What is the optimal "default" aperture value in AV mode?

There is no optimal default as every situation will be different - but if there was a BEST setting, specifically if you are using the 18-55 kit lens, it would be to use the sharpest aperture of the lens at the focal lengths you are using. For the Canon 18-55 use f5.6 at the wide end, F8 mid-range and f11 at 55mm setting. Lens resolution tests prove these to be the sweet-spots for this lens.

And for those people that will post afterwards that this doesn't take into consideration Light levels, hand-hold ability or desired depth of field, well DUH! It is simply an attempt to reduce aperture selection to the lowest common denominator of desirable settings, maximum center to edge image sharpness.

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