Tuesday, January 30, 2007

Not so simple

I've processed all the photographs of the wedding now. Thankfully I use Adobe Photoshop CS which enables me to make adjustments otherwise some of my photos would be unusable.

So why did I have to do this? The Sigma EF-500 DG ST flashgun I bought uses the E-TTL II system to automatically adjust the amount of light required to get a perfect exposure. So I set the camera to ASA 200, the aperture to f8. The shutter speed is automatically set to 1/200th sec. when the flashgun is attached. In theory it should then work flawlessly and produce correctly exposed pictures.

The flash has a guide number of 50 in metres at 100ASA. As I said the camera was set to 200ASA so that would give me a guide number of 100. At my f8 aperture that would give me a range up to 12.5m but only with a 105mm lens. The flashgun has an autozoom facility which adjusts the width of the light beam according to the setting of the lens. With wider angle lenses the guide number is reduced.

I was using my 18 - 55mm lens. So at the widest angle the actual guide number was 60 which meant the range was reduced to 7.5m. That would still have been adequate if the room hadn't been so large with a high ceiling i.e. nothing to bounce the flash back. Changing to 400ASA for more distant shots made some improvement but the pictures were still a little underexposed.

I wonder what if anything I could have done if I had been using a film camera instead of a digital. Without the ability to adjust the pictures I could well have been stuffed.

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