![]() No matter what we are going to discuss below - the focal length and aperture of a lens isn't going so change. The lens aperture is usually specified as an f-number, the ratio of focal length to effective aperture diameter. The aperture determines how collimated the admitted rays are. ![]() More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane. Aperture: In optics, an aperture is a hole or an opening through which light travels.For an optical system in air, it is the distance over which initially collimated rays are brought to a focus. Focal length: The focal length of an optical system is a measure of how strongly the system converges or diverges light.I mention the obvious here because some people are really getting religiously defensive when it comes to a discussion about system equivalence.īefore diving into the details let's make a clear statement first - the focal length and aperture are static physical values. YOUR preferences are the only ones that matter to YOU. All systems compromise on something (size/weight/image quality/handling)- the important thing is that a system is right for YOU. In fact, I am not using full format cameras by choice whenever I can. This article is NOT an article promoting full format cameras. ![]() This will surely not end the discussions but it illustrates our view on this - take it or leave it. For the sake of efficiency, it was about time to write an article about it. Over the last few years we have received several messages asking about and disputing the system equivalence that we are mentioning in our APS-C and Micro-Four-Thirds lens reviews. Equivalent Focal-Length, Aperture and Speed of Camera Systems
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