As I understand the subject, the issue is that the image can be out of focus if the distance from a convergence of the light rays within the lens to the focal plane of the camera is not correct. This can happen because there is a tolerance during manufacturing as to what is acceptable variation from the optimum. This is needed to make it economically practical to manufacture the parts. That variation can occur in the distance between the face of the lens mount on the camera and the sensor (or film) plane. It can also occur within each lens.Steppenwolf1 wrote:This is a technical question related to FF/BF. I always thought this was caused by the camera being inaccurately assembled (i.e. the light path length from the lens to the AF sensor is of a different length than from the lens to the photo sensor). So why is it that camera manufacturers give the option to fine-tune focus relative to each lens. I guess it's because people say that one lens works fine but another back-focuses, etc. but how can this happen - from a technical point of view? How does one lens cause a camera to focus incorrectly while another doesn't?
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