articles/Lens/shift-page4

The Shift and Tilt Lenses - part 4 of 1 2 3 4 5 6 7 8 9

by Mike McNamee Published 01/10/2014

First Decide on the Optimum Aperture

The usual starting position when using tilt is to gain sharpness right through the scene to be depicted. You have to be tripod mounted anyway, and so exposure time is not an issue. Thus you are free to choose the most effective aperture available. Simply slamming the lens on f22 is not the answer because the effect of diffraction is to degrade the image all over and progressively more once the 'diffraction limiting aperture' has been reached and then exceeded. The limit depends upon the chip size, the chip resolution and the aperture (and little else of consequence).

Technically diffraction limits quality once the, so called, Airy Disk reaches between two and three pixels wide - it is an exactl number so we have chosen to use 2 half pixels for our illustration. The calculations for 'typical' camera are shown in the table. In practice you can vary around the f-number just a little. Going to a smaller aperture will give slightly more depth of field (DoF) with an overall softening; going wider will most likely be using the lens at closer to optimum performance at the expense of DoF.

We are now in a position to think about setting up to shoot a scene but first we practise in the garden. This is recommended for beginners; a windy mountainside might not be conducive to clear thinking!

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