I see most professional photographers use lens hoods. Why is that? Do they use them for something apart from preventing scratches, etc.? Are lens hoods a must for all lenses? What should I consider before buying them?
Flare in an optical system results when stray light (non-image forming) comingles with the image-forming light rays. A lens hood diminishes the ill effects of flare by shielding the lens from unwanted peripheral light rays. Every picture will benefit if a lens hood is mounted. This is because stray light enters the lens barrel, and is reflected off the polished surfaces of each individual lens within the optical system. This stray light bathes the image sensor (or film), and this reduces the contrast of the visual image. Flare light has many sources -- thus mounting a lens hood won’t eliminate flare.
The chief countermeasures are flat-black paint applied to the inter surfaces of the camera and the lens barrel together with a coating on each lens. A thin mineral coat is applied to all polished optical surfaces. It is the thickness of the coat that does the trick when it comes to mitigating internal reflections within the optical system. A coat thickness of ¼ wave length of light will dampen reflections. Often each lens will have multiple coats, one for each color (frequency) of light.
In summation, flare is devastating; it robs our images of contrast. The lens hood is just one additional measure we can bring to bear.
Using a properly sized and oriented lens hood reduces the probability of negative circumstances such as lens flare and some forms of physical damage to the lens (a lens hood could possibly increase the probability of other forms of physical damage so each photographer should conduct their own risk assessment).
As is typical for most photographic equipment, a manufacturer's lens hood designed specifically for the lens is likely to provide the reasonable performance in common use cases and it may require less research regarding the technical lens hood requirements for a particular lens. Compared to a third party lens hood, a manufacturer's lens hood is more likely to utilize a lens specific mounting rather than filter thread mounting.
Things I look for:
Attachment The generic attachment is screw on via the filter threads of the lens. However, manufacturers may incorporate bayonet or other mounts on the body of a lens allowing quicker attachment of the hood and avoiding the need to share the filter threads with filters or mount filters in the lens hood.
Shape Wide angle zooms may require a petal style lens hood to avoid capturing the lens hood in the corners of the image because this will create vignetting.
Size A screw on lens hood needs to be sized to the same diameter as the lens threads (without opening the adapter can of worms). Some screw on lens hoods have different diameters at the rear (male) and front (female) and therefore will require different lens caps and filters (or adapters).
Material This is a matter of personal preference (tradeoffs). Hard materials like plastic or metal may provide more protection from some forms of physical damage. Collapsible rubber lens hoods take less space in a camera bag when left attached.
Price More or less, there's some correlation between quality and price but that's on average and like everything, it's a tradeoff. A cheap rubber hood may actually meet a particular photographer's needs more readily than a bespoke hood that is objectively better by many common metrics.
Aesthetics There's nothing wrong with selecting camera equipment based on aesthetics. Typically there is a lot of hard work and deliberate effort behind the design of camera equipment and appreciating that is no different from appreciating the work and effort that goes into the aesthetics of a particular photograph or painting or piece of music or building or garden.
Many full-time pros I know don't use hoods. I mostly don't use them myself.
However, there are times I do use them, these are: (1) hoods are indispensable in inclement weather, although on a wide-angle lens such as the Canon 16-35 f/2.8, a hood offers little rain cover; (2) hoods stop flare caused by strong oblique lighting; and, (3) hoods offer some mechanical protection from knocks and scratches.
The disadvantages of hoods are: (1) They add length and bulk to the lens. This can be a pain when trying to work quickly in fast moving situations with multiple lenses as you may need to remove the hood, or reverse it, to get the lens into your bag or belt-pouch. (2) As I often work very close up to people, I think the camera is intimidating enough without some large contraption added to front, so for the sake of being less intrusive, I'll often avoid using a hood. (3) I find flare caused by strong oblique light is rare, so I often don't feel there's any need to use a hood. Of course, being in the UK, may have something to do with that. If I lived and worked somewhere sunnier, I may have a different experience. (4) I think modern lens coatings go a long way to eliminating flare meaning hoods can be redundant. Some lenses suffer more than others, so let experience be you guide here. (5) I'm a minimalist by nature and hate fiddle-faddle. So if I can do away with something in my workflow, I do. Having said that, I've not thrown away any of my hoods, and normally carry them, but only use them if I feel there is a real need to. (6) And finally, sometimes flare actually adds to the shot.
My rule, if I had one, would be not to use a hood, unless there was good reason to use one.
They are an equivalent of you shielding your vision against a bright light using your hand or by wearing a cap, resulting in a better image quality. You can't put any lens hood on any lens, so always search for hoods made for your specific lens and you should be fine.
They are not a must, but they can help a lot, as they offer a good physical protection against scratches and bumps, and against flare caused by bright sources of light (sun, studio lights, etc.).
For me, one of the joys of photography is capturing the pureness of kids at play, which often means a long lens (so they are less aware and don't ham it up) and being down at their level, often sitting on the ground. Over the course of nearly 40 years of photography, I can't tell you how many times the lens hood has prevented a misbehaving child from reaching my lens with his icing- or whip cream-covered fingers. When you are hunched down, and looking through a long lens, your situational awareness of little Bobby winging his way in from the side is non-existant. The first you know something requires quick action is when that dark object is coming in on the lens barrel.
As an eyeglass wearer, I am constantly reminded that light sources close to the plane of my eyeglasses cause haze unless I clean the lenses diligently.
So whenever I'm asked the age-old question: "filter or hood?" I answer "hood."