Aperture Priority Mode – Controlling the Depth of Field

[yelvertoft]

Ok, you’ve got your first camera that goes beyond “point and shoot”. There’s a lot of buttons and dials on it that you really would like to know about but feel a little intimidated in case you break your new toy or get stuck. Your pictures seem ok when you stick to “Program” mode so you stick with what your comfortable with. Why should you change? Read on……

A lot of users, when they get their first ‘serious’ camera find themselves in the situation described above, there is nothing wrong with using Program mode, but you’re going to find it a lot more difficult to get the picture you see in your mind’s eye if you don’t understand the settings the camera is choosing for you.

Where to start with our adventure beyond the program mode? Let’s begin with Aperture Priority mode, this is usually marked on the camera’s mode dial as Av, this stands for Aperture Value.
What is the aperture? Put in its simplest form, it’s the hole in the lens that the light comes through. Why do you need to change it? Because it affects many things, most notably the Depth of Field. Before we go any further, look at the definition of Depth of Field given here:
http://www.worldphotographyforum.com...read.php?t=289
So what effect does changing the aperture value have on the depth of field? The basic rule to remember is that a bigger hole will give a smaller depth of field, i.e. less of the picture (in the front-to-back dimension) will be in focus. A smaller aperture will have the effect of more of the picture from front to back being in focus. Something else to remember is that a smaller aperture is shown as a bigger number, f/22 is a smaller aperture than f/4.5.

Why should you want to change the depth of field? This effect can be used to change the emphasis of the subject. A shallow depth of field may be used to throw the foreground/background out of focus, thus drawing the viewers attention to the (hopefully in focus) subject. A long depth of field can be used to put the whole of the picture in focus, allowing the viewer to look see a range of objects at different distances. This can be very useful for providing additional context to the main subject.

If you want a large depth of field, you will select a smaller aperture, the hole is smaller so it has to be open for a longer time to ensure the correct exposure. In other words, the shutter speed will be slower. This may introduce camera shake/blur unless a tripod or other support is used.

If you want a small depth of field, you will select a bigger aperture, the hole is bigger so it has to be open for a shorter time to ensure the correct exposure. In other words, the shutter speed will be faster. This may have the effect of “freezing” any subject movement.

Another reason you may want to change the aperture is because many lenses will provide a sharper image at smaller apertures. Indeed, a pinhole camera can provide a sharp image without even having a lens, simply because it has a very small aperture indeed.

Right, now having outlined the basic effects of changing the aperture, how do you put this into practice? When you switch your camera to Av mode, you have control over the aperture. You can set the aperture to a large f-number (small hole), providing a larger depth of field, or a small f-number (big hole), providing a smaller depth of field. The camera will then automatically set the shutter speed to a value that it considers will give the correct exposure.

As well as the aperture setting, the depth of field is also affected by the distance of the subject. On a given lens, an aperture of f/8 may give an in focus depth of field from 2.54m to 3.66m when the subject distance is set to 3m. The same aperture of f/8 on the same lens may give an in focus depth of field stretching from 6.23m to 25.3m when the subject distance is increased to 10m. Note the proportions given in these examples. The depth of field behind the point of exact focus is greater than the distance in front of point of exact focus. In the past, many lenses used to be marked with scales to show the depth of field at any given aperture and focus distance. Unfortunately this feature seems to have gone out of fashion. It should also be noted that the focal length of the lens in use has an effect on depth of field, a longer focal length lens will give a shallower depth of field, all other things being equal.

I hope this helps to demystify the Av mode on your camera and gives a greater understanding of the effect of aperture even if you are using Program mode.

Regards,

Duncan.

[Nogbad]

Thanks Duncan a useful intro for Av. I tend to use Program most of the time with a few experiments on Manual but with varying degrees of succes.

I aim to try all shots in AV mode when I next go out so I can see the difference in the depth of fields and effects.

Incidentally what is the Hyper focal distance? (Have I got this right?)

I read about it in my D70 manual but cant seem to figure out what it is saying or do it?

Perhaps you could do another explanation on this at some time?

Regards

Nogbad

[yelvertoft]

Originally Posted by Nogbad

Incidentally what is the Hyper focal distance?

Hyperfocal Distance: The shortest distance at which a lens can be focused to give a depth of field extending to infinity. In fact, the depth of field will then extend from roughly half the hyperfocal distance to infinity.

Duncan.

[Nogbad]

Thanks, think I understand it. Next question what is it useful for and when is it best to use it.

Nogbad

[yelvertoft]

It is useful for when you want the whole picture, from front to back to be in focus. This would typically be used for landscapes.

I wouldn't get too hung up about it. Just use the depth of field preview feature on the camera to check that all the bits you want to be in focus are in focus. If not enough is in focus, turn to a smaller aperture and/or refocus on a different point and check again.

Duncan

[Nogbad]

Thanks its always useful to know these things even if i dont need them just yet.

I'll leave it for another time.

Nogbad

[crazee horse]

wow, that was written well, i usually struggle understanding it all and get all confused after the first paragraph lol. ive just gone from a bridge to dslr and its all a bit much at once for me. buttons and features everywhere. hopefully i"ll get to grips with it in time lol

[Birdsnapper]

Nicely explained, Duncan. With regards to hyperfocal and getting most of a landscape in focus, there are ways of calculating the focus point, but the 20D and 350D have a setting 'M', which sets the focus to maximise DOF for landscapes.

[robski]

A couple of on-line DOF calculators

http://www.dofmaster.com/dofjs.html

http://www.cambridgeincolour.com/tut...calculator.htm

[sassan]

Thanks Duncan.
Very useful as usual.

AV is my preferred, most of time used mode. Now I can rationed it more too.