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We clicked with:
Excellent white balance tool
Portrait version adds some welcome warmth...
Turn your camera into an incident light meter, too
Addictive properties - no more auto WB
Forces more deliberate picture taking
Shots in the dark:
You will also need a grey card sometimes
Er... that's it!
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Designed long before digital cameras became commonly available, the Expodisc was designed by George Wallace. In the 1940s, Wallace was a student of Ansel Adams, Minor White and Edward Weston. As the name might suggest, the Expodisc was (and still is) designed to act as an aide to exposure, but since the dawn of digital, it has found another use - accurate white balancing.
The idea is simple, but elegant. The Expodisc is designed to slip on the filter thread of your lens, with flat side facing the front element of the lens and a series of prisms facing outward. It makes your lens look a bit like a fly's eye. The combination of the two effectively remaps all the light falling on your sensor as one even monotone. If you use your camera's custom white balance setting, the Expodisc will do wonders at averaging out the different light sources. It will also do precisely the same with the exposure.
There's a 'but' here. Yes, the Expodisc effectively turns your camera into an exposure and a colour temperature meter, but both take incident, not reflective, readings. In other words, if you want an accurate colour temperature or exposure reading, you need to position the camera (with the Expodisc over the lens) at the point where you will be photographing your subject matter and point the lens back at the spot you intend to photograph from. You also need to be careful to get the right position, if you are working with mixed lighting sources; point the camera too high or too low and you change the mix. A good tip for tripod users is to set up your shot and then point the camera back at the tripod head.
If this seems like too much trouble, think again. Incident light readings return extremely accurate metering, because they aren't influenced by the nature of the subject matter. And the white balance control is little short of stunning. OK, under normal daylight conditions, the auto white balance should get very close to the Expodisc result, but as soon as we move out of the ordinary, the Expodisc quickly pays for itself. White balance systems struggle to create an accurate image when the subject matter is strongly coloured in and of itself and under low brightness tungsten light. Also, the new 'green' lightbulbs that are replacing traditional tungsten designs have a weird balance somewhere between tungsten and fluorescent light - the only way of accurately rendering both situations is to use something like the Expodisc.
There's also the exposure issue, and this is where the Expodisc scores a double bonus. Despite the efforts of all camera makers, no metering system is 100% infallible. Say you are photographing your subject matter is either is a white wall; most TTL meters will render the wall 18% grey or blow out the wall as a highlight. By taking a reading of the light hitting the wall instead of the light reflecting off a wall, an incident meter reading will make the wall appear white… but not so white it's just one big highlight. Just remember to keep your camera in manual mode throughout and switch off your autofocus.
There are two types of Expodisc; the standard and the 'Portrait' or 'Warm' and you can tell them apart by their lanyard. Both give precisely the same exposure, but the blue-lanyard standard Expodisc gives an essentially neutral balance (excellent for landscapes) while the yellow-taped Portrait adds a subtle warmth to the image, akin to adding an 81A filter (which, again as the name suggests, makes it great for portrature).
There will be times when it's impossible to take an incident reading, and at that point the Expodisc becomes redundant and a grey card is a better concept (be warned - not all grey cards are created equal). There are other times when the automatic or preset white balance settings are so close to the custom setting as to make no real difference. For everything else… there's Expodisc.
