If I could place the RX100 sensor in A500 for bird shooting, .........

Allan's post is an example of the kind of confident misinformation which pervades forums. I'd love just to be able to erase it permanently - because you never know who may read it, and have insufficient technical knowledge to realise it's a completely fallacious argument. Doesn't seem to matter how many other intelligent people point it out, this ridiculous position about 'less light' reaching smaller sensors (in overall dimensions) is a myth which seems to persist.

DPR's comparison refers only to equivalent depth of field, and even then this comparison only applies when the scene is scaled in all respects - that is, a 2.7m high subject photographed at 2.7m away (on full frame) compared to a 1m subject 1m away etc.

This is one of the things I love about small sensors - being able to shoot with 'miniature as full scale' effect, placing the camera on the ground, inside dolls-houses, within architectural models, inside a guitar, under the floorboards - or just to make an interesting plate of food into a full landscape foreground for a restaurant view.

There are specific cases where format change does affect relative lens aperture, and it's all do with subject scale. You can photograph an egg in an eggcup using a 2/3rds sensor camera, fill the frame, and your working aperture will be pretty much as marked; try it with 10 x 8 sheet film to fill the frame, and the required 2X magnification will turn your marked f/8 into a real f/32.

But at long distance, telephotography of birds etc, this is not significant even for extreme differences in film or sensor size. f/5.6 is f/5.6 is f/5.6 and never becomes f/15 - and the luminous flux, photons per square millimetre reaching the sensitive surface, remains identical.

David