This is the seventh in a series of articles discussing some of the photography techniques I’ve learned and employ when I’m out on a shoot.
I’ve been dreading writing this article because you can write an entire book about exposure. And in fact many people have, some I would recommend are:
- The Perfect Exposure by Jim Zuckerman
- Understanding Exposure… by Bryan Peterson
- Night and Low Light Photography: The Complete Guide by Lee Frost
If you really want to master the art of exposure, you should pick up a book and read it as it can be complex and subtle subject. I’ll give a brief overview of aperture and shutter speed, then I’ll talk about why I always use manual mode on my camera and how I use it to get the exposure right. Following articles will build on what I discuss below.
First of all, the two main components of exposure are aperture and shutter speed (I’ll not get into the others at this point like ISO as it’s just going to be confusing). Generally, a wide aperture is going to let more light in (think of a big window in a room – a lot of light will be allowed in on a sunny day, whereas if you have a port hole instead of a window, not a lot of light will get in at all). However, to complicate things, the wider the aperture, the shorter the depth of field. If you want to take a photo of someone where they are in sharp focus and the background is very blurred, use as wide an aperture as possible. If, however, you want the background and the person in the foreground in sharp focus, use a narrower aperture.
The same composition taken with a narrower aperture will need the shutter to be open longer to let the same total amount of light in as before. So you can take two photos of the same scene with the same amount of light being captured by the camera, but in one the aperture is wide and the shutter speed fast, whereas in the other the aperture is narrow and the shutter speed much slower. In the former the depth of field is short and in the latter the depth of field is much greater. You still with me?
One more confusion is that a wide aperture is denoted by a low f-number on a camera (such as f1.8) and a narrow aperture has a high f-number (like f22). If you want the full run-down on f-numbers, check out this Wikipedia article.
As I mentioned when I talked about ND Grad Filters, the human eye can capture huge differences in brightness from the reflection of the sun on a rain-soaked road to the darkness of the ground under the shadow of a tree – and all at the same time. A camera is nowhere near as good, only being able to resolve a few stops of light (remember that a one-stop increase in exposure equates to double the amount of light, so 2 stops is 4 times the light, 3 stops is 8 times and so on). This means that the brightest and darkest things in your picture don’t want to be more than a few stops apart. But how can you tell?
Well your camera has a light meter built into it (I’m assuming you’re using a DSLR of course). When you point your camera at something and half-click the shutter, it’ll activate the light meter and it’ll show you something like this (which is the display from a Canon EOS 400D):
If you use spot metering (so the needle is telling you what the exposure is just around the centre of the viewable area) and point the camera at something and the needle is in the centre, then that portion of the picture is correctly exposed. However, that’s only a small part of the story. Say you point it at the brightest part of the shot (maybe the sky) and change the shutter speed or aperture until it’s just under the +2 marker. Then point it at the darkest part, say the ground. If it’s above -2 then your whole picture ranges across just under 5 stops of exposure (rather confusingly). It probably means it’s quite a dull day or you’re using ND grads like a good photographer! Take the picture and the shot probably looks pretty good. Now change the settings again so that the brightest part is at zero – so you’ve exposed for the sky. Now if you point at the dark ground you’ll notice it goes off the bottom of the scale. Take the picture and you’ll see that the sky will look pretty good, better than your first shot, but the ground will be completely black. The shot is underexposed. The histogram function of your camera can explain what’s happening.
A histogram shows the total number of pixels and ranks them by lightness – the dark pixels towards the left and the light towards the right. If you take a picture of a black cat in snow, there’s very little black and a lot of white – so you’d see a small spike on the left and a big spike on the right. If you take a well exposed picture of a landscape with lots of colours, you’ll see a nice curve going from low on the left, rising up in the middle and going back down towards the right again.
From the histograms above you can see that the left-hand one is perhaps a little underexposed, most of the pixels are towards the left. The right-hand one is over-exposed so most of the pixels are towards the right axis. The centre one looks about right and has a nice spread of dark-to-light. However I tend to prefer the left-hand one as although it’s a bit darker, the colours are richer (you might notice a lot of my photos tend to be darker for this exact reason).
There are hard and fast rules about how to expose correctly, such as finding the middle-grey in the picture and exposing against that. If you put a camera on auto mode it’ll take a sample of pixels across the image, average them out to middle grey (which is a particularly drab shade of grey) and set the exposure accordingly. This means it almost always gives you a wishy-washy exposure because it’s guessing and doesn’t know what you’re actually trying to take a picture of. If you go to the extremes like a black cat in a coal cellar, it’ll always over-expose the cat and for the counter-example of a polar bear in the snow it’ll always under-expose. For this you need to either use exposure-compensation or manual mode because only you know what you’re taking the picture of.
My friend Ade sets up exposure by assuming that -2 is black, -1 is soil, 0 is grass, +1 is sand and +2 is white. This means no matter what the lighting of the subject he’s taking a photo of, if he wants the grass to be perfectly exposed, he meters off the grass and adjusts his settings until it reads 0 (by metering I mean pointing the centre of the viewfinder at the grass, half-pressing the shutter button so the meter comes to life and adjusting the aperture or shutter speed until the meter reads 0). If there’s sand and he wants that perfectly exposed, he changes the exposure until it reads +1 on the meter. And so on.
I try to keep that in mind, but sometimes I just experiment and see what I get. The best way to tell how well exposed the shot is isn’t to look at the picture on the camera display, it’s to look at the histogram and see what the distribution looks like. More to the left leans towards under-exposure, more to the right is over-exposure. However, you don’t always want a perfectly exposed shot. Maybe you want to under-expose to draw your eye to something brighter than the rest of the picture. Maybe you want to over expose a background to give a dreamy look to the shot. It all depends what you’re trying to do and I could write a book on the subject and still not explain it fully! For example, the following two shots demonstrate that sometimes you want to underexpose and sometimes you want to overexpose:
|By overblowing this shot (the meter was telling me it was overexposed), the details in the room are lost but the eyes really stand out. On automatic mode you’d have a more detailed and dark photo, but without much impact.|
|Here I wanted to produce something dark and mysterious so your imagination fills in the black. If I’d gone on auto-mode it would have taken a longer exposure and brightened things up somewhat – which wouldn’t have had as much impact.|
I’ve really just given a brief overview of the subject, but I urge anybody who wants to improve their photography to stop using the automatic settings. To use manual effectively you need to understand exposure, and exposure is one of the main keys to taking great photographs. It’s tricky to figure out at first but once you do there’s no stopping you.
Personally I never use any of the auto-settings on my camera. The one time that I tried was on a night shoot when I was curious to see what exposure it would set for the picture of a skyscraper I was taking. I half-clicked the shutter to see what it would do and the flash popped up which made me laugh given that the building was half a mile away! So no use at all then! 🙂
Next: Everybody Loves Waterfalls.