Amateur Photographer – 06 July 2019

subscribe 0330 333 1113 I http://www.amateurphotographer.co.uk I 6 July 2019 19

AF mysteries

explained

1

What is the focus

limiter switch for?

If your subject is going to be within a

specifi c range it can be helpful to use the

lens focus limiter to prevent it from hunting

across the entire focus range.

2

What is focus stacking?

This technique involves shooting

a series of images, each with a slightly

different focus distance. The images are

then composited into one that has much

greater depth of fi eld than an individual

shot. It’s a very popular technique with

macro photographers.

3

Why does AF depend on

the lens I am using?

Different lenses have different motors that

drive the focusing and different weight

elements. Also, lenses with a wider

maximum aperture let in more light,

which helps the AF system.

4

Why are full-frame DSLRs’

AF points near the centre?

Vignetting as well as the main and

secondary mirror arrangement of a DSLR

mean that the AF sensor is smaller than the

imaging sensor. As a result, the AF points

are central.

5

Can f/5.6-sensitive AF

points be used at smaller

apertures?

Yes. The focusing takes place with the

aperture wide open, so as long as you use

a lens with a maximum aperture of f/5.6 or

greater, it will work.

Some cameras have a series of customisation
settings for the AF system. These affect aspects
such as how the AF responds to a change in

the subject distance or if the subject moves
from the selected AF point.
Many photographers assume that it’s best to
set the camera to react quickly to any change,

but a little pause can be a good thing. For
example, if you’re photographing a swimmer
doing the breaststroke towards you, with a fast

setting the camera will refocus on the far end

of the pool every time they bob underwater.

Then when they reappear it will refocus again,

making a major adjustment. A slower reaction

will help the camera stay on the subject.

To get the best from these customisation

systems you need to consider what you’re

photographing as well as the shooting

conditions – for example, whether any objects

are likely to come between you and the subject.

Focusing systems need light and contrast to
work. It’s the lack of contrast that can leave a
camera struggling to focus on a well-lit sheet
of paper or a smooth wall. If you can fi nd some

point of contrast, even if it’s just a join in the
wallpaper, it will often snap into focus quickly.
Because the phase-detection sensors in a
DSLR are often arranged in lines, the detail

needs to cross a line rather than run along it for
it to be ‘visible’ to the AF system. Consequently,
it’s a good idea to understand your camera’s

AF sensor arrangement. It’s also why cross-

type AF points are useful – they can detect

details in more than one direction.

If your camera is struggling to focus, look for

an area of contrast or consider switching to the

central AF point. As well as being cross-type,

this is usually the most sensitive in a DSLR.

Cameras sometimes have multiple cross-

type AF points and it’s worth knowing where

these are, especially when shooting action

in low light.

Knowing how your

subject moves can help

you decide on the ideal

AF customisation settings

Most cameras struggle to focus on a
silhouette unless you put the active AF point
over the edge where there’s some contrast

My AF system is struggling to lock on – what should I do?

Is it worth customising my AF system?