An Eastern Dwarf Tree Frog nestled into the vividly coloured plumage of a Pink Cordyline plant. This photo was taken hand-held using natural light.

Attaching a UV filter provides some protection for your lens. You’ll need to make sure the diameter of the filter matches your lens or else it won’t screw on. Once it’s attached you can leave it on. Your lens cap should clip right over the top of it.

Fully Auto?

That fully-automatic setting which turns your SLR into an aim-and-shoot box is smart, but unfortunately it doesn’t get it right every time. That’s because it can’t always guess what you want. But no worries — by the end of this article you might not even be wanting to use it any more!

Multiple focus points

Modern SLRs use a whole bunch of auto-focus points to help you take better pics. That is, when they’re not helping you take worse ones.

If you have all the focus points activated and you take a shot of something surrounded by foliage, like this Red-backed Fairy Wren, then you’re almost certain to end up with the wrong parts of the scene in focus.

What I should have done: here I’ve activated only the centre focus point and so this time the bird (Red-browed Finch) is in focus while the other stuff is blurred.

When I took this picture, I aimed the focus point (it was the centre focus point) over the wallaby’s eye and pressed the shutter button half way down. That locked the focus on the eye. Then, keeping the button held half way down I aimed down and to the left to recompose the shot. When I was happy with the composition I pressed the button the rest of the way down to take the photo. Locking the focus is a handy trick, although it won’t work in servo focus mode because in servo focus mode the focus is constantly correcting itself.

Here’s an example. That blurred thing in the picture at right is a wren. No, really. But the wrong parts of the image are in focus. That can happen a lot when you have all focus points activated. Which makes sense when you think about it — all those focus points are more likely to latch onto those many bits of foliage rather than that one little bit of bird.

That’s why one of the first things I do is set which focus point I’m going to use. I select it and then the camera remembers my choice until I change it again. Now, to select a focus point will probably mean you need to work in something other than fully-Auto mode. Because we’re only learning here, then aperture priority mode (AV or A, depending on your brand of camera) should do fine. You can set your aperture to f/8 for example, and then choose which focus point to use. (You might need to check your camera’s manual to find out how to select focus points.)

My most common choice is to activate only the centre focus point. That means the camera focuses on whatever’s in the middle of the frame when I press the shutter button. As you get more comfortable with things you’ll find situations where some other option is better. But the important thing is that you’re the one deciding which bits are in focus, instead of leaving that decision to the camera.

And now we get to the good stuff, the stuff at the core of SLR photography. This is where your digital SLR really shows its strengths as a fantastic tool for making images.

Depth of field, f-stops and shutter speed

Why am I lumping all these things together? Because they’re all connected.

When you understand this bit then the rest of photography, especially the bits which make SLRs such fantastic cameras, will start making a lot more sense.

In this shot of a Darter, the water was creating lots of distracting ripples in the background and I wanted those ripples to disappear. So I blurred them away with a small depth of field

The smaller the f-number, the bigger the hole letting in light.

I’ll start with depth of field

Depth of field is a term photographers use to describe the amount of distance between the closest and farthest objects in focus. So a landscape that has everything nearby and in the distance in focus would have a big depth of field. Or the bird picture shown at right, where only the critter is sharp, would have a small one.

Well, your clever SLR makes it easy to control that. Now, some of the better compact digital cameras can too, but not as well as a digital SLR. The reason’s a bit tricky to explain but for now you can be happy knowing it involves the larger sensor used in digital SLRs.

So, how do you do it?

To get a small depth of field you go to aperture priority mode (AV or A on your mode dial) and you choose a small f-number.

Now it’s important I go off on a bit of a tangent here so you’ll understand what I just said. A small f-number means you are opening up the hole in your lens — the hole letting the light into your camera — into a big opening. And a big f-number results in a small opening.

Why does the big f-number equal the small hole?

Because apertures are expressed as a fraction, and the f-number is just part of that fraction.

Huh?

Okay, I’ll make this heaps easier to understand by talking you through an example. The “f” in aperture numbers stands for “focal length”. So for a 50mm lens at f/2 the aperture hole would measure (50/2)mm, or 25mm across.

So you can now see that f/2 means a different sized hole every time you change your focal length. For example, f/2 in a 30mm lens would give you a 15mm aperture hole.

You can hopefully see now why apertures are written as a fraction, complete with that little slash symbol. An f-number is just camera-speak for that fraction. And just like with other fractions you’ve seen, where for example 1/22 of something is a smaller amount than 1/2 of it, likewise an aperture of f/22 gives you a smaller aperture hole than at f/2.

Some people prefer to leave out the slash symbol when they describe their f-stops. For example, they might talk about f2 instead of f/2. But the important thing is that it means the same thing.

Some people think f/2 lets in twice as much light as f/4, but the truth is it lets in four times as much. The reason why is because the aperture hole at f/2 will have twice the diameter of f/4. And when you double the diameter of a circle you quadruple its area. Which means four times the amount of light!

Getting back to our depth of field

Here’s what you need to know: a small f-number will give you a small depth of field. Which means your subject is in focus but other bits can be blurred and hard to see. That kind of effect can be especially effective (and flattering) when taking portraits, and it also gives a very pleasing effect of matching the way a human eye works.

And of course, the opposite rule applies. A big f-number means a big depth of field.

Congratulations! You’ve now survived all the complicated stuff. From now on it gets heaps easier as we bring it all together.

Just like I said, it all comes together

We’ll use f/16 as our example here. You chose that big f-number because you wanted a big depth of field. Because you’re letting light into your lens through a small hole, there’s not much light reaching your camera’s sensor. It’s like being in a house with the blinds mostly closed, making everything dark (underexposed). To make up for that, your camera’s going to want to brighten the picture back up again by giving itself more time to capture photons. And it does that by slowing down the shutter speed. Which is why depth of field, aperture and shutter speed are all connected.

Have a look at the graphic below to see a rough guide to this whole thing.

So that’s it. Now you know it! Photography becomes a balancing act between depth of field, aperture and shutter speed, with you, the photographer choosing which aspect to give priority to, according to what you’re photographing at the time and how you think it will look best.

You can see in the pics of the toy dinosaur above what a big creative difference depth of field (or a lack of depth of field) makes to a shot. So, if your main concern is providing a soft background, then you give priority to your aperture setting, because that’s where you control depth of field. You choose aperture value priority mode (AV or A mode) and select the best f-stop for the amount of depth of field you want. Your camera will then look after the exposure for you by choosing an appropriate shutter speed.

But what if that dinosaur was running? In that case you would be more concerned about motion blur in your photo. So you tell your camera to give priority to its shutter speed. That means you go to shutter speed priority mode (also called time value priority mode — it will be marked as TV, or S on the mode dial, depending on the brand of camera), then once you’re in that mode you select a shutter speed fast enough to freeze the dinosaur’s movement. Your camera will then pick an appropriate aperture to get the exposure right.

So that’s why they call them priority modes

You’ve probably now guessed why your camera calls them priority modes. In shutter speed priority mode (time value priority or whatever your brand of camera calls it) your camera gives priority to your shutter speed when it figures out the exposure settings. When I say it gives priority to it, what I mean is that it uses it as a starting point in figuring out how to get the right exposure.

And likewise, in aperture priority mode the camera gives priority to the aperture (f-stop) you choose, while you give it the flexibility it needs to choose an appropriate shutter speed.

When you want to take control of both of those settings yourself then you switch to manual mode.

Because the amount of depth of field makes such an enormous difference to the appearance of a photo, a lot of photographers I know prefer to keep their camera in aperture priority mode all the time. However, with wildlife or sports photography, where your subjects are often moving quickly, then they might prefer to work in shutter speed priority mode instead to ensure they don’t get motion blur.

An easy rule

I think it’s time to throw in one of photography’s rules because you’re ready for it now: Whenever you halve the area of your aperture hole you have to double the exposure time set by the shutter speed. And vice versa.

Is there anything being left out here?

Yes, of course there is.

Sometimes the light just isn’t bright enough for your camera to get a decent exposure, even after doing all the clever stuff I’ve been talking about.

Back in the old film days, we used fast film for low-light situations. The same rules apply with digital.

By increasing your camera’s ISO setting you will increase the sensitivity of your sensor. Which means suddenly your camera is getting enough light again.

I haven’t discussed ISO until now because I figured you had enough to think about, and I won’t say too much more about it right now for the same reason. But here are a few things about ISO which are worth noting:

This photo of a Superb Fairy-wren has been badly degraded by noise.

• By default, your camera will probably be set up to be taking care of its ISO settings automatically.
• Increasing your ISO settings will allow for high shutter speeds, or small apertures, in less light.
• The down side of relying on ISO happens when you push things to their limits. You see, very high ISO settings tend to create more noise in your pictures. By that, I mean that the photos tend to be speckled with incorrectly-tinted pixels, especially in the shadow areas where the poor-light issue is at its worst. Still, you will probably find that you can take your camera’s ISO settings up a bit without any noticeable loss of image quality, and the pros often do exactly that.

All the other rules still apply, with depth of field and how it relates to aperture and shutter speed and so on, but just understand for now that higher ISO allows you to do the same things in weaker light.

To learn how to crank up your ISO you might need to glance at your camera’s manual. But I’ll stress again that you should be wary of maxing out your ISO settings, or else you could get some unusable images.

There are lots of variables in photography and I could go on and on about them. For example, a telephoto lens (one with a very long focal length) will blur your background more than a wide angle lens at the same f-stop, and if you’re interested in why that happens then I explain it here. (Don’t worry — it’s not nearly as complicated as what we’ve just learned!) But once again, the same general rules about depth of field, aperture and shutter speed will apply.

What next?

So you’ve made it through the basic concepts behind camera exposure. How you apply this is the trick behind using your SLR camera and that is surprisingly straightforward. I want you to be nice and clear about what we’ve already learned. It’s at the heart of knowing when to choose things like aperture value priority mode or time value (shutter speed) priority mode when you step outside to take your shots.

And the easiest way to make sense of it all and help you remember it, is by talking about some different situations out in the real world in Part 2 of this guide. You’ll see how we’re using this stuff to make decisions which drive your SLR camera towards much better photography.

PART 2

You might also be interested in …

Beginners’ series on digital SLR photography

 

Other photography stuff

Copyright © Mark David. All rights reserved