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naturesunnylensflare7531Lens flare

What causes lens flare, how does it affect your pictures, and how can you overcome the problem? We look at this common problem, help you troubleshoot the causes and find a solution.

“Always shoot with the sun behind the camera.” That's likely to have been the advice given to you when you started out with your first camera. There were two reasons for this. First, the dynamic range of early camera film and digital sensors was low, so it was difficult to capture good detail in highlight and shadow areas at the same time. Keeping the sun behind the camera meant that few heavy shadows were visible, making it easier to record the full range of tones. Second, many early lenses had serious problems with flare.

What is flare?

Flare is unwanted reflection: it's the effect you can see in the image at the top of this article. A very bright source of light in front of the camera creates multiple ghost images, typically circular spots, in the photograph and often affects the contrast, making an image look flat and lifeless. It happens most often with the sun, but floodlights and spotlights can give similar results.

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However, having the sun behind the subject, rather than behind the camera, often gives better lighting effects. Solid subjects display a rim of light which makes them stand out from the background (above left). Translucent subjects show stronger and more vibrant colours as the light shines through (above right). This means that we need to find ways to overcome the effects of flare.

Fighting flare

There are different ways to deal with flare. First, you can avoid situations which will create the effect – this often means shooting with the sun behind the camera. Second, you can shield the lens (with your hand, for example, or a lens hood) so that direct rays from the sun do not reach it. Third, you can use a lens which has been treated with special coatings to reduce the effects of flare – this includes all Canon lenses for EOS cameras. Fourth, you can embrace the effects of flare and use them to enhance the creative aspects of your images.

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Move your feet

There is another way to beat the flare – change your position. The difference between a photograph ruined by flare (left) and one which has good contrast (right) can be just a few feet. Changing the viewpoint might be all you need to do. It is difficult to predict when flare will strike, but the effects will often be seen in the viewfinder before you take a picture. You can check for flare easily after taking the shot – just take a look at the image on the LCD monitor. If flare is apparent, move a little to the left or right and try again. Or use a lens hood.

How a lens hood works…. and when it doesn’t

a001hoodet65bThere is no mystique to a lens hood. It is simply a shield which stops rays of bright light from the side reaching the surface of the lens. This reduces flare.

There are occasions when a lens hood doesn’t help, but it never does any harm either, so it is always worth using a hood on your lens. The main reason many photographers do not use a hood is that it is not usually supplied with the lens and they never get round to buying one. Big mistake! You will never know how good your lens is until you use it with a lens hood.

Lens hoods and zoom lenses

The length of the hood needs to be matched to the focal length of the lens. It should be long enough to give maximum protection, but short enough to remain just outside the field-of-view. This is simple with a fixed focal length (prime) lens. But what do you do with a zoom lens? Here, the focal length changes. The field-of-view is reduced at the longer focal lengths, so the lens hood can be longer. But at the shorter focal lengths the field-of-view is wider and a long hood will be visible at the edges of the frame.

Unfortunately, no compromise is possible. The length of the hood has to suit the shortest focal length of the lens, which means that the hood is less efficient at longer focal lengths. However, a short lens hood is better than no hood at all.

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Lens hoods for longer focal lengths tend to be circular. However, if you think about this, it is not the ideal shape for the camera format. A rectangular hood would be more effective. In practice, a circular hood is fine for longer focal lengths, but a so-called ‘petal’ shape is used for hoods designed for ultra wide-angle lenses. Think of this as a circular hood with four cutouts relating to the four corners of the image. It offers good protection from a compact design.

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Why lens flare occurs

LensflarediagramWhen a ray of light moves from one medium to another it is refracted (changes direction). This happens when light moves from air to glass – and also when light moves from glass to air. So a ray of light travelling through a glass element is refracted twice. All lens design is based on this principle.

Lensflarediagram1Not all the light passes through the element. Around 4% to 10% of the light can be reflected at each air-glass or glass-air boundary. This can result in significant light loss, especially when a lens is composed of many elements (the number of elements in an EF lens is between 5 and 23).

Lensflarediagram2If all the reflected light passed back through the front of the lens, the only problem would be light loss. But a percentage of the light reflected from the rear glass-air boundary is reflected back into the lens from the front glass-air boundary. Some of this light will now pass out of the rear of the lens.

This is where it begins to become complicated! The internal reflections multiply, giving rise to multiple rays Lensflarediagram3emerging from the rear of the lens. Remember, all of this is from a single ray entering the lens. The potential is there for multiple images of the subject to be formed. But this is only the start. Every camera lens has more than one element, so the whole process is repeated as the multiple rays reach the next element, and the next... Fortunately, most of the secondary rays are quite weak and many cancel each other out. Secondary images only form where areas of light in the subject are strong enough to create bright secondary rays.

 

 


 

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Here to help 01869 331741

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