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I tend to use a lens hood routinely when taking photos simply because it makes sense. Ultimately the lens hood helps to keep out stray light and so you do not get washed out pictures or reflections which means that the image has deeper contrast and is likely to he clearer with less aberations. In addition to this, the lens hood helps to keep dust off of the lens so that you don't have to keep cleaning your lens in the field, even if very dusty conditions. When it rains I like that it also helps to keep the rain off of the lens itself and so photos are still possible even in bad weather.
The Canon ET-65 III hood was bought for my Canon 100-300 f/4.5-5.6 USM lens (it is recommended specifically for this lens). It cost me just £17.99 from Warehouse Express.
The lens hood made of black plastic. It is slightly narrower at the part where it attaches on to the lens and then tapers out as it moves away from the lens. The official dimensions stated are 3.7 x 3.6 x 2.2 inches. Because of the design of the lens, there is no vignetting on the image, even when I'm using a full frame sensor.
Compared to other lens hoods of this type, I find this hood seems significantly longer which seems to have the added benefit of blocking more light.I also like this hood because on the inside of the hood there is a rough lining which acts to absorb more stray light and to decrease lens reflection.
This lens hood does not screw and lock onto the lens as I have generally found with other lens hood I've used. Instead it uses spring loaded clips to mount onto the lens barrel - which was fairly simple. To release the lens hood there are two release buttons which will allow you to remove the hood from the barrel. Although I found this mechanism simple to use, I do prefer the traditional twist and lock mechanism, simply because it felt more secure and that the hood was more firmly attached.
When I am not using the hood, I can reverse the hood so that it fits backwards onto the lens. However, I do find the lens itself harder to hold with the hood mounted on it in this way and so I tend to store it in my bag when I'm not using it. However, the fact that it mounts onto the lens for storage is good for saving room when storing equipment in my camera bag.
All in all, I recommend this lens hood for those with the appropriate lenses (look at the Canon website).