[Tigerpelle]

Why are wide angle hoods tulip-shaped while tele-hoods are circular? The sensor have the same proportions regardless of focal lenght I guess...

[Anvh]

Not all telehoods are circular, i only have one and that is on the DFA 100 macro... and well the DA40 if you count that.

Anyway here is all you want to know.
toothwalker.org/optics/lenshood.html

Short answer is , telehoods are longer while wide angle hoods are shorter so they need to be shaped in a more efficent form to prevent flare.

[Tigerpelle]

Not all telehoods are circular, i only have one and that is on the DFA 100 macro... and well the DA40 if you count that.

Anyway here is all you want to know.
toothwalker.org/optics/lenshood.html

ok, not all but almost all...

[DeadJohn]

Wide angles are more prone to flare. The manufacturers are trying to squeeze in as much hood as possible without causing vignetting. The top and bottom can be made longer than the sides.

For a telephoto, even a less-than-maximized hood will block most bright lights.

[aoeu]

If the front element rotates

the hood can only be circular.

[Eulogy]

Two other petal teles: DA 50-135 and DA 60-250.

Basic Breakdown for the OP:

Tele Hoods can often cover the front of the lens from 99% of stray light by being about four inches long(Excluding 100mm+ front elements). Wide angles use petal hoods to cover as much as they can, at the lenses widest setting without having vignetting. Normal-Short Telephotos use Petal hoods as they tend to have a semi-wide view, and can benefit from the extra length in parts.

Basically, if it's over 100MM, you can probably get away with a circular hood. 35-100, you *could* use a rubber collapsible hood for a zoom, adjusting it's extension with the lens. Under 35, petal hoods are a must. If you're using a straight hood with a wide angle, I'll just presume it's either to merely protect the lens(As a straight hood would need to be quite small, or absurdly wide at this point), or because you don't entirely grasp what a hood is for.

In theory, though, a Tele would still benefit more from a petal hood. But let's put it this way, you bought a DA 560. In order to cover all unneeded light, you'd need a over hood half the size of the lens. A bit bulky, to say the least. Best to stick with 99% blockage at 1/4 of the size.

If the front element rotates

the hood can only be circular.

This doesn't make sense in all cases. Bayonet hoods aren't affected by the filter ring.

[Anvh]

If the front element rotates

the hood can only be circular.

Some hoods are fitted to the body instead of the front element.
The Macro's from Pentax are like that.

[aoeu]

It isn't supplied but a friend has one and I must say it looks too short to be effective.

My most recent hood purchase is a Takumar 1:3.5 28 mm with lens and case. I wonder whether it would be more effective if it were actually rectangular or whether the slight bit of vignette is unhurtful.