Lesson 1 - Understanding depth of field and aperture


Get to grips with what is and isn't intentionally sharp in your images by understanding aperture and your images will immediately improve explains Andrew James.



Essential Know how

Here at FotoBuzz we'd rather keep focused (pun absolutely intended) on the things that really matter and try to give you the important basics, so we're going to concentrate on aperture because this is the way we control depth of field .

At its most basic, an aperture is simply the hole through which light travels. Without light, no photo. Unless you like really dark photos, that is! We need to be able to increase or decrease the aperture to let in more or less light so to measure that we use a system known as f-stops. Each f-stop is a different aperture size. Before you start screaming at us about how we promised not to get too complicated, bare with us; it's all pretty simple stuff. And if, as I suspect, many of you know this already – well as I've already said, a refresher does no harm.

What you need to know is that an f-stop is simply a measurement that allows you to know how big or small the hole that lets the light into your camera is. For example, a standard 50mm lens like the little nifty fifties from Canon (left), give you the opportunity to change aperture from a maximum (when the hole is at its largest) f/1.4 to a minimum (when the hole is at its smallest) f/22. Between maximum and minimum there are seven other f-stops… f/2, f/2.8, f/4, f/5.6, f/8, f/ll, and f/16.

Ignoring (but accepting) the fact that most digital cameras also allow a division greater than a whole stop – one-third stops being most often used – what we really need to understand is that within our nifty fifty lens' f-stop range of f/1.4 to f/22, the least depth of field occurs at f/1.4 and, in principle at least, the most at f/22.
I say in principle because as you more experienced fotoBuzzers out there will know, a lens has a sweet spot. This is the aperture at which a given lens performs best in the sharpness stakes and just to confuse us all, it might not be its minimum aperture size. But I don't want to stray away from the path in this article, so let's save sweet spots for another day and agree in principle that a wide aperture of f/1.4 equals minimal sharpness and a narrow aperture of f/22 means lots of sharpness.
If you look at your lenses they'll all have an f-stop range. It may not start with f/1.4 and it may not end with f/22, but you understand that what ever range your lens has, the potential sharpness alters as you change f/stops. The incredibly simple rule to always remember is: Small f/number equals less sharpness while big f/number equals more sharpness.

It's important to note that your choice of focal length and the size of your digital camera's sensor also have a large influence on the apparent ‘sharpness' through an image. Put plainly, you'll have probably already discovered that a wider focal length offers greater visible sharpness through an image than does a telephoto. Let's compare two example shots. The poppy shot has been taken on a short telephoto lens using a wide aperture while the boat picture has been taken on a wide-angle lens and a narrow aperture. Click on the I-button for more info...

Creative choice

All the technical pre-amble and information is here to help you do one thing and this is get creative. Anyone who knows anything about photography and, in fact from my experience a lot of people who know nothing about it, all agree we want to get our subject as sharp as possible. That's right, isn't it? Um…well in principle this is true but like all things true, sometimes it isn't! Sharpness is hugely important to us. But so is what's out of focus. In fact, we need to think as much about what is blurred as we do about what is sharp. It's the relationship between the two that helps drive us along the road to creative depth of field, better pictures, fame, fortune, and a happy photographer's life. Careful selection of aperture, along with other aspects of good photography like getting the exposure and composition right, will make or break your photos. So, whereas in some landscape photos we might really want that through-the-image sharpness that is so popular, this really isn't always the case.

Look at the two images of the white boat on the right. The image on the top has been taken using a short telephoto lens and an aperture of just f/1.8. A lot of lenses don't have such a wide aperture but it just goes to show that rules are there to be broken and even landscape images can be taken using a very shallow depth of field. The second image was taken with the same lens but with an aperture of f/8. We can see that the background is a lot sharper. Which one do you prefer? I'm taking an educated guess that a large number of you will prefer the through-the-image sharpness of image number two but there will also be a significant number of people who prefer the first image and the way the more out of focus background throws attention onto the boat itself. My point here is simply to remind you that it's all about creative choice. If you know how the different apertures coupled with your lens will affect a scene, then you can decide how you want that scene to actually look.


Now you understand how aperture works you can start to apply it to every subject. Let's take a look at how it works when you shoot a portrait image.

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