kortina.nyc / notes / devlog
30 Jan 2023 | by kortina

film devlog #3 // Speed Boosters, Image Sensor Crop Factor, FOV, F-stop, and Focal Length

Here’s a handy spreadsheet I finally put together to make sense of focal length conversions for my bmpcc4k camera – feel free to grab a copy and set one up for yourself!

35mm equivalent focal length conversions My lenses and their 35mm equivalent conversions.

If you’ve ever used a camera with a sensor that’s not full frame, you’re almost certainly familiar with phrases like “the 35mm equivalent focal length for this lens is is 20mm.” If you weren’t immediately scratching your head when you first heard this sort of thing, what can make this even more confusing is (1) the real point of this equivalency often has much more to do with FOV (field of view or angle of view) than with focal length and (2) in addition to worrying about the crop factor of your image sensor you also need to worry about lens adapters that are also “speed boosters” (again, a confusing name, because although they do boost the lens speed, the real multiplier you care about for the purposes of framing and composition is the multiplier on the FOV).

Confused? I was. After many Google and YouTube searches and re-searches over many months, I think I have finally begun to internalize this and wanted to write something up as a reference for myself (and anyone else confused).

Why care about Focal Length?

What really got me thinking about Focal Length was how Field of View (Angle of View) affects composition. Take a look at this shot with a 57° FOV (33mm lens) in Storyboarder:

story boarder shot - 33mm Storyboarder Shot Generator, 57° FOV.

vs the exact same camera setup with a 29° FOV (70mm lens):

story boarder shot - 70mm Storyboarder Shot Generator, 29° FOV.

The longer lens has a much narrower horizontal FOV, cropping out nearly all of the foreground objects, resulting in a very different composition.

To make this particularly fun (ie confusing) to deal with, we’re talking about the FOV produced by these focal lengths on a 35mm full frame image sensor (which has a width of 36mm – more fun!).

My bmpcc4k camera happens to have an image sensor width of 18.96mm (not quite the 18mm of a standard MFT sensor, though it does have an MFT lens mount).

To get the former image, I use a Nokton 17.5mm lens, which produces the picture you’d get using a 33mm lens with a 36mm wide sensor (when you apply the 1.9x crop factor to account for the difference in sensor size)

To get the latter image, I use a Helios 58mm lens. This has an EF mount and I fit it to the MFT mount using a Metabones 0.64x Speed Booster, which also modifies the crop factor (0.64x) of the final image, the equivalent of using a 70mm lens with a 36mm wide sensor. (NB: lens adaptors don’t necessarily always affect crop factor – only when they are also a speed booster).

As you can imagine, all of these conversions quickly get to be a headache – when you’re composing shots or lens shopping, which is why I made this handy little Speed Boosters, Image Sensor Crop Factor, FOV, F-stop, and Focal Length spreadsheet.

In it, I’ve got the complete list of lenses that Rob and I have between the pair of us, the labeled focal length of the lens, and the 35mm equivalent focal length and field of view, which are computed by selecting a Speed Booster and Image Sensor.

To the right side of this sheet you can add your own Image Sensor and Speed Booster options. Just make sure to fill in the sensor Width and speed booster Multiplier, and the conversions will all happen for you automatically in the main table when you select these from the dropdowns.

I reference this sheet constantly and now refer to our lenses more often by their 35mm equivalent focal length. If you think it looks useful, make your own copy, and put in your lenses, cameras, and speed boosters.

Horizontal FOV vs Focal Length Lookup Table

In the second tab of the sheet, I plotted Horizontal FOV vs Focal Length:

plot Scatter plot of FOV vs Focal Length.

Artemis Pro Viewfinder App

The other handy tool that I use in conjunction with this spreadsheet is the Artemis Pro app. It’s basically a director’s viewfinder as an iPhone app, that allows you to save your camera (image sensor size) and lenses, so you can quickly preview what shots you’ll get from different lens when you’re on location:

42 degrees 42° reference photo from Artemis Pro.

56 degrees 56° reference photo from Artemis Pro.

If you want to learn more about all the nuances of focal length, image sensors, and Field of View, here are some links I found useful:

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