I’ve compared the two lens to each other on my Pentax Q7 using a standard C-mount adapter. (This one: https://www.ebay.com/itm/C-mount-16mm-Cine-Film-Lens-to-Pentax-Q-P-Q-PQ-Camera-Mount-Adapter-Q7-Q10-Q-S1/181579402443 . Note that there is another adapter for the Pentax Q, which is about two millimeters shorter, which means it’s in no man’s land: it’s not long enough to be a proper C-mount adapter but not short enough to adapt CS-mount lens either. The latter would need 5mm distance. An example seller: https://www.ebay.com/itm/C-mount-Cine-Move-Lens-to-Pentax-Q-Mount-Adapter-For-PQ-Q10-Q7-Q-S1-Camera-C-PQ/263589562065?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649 . This also has a review “The adapter doesn't let the lens sit at the right distance”, which is perfectly true - others have also run into the same problem.)
The following tests have been done:
I’ve covered all the, on the lens, merked apertrue values at every tested focal length. There are nine of them on the 6-80 (1,4 / 2 / 2,8 / 4 / 5,6 / 8 / 11 / 16 / 22) and eight on the 8-64 (1,9 / 2,8 / 4 / 5,6 / 8 / 11 / 16 / 22)
I’ve tested the extreme wide and tele ends of both lens; with the wider and longer 6-80, also trying to aproximate the wide and tele ends of the 8-64. (Unfortunately, these aproximations delivered somewhat worse-than-expected results.) This means I’ve tested the following extremes:
a, 6mm (only available on the 6-80, obviously; there, the wide end),
b, 8mm (the widest end of the 8-64),
c, 64mm (the tele end of the 8-64),
d, 80mm (only available on the 6-80, obviously; there, the tele end).
I’ve also tested two intermediate focal lengths: the “normal” 15mm and the “short tele” 30mm. Obviously, these tests were done on both lenses.
Note that, with some of the 6mm and 8mm tests, I’ve also tested the subject being far closer than the minimal focusing distance of the lenses (0.9m for the 8-64 and somewhat (about 5cm?) lower for the 6-80) as, on the wide(r) end, the large depth of field allows for shooting subjects not entirely in focus.
I’ve uploaded all shots to https://www.flickr.com/photos/33448355@N07/albums/72157691200379483 . The individual shots have the following name convention:
<test serial number> - <lens type> at <focal length>mm - <film plane (here, sensor) distance from subject, measured with a tape measure> - <aperture value * 10 with a 0 prefix when needed to avoid having to use non-integers messing up file sorting>
That is, for example the photo at https://www.flickr.com/photos/33448355@N07/47744075651/in/album-72157691200379483/ has a name “014 - 8-64 at 8mm - 90cm - 040”, which means:
1, Test series 14
2, the tested lens is the 8-64
3, the focal length is 8mm
4, sensor distance from subject is 90cm
5, aperture was 040/10, that is, f4.
The individual series have been as follows:
Series 1-5: 6mm and 8mm tests with the subject being far closer than the minimal focusing distance of the lenses (see the note above). This was just an additional (nonstandard) test. Obviously, particularly with the aperture wide open, the softness caused by the lack of focus is certainly visible in the shots. I’ve also made “proper” wide-end tests with the sensor and subject distance being 90cm (the shortest distance the 8-64 can focus to); see below. With these tests, the printed test chart occupies just the Super8 frame (with the 8mm; in the 6mm tests, even less area).
Series 6 and 7: 30mm (subject distance: 157cm)
Series 8 and 9: 64mm (subject distance: 299cm)
Series 10 and 11: 15mm (subject distance: 90cm)
Series 12: 6mm (subject distance: 90cm) on the 6-80
Series 13 and 14: 8mm (subject distance: 90cm)
Series 15: 80mm (subject distance: 438cm) on the 6-80
Note that the album has another photo at https://www.flickr.com/photos/33448355@N07/47746340931/in/album-72157691200379483/ , where I’ve shown the Super8 frame inside the 1/1.7” frame of the Pentax Q7 camera.
General observations with regards to the difference between the lens:
1, At lower focal lengths, the 6-80 can let in up to two times more light than the 8-64 if you need. An example of both lenses’ aperture wide open AND at 8mm (the widest focal length for the 8-64 and still wide for the 6-80):
014 - 8-64 at 8mm - 90cm - 019: 1/100s, ISO125
013 - 6-80 at 8mm - 90cm - 014: 1/320s, ISO200; converted to ISO125: 1/320 / 1.6 = 1/200s; that is, exactly twice of the 8-64 value
2, At higher focal lengths or at smaller apertures, the 6-80 still delivers more light at exactly the same aperture setting than the 8-64. An example: at f8 and 64mm, the (compare the EXIF data of “008 - 6-80 at 64mm - 299cm - 080” and “009 - 8-64 at 64mm - 299cm - 080”) shutter speed of the 6-80 shot is 1/40, while that of the 8-64 is 1/25. That is, the larger lens let in 1.6 times more light even at f8.
The advantage gradually diminishes with increasing aperture values; for example, at f16, the shutter times are 1/8 (smaller lens) and 1/10 (larger lens); that is, here, the 6-80 lets in 20% more light. And at f22 (which should otherwise NOT be used because of the diffraction!), there’s absolutely no difference any more.
This also means that, at lower focal lengths and apertures wide open, the 6-80 has a tremendous advantage over the 8-64 with regards to image quality. In order to get the same amount of light projected to the film, for example at 8mm, the 6-80 can use f2.8, while the 8-64 still needs to use the widest setting, f1.9. The sharpness, CA etc. difference between the two lens is very large. Just compare the images “013 - 6-80 at 8mm - 90cm - 028” and “014 - 8-64 at 8mm - 90cm - 019”. (Incidentally, if you take a look at “014 - 8-64 at 8mm - 90cm - 028”, you’ll also notice it has been shot at 1/100s, while the 6-80 f2.8 image at 1/160s. That is, the 6-80 let in 1.6 times more light at f2.8 than the 8-64. And if you account for that and use around f3.5 on the 6-80, the image quality difference becomes even more pronounced - the 6-80 produces way better image.)
All in all, if you don’t always need f1.4….f1.9 (which the 8-64 is obviously incapable of) but would often operate your lens under, say, f5.6, the 6-80 is a significantly better choice. The advantage diminishes at significanly higher aperture values, of course.
3, If you plan to use as much area as possible outside the Super8 frame (that is, you want to use the lens on your, say, Samsung NX Mini, Pentax Q, Fuji etc. (note that even the 6-80 is compatible with most non-Mini NX/Q adapters, it being pretty narrow at the end, let alone the 8-64!)), you will want to prefer the 8-64 for the following reasons:
The chromatic abberation and the sharpness falloff outside the S8 frame is a serious issue on the 6-80. Practically, it’s only acceptable arounf f14…f16. The 8-64 is definitely better.
at lower focal lengths, the 8-64 renders a far larger image circle than the 6-80. It’s only around 30mm that the latter lens starts to fill the entire 1/1.7” frame of the later Pentax Q cameras.
Otherwise, the 6-80 is also a bit more contrasty.
Finally, you want to avoid f16 and particularly f22 not only because of the diffraction, but also to minimize lens imperfections (dust etc.). An example: https://www.flickr.com/photos/33448355@N07/47694810482/in/album-72157691200379483/ . In this shot, I’ve annotated a dust particle with a green arrow. If you compare this shot to the other photos in the same series using wider apertures, you’ll see the particle is fully invisible up to around f8 and increasigly becomes an issue starting with f11.