Pentax 6mm F1.2 – the perfect lens for GH2 crop mode

For full samples by Seb Farges taken on the GH2 click here

The Pentax 6mm F1.2 c-mount lens is the cheapest fast wide lens for DSLR filmmaking currently available. To use it you will need to have a Panasonic GH2, and to enable 1080p Ex-Tele mode (commonly known as 1:1 crop mode). It gives a feel similar to 8mm or 16mm film on the GH2.

At F1.2 it maintains a relatively sharp image but contrast suffers and bright edges have a slight glow. When stopped down to F1.8 or F2 this disappears. The 6mm focal length works out at around 30mm in real terms when the 2x crop of the Micro Four Thirds sensor combined with the roughly 3x crop of the ETC mode is enabled. So it is not quite the look of a 24mm F1.4 on a 5D Mark II for $20, nor does it have the shallow depth of field of that focal length and aperture. It is however great for low light filming in 1920x1080p and has a similarly shallow depth of field as the 14mm F2.5 pancake lens.

The lens vignettes just to the right extent to allow the cropped sensor area to fit neatly in the image circle. Here is the full Micro Four Thirds frame, and I’ve added the red box to show what ETC (Ex Tele Conversion) mode crops out at 1:1 pixels, to produce a 1920×1080 video frame.

The lens is extremely cheap, around $20 and available on eBay. Unfortunately some modification is required to get infinity focus. I removed the aperture ring and put the rear mounting plate from my 25mm F1.3 Computar (itself a fine lens) on it to achieve infinity. Seb Farges had the mount ground down by a millimetre or so at a machine shop for 20 euros. This has to be done carefully otherwise you will end up way beyond infinity, or with a weakened thread. Focus really is a matter of millimetres on this lens.

It does have some barrel distortion, but that’s to be expected with such an extreme focal length left uncorrected by firmware.

It’s not a better lens than the 14mm F2.5 pancake, but it does have that critical advantage of being able to produce an image at F1.2 and at a wide angle, at least for video. For that reason it has earned a permanent place my collection.

However, right now – the combo of ETC and 6mm c-mount does have quite a large flaw. That is, although the main reason to use it is for a faster aperture, you cannot go as high with ISO because crop mode is noisier at the same ISO as full sensor mode. Sometimes even ISO 640 is unacceptable in crop mode over shadow detail and in dim light.

It is a trade off between aperture and noise. I’d rather shoot with the 14mm Lumix pancake at F2.5 in full sensor mode, and bump the ISO up to 1600 to compensate, rather than shoot at F1.2 with a softer image with the Pentax and have to keep ISO below 640 anyway. I’ll do some more tests to illustrate this in part 2 of my Pentax 6mm article next week.