Canon EOS mag

Technique Flash factors

How the ISO setting affects flash exposure

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.

nina bailey

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.”

ISO 100

ISO 800

ISO 6400

ISO ranges of EOS cameras (Standard range in BLUE ; expanded range in RED )

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

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.

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.

EOS D30 EOS D60 EOS M

30 Reproduced from EOS magazine October-December 2012

Reproduced from EOS magazine October-December 2012 31