Depth-Of-Field
Modern cameras are jammed solid with automatic functions, but the real creative control comes when you switch to manual and take the decision-making into your own hands. Understanding depth-of-field is the key to unlocking selective focusing as a powerful
In response to requests from quite a few readers, we’re going to revisit some of the key technical elements of photography so you don’t have to always rely on the automatic systems. We’re kicking off with one of the trickier concepts to grasp – depth-of-field.
While we’ve never had it better when it comes to cameras that simplify the technicalities of photography – all those special effects and digital filters, for example – but if you really want to be in control of the end result… well, you have to take control. Modern digital cameras retain a full suite of manual controls and, if you’re still flying on auto pilot, you’ll be amazed what you can achieve with them. It’s really not as difficult as it might look and, once you’re more involved with the picturemaking process, you’ll also be surprised at how much more enjoyable and rewarding your photography will become.
Depth-of-field is often perceived as the photographic technicality that causes the most confusion and looks the most difficult to comprehend, but once you grasp the basic principle, it’s actually not all that mysterious and certainly very useful in all areas of photography. Importantly, depth-offield may be one of photography’s key technical considerations, but it also has many creative implications.
Starting With F-Stops
Depth-of-field is the area of an image that’s sharply rendered from the foreground to the background or, more simply, the front to back depth of sharpness. Depth-of-field is affected by a number of factors which are all lens-related, starting with the aperture settings.
The aperture mechanism in a lens (also called the ‘diaphragm’) creates an opening which varies in size, thereby allowing progressively more – or less – light to pass through and reach the sensor. Along with the shutter speed, the aperture setting is how an exposure is controlled. The size of the aperture is expressed as an ‘f-stop’ which is written as the letter ‘f’ followed by a number. These numbers are determined by a simple equation; the focal length of the lens divided by the diameter of the aperture. You don’t really need to know this because f-stops are always marked on the lens or shown in the camera’s readout displays, but it will help you understand why aperture f-stops are numbered in the opposite way to what would seem logical – the larger apertures have smaller f-numbers.
Typically, a modern lens has an aperture range from f2.8 – its largest or widest diameter opening – to f22, its smallest aperture. However, some lenses, may have a larger maximum aperture – say f2.0, even f1.4 – and some may have a smaller minimum aperture of f32. Zoom lenses may have a variable maximum aperture which is related to its focal or zooming range - such as f3.5-5.6 – but some zooms are constant-aperture which means there’s no variation in an aperture setting across the zooming range.
Visual Effects
As noted previously, the aperture has an effect on sharpness so you can increase the depth-of-field by using a smaller setting (for example, f16 or f22) or decrease it by using a larger aperture (say f2.8 or even f1.4).
Obviously, this requires manual setting of the aperture, either in the aperture-priority auto exposure control mode – in which case the
camera will automatically set the shutter speed needed to ensure a correct exposure – or in the fully manual mode.
Controlling the depth-of-field in this way has visual implications. A shallow depth-of-field is desirable if you want to throw the background out of focus so it doesn’t create any distractions or conflict with the main subject in your picture. Alternatively, longer depth-offield may be required if you want details in both the foreground and background to be in sharp focus such as may be required in landscape photography where both the details in the foreground and background are important.
A very shallow depth-of-field allows for selective focusing so the viewer’s eye is only drawn to the very small area of the image that’s sharp. This can be a compelling way of telling a story, particularly if the lens’s design gives a nicely progressive fall-off in sharpness. A very fast prime – or fixed focal length lenses – with a maximum aperture of f1.0 or even f0.95, will reduce the depth-of-field to a mere sliver when shooting at these settings, giving the potential to create very dramatic visual effects contrasting the sharp with the blurred.
Depth-of-field is also affected by the focal length of the lens. Wide-angle lenses (which have a short focal length, typically in the range of 14mm to 28mm in 35mm format terms) have an inherently greater depth-of-field — at any given f-stop — than telephoto lenses (which typically have a focal length of 200mm to 600mm). Depth-of-field also decreases as the camera-to-subject distance decreases so with macro photography – where sharpness can be very important – it’s often a challenge to obtain a wide enough depth-of-field.
Dealing With Exposure
Because changing the aperture also has an effect on exposure, it may not always be possible to just select a larger or smaller setting without making other corrections. In very bright conditions, opening up the aperture may result in overexposure even when using a very fast shutter speed such as 1/4000 or 1/8000 second. In this situation, a neutral density (ND) filter may be needed on the lens to help reduce the amount of light reaching the sensor by another means. These days ND filters are available in a variety of ‘strengths’ – some are even adjustable – reducing an exposure by as much as ten stops or more.
The alternative problem is the risk of underexposure if you’re shooting in lower light conditions and still want to use a small aperture setting to maximise the depth-of-field. There is, of course, a limit to how much you can slow down the shutter speed – as a way of letting more light into the camera to compensate – before camera shake becomes an issue and you’ll need to use a tripod. However, with in-camera image stabilisation systems becoming more effective – some now give up to five stops of correction – you may still be able to shoot handheld in some situations.
Alternatively, you can increase the sensitivity or ISO setting as a way of increasing the exposure, but noise may become a problem at settings of ISO 1600 or above. If you want very fine details to still be crisply reproduced, you may still have to resort to using a tripod to eliminate any risk of softening caused by the noise reduction process (or blur caused by camera shake).
Another option is to add more light and one of the benefits of digital capture is that this doesn’t necessary have to be via using electronic flash. Small to mid-sized LED-type continuous
“Modern digital cameras retain a full suite of manual controls and, if you’re still flying on auto pilot, you’ll be amazed what you can achieve with them.”
light sources can be used very effectively, especially for macro work, although the challenge is to make your artificial lighting look as natural as possible.
But Wait, There’s More…
Let’s talk about the hyperfocal distance. Aaargh! The what? Don’t panic, the hyperfocal distance is actually quite an easy concept to grasp and it will help you make the most of the depth-of-field available with a particular lens focal length and aperture setting. To get the idea, think of the depth-of-field as a fixed length, say a ruler for example. However, it doesn’t have to stay in a fixed position, you can move it forwards or backwards to alter which parts of the subject or scene are in focus. When you focus on a point, roughly onethird of the available depth-of-field extends in front of that point and the remaining two-thirds extends behind it. If you’re focusing at or near infinity – the longest distance setting on any lens – that twothirds of the depth-of-field behind the focusing point is effectively wasted while the nearer parts of the foreground will be out-of-focus.
The trick here is to instead focus on a nearer point – i.e. in front of your main subject – so the depth-of-field effectively shifts forwards to include more of the foreground, but without losing any sharpness in the background up to infinity.
How do you determine the focusing point which will give you this greatest depth-of-field and which is, incidentally, the hyperfocal distance? Inevitably, there’s an equation, but applying this in the field isn’t really practical, especially if you’re not so hot at maths. In the ‘olden’ days when lenses all had depth-of-field scales – for a selection of aperture settings – marked above the focusing control collar, you simply adjusted the focusing forward until the infinity symbol was positioned over the appropriate f-stop marking in the scale (rather than at the central focusing index mark). Some modern lenses still have depth-offield scales, but many zooms, in particular, don’t. Fortunately, there’s a sure-fire way you can determine the hyperfocal distance in the field. First find the nearest point in the foreground that you want to be sharp and estimate its distance from the camera… or simply focus on it and check the distance setting on the lens. Doubling this distance will then give you the hyperfocal distance as the rule at play here is that everything from half the hyperfocal distance to infinity will be sharp… at a given focal length and aperture, of course. So, for example, if that nearest point is 2.5 metres from the camera, the hyperfocal distance is five metres and focusing at this distance will give you all the depth-of-field you need (i.e. from 2.5 metres to at or near infinity). As before, you’ll have more depth-of-field to play around with in the foreground when using a wider-angle lens and/or a smaller aperture setting.
A digital camera provides a back-up, of course, as you can immediately check the image – using the magnifying function – to determine whether everything you want to be sharp, especially in the foreground, actually is. Alternatively, you could also use this method to help determine the hyperfocal distance. Focus on something at or near the background of your scene and take a shot. Magnify the image in the monitor screen and move the enlarged view slowly from the background to the foreground of the scene while checking the sharpness… the point at which the foreground becomes out-offocus is the hyperfocal distance. While the camera will almost certainly perform some sharpening of JPEGs – and hence the onscreen image – you should still be able to see when details become out-of-focus. And you can always experiment by taking a few images with the hyperfocal distance moved a little closer and/ or further away.
It’s advisable to use manual focus in these situations, but you can use autofocusing provided it’s set up to allow the precise selection of a particular focusing point and its active area provides sufficient coverage of the scene or subject.
No Cost, Plenty Of Benefit
So, depth-of-field isn’t really scary at all, but a really useful weapon when you need to take charge of what’s in focus in a picture or – perhaps more importantly – what’s not. As such it’s not only technically important, but has many creative implications. Using the principle of the hyperfocal distance simply extends the scope of what’s possible when maximising the depth-of-field available at a particular lens focal length and aperture setting.
What’s more it’s a control that’s right at your fingertips, ready to be exploited at no extra cost, but with the potential to greatly enhance your photography.