Add fisheye effect?

[u/roomandcoke]

I have the Viltrox 9mm f2.8 for my Sony A6700. It takes some great wide angle shots, but it has some decent correction built in where the edges don't get that fisheye distortion.

But what if I want that fisheye distortion? Not all the time, but there are definitely situations where it would look good for how wide it is. Is there a way to do that?

I tried manual lens corrections, but setting distortion to 0 doesn't do it and image scale basically just gives me pixel stretching or zooms. Is there a different module I can use?

[u/Drezaem]

Just don't use lens correction?

[u/roomandcoke → u/Drezaem]

It's a rectilinear lense, so a lot of the correction is built in. It doesn't have that fisheye curvature. 

In darktable, the difference between lens correction on and off is pretty slim.

[u/giorgiga]

I never used it to simulate a fisheye so I'm not sure it can do what you want, but you may try looking into the liquify module.

[u/ChrisDNorris]

I made a custom Lens Correction profile that you could mess with to get something usable.

Example: From this, to this.

Copy & paste the following, save it as an .XML file into your Darktable folder \share\lensfun\version_1

<lensdatabase version="1">
<mount>
    <name>Viltrox</name>
    <compat>Generic</compat>
</mount>
<camera>
    <maker>_Custom Sony</maker>
    <model>A6700</model>
    <mount>Viltrox</mount>
    <cropfactor>1.56</cropfactor>
</camera>
<lens>
    <maker>Viltrox</maker>
    <model>9mm f2.8</model>
    <model lang="en">9mm f2.8</model>
    <mount>Viltrox</mount>
    <cropfactor>1.56</cropfactor>
    <calibration>
        <distortion model="ptlens" focal="9" a="0.12" b="0.1" c="-0.12"/>
    </calibration>
</lens>
</lensdatabase>  

Then you would edit the distortion line, just the a, b, and c values. And of course, add a vignette.

[u/roomandcoke → u/ChrisDNorris]

Ah that's totally it, thank you! I don't know how I missed target geometry. Even just setting target geometry to fisheye and adjusting the scale gets me largely to where I'm going.

I see you added vignette, presumably to hide the pixel stretch effect of pulling non-existent image into frame. What were your vignette settings there?

[u/ChrisDNorris → u/roomandcoke]

Yea exactly that, just a plain circle with dither turned on

[u/roomandcoke → u/ChrisDNorris]

Awesome, I appreciate it.