De-Mystifying ‘Crop Factor’

The crop factor conundrum 5d,scarlet,epic,arri

The Crop Factor Conundrum

There has been quite a bit of discussion lately on the crop factor of cameras like the Scarlet. There is a recurring phrase that I’m seeing which goes something along the lines of, “You will need to shoot at 30mm because the lens will become a 48mm.”. Yes, that’s true – the lens will have a 1.6x magnification factor, and 30 multiplied by 1.6 is, in fact, 48 – but guess what? That 30mm will still behave like a 30mm, regardless of if your field of view changes or not. However, let’s take a step back for a second, because we’re getting ahead of ourselves.

Back when I first began making films there was a de-facto camera for indie filmmakers – the DVX100. A plastic fixed zoom lens gave you plenty of framing options, allowing the camera to shoot anywhere you wanted to in glorious 24p. Jump forward to a couple years ago, and we had the 5DMKII, a camera which changed the indie landscape and gave people the added bonus of using custom glass. Here’s the snag, a lot of people that have shot on the full-frame 5D are now moving on to shooting with other cameras and becoming flustered when their 50mm is now an 80mm equivalent in terms of Field of View (FOV). This is nothing new, in fact many cameras have an ASP-C crop sensor, oftentimes this means there is a 1.5-1.7x magnification factor.

The reason your lenses are being cropped is because, as seen in the picture above, the circle of light that is projected from your lens is only hitting part of your sensor. As the sensor gets smaller, you get less and less image. This is why the field of view becomes smaller, and more telephoto as the physical film or sensor size changes. This is also the reason that there are 35mm lenses, 16mm, and 8mm lenses. They all will only cover their size film back. If you put a 16mm lens on a 35mm camera, you will get vignetting, where the edges of your frame darken, since the circle of projection isn’t covering the whole film back. It is only covering 16mm worth of film.


Field of View Illustrated

There is no “crop factor”.

I mean, there is but it’s not what you might think. It’s no secret that most motion cameras are not 5DMKIIs, meaning it’s not often you find a true full-frame camera that shoots motion – since the 5D was essentially an accident. When it comes to using these lenses for judging focal length to field of view – the sensor is king. The Scarlet is no exception. You are dealing with a 1.6x magnification on your lenses. In other words, a 50mm becomes an 80mm in terms of field of view. The sensor native on the Scarlet means only a 1.3x magnification @ 5K resolution, but the issue is that we can only shoot 12fps at that resolution. In order to shoot 24fps, you have to move to 4k, which means windowing the sensor. So for the sake of argument, I am putting it in the 1.6x category.




Feeling confused? Don’t be. Check this list, it should help put things into perspective:

Format height width Diag Area Crop Examples
mm mm mm (mm2) factor
Medium Format 44.0 33.0 55.0 1452 0.7 Pentax 645
Full-frame 24.0 36.0 43.4 864 1.0 FF SLRs
Red Epic 14.6 27.7 31.3 404 1.3 Red Epic/Scarlet
35 Cine 13.7 24.4 28 334 1.4 Red One
Super 35mm 13.8 24.6 28.0 339 1.4 Canon C300
APS-C crop** 15.0 22.0 27.3 329 1.5 crop SLRs
1.5″ 14.0 18.7 23.4 262 1.9 Canon G1X
4/3 13.5 18.0 22.4 243 2.0 Four-thirds
Nikon CX 8.8 13.2 15.8 116 2.7 Nikon J1/V1
Super 16 7.4 12.5 14.5 93 3.0 film only
2/3″ 6.6 8.8 11.0 58 4.0 Fuji X-10; camcorders
1/1.7″ 5.6 7.4 9.5 42 4.6 Best P&S
1/1.8″ 5.3 7.2 8.9 38 4.8 Best P&S
1/2″ 4.8 6.4 8.0 31 5.4 camcorders
1/2.5″ 4.3 5.8 7.2 25 6.0 P&S
1/2.7″ 4.0 5.4 6.7 21 6.4 P&S
1/3″ 3.6 4.8 6.0 17 7.2 camcorders

As the sensor gets larger, more of the lens gets used, and your Depth of Field becomes shallower. Clasically, you could compare 35mm, to super 16, to super 8 – and each had its definitive look. Now – with so many different types of sensor sizes and combinations, it becomes a bit more complicated when trying to visualize the final look of your shots before you’re on set.

My point is that you could theoretically shoot something on a 2/3″ chip, with a 20mm lens, but even though your field of view would be 80mm, the lens will still resolve as if it were a wide-angle lens, meaning bulbous heads and warped exteriors. The only difference is your field of view and depth of field. This is why you should be wary about shooting with heavily cropped sensors. There can be issues with field of view versus lens characteristics that can be jarring to say the least. In addition, it’s another reason to start thinking about pre-visualization before shoots, to help mathematically plan the dimensions of your stage with what camera and lenses you’ll be using. Keep an eye out for an article on that in the near future.

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