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Is it a good idea to use a full frame fisheye lens, like a Canon EF L 8-15 f/4 USM Fisheye (or any other similar) as ultra wide angle lens on an APS-C camera? In my understanding, the APS-C will just crop away the most distorted parts of the image, we use the best part of the lens and will have more or less the same image style as if we were using an Canon EF-S 10 - 18mm 4,5 - 5,6 IS STM?

I am not really satisfied by the EF-S lenses available cause of missing small f-numbers, weather sealing etc.

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    You can see how it would look by just taking any FF photo done with a Fisheye and then cropping the image to roughly what APS-C looks like. – Mike Sowsun Apr 10 '18 at 12:50
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A fish-eye lens is not just a lens with a very short focal length, it also has a specific distortion (cf. fish-eye projection vs. rectilinear projection).

So while you can use a full-frame fish-eye on an APS-C body, it will still be a fish-eye lens.

  • Thank you for pointing out that fish-eye has a different projection. I always thought it is just a more extrem wide angle. – this.myself Apr 10 '18 at 15:33
  • Well, it is that, too. But the fisheye distortion will be present in a way that a wide angle (with a similar angle of view for APS-C) would not. I would buy a full-frame lens for an APS-C body if I were already planning to upgrade to the full-frame model of the same system. That said, I sold my one Canon full-frame lens along with my own Canon crop-frame camera, and moved to Sony A7-series (instead of moving up to a Canon 5D -series model, as I had thought I would when I bought that lens). With a macro lens (what I bought), there is less compromise buying full-frame to fit APS-C. – Lowell Montgomery Apr 11 '18 at 1:07
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No, it's not a good idea. Most fisheyes use an equisolid or equirectangular projection, not a rectilinear projection like an ultrawide will. There is more distortion, and while that distortion might be more extreme at the edges, it is still different from the typical barrel distortion of an ultrawide zoom lens.

In addition to this, the Canon EF 8-15mm f/4L fisheye lens is not a diagonal fisheye at the 8mm end of the zoom range; it's a circular fisheye; it's only diagonal at the 15mm end (see: the-digital-picture.com review of 8-15L). What this means is that the entire full-frame sensor is not covered by the image circle of the lens, instead, the entire lens's image circle fits inside the frame. When used on APS-C, this means that you'll get black vignetting at the corners: the image circle will still be too small to cover APS-C, either. You will only completely cover an APS-C sensor corner-to-corner if you zoom in past 10mm.

I am not really satisfied by the EF-S lenses available cause of missing small f-numbers, weather sealing etc.

Have you looked at the Tokina 11-16/2.8 DX II? It's not weather-sealed, but it is f/2.8 and does have a rubber gasket, and is designed for crop.

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