the shot. Image by Matt Reinbold, used under Creative Commons licence (commercial) How to use spot metering First of all, you must be on P (Program), A or Av (Aperture Priority), S or TV (Shutter Priority) or M (Manual) on your Mode dial. You can t usually change metering modes on Auto or in Scene Modes. If you re still at the beginner stage but want to try the effects of spot metering, then use P. Set the camera to single-point AF mode you want your camera to take its reading from just one autofocus point and not all over. Consult your manual to find out how to do this. (Confusingly, on some cameras notably Sony single-point autofocus is called spot autofocus. This is different from spot metering.) For the moment, make sure the active autofocus point is the one in the centre. (We ll look at other options later.) 3
Now switch to spot metering, either by going to the shooting menu, the Quick menu or pressing and toggling the metering switch on your camera if you have one. If your subject is right in the middle of the frame, it s now very easy just place the central focus point over your subject, half press the shutter button to lock the focus and exposure and take the shot. In most situations your subject should come out perfectly exposed. However, if your subject is very bright or very dark, there is something else you need to think about your camera is likely to over- or under-expose because of the extreme light or dark and you ll need to compensate for this by using exposure compensation in the usual way ie, lighten exposure for very light subjects, and darken it for very dark ones. (You might wonder why you shouldn t just use exposure compensation in the first place well, spot metering will get the initial exposure closer to what you want, and exposure compensation is then used to fine tune it.) If your subject isn t in the centre of the frame, then you need to place the central autofocus point/metering point over your subject, half press the shutter button to lock the exposure and focus, and then recompose and shoot. The problem that can arise using the above method is that the focussing and the exposure settings are tied together, so when you place the central autofocus point over your subject and half press the shutter button, you re locking both the autofocus and the exposure setting at the same time, for the same thing. Most of the time this isn t a problem - usually the part of the image you want correctly exposed is also the part of your image on which you want to place sharpest focus. However, if you re going to significantly recompose, and are using shallow depth of field, your focussing may end up being slightly off. The way to get round this is to separate the exposure locking from the focus locking by using auto-exposure locking. Using Auto-exposure Locking By default, your camera locks both the focussing and the exposure at the same time, using whatever focussing point you ve selected. However, it s possible to lock the exposure separately from the focus, and to do this you need to use the auto exposure lock AE-L. You ll normally find this option on a button on your camera, usually on the back panel near the top. On a Canon it will be and asterix symbol *. Typically assuming that your spot metering reading is taken from the centre of the frame you place the centre point over the area you want to meter for, press the AE-L button which locks the exposure settings in and then recompose and take your shot as normal. Once the shot has been taken, the AE-L will switch off again. Some cameras allow you to press the AE-L button and let go, and it will hold the exposure for that shot. Other cameras might require you to keep holding the button down until you ve taken the shot. Check your manual to see how yours works. When the AE-L has been activated, you ll usually see a small * next to EL (exposure lock) on your LCD screen or viewfinder display. Because your focussing is now separate from your metering, you can zoom in closer to your subject in order to set the AE-L moving closer is likely to get you more accurate metering and then zoom out again, focus, and take your shot. (If you tried to do this with the focussing and metering linked together then your focussing would be all wrong once you d zoomed back out.) This is actually the most effective way of using spot metering. Spot metering isn t always fixed to a central point 4
On entry-level cameras the spot metering function is more often than not limited to a single central location, and most of what we have said so far assumes that this is the case. However, higher end models usually allow you the choice of having the metering linked to the active AF point (which can be anywhere in the frame). This is a great option as long as you want your focussing and metering points to be in the same place. Up till now we ve assumed a situation where your spot metering point is in the middle of the frame, and you ve set your focussing to single point and placed that point in the centre (ie, the focus point and the spot meter point are then both in the centre). However, if your camera allows the spot metering point to be linked to the autofocus point, then you can move your focus point from the centre to anywhere in the frame and the spot metering point will follow. The biggest advantage of this comes when you re using a tripod, as placing the central spot metering point over your subject and then recomposing isn t an option when the camera is fixed in place. Lastly, and just to make things more confusing, we should mention that there are some camera models that offer a mode that behaves like matrix metering while at the same time placing greater importance on the area containing the active AF point, thus giving you a sort of compromise between matrix and spot metering modes. You might find this useful, but until you get a sense of how spot metering works it s probably best to avoid it for the time being and experiment with it later. Finally, the rough and ready guide to spot metering If you re new to this whole spot-metering thing, you may be feeling by now that it s very confusing and more trouble than it s worth. In that case, don t worry about the subtleties just give it a try in the simplest possible way. Here s a rough and ready guide for metering beginners: Turn your mode dial to P Make sure your focussing mode is set to single point, in the centre Change the metering mode to spot metering Place the centre spot over your subject, half press the shutter, recompose if necessary, and take the shot. Most of the time this will work very well without you having to worry about the finer details. Experiment with it take the same picture with and without spot metering and see just what it can do for you. 5