Confused about EF Lens FOV vs EFS when both are on APSC.

Started 2 months ago | Discussions thread
Robert Krawitz Contributing Member • Posts: 748
Re: Confused about EF Lens FOV vs EFS when both are on APSC.

Bionic963 wrote:

I thought I understood that when you mount an EF lens to a 1.62 Apsc crop camera the sensor will not record the same FOV as if you would have had the same mm lens but in EF-S.

For instance.. If you had a EF 50mm 1.4 mounted to apsc and a EF-S 50mm 1.4 mounted to an apsc.

My theory is that the fov of the image recorded on the apsc sensor when using the ef lens will not match. The image recorded when using the EF 50mm lens on an apsc will have similar FOV to what an image recorded with an 81mm EF-S lens on an apsc would have (50mm x 1.62 = 81mm).

No.  The EF 50 f/1.4 and the EF-S 17-55 f/2.8 zoomed to 50mm will have the same FOV on an APS-C sensor.

I thought this was based off the fact that the crop camera has smaller sensor, which results in a narrower field of view equivalent to what you'd get with a longer (1.6 times longer) focal length lens on a full frame camera.

The field of view of a 50 mm lens -- regardless of whether it's an EF or EF-S lens -- mounted on an APS-C sensor will be the same as that of an 80 mm EF lens mounted on a full frame sensor (well, "mounted on a sensor" isn't right, but you know what I mean).  But if you could mount a 50 mm EF-S lens on a full frame body, you'd have the same image as a 50 mm EF lens on that body -- except that the EF-S lens's image would be cropped at the image circle.

Apparently my understanding of how this works is all wrong. If indeed my understanding is incorrect, then what does it mean when I see members post about liking to use apsc with there zoom lenses because they get "further reach"?? If the field of view is the same, then how do they get a more zoomed in image simply by putting an EF lens on a crop body?

The answer is that you don't get more reach; statements otherwise are not correct.  You do get a crop of the full frame image, but that's not actually further reach, since it's a smaller sensor.  If the distance between pixels (pixel pitch) is greater on the full frame sensor than on the APS-C sensor -- which it usually is; you'd need 51.2 MP on a full frame sensor to yield the same pixel pitch as a 20 MP APS-C sensor.  So you might well get more pixels on subject with the APS-C sensor.  With the 90D (32 MP), you'd need an 82 MP full frame sensor to yield the same pixel pitch, so if your lens is very sharp and you don't have a lens long enough so that your subject won't fill the APS-C sensor, you may well get better results on the APS-C body.  But that's not due to "greater reach", it's due to more pixels on the subject.

I shoot with an APSC body. I have the EF-S 17-55mm, and the EF 50mm. Tommorw I plan on mounting the 50mm EF lens and take a photo, then I will mount my EF-s 17-55 and zoom to 50mm on the apsc and take a photo, them compare them..

You'll get the same result (subject to minor errors due to distortion, the focal lengths not matching exactly, and that), but it won't be grossly different as you expect.

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