Slow Motion & Flickering >> Red Epic vs Alexa
Just because your camera is shooting slow motion doesn’t mean that it’s magic. Just like any other frame you’re putting together, it needs light. The higher your frame rate, the more light it needs. This translates into the need for fast lenses, well lit areas, and higher ISO. I find myself in a shooting situation where the production is asking for 300fps with the RED Epic and there’s little to no light in the scene. If you don’t have proper lighting for your slow motion shot, you should expect a corresponding result…
Let’s do some quick math. If you’re shooting a 180° shutter (standard for most cinematic shooting), your shutter speed will be 1/48 for regular 24fps shooting. Correspondingly, your light needs increase as you raise your frame rate:
24fps = 1/48 (benchmark light level)
48fps = 1/96 (1 stop light loss)
60fps = 1/120
96fps = 1/192 (2 stops light loss)
120fps = 1/240
192fps = 1/384 (3 stops light loss)
240fps = 1/480
300fps = 1/600 (just under 4 stops light loss)
Know your shutter angles
and to what extent you want them for your shots and the corresponding amount of motion blur – this can help you in a pinch. If you’re shooting 300fps and can pull off 360° shutter angle instead of 180° – this will allow in twice the amount of light. When you’re talking about extremely high frame rates – this can be a true blessing as long as your image works out. Do some tests at 360° and review the footage. If those motion blur for your action is acceptable, you just gained a full stop off light compared to 180° shutter.
If you’re working around practicals that are fluorescents, there’s a good chance you’re going to see flickering. If you’re shooting narrative and it’s not horror genre, this is very likely to be unwanted. When it comes to current cycles and older wiring, fluorescents are most often not flicker free. Bring your own lights. Do camera tests to make sure things look how they should in playback. Even flicker-free HMIs can fall short when it comes to shooting high speed. Their flicker free ballasts usually have a switch between flicker-free and normal running and when switching you will see a reduction – but often still see a rolling band through the footage.
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Natural Light vs Artificial Light
There is two simple equations to give you rough calculation of how much light you’ll need. X (special frame rate) divided by Y (natural frame rate – 25 fps) = Z (how many times more light you need) So In our case: X (200) divided by Y (25) = Z (8) You’ll need roughly eight times as much light as you would shooting at 25 fps. Also worth knowing is that every time you double the frame rate you have to go up one f-stop. So 25 fps to 50 fps equals 1f-stop up , 50 to 100 equals 2 f-stops up, 100 to 200 equals 3 etc.
In the process of setting up the camera each setting play its role. But there is one I haven’t mentioned until now. Shutter Speed. The default setting on the RED is 23,98 fps time base so don’t forget to change it to 25 or you’ll have trouble with the shutter speed and in post production, especially if you’re used to working at 25 fps time base.
When you shoot at 25 fps the shutter speed is 1/50, but when you change the frame rate to 200 fps the RED automatically sets the shutter at 1/200. That could work but I prefer to double up 1/400 in order get ride of the blurring caused by camera movement or camera shake. In addition, it gives me a more precise focus point and an image with more peaking and detail as well as compensating for any faults which might appear due to the focal length. When you shoot at this frame rate, you cannot be out of focus.
These settings continue to make the camera less sensitive. So it allows me to compensate by re- opening the diaphragm a little. It re-establishes some depth of field in the shot (so that not everything is in focus, even with the 18 mm). It also allows me to take pity on my FP (not that he needs it) but it’s a team effort after all! Which is how we were able to pull off a tracking shot using an 85mm lense on a homemade dolly.
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- Flickering. Artificial lighting is much more likely to flicker at higher frame rates; always try to shoot under bright daylight or continuous studio lighting when possible. See the tutorial on flicker-free shutter speeds for additional tips.
- Motion Blur. Viewers typically expect to see minimal motion blur with slow motion, so shutter angles smaller than the standard 180° may be necessary. However, without extra lighting this may require larger apertures and higher ISO speeds, which may come at the expense of sharpness and noise.
- Detail. Shooting at very high frame rates typically requires lower resolution. It’s therefore more critical than ever that these images have the highest quality possible. For example, light permitting, stopping the lens down by 1-2 stops will improve sharpness. In post, using the unsharp mask and denoise tools can also help. Most importantly though, use the highest quality lenses available.
- Wide Angles. With RED cameras, shooting at lower resolutions means the outer portion of the frame gets cropped out — causing the image to appear as if it were taken with a longer focal length. For example, when shooting at 2K as opposed to 5K resolution, the effective focal length will increase by 2.5X (causing a 20 mm lens to appear as if it were 50 mm). See the interactive RED crop factor tool to see what happens with your specific settings.
Alternatively try Twixtor and Flicker Free plug in for After Effects