Except the sensor isn't square....but then neither is the hood. Petal shaped hoods are the most efficient and we can test this with the 12-24mm hood. When I first got this lens the hood wasn't clicked totally into position and sure enough I got vignetting. It was utterly precise. When clicked fully home, no vignetting.

A squared off hood (longer in one dimension than the other)is also quite efficient, following the dimensions of the film gate.

Round hoods tend to be quite long tubes, for telephoto lenses.

As for the man from National Geographic, I don't know what he was saying.

I found the SMC Pentax-DA 70mm f/2.4 Limited was quite prone to flare and the hood made a dramatic difference.

Many other designs don't seem at all prone to flare.

Thanks Brian, I'd missed that.

The new DA645 28-45mm lens is very interesting - the first Pentax lens with SR in the lens. There don't seem to be many fanfares about this, but maybe it's because of the size of the medium format sensor or maybe it's because medium format DSLRs and wide angle lenses need different solutions.

I wonder why?

I'm not the only one who interpreted your post in another way Mike!

Yes, but we're talking Pentax here, not Canon. It is true to say that, apart from a few low cost lenses, Pentax lenses are generally supplied with a hood.

Because the field of view is narrower as only the centre of an imaginary film frame is used. So the Pentax 50mm snap on lens hood can be used quite happily on the 35mm DA lens. They both have the same field of view, therefore need the same amount of shading.

First deletion because it's out of time frame.

Cheap and sharp don't usually sit well with each other when it comes to teleconverters.

Why not look out for a Pentax Rear Converter K T6-2X - I have one here that cost me about £40. No electronics, so manual focus and manual stop down metering, but better quality than most of the cheap converters.

If you keep your eyes peeled one will turn up sooner or later.

I prefer your own website Michael, there are some lovely images on there. They also load quickly, which is fantastic.