So a 50mm lens is a 50mm lens, is a 50mm lens, right? Well that’s not exactly true. The focal length of a 50mm lens is always 50mm, regardless of the system it is associated with. The focal length of a lens is independent of the camera system. So a 50mm lens on an SLR will have the same focal length as a 50mm lens on a DSLR, which is the same as one on an APS-C sensor, or a medium-format sensor. What is different is how they behave in terms of angle-of-view (AOV), with respect to a particular sensor size.

Table 1 shows the behavioural differences of 50mm lenses on various systems. For example a 50mm lens from a 35mm rangefinder camera has a (horizontal) AOV of 39.6°, whereas the AOV of an APS-C camera, is 26.6°. This is because due to crop-factors, a 50mm lens on an APS-C sensor is equivalent to a 75mm on a full-frame camera (from an AOV perspective). To get a 39.6° equivalent AOV on an APS-C camera, you need roughly a 33mm lens – but the closest lens to this is a 35mm APS-C lens (35mm×1.5≈52mm).

System	AOV (diag)	AOV (hor)	Crop-factor	FF equiv.
16mm cine	14.5°	11.7°	×3.4	170mm
1″ sensor	18.2°	14.6°	×2.7	135mm
Micro-Four Thirds	24.5°	19.6°	×2.0	100mm
APS-C	31.7°	26.6°	×1.5	75mm
film SLR	46.8°	39.6°	×1.0	50mm
film rangefinder	46.8°	39.6°	×1.0	50mm
digital SLR (full-frame)	46.8°	39.6°	×1.0	50mm
digital Medium (44×33mm)	57.4°	47.5°	×0.8	40mm
6×7 (72×56mm)	84°	67.6°	×0.5	25mm
4×5”	117°	104°	×0.27	13.5mm

Note that because a 50mm lens on a Micro-Four-Thirds camera behaves like a 100mm FF lens, most manufacturers won’t sell a native 50mm MFT lens, opting instead for the 50mm FF equivalent – the 25mm. That’s because a 25mm MFT lens provides the “normal” angle-of-view, just like a 35mm APS-C lens, or a 100mm 6×7 lens. A vintage 50mm SLR lens used on an APS-C camera will behave like it was designed for APS-C, i.e. it will have a horizontal AOV of around 26.6°. The remaining 6.5° either side is just cut off because of the smaller sensor (as shown in Figure 2).

Invariably, all focal lengths are treated similarly. A 35mm is always 35mm, an 85mm is always 85mm. It’s just their behaviour, or rather their “view on life”, that changes.