3 April 2009

Shutter speed choices

Camera shake can be the ruin of any image. Fortunately, with a little thought it's very easy to overcome. There is a rule-of-tumb that states to avoid camera shake, you should use a shutter speed that is no slower than the reciprocal of the focal length of the lens. This is based on a standard print size, which in the days when it was pronounced, was around 10x8inch. Now, in today's world where A4 seems to be the norm, that fortunately still applies. However, if you plan on printing bigger, then you might just want to reconsider those figures and perhaps make them a little faster. As with everything, it's worth doing some testing yourself.

What is true though, is that as resolutions increase, pixels get smaller and this can have an effect on camera shake or the appearance of subject movement in your images.

using a shutter speed no slower than the reciprocal of the focal length of the lens will avoid camera shake

Take for example an EOS 50D. This camera packs in 15.1 Megapixels onto an APS-C sized sensor. Clearly, those pixels need to be really quite small (a little under 5 micrometers - 4.7 to be exact). These smaller pixels mean that any movement during the exposure will occur over a greater number of pixels than if they were larger, and this will increase the apparent blur from either subject movement or camera shake. Now with the apprent increase in focal length due to the narrower field of view afforded by the APS-C sensor, and the smaller pixels, it's no wonder you might be in for some trouble if you stick to the old rule of thumb and work with the actual focal length to make your calculations.

For example, a 300mm lens on a camera should, in theory, need a shutter speed of 1/300sec (or 1/320sec since there is no 1/300sec setting). However, that 300mm lens on an APS-C sensor has the field of view of a 480mm lens. Although the magnification is no different, the lens needs to move less distance for it to be as apparent on the sensor, due to the effects of trigonometry. Effectively, for the same angular distance moved, it forms a greater percentage of the overall image on a smaller sensor than it does on a bigger sensor and therefore motion will be more apparent.

So how to get around it. Well from the camera shake side of things, Image Stabilisers are a great help giving 3, 4 or 5 stops of extra shutter speed. In other words if you need a shutter speed of 1/125sec without IS, you should be able to get away with 1/15sec with a 3-stop IS system - but you should do your own tests as everybody differs in how well they can hand-hold a lens. If you don't have IS, then you can use the same rule of thumb as before, but this time use the apparent focal length of the lens rather than the actual focal length:
For example: EOS 50D with APS-C sensor
100mm = 1/160sec
200mm = 1/320sec
300mm = 1/500sec
400mm = 1/640sec

For subject movement though, IS has no effect. You'll just have to up the shutter speed at which you shoot to ensure you freeze the action.



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