Use of lens hoods

[Lardog888]

I've looked around for articles about the proper use of lens hoods, but it seems that knowledge of proper hood usage is implied. I have a 17-85 IS and just purchased a 70-200 2.8 IS and have hoods for both. The hood that came with the 70-200 is quite large so it must be very important to use it. I've been using the hoods anytime I'm outside in bright conditions, assuming that I'm attempting to reduce or eliminate flare spots on the images.  Should I also use the hoods in other conditions?  What is the true purpose of the hood?  Are there any disadvantages of hood usage?

Any information would be greatly appreciated, as I would like to be able to speak knowlegeably about the proper use of a lens hood when someone asks why I'm using one indoors (not that I've done that, ahhem )!

[Bob Atkins]

As long as the lens hood is well designed, it never hurts to use a lens hood. A badly designed hood could cause vignetting, but if a hood is specifically made for a particular lens, that's unlikely.

The technical purpose is to reduce the possibility of flare, which can result from light getting into the lens that doesn't actually form part of the images (i.e. from outside the field of view of the lens). Such light should mostly be absorbed by internal light absorbing coatings (black paint , black plastic or black flocking on the inside of the lens barrel) and so called "flare cut" aperture stops inside the lens, but if it is intense enough (e.g. direct sunlight), it can bounce around enough and be reflected off the surfaces of the lens elements and some of it can end up hitting the sensor/film. It's not just flare spots, but also what is sometimes called "veiling flare", which is a general washing out of the image resulting in lowered contrast.

Lens hood also offer the front element of the lens some physical protection. They can keep rain drops off the lens if it's raining, or they can offer some limited protection if you accidentally bang the lens into something.

They are most useful when shooting outdoors in bright sunlight. They're less necessary indoors, but could still possibly be useful if there's a very bright light just outside the frame.

Of course lens hood can't stop flare when the light source (e.g. the sun) is actually in the frame.

[Lardog888]

Thanks Bob. I always find great information on your site!

[KeithB]

The reason that the lens hood on the 70-200 can be so big is that the field of view is much narrower.  The hood as to block any light *just outside* the area the camera sees so that there is no vignetting.  Long telephotos can have long relatively small hoods since the field of view is so narrow.  Wide angles, like at the 17 mm end of our 17-85 have a wide field of view and the lens hood must be correspondingly smaller/wider.  This leads to the "petal" type of hood that uses the fact that you use a rectangular part of the field of view.

Ideally. the hood would be a tube that extends all the way to the subject, but that would be a bit impractical. 8^)