Thursday, 7 January 2010

Tips for Sharper Shotz (2)

Aperture as a creative tool
Cameras, point-and-shoot or D-SLRs, come with full auto focus (AF) capabilities. This means that, in theory at least, we just need to be able to point the thing in the direction of the subject and the camera meters (regulates) the light passing into the camera and it automatically focusses on the subject too. Unlike the shutter though, the aperture also affects how much ‘stuff’ appears sharp around the bit that the lens initially focused on. Wer'e all familiar with depth of field focussing effects. Every Donna Hay cookbook has illustrations shot using a wide open aperture, not because the photographer needed to get more light but because a side effect of using a small number, or wide open aperture, is that only the bit focused on actually comes out sharp. It’s a great effect that works well for many subjects (see illustration here). To get more stuff clear in the same frame, the photographer has to choose a big number f-stop (that has a correspondingly small aperture number) however, this's not always good because, if the aperture is made smaller, you need a correspondingly brighter light source, or a slower shutter speed to collect the same amount of light into the frame as before. Here’s where problems occur. A longer shutter speed means slower exposures which, you guessed it, means camera shake.

Telephoto lenses produce markedly shallow depths of field, especially once zoomed in to full magnification. This can work for, or against the photographer, depending on how you interpret the results. Here I focused in the centre of the viewfinder and, at a magnification of 200mm, only got a small fraction of the frame sharp. It’s important to note that this is not so much out of focus or blurred, it’s selectively focused.

Understand light metering
All cameras have a light meters. Some store memories of thousands of ‘typical’ photos so the current scene can be evaluated against its memory bank in the hope that the perfect exposure is the result. Sometimes this happens, sometimes not. I don’t care what the camera designers and tech geeks claim this new technology can do, inconsistencies will always occur simply because of the variables involved. Give ten photographers, ten similar cameras and similar subject matter, and you’ll get ten different results.
Once you understand how a meter works, you’ll be able to control the results much better. The important bit is to appreciate is that the meter’s job is to average out the light to a point somewhere in between light and dark. Typically, a white wall will be metered as grey. A black predominently dark subject will be metered as grey. This happens because light and dark is averaged to get a more consistent result. Compounding this is the way that the meter prioritises its (shutter and aperture setting) decisions. All cameras prioritise shutter speed over aperture, up to relatively fast speeds of 1/125s, or more. Use a Photo Browser and look at a bunch of your shots. I can guarantee that, reading off the Metadata for each, you’ll see most have been shot at the maximum aperture unless the scene was shot in very bright light. This is because the programmers want the fastest shutter speed, to prevent camera shake. This also explains why sometimes stuff appears to be soft in the shot, especially if it’s located physically close to the point where the lens initially focused. Because it’s chosen a big aperture, the depth of field is small, therefore stuff looks fuzzy. The photo isn’t necessarily out of focus, it’s just the bits that fall outside of the depth of field are not clear. Change the exposure mode from Auto to Aperture. This enables you to dial-in an f-stop of your choice. Great for controlling depth of field, something that’s needed by advertising, landscape and architectural photographers. So, if you override the light meter by forcing it to set f22 instead of f2.8, the shutter speed increases by six steps. Thus an original exposure of 1/60s turns into one, or two seconds. Impossible to hand hold and still guaranteed to produce inferior results unless a tripod is used (cont'd).

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