Tech:Basics of DSLR photography

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Introduction

Just bought a DSLR and don't have a clue what to do with it? Or maybe you've been using the auto mode and want to experiment a bit more? Or ... well, there are lots of reasons why you might want to know a bit more about taking a photo with a DSLR. There are a lot of books written about using a DSLR and it would be exceedingly difficult for me to compress it all into an article, so consider this article as a basic introduction rather than a complete guide.

The elements of a photo: aperture, shutter speed and ISO

There are a few elements that come together to make a picture, and the three above are the most important ones. Understanding how they work and how they affect a photograph is the foundation of DSLR photography.

Any camera is essentially a box that captures light. There is a sensor which actually records the light and most of the rest is dedicated to getting light to this sensor in appropriate amounts. The aperture, shutter speed and ISO all work to control the levels of light.

Aperture

The aperture is how big the hole that allows light through to the sensor is. It is given in terms of 'f-number' and the lower this f-number is, the bigger the aperture is. For example, an aperture of f4.5 is bigger than an aperture of f16. A big aperture (small f-number) gets more light to the sensor than a small aperture (big f-number.)

The aperture controls the depth of field, or how much of the image appears to be in focus. A small aperture has a wide depth of field, so a lot of the picture will appear to be in focus - foreground through to background; landscape photographers tend to use small apertures for this reason. A big aperture has a narrow depth of field, so not much of the picture will appear to be in focus - just the part you focussed on; this is how photographers achieve the blurry background photos you get on portraits.

Shutter speed

The shutter speed is how long the aperture is open and the sensor is recording the light. It is given in seconds or fractions of a second; the smaller the fraction is (quick brush-up on fractions for the non-mathematicians: 1/500 is smaller than 1/250, it's divided the one into more pieces), the quicker the photo is taken. A slow shutter speed (big fraction or full seconds) gets more light to the sensor than a fast shutter speed (small fraction)

A fast shutter speed has the effect of 'freezing' motion - something that was moving will appear to be still; sports photographers use fast shutter speeds to capture action. A slow shutter speed blurs motion - movement will appear blurred, including movement such as leaves blowing in the wind; if you've seen photographs where flowing water looks milky, that is achieved by using a slow shutter speed.

ISO

The ISO is how light-sensitive the camera is. It is given in terms of multiple of 100 - 100, 200, 400, 800... The larger the ISO number, the more sensitive to light the camera is and therefore the better it is at taking photographs in low light. However, the compromise is a high ISO can add noise to a photograph.

A low ISO such as 100 means the camera isn't very light-sensitive, but there is no noticeable noise in the image - this is used in light conditions, such as outdoors on a sunny day. A high ISO such at 1600 means the camera is very light-sensitive, but there will be more noise in the image - this is used in dark conditions, such as a badly-lit pub.

How the elements are related

Aperture and shutter speed are each dependent upon the value of the other, and there rules of thumb that state how their relationships work. ISO value depends on the lighting conditions available, but can affect the relative size of the aperture and length of the shutter speed; the more light sensitive the camera is, the less light you physically need to produce the same result, and vice versa.

• big aperture means fast shutter speed.
• small aperture means slow shutter speed.

• fast shutter speed means big aperture.
• slow shutter speed means small aperture.

• high ISO means that the aperture can be smaller and the shutter speed can be faster than in the same conditions with a low ISO.
• low ISO means that the aperture can be bigger and the shutter speed can be slower than in the same conditions with a high ISO.

Exposure: when it all comes together

A lot of people write books and spend time on making exposure very complicated. Really, it boils down to one very simple statement: exposure is the amount of light that gets to the sensor. As you've already seen, you can control how much light gets to the sensor by changing the aperture, shutter speed and ISO ... therefore, you control the exposure.

There are three broad camps for the exposure of your photograph to fall in to:

• well exposed, which means the right amount of light got to the sensor - this picture looks good.
• over-exposed, which means too much light got to the sensor - this picture looks too bright and white.
• under-exposed, which means not enough light got to the sensor - this picture looks too dark and black.

There are no universally correct exposure values; what works for one scene won't work for the next. In addition, the 'correct' exposure for some scenes is under- or over-exposure, the classic example being silhouettes on a beach at sunset, which is an example of an under-exposed picture that works effectively.

How do I know if I've got the exposure right?

An experienced photographer can look at a scene and judge the right settings needed to exposure it correctly. However, that is a skill you gain with many years of experience; as a beginner in the world of DSLR photography, your best friend will be the histogram

Quick introduction to the histogram

The histogram is a graph that shows the distribution of light across your picture. It shows as a series of spikes, a bit like a mountain. They often come accompanied with something very technically known as 'blinkies' - a flashing light on parts of your photograph that are over- or under-exposed in relation to the rest of it. Different cameras access the histograms and 'blinkies' differently, so check your manual for how to find them.

Reading the histogram

The left of the graph represents dark tones and the right of the graph represents light tones.

Where interpretting your histogram, there are two important things to look at - the spread of the graph and the peak of the graph.

• Spread: is the entire spread of the histogram contained within the chart or would it continue off the chart? Does the histogram touch the horizontal axis at two points, one in the dark range and one in the light range?
• Peak: is there a distinct peak on the histogram, or are there multiple peaks or no peak? Is the peak in the middle of the chart or to one side of it?

As there is no one correct exposure, there is no one correct histogram. In general, a well exposed photograph has a histogram with the following attributes: touches the bottom in both the dark and light regions, therefore entire spread is on the chart; there is one distinct peak in the middle of the chart.

Knowing exposure: a summary

Always check the histogram and the blinkies. In the beginning, you'll get a lot of badly exposed photos - don't be disheartened. You soon get the hang of it with practice and start getting the majority well exposed or, if not quite well exposed, not far off and recoverable in software.

Modes of a DSLR

A DSLR has several modes that can be used to take a photograph. Here's a run-down of the modes available:

• Auto: this mode allows the camera to assess the scene and decide on all values to be used to take a well exposed photograph.
• More specific auto: a more specialised auto mode, you tell the camera what you're taking a photo of (for example, a portrait) and it decides what settings it would use to take that photo.
• Aperture priority: you choose the aperture to be used and the camera chooses the appropriate shutter speed to correctly expose the scene.
• Shutter priority: you choose the shutter speed to be used and the camera chooses the appropriate aperture to correctly expose the scene.
• Manual: you choose both the aperture and shutter speed to be used.

As for ISO, you can always choose whether to have that on automatic or choose your own value of it (remember, it only relatively affects the other values); some newer models come with an option to set the maximum value for automatic choice.

When you use what mode

I'll assume that, as you're reading this, you want to come off of using auto and more specific auto modes; therefore, I will focus on the uses of aperture priority, shutter priority and manual modes.

Manual mode should only be used when you understand the relationships, have become good at judging the right settings and, even then, when you have all the time in the world. As for the other two, you have to decide what the important part of your photo is: the depth of focus or the action. In some cases, they will compliment one another: when taking someone's photograph, you want what they're doing frozen and the background blurred. In other cases, it is a compromise; you might want to have all the cars at a race in focus, but decide that the action freezing is more important.

If the depth of focus is the important part (for example, a landscape), control the aperture and let the camera choose the matching shutter speed. If the action is important (for example, a football match), control the shutter speed and let the camera choose the aperture.

There are no hard and fast rules for which situation calls for which priority. In general, scenes involving animals (including people) or movement wants shutter priority, and scenes involving objects or still views wants aperture priority.

In summary...

• Taking a good photograph is all about getting the exposure of a scene correct.
• You can control the exposure by changing the aperture, shutter speed and ISO.
• Different values for the above will affect the photograph in different ways.
• Decide the important part of your scene and choose the best mode for that.

Test what you've learned

There's a lot in this article, but knowing this stuff really does make a photographer ... so I've prepared a few questions just so you can check that you've understood the principles and are ready to get out there.

1. Using a big aperture would mean having a...

• slow shutter speed?
• fast shutter speed:

2. High ISO would allow...

• the relative aperture to be smaller and relative shutter speed to be faster?
• the relative aperture to be larger and relative shutter speed to be slower?

3. A generally correct histogram has...

• two touch points - one in the dark area and one in the light area - and a distinct peak roughly in the middle?
• two touch points - one in the dark area and one in the light area - and many peaks over to one side?

4. If you were taking a photograph of your dog playing the garden, it would be best to use...

• aperture priority?
• shutter priority?

Answers: 2, 1, 1, 2.

If you'd like to have a play around with the settings - how they come together to produce different exposures and how they affect the outcome of the photograph - I heartily recommend (although am not affiliated with) this camera simulator.