When RED announced the Epic-W with Helium sensor this year, I knew I had to make the jump to the DSMC2 line. My reasons for making the switch from Epic Dragon to Epic-W are for another post, but my biggest concern lay with the frame rates and the cropping of the sensor.

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The Epic-W shoots 8K resolution, but only up to 30fps at full field of view. For narrative work, this is perfectly fine as I’m usually never shooting beyond 23.98 or 24 fps. However, I often shoot at 48fps and 60fps for commercial and music video projects. In order to get to those frame rates, one needs to jump down to 6K resolution. According to a handy tool that RED/Phil Holland developed to calculate the crop factor for different resolutions, I could easily see what the crop factor is if going down from full FOV 8K down to 6K and lower.

http://www.red.com/tools/crop-factor

Who can tell me?
I asked many colleagues how the crop factor would affect the compression ratio of the lenses, and if a “50 is a 50 is a 50” would hold true – that is, does only the FOV change? Or does lens compression ratio and depth of field change when utilizing a smaller portion of the sensor? I got varying answers, so we (my colleague Brian and I) decided to do some real world tests of our own to see how the crop factor affected our lens choices.

Coming from the DSLR world, I had a good feeling for how crop factor affected lens choice. However, for the first couple of years I was focused more on learning lighting and camera movement, and didn’t “see” the effects of lens compression ratio yet, unless it was a long telephoto or ultra wide angle. I wasn’t shooting enough to see the differences between 28-75mm by just looking at an image.

Who cares?
Working with directors who know what they want, you add an incredible amount of value as a DP by being able to give a recommendation on a lens for a shot or to have a conversation about why the director should consider using a specific focal length rather than the one they were thinking. You have to know what each lens ‘feels’ like, both in the compression of space as well as FOV. This is why figuring out which lenses I should use while cropping is important to me – it would affect my recommendations.

I don’t want to get too technical on this post because quite frankly, I was looking for which lenses I needed to use if we changed the distance from the subject to retain the same field of view. Here are various frames with accompanying explanation.

24mm CN-E at 8K, 6K, 4K
I think this test is the most telling. We often use the 24mm close up to talent to make a space seem larger than it is (but try to avoid edge and face distortion). We wanted to see how the cropping of the sensor down to 6K and 4K would look to the camera.

8K, 24mm, f/2.1, 2′ 1″ from subject. This frame just screams 24mm just by looking at it. Most experienced DPs know this look and can tell you the approx focal length just by looking at it. I love this look for certain projects. Slight distortion and a smaller background makes the subject seem a little large than life. We normally try to avoid the facial distortion you see here from being too close, but sometimes you want to exaggerate this look.

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6K, 24mm, f/2.1, 3′ from subject. With a 1.33x crop from 8K to 6K, our lens effectively becomes a 32mm focal length. While it retains the property of a 24mm lens (because the lens doesn’t change, only our FOV and crop factor), to achieve a similar frame above, we moved the camera back. Notice that because it’s now a 32mm focal length to the camera, moving the camera back to achieve a similar frame has the crop factor increasing the compression ratio in camera (again, the lens itself does not change properties). You’ll notice the building and the trees are much closer to subject now that previously, even though we haven’t changed the lens.

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4K, 24mm, f/2, 5′ 8″ from subject. We then changed to a 4K frame, which is a 2x crop, making our 24mm lens at 4K effectively a 48mm lens. The jump from 8K to 4K is pretty obvious with the compression ratio of the background to foreground elements now looking much closer together. Also, we begin to lose the distortion of the subject while entering the ‘normal’ lens field of view.

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The following images display the same behavior with the 24mm at 8K, 6K, and 4K respectively, with the backpack as our reference point. You can see that the lens compression ratio changes based on the crop and where we moved the camera in relation to the backpack to match the frame for each shot. The lamp post creeps closer and closer in the more we’re cropped on the 24, even though the backpack is in the same general size to camera.
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Stationary camera – “a 50 is a 50 is a 50”

Here we compared 8K, 50mm, f/2, 11′ 2″ from subject to 6K, 35mm, f/2, 11′ 1″ from subject (Backpack). The 6K 35mm is equivalent to 46.55mm. We can see here that when we don’t move the camera, there is a difference in the lens view and space compression even though our FOV is approximately the same. We wanted to do this test to see what a lens would behave like if we didn’t move the camera (they’re about an inch difference from one frame to the next in terms of distance from the subject). This effectively shows that a “50 is a 50 is a 50” – if you’re not changing the distance of the camera from the subject, the lens properties don’t change, lens compression ratio does not change, only FOV changes. As a sidenote, the depth of field is greater on the 50mm as well*. It’s when you move the camera to match your frame where we see changes in the compression of space.

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Conclusion and Caveats

The takeaway here is that when there is a crop factor, on the sensor, lenses can be somewhat interchanged when working with ‘normal’ focal lengths, it’s okay to use an equivalent focal length to compensate for the crop factor. I’m comfortable swapping a 35 for a 24, a 50 for a 35, and a 85 for a 5o on a cropped sensor. We did not test what happens when we shot at 24mm 8K, jumped down to 2K, to mimic a telephoto lens – first of all this was not a technical test, and secondly we would not do that for the type of work we do. This was a practical test to see how we can compensate for the loss of FOV when cropping by using different focal length lenses.

See anything that you think needs to be corrected or looked over again? By all means, reach out to me at arthur@arthurwoo.com – I would love to hear your thoughts!

*This article has been reviewed and critiqued by various cinematographers – thank you for your help in keeping me honest with the technicalities.