How to use a DSLR: Understanding Exposure & Shutter Speeds

This week I’ve got some information on exposure and shutter speeds, and why you should know what they do on your DSLR. If you missed my last post – Understanding Aperture – you can find it here. (Worth checking out before you read on!)

How to use a DSLR: Understanding Aperture

What is Exposure?

Exposure is the unit of measurement for the total amount of light that is permitted to get through to the sensor when taking a photograph. So from my last post, we already know that the aperture controls the hole that the light can get through. Well the shutter speed controls how long or short that hole is opened for, and together these two control the exposure of the final photograph. Getting the exposure right needs a balance between the aperture and the shutter speed. Too much light will result in over exposure, and too little will result in under exposure.

Long/short Shutter Speed?

Okay so your shutter speed number will look like a fraction on your camera. These numbers are basically fraction of seconds, and the higher the number, the faster the shutter speed will be. This just means that the shutter will only close for a tiny amount of time, say 1/80 of a second. These numbers will roughly double as they go on, so, starting from the slower speeds, you’ll have 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500. As you move to the slower end of the shutter speed, your numbers will just be normal numbers, and measured as seconds. My canon shows “2, “10, “30 and so on. You will also have a bulb (B) setting, and this just keeps the shutter open for as long as you hold it.

As a general rule of thumb, 1/60 is a good speed to shoot at with a regular lens, say a 50mm or your 17-55mm kit lens. However, the focal length can affect the shutter speed you need because of the camera shake that you’ll get from handheld shooting. Longer focal lengths – for example a 200mm lens – will accentuate the amount of camera shake you have, so a faster shutter speed will be needed. A good ‘rule’ to stick to is to choose a shutter speed where the number is larger than the focal length. So with that 200m lens, shooting at around 1/250 will give sharp photo.

How to use a DSLR: Understanding ApertureNow, let’s look at exposure. Here you can see three different exposures. The photos have not been edited at all, so these are straight out of camera (SOOC).

When looking in your camera’s viewfinder, you will see a little graph. This is also visible on the screen, so you could use that one too if you’re using a tripod and have the subject in frame and set up. This graph shows what your exposure is like. Ideally, you want the little moving line to be right in the centre – that is the perfect exposure as far as your camera is aware. This is decided on the whites and blacks in the photo, checking that none are too dark or too light. In the photos on the left, the middle photo had perfect exposure.

To overexpose the photo, I slowed my shutter speed down 3 places. So I went past 1/200, 1/160 and stopped on 1/125. As the number has decreased, it means that the shutter is left open for longer when taking the photograph. This lets in too much light, and you can see that it is quite bright. Now a lot of bloggers like this ‘white’ look in a photograph, and to get the bright white background overexposure is the way to go. I would just give the photo a little contrast and perhaps add a little more shadow in photoshop and then it’d be done. You have the brighter photograph that you want.

Underexposure is a different matter. It can work for some things, but I would say very rarely used as a technique. Now for the last photo I moved the shutter speed up 3 places. I went past 1/320, 1/400 and stopped on 1/500. You can see that this photo is pretty dark, but it’s not too bad really. If the shutter speed was any faster, it would definitely be too dark.

Now to get the perfect balance, your aperture will need to move as the shutter moves. Let’s take the perfectly exposed photo in the middle. It was taken at f1.8 and 1/250. If I wanted to have a faster shutter speed, say one up at 1/320, I would need to widen the aperture because the shutter is staying open for less time. So I would move that to f1.4. Say my perfect exposure was at f6.3 and 1/80, I would move the shutter to up 1/100 and the aperture down to f5.6 to get that same exposure. This is the same if you want to change the aperture, the shutter will need to either be made faster for a wider aperture, or slower for a narrow aperture. I would try shooting in shutter priority to see how this works (Tv on a Canon, and S on a Nikon).

When shooting in RAW, exposure can be an easy fix, depending on how much you’ve over/under exposed. The photos above would be easy to fix, but when your highlights are completely white they’re blown out and there’s nothing you can do to bring those back. Same with the blacks, if they’re too dark then there won’t be any detail to recover. I am going to do a post on shooting in RAW and the fixes you can make.

I would always recommend getting the exposure that you want in camera. Too much fiddling with editing can really ruin the file, especially when editing jpeg. The quality will decrease, and we don’t want that do we?!

Examples of slow/fast shutter speeds

I’ve found some old photos that I’ve taken where I’ve experimented with the shutter speed so you can see what kind of effects you can get.

If you’re using a fast shutter speed, your only aim really is to freeze motion. Here you can see that I’ve managed to completely freeze the Robin. A fast shutter speed is pretty necessary if you’ve got a fast moving subject (such as a child or an animal!) or if you have a really shaky hand and bad lighting available.

How to use a DSLR: Understanding Exposure

Here you can see the difference between a fast shutter and a slow shutter. On the left I’ve managed to catch the movement of the Starlings, and on the right the birds are completely still.

How to use a DSLR: Understanding Exposure How to use a DSLR: Understanding Exposure

This one was part of my major final project in college. All I did here was set a slow shutter speed – I’m sure it was around 1/4 or 1/8 – and moved my camera upwards (as straight as possible!) quickly as the photo was taking. You’ll probably need to do it a few times to get the lines perfect, but it gives a cool effect right?

How to use a DSLR: Understanding Exposure

Another scenario where you might need a slow shutter speed is when doing night photography. For landscapes, a slow shutter speed doesn’t matter for focus because you generally won’t have a moving subject – camera on tripod, set the focus and you’re good to go. The photo on the left is lit up by just the street lights, but if I had used a fast shutter speed, those lights wouldn’t have been enough to light the scene.

How to use a DSLR: Understanding Exposure

So you can see that the shutter speed is a great tool for experimentation. Of course, for blog photos you will generally be wanting all movement frozen, so a fast shutter speed is what you’ll need.

I hope this has been helpful for you and please let me know if you have any questions!