Sony NEX Camera + E-Mount Lens = Crop Factor???

Started May 2, 2013 | Questions thread
boardsy Senior Member • Posts: 2,215
Re: Sony NEX Camera + E-Mount Lens = Crop Factor???

Salamihawk wrote:


I just ordered the SEL50F18 50mm prime lens (it should be arriving today or tomorrow) and I've been doing a bit of reading about it. A lot of people insist that when talking about the 50mm focal length of this lens, that one needs to take into consideration the fact that the Sony NEX-5N has an APS-C sized sensor and therefore would only receive a smaller portion of the total image produced by the 50mm lens, giving an effective FOV of 75mm.

That's exactly right.

That doesn't sound right to me, though, because, as I understand it, crop factor only comes into play when using a lens designed to put an image onto a 35mm piece of film or image sensor, so if you have a camera with an APS-C sized sensor but it has a mount that's been in use since film DSLRs (like Canon's EF mount or Sony's A mount), then you have to calculate the crop factor, because physically the back of the lens will be the same distance away from the APS-C sized image sensor as it would be a piece of 35mm film.

It would seem to me that if the SEL50F18 is noted as having a 50mm focal length, then the FOV it will produce on a Sony NEX-5N will be (roughly) the same as a 50mm Prime lens designed for a 35mm camera used on a 35mm camera.

I had an old Canon Digital Rebel and the mount was an EF-S mount, and it could take EF or EF-S lenses, but according to the manual (I think... it was 10 years ago), the crop factor was only an issue using EF lenses, since the EF-S lenses were designed for the smaller sensors.

So what's the real truth here? Does 50mm == 50mm when using lenses designed for E-Mount cameras? I know there's a Sony NEX video camera out there with a full-frame sensor, but if you attach a regular E-Mount lens that's designed for an APS-C sensor, it should go into APS-C crop mode, so that tells me that 50mm == 50mm...

Well, the 35mm FF convention is still there, so everyone gets confused! Simply put - imagine your 50mm lens projecting an image circle. That circle covers the 35mm film/sensor area. Now crop a smaller APS-C-sized portion out of that circle - that's as much as your APS-C sensor sees. The lens doesn't know or care what size sensor or film records its image, so nothing else magically changes.

Even if made for APS-C, M4/3 etc every lens's focal length relates to 35mm, so x1.5 for its equivalent field of view on APS-C. 20mm x1.5 > 35mm, 50mm x1.5 > 75mm etc.  (x2 for 4/3)

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