Using a lens hood is important.
It is not always clear why we need a lens hood. Why do we need them and what are they for? Actually they are pretty important and can help prevent some nasty visual artefacts.
What is going on in the lens
When light hits the lens from the side the situation is slightly different. Some of the light is refracted through the lens correctly. Some of the light however, is reflected off the surface and lost. But there is a percentage of light that goes astray in the lens. It can be bounced around in the lens – reflecting around from the surfaces of different lens elements internally. If it does so, on each reflection some of the light will get through to the sensor. Each time that happens there will be a slightly side-shifted ghost image. All sorts of light aberrations can be created by this internal reflection in the lens. It is these that cause the artefacts you see in the image.
Light is also scattered by inconsistencies in the lens glass. Chemical, and structural variations in the glass can impact on the way the light travels through the lens. This scatter contributes to the problem. These artefacts, and often an associated haze, are called lens flare and can be worse the further to the side that the light enters the lens. Flare and haze will not only form a distraction but also act to wash out the colours in your shot and reduce contrasts. These will make your picture look flat and lifeless. Unintended flare can simply kill the effectiveness of the shot.
A sharp angle of side-light can therefore cause all sorts of visual image ghosts which are not there in the scene. You can see a range of different types of flare in this Google image search…
Google images: Example lens flare images
In general, poor quality glass and multiple elements in the photographic lens will tend to create more flare. Of course better lenses (read: “more expensive”) can help to reduce the problem. Higher quality lenses will incorporate a range of ways to reduce the problem. These include optimised lens-element design, surface coatings and non-reflective surfaces/parts internally as well as high quality glass. Despite that no lens is immune to flare.
What does a lens hood do?
The most common reason we use a lens hood is to reduce the incidence of a bright light source hitting the lens from the side. This will act to reduce the chance that the lens will suffer from the flare problem. In other words, the hood will help to keep the light coming in from the front of the lens in parallel rays.
It is simple really. Put up a wall at the side and the side light is cut out. However, it is not so simple to design a lens hood that will do that without obstructing the lens. The field of view of the lens cannot be allowed to catch the hood sides. If it does it will leave its mark on the image. So all sorts of hood shapes and sizes are required to match the visual characteristics of the lens. Lens hoods are quirky shapes because they have been designed to optimally reduce the side incidence of light and not interfere with the field of view.
Common design elements include “petal” shaped edges. These allow the corners of the sensor a wide field of view without interference in the corners from the hood. The long petal shape must be along the long side of the sensor (landscape view). If you put it on the other way the side of the picture will show the edge of the hood and light will also get in from the top (or bottom) because the cover is insufficient there.
There are conical shapes and cylindrical shapes too. When these don’t have petals they are designed to accommodate the full field of view of the lens from any angle. You may find that these types of hoods are common on lenses where the front of the lens extends when changing focal length. As the extension of the lens will change the field of view the hood shape must be wide enough for the widest angle of view. But it cannot have petals because they would rotate with the lens and at some focal lengths would interfere in the picture.
Lens hoods do more…
As you can see you should buy a hood for your lens that has specially been designed for it. If you don’t, you risk the hood intruding in your shot, or not providing sufficient protection against flare.
Hoods can help in other ways too. When you have a lens hood on the lens it acts as a primary protection for your vulnerable front element. Once, when I was panning to follow a bird with a heavy/expensive pro-zoom lens I whacked the lens hood off the glass of my car window. I am convinced I would have broken the window and damaged the front lens element if I had not had a lens hood on. It harmlessly bounced. Phew! I use hoods whenever I can these days.
Lens hoods also help to reduce over exposure generally from incident reflected light on bright days. There may be no direct bright light source shining into your lens. But it helps to reduce the high levels of brightness from the side regardless. That helps to reduce the overall high light levels and especially the contrast.
Lens hoods are worth the effort
Lots of learner photographers forget the lens hood. Yet it can have quite a significant impact. Even if it is not significant, when you are trying to get a sharp image and reduce the colour wash-out in bright light every little detail counts. Great images come from the attention to detail.
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