Does a mirrorless camera need a shorter focal length than a DSLR for the same field of view?
Asked 12/27/2023
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If I use a 50mm lens on a full-frame DSLR and compare it with a full-frame mirrorless camera, does the mirrorless camera need a shorter focal length to get the same field of view because its flange distance is shorter? Or does the shorter mount-to-sensor distance only change the lens design, not the focal length needed?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
2y ago
2 Answers
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No, we don't need a different focal length – a lens designed for a mirrorless camera is just intrinsically different to a lens designed for a traditional (D)SLR camera. The image that a mirrorless system lens projects out the back and onto the sensor is designed for the shorter flange focal distance.
I feel like this schematic (taken from DPReview's very old Mirrorless camera buying guide) shows it quite clearly. You can see how the lenses are designed to project the image differently for the respective flange focal distance.
That's why/how you can mount a traditional (D)SLR lens on a mirrorless camera – all you need to do is mount it further from the sensor (with the help of a spacing adapter).
Originally by user38159. Source · Licensed CC BY-SA 4.0
user38159
2y ago
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No. For the same sensor size, the same focal length gives the same field of view on DSLR and mirrorless cameras.
A mirrorless camera’s shorter flange focal distance does not change the focal length required. It only changes how the lens is physically designed so the optics end up the correct distance from the sensor. In other words, a mirrorless 50mm lens is still a 50mm lens.
What changes is the lens construction: mirrorless lenses can place optical elements differently because there is no mirror box in the way. This matters most for wide-angle designs, which often need special retrofocus designs on SLR/DSLR cameras to keep the rear of the lens far enough from the mirror.
This is also why DSLR lenses can often be adapted to mirrorless bodies with a simple spacer adapter: the adapter restores the extra distance the DSLR lens expects between lens mount and sensor.
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