More on Depth of Field

Background
I've already mentioned Depth of Field briefly, in my tutorial on Apertures and F-Numbers. But it's not just the aperture you are using which determines how much depth of field your pictures have.

Theory
Three factors influence the depth of field of an image. They are:

• Aperture of lens - the f-number you use at the time of taking the image
• Focal length of lens - eg, 28mm for a wide angle, 300mm for a telephoto.
• Distance to subject - how far you are from the main focus of the image.

Again, this might seem complicated, but the important things to remember are these:

smaller aperture = greater DoF than larger aperture
with the same focal length and distance to subject

wide angle lens = greater DoF than telephoto lens
with the same aperture and distance to subject

greater distance to subject = greater DoF than smaller distance
with the same focal length and aperture

Examples
The following show three pairs of pictures, each where two of the three parameters (aperture, focal length, distance to subject) have been kept constant, and the third is varied. Hopefully, this will demonstrate the effect of each. They were taken at a recent rugby match (not specifically for this tutorial, but they illustrate the points well). Whilst I will quote the shutter speeds and ISO setting used, they actually bare no relevence to the depth of field, so are just included for interest.

Changing The Aperture
Same focal length = 400mm
Similar subject distance = about half way across the pitch

[small aperture - f/10, 1/800th @ ISO 400 - most of the image is in adequate focus, giving quite a busy appearance]

[larger aperture - f/6.3, 1/400th @ ISO 400 - background is more blurred, giving better separation from the players]

Changing The Focal Length
Same aperture = f/7.1
Similar subject distance = close to the nearest touchline

[shorter focal length - 130mm, 1/250th @ ISO 400 - everything from touch judge on left (green) to furthest player is pretty sharp]

[longer focal length - 375mm, 1/400th @ ISO 400 - nearest player is blurred (he's about the same distance as the touch judge was in the above) and the background is also very soft]

Changing The Distance To Subject
Same aperture = f/5.6
Similar focal length = about 250mm

[further away - far touchline, 1/4000th @ ISO 400 - most of the image is in sharp, including much of the background. The action was about 70m away]

[closer distance - near touchline, 1/400th @ ISO 400 - background is more blurred, giving better separation. The action was about 10m away]

Checking Depth of Field
Some cameras allow you to check the depth of field you will get before you take the picture. This is usually called Depth of Field Preview. Usually, when you look through the viewfinder, the camera is using the maximum aperture of the lens to show you the image. Pressing the DoF Preview button shuts the lens down to the size of aperture it will use at the time of taking - this often results in a darker view in the viewfinder (the lens is letting in less light) but you can visibly see just what will be sharp.

Maximizing Depth of Field
There is one special point in the image, which if you focus on it, will maximize the depth of field you get with any given combination of focal length of lens and aperture. This is known as the Hyperfocal Distance - and is approximately 1/3rd of the way into the picture, from where you are standing to infinity. There's another complicated formula telling you what will be sharp, but basically, if you set focus at the hyperfocal point, everything between half way to that point and infinity will be in focus. There is more sensible reading on the subject here.

As good practice, why not experiment with apertures, focal lengths and distance to subject, to see how you can go about controlling the depth of field in your pictures.

Putting It All Together - Exposure

Background
I've already explained the basics of ISO speed, apertures and shutter speeds. Now it's time to put them all together and see how they relate to the overall exposure of an image. The EV or Exposure Value for any given scene and lighting is always a constant.

Theory
The correct exposure for an image is governed by a complicated formula. However, all you really need to know is what effect altering any of the three options has on your pictures. So, for a particular scene and lighting conditions, your camera might suggest this exposure:

ISO 200, 1/250th @ f/5.6

If we were to change the ISO setting to 400, the film or sensor is now twice as "quick" at gathering the same amount of light, so the exposure time can be halved:

ISO 400, 1/500th @ f/5.6

Or, the same amount of light will be let in using 1/250th of a second as before, but decreasing the size of the aperture (increasing the f-number):

ISO 400, 1/250th @ f/8

Assuming we leave the ISO setting as it was originally, the following are also equivalent exposures to the first:

ISO 200, 1/125th @ f/8

ISO 200, 1/500th @ f/4

To see more equivalent values, you might like to look at this interactive EV calculator.

What does this mean for your pictures? Well basically, the combination of shutter speed, ISO and aperture is always a trade-off, depending on your priorities - or what you can change. Sometimes you may have low light and hit the widest aperture of your lens, which will give you slower shutter speeds. If you still want pictures which don't suffer from camera shake, you will need to increase the ISO setting.

Other times, you might have plenty of light (on a sunny day), so use a slow ISO setting, and choose the aperture/shutter speed to either freeze movement (fast shutter speed) or maximise depth of field (small aperture).

Examples
Here are some shots I took of the moon, in October 2004. With this sort of subject, you often don't know what will be the "correct" exposure, so by keeping two of the three parameters the same, and just changing the third, you can bracket the exposure to see which is best.

In this case, I used ISO 200 (to preserve fine detail) and the lens was set at it's widest aperture, f/5.6. The only option left was changing the shutter speed accordingly. Apart from the blue hue, see what effect this has on exposure:

1/250th

This version looks a little bit too bright, it's overexposed.

[1/250th @ f/5.6 with ISO200 - overexposed. Loss of detail in the highlight areas, bottom right]

1/400th

This version looks about right, it's correctly exposed.

[1/400th @ f/5.6 with ISO200 - nicely exposed. Enough detail in the highlights and shadows]

1/640th

This version looks a little bit too dark, it's underexposed.

[1/640th @ f/5.6 with ISO200 - underexposed. Loss of detail in the shadow areas, centre]

As good practice, why not take a few pictures of the same subject, but using a combination of different shutter speeds, apertures and ISO settings. Change one at a time and take a series of shots, then see what effect this has on your subject, and the way the camera records it.

All About Apertures & F-numbers

Background
What is meant when referring to the "aperture" (or f-numbers) of a lens?

Photographic lenses contain an iris, made up of solid vanes, built in a similar pattern to those on the left. The vanes can move in and out, thus making the central hole larger or smaller - and so letting more or less light through the lens. This is what's called the aperture, and is a guide to the speed of the lens.

The f-stop, (f-number) or aperture is not an absolute number, it's also related to the physical diameter of the lens. So we can compare the relative apertures of a small compact camera's lens with that of a large telephoto, and still get a sense of which is better at gathering light.

Theory
The main series of f-numbers goes as follows (a sequence of root 2): f/1.8, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32

This seems rather complicated and counter-intuitive at first, but the basic thing to remember is this:

lower f/number = larger hole = faster lens = better in low light
higher f/number = smaller hole = slower lens = better in bright light

Most SLR lenses will have their maximum aperture written on the side. This is the largest size of the hole, when the iris is fully retracted.

More expensive lenses will have a lower f-number, such as f/2.8, which means they have a larger maximum aperture - and will be better for taking photos in low-light conditions.

Things get complicated with zoom lenses - which often have a different maximum aperture at each end of the zoom range, eg. 28-105mm, f/4-5.6. This is saying:

at 28mm end, max aperture is f/4
at 105mm end, max aperture is f/5.6 (ie smaller than f/4.5)

How this affects your pictures - Depth of Field
The other thing which needs considering at this point, is something called Depth of Field. This means what proportion of the image is sharp, in front of and behind the actual point where you focus. I will cover depth of field more fully in another post, but for the purposes of apertures, in general, we need to remember the following:

lower f/number = smaller depth of field = less of image in focus
higher f/number = larger depth of field = more of image in focus

Examples
Let's compare the effects of using two extremes of f/number:

f/2.8

• Large aperture
• Good in overcast or low-light conditions
• Will tend to require using faster shutter speeds
• Smaller depth of field - less of the overall image in focus
• Better for portaits or closeups, because backgrounds become blurred and do not intrude

[large aperture - in low-light conditions, giving small depth of field (only the first couple of blooms are in focus). The background is unobtrusive]

f/16

• Small aperture
• Good in bright, sunny conditions
• Will tend to require using slower shutter speeds
• Larger depth of field - more of the overall image in focus
• Great for landscapes, where you want everything, from a few meters to infinity, to be sharp

[small aperture - in sunny conditions, giving a large depth of field (almost everything is sharp), great for landscape work]

As good practice, why not turn your camera to AP (aperture-priority) mode and take a few pictures of the same subject, but using different aperture settings. This will show you exactly what the effects of controlling the aperture can do for your photographs.