The story

Back in the 1990s, 35mm film was the most predominate film standard in the world. The standard frame size on this film is 24x36mm, and lenses designed for this system had an "image circle" just large enough to cover this area. This standard was so primary, that the world continues to refer to lenses' focal lengths by their "35mm equivalent" focal length. Just to illustrate, the iPhone 6 Plus, which has an image sensor much smaller than a 35mm frame of film, is marketed as having a lens equivalent to a 29mm lens's field-of-view in the 35mm film world. (This is a little confusing, because the "35mm" part of that sentence is referring to a frame size, whereas the "29mm" is referring to the focal length of the lens.)

Going back to film SLRs in the 90s, all the major camera makers had complete systems of SLR cameras, lenses, and other accessories designed around 35mm film. But then digital happened. Making a digital image sensor is expensive. I won't claim to be an expert, but from comparing retail price alone, you can tell that the primary cause of a higher price on a camera is a larger image sensor. So, in order to make a digital SLR that was compatible with existing film SLR equipment, yet was affordable, the camera companies had to make a compromise. The result was what came to be known as the "crop sensor" -- a sensor significantly smaller than a 35mm frame of film, but in a camera body which was otherwise designed around 35mm film standards.

Canon was the first camera maker to run with this in a big way. Canon's main lens line at the time (and still is) their "EF" lenses. With an EF lens mounted on a Canon digital SLR, everything worked identically to how it would on a Canon film SLR, except that with the sensor being relatively smaller than a film frame, the image circle's outer portions would be "cropped" off, leaving much of the image circle unused. So an EF 50mm lens would have a "normal" field-of-view on a Canon film SLR, but on a Canon digital SLR would have a field-of-view equivalent to using an 80mm lens on film.

Most of the time, this is not a problem. But there's two areas where this compromise hits. First, since there's a lot of the lens's image circle being unused on digital, there's a lot of glass in the lens that, if you only ever use this lens on digital cameras, doesn't need to be there. This means your lenses are heavier, bigger, and more expensive than are needed for your photography. Secondly, since the physics of making a (low-distortion) lens with a large image circle get increasingly complicated the wider the angle of the lens (the shorter the focal length), that having a massive, complicated 16mm lens, then throwing away half the image data just to get the equivalent of a 25mm lens's view on your digital camera is getting the worst of both worlds.

Hold on, I'm getting the point, I swear

So, to address these compromises, Canon introduced a variation on their EF lens line: the EF-S line. The new EF-S lenses were designed specifically for the smaller digital sensors. They took advantage of the smaller image circle needed, and were themselves smaller, lighter, and more inexpensive. Of course, these EF-S lenses, if you were to mount them to your old Canon film SLR, would not draw an image big enough to fill the film's frame. But Canon was proactive, and put a special flange on the EF-S lenses that prevented them from mounting on a film SLR, and only allowed them to be used on their digital SLRs with the smaller image sensor.

Then the other camera makers followed Canon's pattern, but each in their own way. Pentax was one such camera maker, although Pentax, being interested in compatibility and being the king of confusing lens names, never physically restricted their digital-only lenses from being mounted on film SLRs, nor gave their digital-only lenses a naming scheme that would indicate as such. As a result, it is possible to mount a digital-only lens on a Pentax film SLR and not realize you're doing so.

I think I see where you're going with this

So this is how I ended up with my smc PENTAX-DA 1:3.2 21mm AL Limited lens (it's actual name) mounted on a Pentax SF1 film camera. Now, I'm not a complete idiot (only partial idiot) so I did realize what I had done when I looked through the viewfinder and I noticed that the corners of the frame were dark. But, I decided to shoot a roll of film this way anyway. For this, I used Ilford Delta 3200, because when I first loaded the film it was very dark and foggy out.

You can see the results below. Some of them came out well – moody, atmospheric images where darkened corners don't impede the image. Others, not so much. Judge for yourself.