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lens flare is an unwanted light affecting the photograph, usually caused by not protecting the lens from extraneous light . It can also be caused by dust or dirty lenses, direct sun in the lens or a reflection back to the lens e.g. a Mirror on the studio wall.
Firstly, flare is sometimes wanted, it can indeed make some very nice effects if controlled.
To control flare it must be realised that it's level will change with a change in the aperture, so viewing the image or testing are the only ways to see how the flare looks.
lens flare can be added with a tiny amount of grease added to a clear or uv filter in front of the lens. The light can literally be pulled across the image by wiping the grease in the direction required, avoid however covering the whole filter with it. As I have said the more you close the aperture the sharper the lens flare becomes.In the picture below at f2.8 there was flare over the whole image but at f11 as you see the light has been pulled across her in a precise manner.
An example of controlled flare
Anyway here’s how to avoid it. Always have a clean, and grease free lens, even after cleaning a fingerprint from a lens ,there can be a small amount of grease left behind that will cause lens flare
Protect at all times the lens from any direct sunlight by holding a card above the camera casting a shadow over the lens or at the very least fit a lens hood, If there is a stong sun in the shot try adding a Cokin graduated neutral density filter , to cover the sky but not the rest of the suject to cut down the lens flare..
The picture above has an exaggerated amount of flare causing the contrast to drop over the whole image.
studio photography is even more prone to flare. You must know exactly where your light is going.
You have spent a lot of time and money buying your equipment now give it a chance to show you what it’s capable of.
All lights bounce around a studio, in a restricted space it is virtually uncontrollable so add black screens or board just outside the angle of vision to shadow the lens it is probably necessary to add at the top and bottom as well. I would end up with a rectangle, sometimes even a black sheet around the front and back of the camera that just leaves my angle of vision free. It is important to check with the diaphragm closed, as it will bring the cards more in focus causing them to close into the image a little, all this seems a lot of effort but it will help to avoid that unwanted lens flare.
Flare is the biggest problem in amateur photography, you won’t know it but your photographs almost certainly suffer from it. Next time you are shooting inside try a before and after I’m sure you will be surprised.