Canon EOS mag

Technique

Flash factors

Reproduced from EOS magazine October-December 2012

ISO value

50 100 200 400 800 1000 1250 1600 3200 6400 12800 25600 51200 120400 204800

EOS-1D

EOS-1D Mark II

EOS-1D Mark II N

EOS-1D Mark III

EOS-1D Mark IV

EOS-1Ds

EOS-1Ds Mark II

EOS-1Ds Mark III

EOS-1D X

EOS 5D

EOS 5D Mark II

EOS 5D Mark III

EOS 6D

EOS 7D

EOS 10D

EOS 20D

EOS 30D

EOS 40D

EOS 50D

EOS 60D

EOS 300D

EOS 350D

EOS 400D

EOS 450D

EOS 500D

EOS 550D

EOS 600D

EOS 650D

EOS 1000D

EOS 1100D

EOS D30

EOS D60

EOS M

ISO ranges of EOS cameras

(Standard range in

BLUE

; expanded range in

RED

)

Many EOS cameras have what is known as

‘ISO expansion’. Check the table below to see if

this feature is available on your camera. It takes

your standard ISO range and expands it a little

in one or both directions. But the extra ISO

values need to be unlocked before you can use

them. Why?

When light reaches the sensor the photo-

sensitive pixels each generate a tiny electrical

current. The level of current is proportional to

the brightness of the light.

In the standard range, ISO values are

increased or decreased by changing the

amplification of these tiny currents. At higher

amplifications the sensor becomes more

sensitive to light.

Canon provides very little information on

the subject, but we think that the expanded

ISO values are not the result of increased

(or reduced) amplification, but are created

by algorithms in the DIGIC processor. If you

salvage an under- or overexposed image in

Photoshop, you are doing something similar.

“Look”, Canon is saying, “we’d rather you

did not go into the expanded ISO zone, so we

have bolted the gate. If, despite this, if you

decide to venture forth, please don’t complain

to us if your images are not as good as

expected. We did warn you. It’s not our fault.”

Expanded ISO settings

Left

Opening up the expanded range of

ISO values varies between camera. On

many models it is a Custom Function. Set

to ‘On’ and the expanded ISO value(s)

will become available when you come to

select the ISO. The display shown here is

from the EOS 650D.

EOS 20D, 30D:

C.Fn-08

EOS 40D, 50D, 60D:

C.Fn I-3

EOS 500D, 550D, 600D, 650D:

C.Fn I-2

EOS 1D:

C.Fn 03-1

EOS 1D III, 1D IV, 1Ds, 1Ds III:

C.Fn I-3

If the ISO expansion is not tucked away

inside the Custom Functions, it is a main

menu item. For the EOS 10D, it is in the

shooting menu (red). For the EOS 1D

Mark II, 1D Mark II N and 1Ds Mark II, it

is under the camera tab. The options are

‘on’ or ‘off’.

Left

The ISO expansion for the

EOS-1D X and the 5D Mark III is also a

menu item under the camera tab, but

offers more options. The first screen

shows the menu for setting the ISO. This

is done by turning the Quick Dial and

pressing the SET button. The greyed

out values cannot be selected. Opening

up the ISO range is done by selecting

’ISO speed range’ from the ‘ISO speed

settings’ menu. This allows you set the

minimum and maximum values for the

range (second and third screens). You

can also set a restricted range – ISO 400

to 3200, for example.

ISO 6400

ISO 800

ISO 100

There is a strange logic at work when

photographers use flash. It goes like this. “I do

not want to use a tripod, so I will use flash. As I

am using flash, I have plenty of light and can set

a low ISO to get the best image quality.”

Years ago, this approach might have been

valid, as the image quality from fast films was

poor. Using a film with an ISO rating above 400

was considered adventurous. Even ISO 400 film

was avoided, except for low-light work.

The problem is that to make use of low

ambient light with slow ISO speeds often

requires long exposure times. Even with the

lens aperture wide open, shutter speeds of 1/4

second or longer are not uncommon. This is not

ideal when shooting people – subject movement

during the exposure can blur the image.

Digital cameras are optimised for ISO 100

or 200 and results from ISO values between

100 and 400 are virtually indistinguishable in

image quality. If you are using the camera on a

fully automatic mode, or with the ISO on Auto,

the default setting when the flash is active,

regardless of whether it is a built-in flash or an

external unit, is ISO 400.

Even going up to ISO 6400 gives acceptable

quality on EOS models with a maximum ISO

of 12800. This is providing you are shooting

raw files, or JPEG files with the noise reduction

function switched on.

How the ISO setting affects flash exposure

Top left and right and

above

This sequence

of images shows the

difference that increasing

the ISO can make, even

when shooting in program

mode. As the ISO gets

higher we see more of

the background. When

the ISO is giving a good

background exposure the

flash illumination starts

to reduce to give a better

fill-in flash exposure.

All images shot using EOS

7D, EF-S 18-55mm f3.5-5.6

IS USM lens at 50mm,1/60

second at f5.

nina bailey