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Monday
Mar092009

SCL Photo Podcast 36 - What The F?

SCL PodcastF-Stops, apertures... complicated at the best of times, so this week I have a go at explaining it all.....

(Listen out for a couple of mistakes where I get my f-stops mixed up - my apologies in advance!)

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Show notes:





Related Links:

Canon EOS 50e Film SLR Video Review.

Columbus V-900 GPS Photo-Tagger Video Review.


My Notes:

Feature: What The F?

Cool (better than mine) explanation of aperture.

F-Stops on Wikipedia.

One of the most powerful tools we have in our camera to produce our artistic vision is the aperture, or f-stop, at which we take our photographs, yet I find it a bit confusing, so I thought I'd talk today about aperture, and how it changes the look of our photographs, and how it links in with exposure, blurring, focusing and depth of field.

To put it simply, the aperture, or f-stop, is the size of the hole in the front of our camera that lets the light in, yet by changing this basic property, we can produce beautifully blurred, dreamy shots, or have photographs that are pin-sharp across the whole scene, we can freeze time or suggest the passage of time - surely there's no more powerful photographic tool than the size of the aperture in our lenses.

Lets start with the basics. If we use a bigger aperture more light comes in, so we can use a faster shutter speed to avoid camera shake blur or subject movement blur, like if we're hand-holding the camera, or for shooting moving objects like sports or kids running around, and if we use a smaller aperture less light comes in, so we can use a longer shutter speed to create blur in our photo's - say that misty, milky effect in waterfalls, or perhaps a panning shot of a racing car driving past us. I'll talk about how changing the aperture alters your depth of field a little later - but again, simply, the larger the aperture the smaller the depth of field.

Now, the way that the aperture is measured can be a bit confusing to say the least. The f-stop number on your lens, or on the settings in your camera, is not a physical dimension in mm or inches - rather it is given as a fraction of the focal length of the lens, with the 1 over bit missed off. This is why f8 is actually a bigger hole than f16, because f8 is really saying that the aperture width is 1/8th of the focal length of the length, which is obviously a larger fraction than 1/16th. So the first thing to understand is that smaller f numbers are actually larger apertures, or holes, in the front of your camera or lens. Often photographers will mention they're shooting wide-open - this means they're using the widest aperture on their lens, which will be the smallest f-number.

You can physically see this easily on manual film slr's - simply take the lens off the camera, and rotate the aperture ring to see it getting bigger or smaller. On other cameras you can usually see it by setting a wide aperture - small f-number - then by looking straight into the lens as you take a photo. If you then set a smaller aperture, and look straight into the lens as we take another shot, you should see the difference.

Now if we think about how photographs are exposed, that is how dark or light they come out, this starts to make sense. In order to produce a desired exposure, and for arguments sake lets imagine we're shooting in one of the automatic exposure modes, say program, aperture or shutter priority, there's three things that work together together to influence this exposure - ISO, or how sensitive our camera or film is to light, shutter speed, or for how long our camera lets light in for, and aperture, which is the size of the hole that lets that light in.

If we have a constant ISO, say 200, then if we make the aperture, or hole, bigger by using a smaller f-number, we're letting in more light, so in order to have the same exposure, that is how light or dark the photo is, the shutter speed has to go up, so the camera lets less light in to compensate for the bigger hole.

Lets think about the reverse. We're still on ISO 200, and we dial in a small aperture, which would be a larger f number, therefore letting in less light through this smaller hole. In order to keep a constant exposure, the camera has to keep it's shutter open for longer, to let the same amount of light in to compensate for this smaller hole.

So, straight away, we now know how, by using program mode or more likely aperture priority mode, we can get similarly exposed shots with vastly different shutter speeds simply by changing our cameras aperture. You can argue that why not just use shutter priority mode - but a quick play in manual mode will show you that your camera, or lens, has a lot less options when it comes to changing your aperture than when changing your shutter speed - you've got more chance of getting an acceptable exposure by changing the aperture, and by then letting the camera choose the corresponding shutter speed.

Few! That does sound a bit complicated, and you may well say, well, if we really want to avoid camera shake and motion blur in our subjects, why not just shoot wide open all the time, using small f-numbers, so that our camera's can always use a fast shutter speed? The answer to this is that our lenses aren't at their sharpest optically when wide open, or at their smallest aperture, they often can be a little soft - it's normally best to set your aperture in the middle of the range available for really sharp shots, then increase the size of your aperture if you need a faster shutter speed to avoid camera shake or motion blur, or if we're shooting in low-light conditions - or with digital cameras we can also increase our camera's ISO setting, to make it more sensitive to light, but then we have to be aware that photographs taken at higher ISO's often contain more noise. The other reason is that aperture also controls depth of field, which I'll get onto in a minute.

Before I move it, it's worth talking about the confusing aspect of how different f-stop numbers relate to each other. When we're talking about shutter speeds or ISO, it's relatively easy to grasp. A shutter speed of 1/100th of a second will let twice as much light in as 1/200th of a second, or by increasing our ISO from 200 to 400, we're making our cameras twice as sensitive to light.

However, with f-stops it isn't quite so linear and simple. Because f-stops are a fraction of the apertures diameter compared to the focal length, we've got a problem. The actual aperture is a rough circle - and its the area of that circle, or disc, that controls how much light comes through. But from maths at school we know that if you double the diameter of a circle, the area more than doubles. I won't go into the maths, just realise that the relationship between f-numbers is not a linear one - that is f/8 doesn't let in twice as much light as f/16, as you'd think, as a one eighth is twice as big as one sixteenth, it actually lets in roughly four times as much light!

This becomes really important when buying lenses for dSLR's - because if you just look at the f-numbers it's hard to understand why an f4 lens costs many times more than an f2.8 lens, as they seem fairly close in terms of fractions, but when you realise that the f4 lens lets in twice the amount of light, and therefore you can halve the shutter speed, you can understand why wildlife and sports photographers prize these lenses so much.

Another fly in the ointment is zoom lenses. As we're increasing our zoom's power, we're effectively increasing the focal length - so as we "zoom in" our aperture is getting smaller relatively, even though the actual size of the hole isn't changing - so that's why lens makers charge so much for fast zooms, where the aperture can be wide throughout the zoom's length - they're difficult and expensive to make, but can make a huge difference to the afore mentioned sports and wild-life photographers, or to any photographer who's hand-holding at long zooms and who wants to avoid the camera shake associated with longer shutter speeds and longer focal lengths.

So, I'll finish this bit about aperture with a list of the basic or full f-stops - the numbers where each setting lets in half as much light. Our cameras have half or third f-stops included too, to make things even more complicated, but the basic numbers go like this: F1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 45 and 64. Don't try to memorise them, just be aware of their relationships, and that smaller f-numbers are bigger apertures, which means faster shutter speeds, and vice versa.

All of this talk of f-stops, shutter speeds and maths isn't very artistic, but perhaps the most interesting aspect of controlling aperture is the way that different apertures also drastically change the look of our photographs in terms of what is in focus and what isn't.

Not only does our aperture control how much light our camera lets in, it also controls what is called our depth of field, the amount of the scene we are shooting that is sharp around our initial focus point.

Woah! This sounds complicated, but it's a great technique to understand. Lets start off with an understanding that our lenses are only able to focus on one distance from our camera at a time - and as we move away from that point, either closer to the camera or further away, the sharpness will decrease to blurring. How fast this change takes place depends on three things - how close we are to the subject, how much zoom (also described as our focal length) we are using, and what our aperture is.

Lets talk about aperture then, in relationship to depth of field. Quite simply, the larger the aperture, remember that means a small f-number, the smaller the depth of field, and vice versa.

Ansel Adams was part of a photographers group called f-64, because he and his colleagues wanted to create beautifully sharp and detailed pictures across the whole scene they were looking at - which meant using very small apertures, hence the name.

In modern photography the move is towards the opposite - it's popular to have portraits, for example, where only the person is in focus, and everything else is blurred. This is achieved by using a large aperture, or small f-number, and the quality of the blurred part of the photo is often referred to as Bokeh, and you can find many forum discussions where impassioned photographers discuss which lenses and techniques give the best Bokeh.

One thing that can be tricky when trying to judge depth of field is that when you look through your view-finder the camera sets it's aperture to its maximum, widest setting, to let the most light in to keep the viewfinder bright. To get a real idea of what the actual depth of field will be like it's necessary to take a photo and review it on your LCD view-screen, or use your depth of field preview function if your camera supports that feature.

Again, this is where expensive lenses have the advantage because of their larger apertures, but always remember the other two things that effect depth of field - distance to subject and zoom, or focal length. If you can't get enough bokeh, or blur, by shooting wide open, try getting closer, using a bit of zoom, or the best technique for people, move them further away from the background!

Actually knowing how large the depth of field is, for a particular lens at a specific aperture is beyond me, but if you do a quick Google search for depth of field guides there's plenty of advice and charts out there, for me it's simply a case of if I want the background blurred, I shoot wide open and get closer or use zoom, and if I want everything sharper I'll use a middling aperture, and move further away or use less zoom.

One final think to talk about in relation to depth of field and aperture is something called hyper-focal distance. This refers to the distance at which everything beyond that point is in focus, or the point at which the lens focuses to infinity. Because we know that smaller apertures, or larger f-numbers, give a greater depth of field, and that as we focus on subjects further away from the camera the depth of field increases as well, often you'll find that in order to get everything in focus you don't need to focus on the things that are furthest away. To simplify, all you really need to do is choose a smallish aperture, then focus on something one third of the way into your scene, and you should find that everything is nice and sharp.

So lets talk about some real-world examples. We're out on a photowalk, in aperture priority mode, shooting hand-held at f6.8 to keep the shots nice and sharp, but the Sun goes in, forcing our camera to use a longer shutter speed to expose correctly, leading to camera-shake blur. We don't want to increase our ISO, so in order to keep our shutter speed up, to avoid camera shake, we then make our aperture bigger, by using a smaller f-number, this lets more light in, thereby keeping that shutter speed nice and quick.

We might then see a nice landscape, where we want the whole scene to be sharp. We set up our tripod, so we don't have to worry about shutter speed, tighten up the aperture, then shoot away, focusing on something about 1/3 into the scene.

Later on there's some nice flowers that could make a pretty macro, and we want some blurred bokeh in the background. We get close, shoot wide open, and there's our lovely subject nice and sharp, with the background blurred just how we wanted it.

Lastly, I better explain why we use the term "f-stop" and stops. Generally, it's believed that we use the term f-stops because the first cameras didn't have apertures that could be adjusted - you actually had to drop in a different plate with a different size hole in it, and these were referred to as f-stops, with each stop letting in twice, or half as much light as the previous plate. So if someone says they're going to "stop down", generally they mean they're going to reduce the size of their aperture by one whole stop, letting in less light. Also, a stop is the same as an EV, or exposure value, for the purpose of exposure bracketing.

Few! I hope that all hasn't been too complicated, and you'll be encouraged to experiment with changing your aperture - to play around with depth of field, sharpness and the resultant shutter speed effects. Make sure you send me some comments with how you get on!

March / April Photo Assignment - "Depth of Field".

Long Term Assignment - "Where I Live"

Technique challenges (No Time Limit):

No Sky Landscapes

Fill The Frame!

Dawn / Dusk shots

A Landscape Style Shot With Strong Foreground Interest

Remember to email me your photos if you'd like to me work on them for the Photo Workbench.

To contact me, just click on the link near the top of the page under the big picture.

Thanks for listening, see you on Flickr!

Join the Flickr Group!

Cheers, Rob.

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