Depth of Field.pdf - The DV Show

0308ccu10 Depth of field 22/5/03 3:18 pm Page 22 

FIELD OF 

DREAMS 

Jim Tyler explains the age-old 

conundrum of Depth of Field. 

Learn to use it and add a 

professional touch to your videos 

Everybody thinks they know 

roughly what Depth of Field 

(DoF) is, but only a few people 

really understand what its implications 

are – and believe me, there are 

plenty of common misconceptions on 

the subject. 

A lens is only precisely focused at one 

point, and the further away from that 

single point a subject is, the less sharp 

its focus. Depth of Field is the distance 

between the closest and most distant 

points from the lens at which the subject 

is in reasonably sharp focus. 

This means that when you shoot a 

panorama with a person standing in 

the near foreground, either the person 

or the background can be in focus – 

but not both. Depth of Field can be 

increased by using a smaller aperture 

or by moving further away from the 

subject, provided the focal length is 

constant – that is, you don’t zoom in. 

I’ve just read an article about Depth of 

Field on a camcorder internet site, and it 

included the fact that DoF decreases 

with lens to subject distance (which is 

correct) but suggested that, in order to 

get the same image with greater DoF, 

you simply moved further away from 

the subject and zoomed in to achieve 

the same frame – which is balderdash! 

If you shoot a subject that’s just a 

yard away then the DoF will be very 

small, maybe between 2ft 9in and 3ft 

9in, depending on the aperture. Move 

back another twenty yards and shoot at 

20x magnification, so that the subject 

occupies the same amount of screen, 

and the DoF will be exactly the same – 

3in either side of the point at which 

the lens is focused to give a total DoF 

of 6in. What will change as a result of 

moving further away and zooming in 

is the perspective, making your subject 

matter appear ‘flatter’. 

There is, in fact, only one way of 

altering the DoF for a given subject 

occupying a given area of the screen, 

and that is to vary the lens aperture. 

The smaller the aperture, the greater the 

depth of field. If you vary the aperture 

the time that the CCD is exposed must 

also vary, so that the correct amount 

of light reaches it. To get the correct 

exposure with a smaller aperture it’s 

necessary to lengthen the exposure, 

and vice-versa. Unfortunately, unlike 

stills photographers, who record only 

an instant in time and not time itself, 

we are stuck with 1/25th of a second 

exposure and can’t vary exposure time 

without suffering side effects. 

To decrease depth of field, which we 

do in order to throw undesired elements 

of a scene completely out of focus, we


Depth of Field The Knowledge 

need a wider aperture. This involves 

reducing the exposure time in order to 

keep the same amount of light being 

processed by the CCD. The problem 

with reducing the exposure time is that 

we record only segments of time and, 

if the exposure is very short, this gives 

jerky onscreen movement. 

Decreasing the exposure time to 

1/50th of a second won’t make your 

video look like cine film, but the effect 

on DoF won’t be much to write home 

about either – just one stop. At 1/100th 

of a second the effect on motion begins 

to be apparent, and still you will have 

opened the iris only a couple of stops. 

Whether such small changes in aperture 

will be sufficient depends on what 

you’re shooting and, perhaps more 

specifically, how far away from the 

lens the point of focus is. 

Increasing the exposure time beyond 

1/25th of a second to achieve a smaller 

aperture and so extend the depth of 

field (not all camcorders allow this) 

obviously means shooting at less than 

25 frames per second, meaning that 

onscreen movement is really jerky. 

There is a better way! 

Lighting-up time! 

If you shoot a very bright scene the 

auto exposure will give you all the DoF 

your heart could desire by selecting a 

small aperture for you, and vice versa. 

If you shoot using artificial lighting you 

can vary its intensity at will, but most 

video is shot using natural daylight, over 

which we have no control other than to 

try to select the right day, and time of 

day, to shoot! 

Put simply, if you want, for instance, 

to shoot a panorama and get as much 

as possible of it in passably sharp focus, 

do it on a day with a cloudless sky, 

sometime between mid-morning and 

mid-afternoon. If you want to get just 

one specific element of the scene in 

focus and lose the rest then shoot on 

a dull day. 

Most of the time, we want to get as 

much of the scene as we can in focus, 

Far Left: The rose is 

about a yard from the 

camcorder, which is at 

a wide-angle setting. 

Note that both the 

near and distant 

foliage is out of focus 

– in fact, some is 

barely recognisable 

Left: Taken from three 

yards away at three 

times the magnification 

(so that the rose 

appears roughly the 

same size). The Depth 

of Field is the same. 

Moving back from a 

subject and zooming 

in does not alter DoF – 

to do that you need to 

close down the iris! 

What has been altered 

is the perspective – 

the shot is from a 

different position 

Below: Here’s the 

same shot, taken with 

the iris stopped down. 

Compare this with the 

previous shot and 

you’ll see both nearer 

and more distant 

detail in better focus 

‘There is, in fact, only 

one way of altering 

the DoF for a given 

subject occupying 

a given area of the 

screen, and that 

is to vary the 

lens aperture’ 

23 

Camcorder User & DVD Movie Maker August 2003


The Knowledge 

Depth of Field 

‘Sadly, not one 

current consumer 

camcorder that 

offers manual focus 

has a distance scale 

engraved on the 

lens barrel’ 

and you don’t necessarily have to be at 

the very smallest aperture to achieve 

enough DoF. A mid-range aperture of 

ƒ8 to ƒ11 will give reasonable DoF 

and, perhaps more importantly, the 

resolving power of a lens peaks at 

these apertures. 

If you want to get the far distance 

in focus, different apertures effectively 

give different closest points of focus. At 

ƒ22 you might get everything from the 

horizon down to 6ft in front of the lens 

in focus when shooting without any 

zoom, at ƒ16 the point of close focus 

might extend to 8ft, at ƒ11 it might be 

11ft, at ƒ8 15ft and at ƒ4 nearer 30ft. 

Unfortunately you’ll find that Depth 

of Field figures are not exact. This is 

because there is no hard and fast 

definition of what constitutes reasonable 

focus – it is subjective and will vary 

according the circumstance. It could be 

so sharp that you can see individual 

hairs on a subject’s head, or just sharp 

enough to recognise that the subject 

was a person! Another inexact figure is 

the Hyperfocal distance. This is the 

distance at which the lens should focus, 

in order for both a desired distant point 

of focus, and a desired close point of 

focus (which is half the Hyperfocal 

distance), to be tolerably sharp. 

Focus off the subject 

It might sound daft, but there are times 

when it can pay not to focus directly 

on the main subject. The Hyperfocal 

distance might be theoretical but the 

principle can be used in practice to 

achieve the shots you really want. 

Suppose that your main subject is 

20ft away and that you want both the 

subject and some more distant parts of 

the scene to be in focus. If you allow 

the auto focus to settle on the main 

Left: In order to get mini DV picture 

quality with none of the colour bleed 

of analogue tape, and also keep control 

over focus and Depth of Field, Jim has 

taken the extraordinary step of coupling 

his JVC GR-S707 and Sony TRV20 

subject, the DoF might not extend far 

enough behind it to accommodate 

the background. However, if you force 

focus on 30ft, both the subject and the 

background should be in focus, given 

that the aperture is sufficiently small 

to grant that DoF. 

Sadly, not one current consumer cam 

that offers manual focus has a distance 

scale engraved on the lens barrel. This 

has become a no-go feature for camcorder 

manufacturers in recent years, 

and while my old S-VHS model does 

feature it, you’ll look long and hard 

before you see it on any of today’s 

models. While manufacturers might not 

include a calibrated lens ring many of the 

higher end enthusiast models do at least 

give focusing and exposure details on the 

LCD screen and optical viewfinder. 

The GR-S707 also allows me to set 

aperture in terms of f-stops and so gives 

complete control over not only focus but 

also DoF. The majority of manual focus 

rings are now ‘fly by wire’ jobs that are 

pretty well useless, meaning that – even 

with camcorders that offer manual focus 

– it is often better to manipulate the 

auto focus than to try to focus manually. 

So how do you ‘manipulate’ auto 

focus? Simply give it a ‘target’ at the 

desired distance. This means placing 

the main subject off-centre and having 

something at the correct distance occupying 

centre screen. If your camcorder 

offers focus lock, engage it and re-frame 

or, if your camcorder has manual focus, 

engage that and re-frame. 

24 

Camcorder User & DVD Movie Maker August 2003 

Close encounters 

There is another way in which you can 

get a subject in sharp focus and get 

the background hopelessly out of focus 

– and that is to get very close to the 

subject. This is because Depth of Field 

is reduced as the point of focus gets 

closer to the lens. Unfortunately, in order 

to get really close you need to go wide 

angle, so that the DoF increases! ■

Depth of Field.pdf - The DV Show

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