Know Your Lens - Optimal Focusing Distance
Have you ever wondered what factors cause an image to be in or out of focus, why some parts of an image can seem
sharp while others are fuzzy and blurred? Why taking a photograph of apparently the same subject with different cameras
or lenses may give differing ranges of focus?
There are many considerations when deciding how to compose an image and then setting the camera to achieve the
desired result. Here we will discuss how to achieve a sharp image over the desired range within the scene, whether
this is a deep area providing a clear representation of much of the scene or a very shallow section highlighting a
particular element. The range of this sharp area within an image is referred to as the Depth Of Field (DOF). There are
three main components that affect the DOF:
- the Focal Length of the lens in use
- the distance at which the lens
is focused and
- the Aperture setting on the lens
When considering the focal length of a lens, the larger this value is, the shallower will be the depth of field, i.e.
less of the entire image will be in focus. This indicates that, assuming all other factors are the same, a wide angle
lens, e.g. with a focal length of 20mm, will have much more of the scene in focus than a 300mm telephoto lens.
The further away from the camera that a lens is focused upon, the larger will be the DOF.
The aperture of a lens is the opening through which light enters the camera. This aperture can be increased or
decreased to let more or less light respectively into the camera. The settings of the aperture and hence the amount of
light reaching the film are indicated by the term f-Stop. A typical range of f-Stops is 1.4, 2, 2.8, 4, 5.6, 8, 11, 16,
22, 32, and 45. The lower a number, the wider the aperture, and hence the more light that reaches the film. Each higher
f-Stop in this sequence lets in exactly half the light of the previous number. Some cameras may be able to set
intermediate f-Stops. The f-Stop is a relative measure and is designed such that whatever lens is in use the same f-Stop
will pass the same amount of light onto the film. In reality the f-Stop is the ratio of the focal length of the lens
to the actual diameter of the aperture. Thus an f-stop of f4 gives an iris opening equal to one quarter of the focal
length of the lens.
An example of this is the comparison between a 20mm wide
angle lens and a 105mm lens. With an f-Stop of f8 the actual aperture diameter of the 20mm lens is 2.5mm (20/8) while the
diameter of the aperture on the longer lens is 13.125mm (105/8).
These aperture diameters on their respective lens allow the same amount of light onto the film.
| To increase the DOF |
To decrease the DOF |
- use a shorter focal length lens
- move further back from the subject
- use a smaller aperture (larger
f-Stop number)
|
- use a longer focal length lens
- move closer to the subject
- use a wider aperture (smaller f-Stop
number)
|
Light Behaviour Through A Lens
Light does not travel in a straight line through a camera lens. It is the purpose of the lens to bend the light so that
a large image can be condensed onto a small piece of film. Thus the lens elements within a camera lens change the angle of
the light as it passes through.
A lens focuses at only one distance at any one time. Light reflected from objects at this distance is brought to a
sharp point on the film. These images are thus 'in focus'. All light is sharply focused at some point. The light from
objects not at the same distance from the lens as its current focus distance will be brought to a sharp point either in
front of or behind the film plane. Therefore, as this light crosses the film plane it is not a sharp point but rather a
circle. The wider this circle the more out of focus the object will appear. These circles are referred to as 'Circles Of
Confusion'
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Primarily, to achieve a maximum depth of field, the objective is to minimise the size of the circles of confusion. As
the aperture of a lens is narrowed (a higher number f-Stop) the passage of light through the lens is narrowed. This leads
to smaller circles of confusion and a sharper appearing image.
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Unfortunately, however, light bends around sharp edges (e.g. in this case the aperture iris) at different rates depending
on its colour. This causes a limit of sharpness depending on the diffraction of the light. Thus to achieve the desirable
DOF use an aperture small enough but no smaller.
The Circle Of Confusion needs a touch more explanation. Traditionally a tolerable CofC equates to what the human eye
deems to be a sharp image on an 8"x10" print when viewed from a standard reading distance of say 10 inches
(254mm). This is generally accepted to be approximately 1/100 of an inch, or .254 millimetres. Thus the circle of
confusion for an 8x10 format will be 0.254mm. To calculate the CofC for other formats the ratio of the size of the film
to this 8x10 format must be applied to the CofC for the 8x10 format.
Thus for 35mm format the acceptable CofC would be
.254 / ((8 x 25.4) / 24) = 0.03mm. Here 24 is the length in millimetres of the short side of the 35mm negative.
Different CofC values can be taken from the following table:
| Format | Dimensions | Diagonal Length | Short Side CofC | Diagonal CofC |
| 35mm | 24mm x 36mm | 43.27mm | .254 / (200 / 24) = 0.030mm | .254 / (320.16 / 43.27) = 0.034mm |
| 456 | 41.5mm x 56mm | 69.70mm | .254 / (200 / 41.5) = 0.053mm | .254 / (320.16 / 69.70) = 0.055mm |
| 6x6 | 56mm x 56mm | 79.20mm | .254 / (200 / 56) = 0.071mm | .254 / (320.16 / 79.20) = 0.063mm |
| 6x7 | 56mm x 69.5mm | 89.25mm | .254 / (200 / 56) = 0.071mm | .254 / (320.16 / 89.25) = 0.071mm |
| 6x9 | 56mm x 84mm | 100.96mm | .254 / (200 / 56) = 0.071mm | .254 / (320.16 / 100.96) = 0.080mm |
| 4x5 | 96mm x 120mm | 153.67mm | .254 / (200 / 96) = 0.122mm | .254 / (320.16 / 153.67) = 0.122mm |
| 8x10 | 200mm x 250mm | 320.16mm | .254 / (200 / 200) = 0.254mm | .254 / (320.16 / 320.16) = 0.254mm |
These CofC values will give the same sharpness for each format when that negative is used to enlarge the print to 8"x10".
Depth Of Field Calculations
To achieve the maximum depth of field the camera should be focused at the Hyperfocal Distance for the lens in use.
Examining this formula reveals that different focal length and aperture combinations will have a different hyperfocal
distance. The range of the depth of field for any distance can be calculated using the formulas here. Objects at and
outside these boundaries will begin to appear fuzzy and out of focus. Remember, using the CofC from the table will give
a "standard acceptable" reproduction of print. Many people do not believe this standard is good enough and prefer to
use smaller values. This will give a narrower band of acceptable depth of field. It will not make objects within this
range sharper than the same distances as before but if the whole image is kept within the range then the whole image
will appear sharper than if the whole image were within the wider depth of field range.
The following table can be used to calculate depth of field ranges:
An Alternate DOF Philosophy
When the hyperfocal distance is used as the focus distance it will provide no more than average image quality in all
but a few specific cases. This is simply due to the fact that not every scene is optimally covered by the hyperfocal
DOF range. Using the formula for resolution limit (the size an object needs to be before it will appear sharp), the
photographer can decide what aspects of the image will be sharpest. For example with a 70mm lens with an f-Stop of
f/5.6 we have an aperture diameter of 12.5mm and a hyperfocal distance of 29.167 metres. When focused at the hyperfocal
distance an object two metres from the camera will have a resolution limit (R) of 12.5 * (29167 - 2000)/29167 = 11.64mm.
An object in the near distance, say 1km, will have as its resolution limit 12.5 * (1000000 - 29167)/29167 = 416.07mm.
Although the near distance R value will resolve as sharp relatively small objects, the far R value of over 41cm will
fail to sharpen leaves, twigs, or even branches. If the image is focused instead at the distant area (1km) then the
two R values are 12.5 * (1000000 - 2000)/1000000 = 12.48mm and 12.5 * (0/1000000) = 12.5mm. Although this gives a
slightly greater resolution limit for the close object the distant scene will be resolved much sharper.
| Film Plane: the plane in the camera where lies the surface of the
film onto which the image is exposed |
| Focal Length: the distance from the optical centre of the lens to the
film plane when focus is set at infinity |
| Angle Of View: the angle inside a conical shape extending from the
camera into the viewed scene. This cone is the limit of what can be seen through the lens |
| Aperture: opening of the lens through which light enters the camera.
This can be modified by an expandable/contractable iris diaphragm |
| f Stop: a numeric indication of the size of the aperture. |
| Depth Of Field: the zone of sharp focus in a scene, extending from
the nearest element that is sharp to the farthest |
| Circle Of Confusion: circular area (rather than a point) of light at
the film plane caused by images that are out of focus. Their true focus point being in front of or behind the film
plane |
| Hyperfocal Distance: The distance that when focused upon provides
the greatest depth of field |
| Resolution Limit: The size an object has to be before it can be
considered as sharply resolved in an image |
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