A beginners guide to digital SLR camera exposure settings

Canon 450D - Digital SLR Camera

So you’ve bought yourself a shiny new digital SLR, its covered in buttons, gizmos & a screen full of menus which all have their uses….but what about the basic settings & what effect do they actually have in the real world?

Photography – the basics

What is exposure?…put simply – the process of capturing light onto film or digital sensor. If you can understand this simple concept from the start, it will help you a lot further along the line as you want to improve & take better photographs. A photo consists of stored/captured light from a given moment in time and nothing else. Sure we see physical objects in the image, but it is the light bouncing off these surfaces that has been captured and stored for prosperity.

Our eyes are far more sensitive to wider range of light levels than a digital camera is (or film camera) – this makes understanding the variables involved in capturing a photograph quite important in order for you the photographer to take the best photographic image that you can.

I’m sure that you have a basic understanding of what happens when you take a picture with a camera – you press the button, the shutter opens, light floods through the hole in the camera lens & onto the film or sensor to be recorded….lets look at this in a little more depth.

The holy trinity – shutter speed, aperture & film speed (ISO)

Starting simply – Shutter speed

This is as I’m sure you already know, the length of time that the shutter is open: shorter shutter speeds = less light, crisper/sharper images longer shutter speeds = more light, greater chance of blurred images.

Because shutter speed is easy to understand, it will most likely be the first setting that you play with and rely on the most (it was for me). A good place to start experimenting with exposure is to use shutter priority mode where you set the shutter speed & the camera automatically sets the aperture and film speed (ISO) for you. As a rule of thumb, a shutter speed of about 1000ms will freeze most fast moving subjects sharply.

Aperture & the flow of light

When you first look at aperture, you may wonder why it is there or what it does exactly. So what is aperture? Aperture is essentially the diameter of the shutter opening that allows the light to pass through:

  • lower f stop (eg f1.8) = larger/wider shutter opening & more light can pass through the lens
  • higher f stop (eg f16) = smaller/narrower shutter opening & allows less light to pass through the camera lens

Ok – thats easy enough to understand….but there’s more to it yet.

Aperture & depth of field (DOF)

What is depth of field?…Depth of field is the amount, or depth of your image that is held sharply in focus with the rest of the image being blurred or out of focus. If you want to take a photograph of a landscape, you normally want as much of your photo to be in focus – or to have a large depth of field. If you are taking a macro photograph of a flower for example, you would want the flower, or part of the flower to be in focus, with the background & any foreground objects being blurred and out of focus – this has small or shallow depth of field.

  • lower f stop (eg f1.8) = shallow depth of field, good for subjects closer to the camera
  • higher f stop (eg f16) = deep depth of field, good for subjects further away from the camera.

What you will typically notice when experimenting with the aperture setting is that through the eyepiece it will seem as if nothing has altered. This is because on most cameras, when you look through the view finder, you are looking through the lens with the aperture wide open (the lowest f stop that the lens can be set to)….it is only when you press the shutter that the aperture will actually change. To counter act this, most digital SLR cameras will have an aperture preview button, normally on the lower left of the camera body where the lens mounts (barrel?). When you press this button & look through the eye piece, you will see what the scene actually looks like through the aperture that you have set – this will allow you to check the depth of field before taking the photograph.

The easiest way to experiment with aperture is to use aperture priority mode where the camera works out the rest of the settings for you. Try taking photos of subjects close to the camera & altering the aperture so that you subject is in focus, but the rest of the frame is blurred. Then crank up the f stop & take a photo where the whole of the scene is in focus.

With digital SLR cameras, the lens that you mount onto the camera will define the aperture range that is available to you. I have a Canon 450d which comes with an 18-55mm zoom kit lens. The f stops for this are listed as being f3.5-5.6….this actually means that at 18mm, the maximum aperture is f3.5 while at 55mm the maximum f stop is f5.6. I also have a canon 50mm lens which has a maximum f stop of f1.8 – this allows a lot more light to pass through to the camera than with the kit lens & it also gives a much shallower depth of field as a result. So it pays to think a little about your aperture needs before buying a new lens….lenses with lower f stops, particularly in the case of zoom lenses will be more expensive!

Film/sensor speed – ISO

So we now know that we can alter the exposure using shutter speed and aperture, so what does the ISO sett ing do? The ISO or film speed adjusts the sensitivity of the camera, put simply:

  • slower film speed/lower ISO = darker image, less grain or noise
  • higher film speed/higher ISO = lighter image, more grain or noise in the image

You will see that I have mentioned grain and noise above. This comes back to the sensitivity of a camera vs our eyes. A camera can see a far narrower range of light levels at any one time than we can with the naked eye. When you take a photo with a digital camera, particularly in lower light levels with a higher ISO, you will notice some background noise in the darker portions of the image – if you take a photo in better light conditions and a lower ISO, you will see a much clearer and cleaner image with much less background noise.

Ideally, you will want to try and keep your ISO setting as low as possible for the given light conditions – if you take the shutter speed & aperture into the equation, say with the example of taking a photograph of a bee on a flower. We want a pretty fast shutter speed to freeze the motion (800-1000ms maybe)and a shallow depth of field (maybe f3.5)….so if our resulting exposure is too dark, we can turn up the ISO setting to brighten up the overall exposure…or if the exposure is too bright, we can lower the ISO to darken the overall exposure. This gives us a little extra control of the exposure of the photograph, although the range that you have to play with here really does come down to the camera that you have – a Canon 450d has ISO 100, 200, 400, 800 & 1600…I try not to use ISO 1600 as things really do get quite noisy there – I do use ISO 800 when I really have to….I try and keep at ISO 400 or lower for the best photographic quality.

While I’m not a professional photographer, I am now a reasonably experienced amateur & I hope that some of the above will go some way to help some newbie photographers out there…the next article will be on exposure and how it relates to flash or strobe photography which is a slightly different kettle of fish!

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