Basics

The point on which you focus your camera is sharp, or as sharp as you can get it. Other areas that can be considered to be sharp (or in focus if you like) are contained within the depth of field of the shot. This is a variable amount and unfortunately tends to be small. It varies depending on the magnification and also the aperture used. The table below gives some indication of its size.

Magnification

2

(2:1)

1.5

(1.5:1)

1

(1:1)

0.66

(1:0.7)

0.5

(1:2)

0.25

(1:4)

0.1

(1:10)

f 2.8

0.14

0.18

0.34

0.64

1.00

3.36

18.4

f 5.6

0.28

0.38

0.68

1.28

2.02

6.72

37.0

f 8

0.4

0.54

0.96

1.82

2.88

9.60

52.8

f 11

0.55

0.74

1.32

2.52

3.96

13.2

72.6

f 16

0.8

1.06

1.98

3.66

5.76

19.2

105.6

f 22

1.1

1.46

2.64

5.04

7.92

26.4

145.2

f 32

1.6

2.14

3.84

7.32

11.52

38.4

210.2

Distance from nearest point of sharpness to furthest. N.B. a magnification of 1:1 means a 35mm subject will appear 35mm on film, 2:1 35mm subject to 17.5mm on film (i.e. half size) etc.

With no flash, in normal sunlight, with an average brown or grey butterfly, using a 100mm lens at 1/125 or 1/90 you can expect about f5.6 -> f11. Giving a depth of field of 5mm or so. This is very tight as the pictures below show.

Three pictures taken in close succession of a blue spot hairstreak. They are all with a 100mm macro lens at 1/125 with an aperture of f7. The film speed was ASA100.
Sharpest tail    
  Sharpest head  
    Sharpest wingtip

None of the pictures cover the three extreme areas of the butterfly perfectly. Each is sharp for one corner, and the middle area, of the picture. How can an image be produced with all of it sharp? Using a flash, changing the ISO rating and changing the aperture are all discussed in the next few pages. Take lots of pictures and hope to get one which is just right is an alternative which the digital age has made possible.

Ideally setting the aperture as small (i.e. large f-number) as possible to maximise the depth of field is logical. However very small apertures produce a diffraction effect which decreases the image sharpness, so in between is best, and about f16 is probably the best compromise.

So the camera is set on f16 and we fire away with good depth of field, oh no we don’t. For the f-number set a shot speed will have to be set depending on the amount of available light,. The shot speed needs to be set so that camera shake does not effect the picture, a guide is generally 1/focal length for a given lens. So for a 100mm lens 1/125s is OK and for a 50mm lens 1/60s is OK. This inevitably means we can’t use the f16 that we want to. So we use a bigger aperture (smaller f-number) and cope with reduced depth of field or we use a flash or we use a tripod. These topics are discussed in a later section.

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All pictures in these pages copyright to Simon Coombes. Permission must be sought and obtained for any use.