Ideally, one would want the light to pass through a lens directly onto the sensor, without bouncing around anywhere. To this end, the interiors of lenses and sensors are painted jet black. The lens mount is designed to be a twist-lock (screw or bayonet) system so that it is not the weak link in this chain. Yet, there is one weak link in this chain.
The weak link is the front of the lens. Light falls into the lens from the scene you are aiming at, but it also spills in from the sides. In general, this effect is called Flare.
Image Courtesy: Photograph by Linus Lövholm 2003
Flare always happens. Sometimes it manifests itself as solar or lens flare, a bright spot of light with circular bands, and is easy to see. Mostly, though, it is not very obvious, and the effect reduces the contrast of an image.
Rule of thumb: You only want a loss in contrast if you’re aiming for it. Otherwise, you generally want the best contrast possible from a lens.
To cut down on flare, you need to cut out unwanted light hitting the front element of a lens. Sometimes it is unavoidable, especially if you’re shooting directly at a light source like the sun or a bulb, etc. Most of the time, though, you can do a lot to minimize its effect.
The simplest way to cut unwanted light is to screw on a lens hood:
Each lens usually has its own corresponding lens hood These are designed differently according to the AOV and other optical characteristics of a lens. Stick to the one the manufacturer recommends.
Its affect is hard to monitor on a tiny LCD screen or optical viewfinder, but it helps nonetheless. Photographers might prefer to leave it behind, considering what image processing software is capable of nowadays. Filmmakers though, don’t want to sit and tweak each frame, especially when there are 24 or more of them per second of footage!
For this reason I recommend you use a lens hood all the time. It won’t hurt your image, and it will also protect your lens if you happen to drop it head first.
So you’ve cut out unwanted light. Good. Let’s talk about the light entering your lens-sensor optical system.
One of the most important pieces of gear that enhances, improves or modifies the light is the filter, usually made of glass or plastic.
Filters can be placed in front of the lens or at the back. It’s a question of whether you want to modify the light prior to its entering the lens of after it has left the lens.
E.g., many cameras have ND filters between the lens and the sensor. As we have seen earlier, some camera mounts have slots for filters if you want to manipulate the light that has left the lens. Sensors themselves have color filter arrays (CFA), etc.
In this guide, I’ll only cover filters that manipulate the light before it hits the lens. I use the term ‘Filter’ to signify a device that can be removed by the professional and used when necessary.
Filters can do many cool things, but for the purposes of this guide, I’ll stick to these major types:
- Ultraviolet (UV) Filter
- Polarizing Filter
- Neutral Density (ND) Filter
- Diffusion Filter
The key thing to do at this point is to decide whether you need filters or not.