I always use the hood on every lens.
They include with some lenses and not with others. What I have noticed is that if the front element is recessed as in my 50mm F1.8 AF, there is no hood supplied, while if the lense is in front as in the 35mm F1.8 DX the hood is supplied.One argument for using is the manufacturers will do any thing to save money yet they still include one,if it wasn't needed they would soon leave it out,there will be times when its no help but will you spot those times before returning to the computer with job done.
Modern coatings and PP opportunities may help but if its not hurting the picture why not use it.
They do help prevent lens flare, and while some lenses are are more prone to flaring, I don't think any lens is immune from it. So yeah, I use them pretty much anytime I'm shooting in sunlight. They also provide some physical protection for the front lens-element.
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Lens hoods are chick magnets, but besides that they do protect the lens somewhat from bumps. My hood saved my lens yesterday from a bump against the dashboard of my car, not hard enough to do any damage though.
Yup...lens hood on.
The worst hood I have is the one on my 28-105. Man that thing is big and wide! Hate it!
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