It is infuriating to see people on the street not use a lens hood or even worst, bring it out and put it on backwards. The lens hood is an essential part of a lens, it shades the front element from stray light that don’t result in any details on the sensor or film. This useless light just bounces around the inside of the lens and some make it onto the sensor to expose it. Causing flare. The whole picture looks lighter in color/shade and loses contrast. This lost in contrast is no good as because you lose information as the stray light overrides light that is carrying information.
Any strong light will cause this flare and it doesn’t have to be the sun, even incandescent lighting can cause problems.
Most lenses come with the corresponding lens hood and in cases where it doesn’t, it should be factored in to the cost of the lens, as it is essential part of the lens’ design. Most Canon, Nikon, Sony, Pentax, Olympus, Panasonic or Leica lenses come with an appropriate hood, but there are some Zeiss-M mount lenses and Voigtlander M-mount lenses that comes with an inadequate hood or none at all.
Many medium format and all large format lenses don’t come with a hood and it can be a very expensive optional extra. When using my large format lenses, I use a special dark cloth (it’s the option Ebony dark cloth) that extends over the lens and acts as a universal hood.
Shading the front element is very important and it becomes much more difficult if you are using 100mm filters from Lee, Cokin or Singh Ray. The enlarged and uncoated surface area of these square or rectangular filters are very prone to flare. Lee has some special bellow hoods with build in filter holders but I find using a large board (usually a A4 grey card) to block the sun manually quite effective. The Lee bellow hoods limit the number of filters you can use. You can get a tripod alligator clip that can hold the shade for you as well. (This clip can hold your macro subject still as well) Here is a link to the clip I use: Plamp