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Aperture and Depth of Field (what does it mean?)

Photography Tips by Ken Finegan

For the last few weeks we have been concentrating on tips that give you more control over what YOU want to achieve with your camera and photography. This applies to both camera phone and DSLR.

This week I want to deal with ‘aperture’, what it basically is and what it does to our images. This may apply mostly to DSLR owners but is also a part of the more modern phones. The ones I have experience of are the Huawei P30 and P20.


Some people get a headache as soon as soon as aperture is mentioned, so let’s break it down. Basically an aperture is an opening, something similar to a tap.

Aperture controls the amount of light going through the lens and is measured in f numbers. Most entry level DSLR’s have a kit lens which have an f stop range from f3.5 to f29, and you control them with the rotary button on your camera. The Nikon cameras have them on the back- top right and the Canon cameras have them on the top right, near your display.

So, go get your camera and take it off the Green setting (Fully Automatic) and put it to ‘A’ in Nikon and ‘AV’ on Canon (AV stands for Aperture Value). We are now taking the first steps on our journey to Manual mode.

The mode we are in at present (A or AV) is ‘Aperture Priority’ and it is a semi-automatic mode. This means that YOU control one part of the camera setting and the camera controls the other. You will get a picture no matter what aperture (f stop number) you choose (I will deals with the consequences of different f stop numbers on shutter speed next week when you have tried out Aperture Priority).

The f numbers control the amount of light getting into the camera but also control a thing called ‘Depth of Field’. ‘Don’t go for the Anadin just yet’!!!

Right, what is depth of field? I’ll give you the definition firstly and then explain. ‘Depth of Field is how much in front and behind your subject (the part of the image you focused on) appears in focus. Therefore a small f stop number has small Depth of Field (DOF) and a large f stop number has large DOF’.

Small depth of field (f 3.5-5.6) has mainly the subject in focus and blurs the foreground, in front of the subject and the background, behind the subject. This is really great when you want to emphasise your subject, such as portraits.

Large depth of field basically has everything in the image in focus. This is ideal for landscape photography.

Let me show you some illustrations. The first illustration is of the tennis balls shot at a small f stop number. As you can see the tennis balls either side of the centre ball (the one I focused on) are out of focus.

The second illustration is of the same tennis balls from the same position but shot at a large f stop number. As you can see, all of the balls now appear in focus.

Have a go at this process and you’ll be very surprised how simple it is. The main thing to keep in mind is to focus on the centre object and go as close as you can to get the best out of the effect. For a start I’d normally do this outdoors as you have better light.

I’ve included another two images which show this effect also. The first image was taken at the Poc Fada. It shows large Depth of Field showing the trophies in the foreground to the mountains and Dundalk Bay in the background all in focus. The other is a portrait blurring out the background and foreground totally, making the subject stand out.

Two other things control Depth of Field as well. These are: How close you are to your subject and the focal length of the lens (the larger the lens (focal length/magnification) the less depth of field appears in the image).

While you are trying out this mode, ‘Aperture Priority’ start to listen to your camera. What? Yes listen to the sound your camera makes.

When you use a small f stop number (f 3.5 for example) your camera selects a shutter speed (remember I said you select one function and the camera selects the other). With a small f stop number your camera shutter and mirror inside make a noise. It will sound ‘fast’. Try it a few times and listen.

Now set you f stop number to a large number such as f 16 or f 22 and do the same. You will notice a change in sound and speed. Try this both inside and outside and hear/see the difference. As I said earlier I will deal with the consequences of this next week.

For the camera phone users these features are usually in the ‘Pro’ settings /menu. They are good and can get you similar effects. They can even automate the process for you.

But knowing what aperture actually does is very important.

Here is something to think about for next week. If you use a tap to fill a glass and not much water is going through the opening (aperture) how long will it take to fill the glass? Will it be slow or fast? But if we open the aperture fully how long will it take to fill the glass? This will make more sense next week.!!!

If you have any queries you can always go on to my Facebook page Ken Finegan Newspics Photography and message me.

So until next week, enjoy your camera and ‘keep safe’.

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