Help with calculating focal length on Helios 44M

Started 7 months ago | Questions thread
Michael Fryd
Michael Fryd Forum Pro • Posts: 15,270
Re: Help with calculating focal length on Helios 44M

Remember, "Focal Length" is not a measurement of Angle of View. It describes a physical aspect of the lens.

The combination of focal length and sensor size determine angle of view.

Do not confuse "focal length", with "equivalent/effective focal length". "Effective focal length is not really focal length, it's simply a cumbersome method of describing angle of view. When we say a lens has an "effective focal length of 50mm" we are saying that the angle of view is the same as a lens with a 50mm focal length that is mounted on a traditional 35mm film SLR.

Thus a 58mm f/2 lens is always a 58mm f/2 lens, no matter what camera it is mounted on, and no matter what camera it was designed for.

If you mount a 58mm lens on a 1.5X crop body, then it will have the same angle of view as an 87mm lens mounted on a 35mm film SLR. Therefore some would say that the combination of a 58mm lens on a 1.5X crop has an angle of view of "87mm effective focal length".

Just to keep things confusing, some manufacturers advertise focal length, and some advertise effective focal length. You need to know which they are talking about. It seems to me, that with interchangeable lenses, the focal length is what they tell you. When the lens is permanently attached to the camera, they tend to tell you effective focal length.

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