Thursday 25 August 2011

Focal-plane shutters and camera-phone slit scan

The principle of slitscans explained and done deliberately in the last post also applies to "ordinary cameras" under certain circumstances. In the "old days" of film photography, this meant focal plane shutters.

Focal plane shutters

Most 35mm film SLR cameras have focal plane shutters. A few compact 35mm cameras have them too (usually the ones like Leicas or Zorkis with interchangeable lenses.) Some medium format cameras have focal plane shutters, and a small number of large format cameras have them. Focal plane shutters have two moving blinds next to the film - one blind opens and the other closes. The advantage for the camera manufacturers is that the closing blind can start to close before the opening blind has finished opening. This makes the shutter look like a slit moving across the film. The slit is narrower the "faster" the shutter is. That's why the manufacturers like these shutters: they don't have to make anything actually move fast, just make the slit narrower. Of course moving slits play havoc with other aspects of photography: they are not good if you want to take pictures with flash, but they are cheaper.

The famous picture by Lartigue is an old one taken with a rather slow focal plane shutter set so the slit is rather narrow. In this picture he captures the car quite sharp (but not the wheels) but it is "slanted" as the shutter or slit is moving from top to bottom. In fact to make it more complicated, Lartigue also pans the camera in the picture, but not enough to capture the car vertically.

If you own a camera with a focal plane shutter you might experiment with it: set it to its fastest speed and take photographs of rapidliy moving objects. Helicopter blades or aeroplane propellers are good subjects. Try holding the camera the nomal way, on its side, or even upside down. Some focal plane shutters (the majority, probably) go from side to side, and some go up and down. For example my Olympus OM1 has a side-to-side movement, whereas my Praktica MTL has an up-and-down movement. (Because of the vertical shutter Prakticas of the 1970s are ideal for these experiments.) I must try this again, but unfortunately I haven't got any nice images from these cameras to show here right now.

Digital slitscans

But in the modern age, some digital cameras work as slitscans too. In this case there is no slit moving but the camera's processor is so slow and is programmed to grab the scene a line at at time, that this moving line as it is "grabbed" is similar to a slit. Try the camera attached to your phone. Or even better get a very old phone and try that. My phone is quite old, and I was pleased to get this image out of the window of a moving car.

Lance charge

With some panning you might get a nice artistic illusion of speed.

Moving bus

Try to work out which way your phone or digital camera scans. I find my images are much better if I hold my phone upside down!

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