Techniques for Selecting Exposure
From Redhax
Multiple viewpoints on the subject of determining exposure for raw images.
Contents |
[edit] Technical Fundamentals
Shutter speed is the only parameter that determines exposure inside the body on RED One. Other parameters of exposure are determined by the lens and anything in front of the lens, such as an ND filters. While there is a gain parameter in the camera menu, this parameter does not affect the sensor or the RAW image that is being recorded. Gain is stored as metadata, and can inform post production software to adjust the exposure digitally or be ignored.
[edit] Choosing the shutter speed
The maximum shutter speed is dependant on the frame rate. The camera will electronically regulate the amount of time that the sensor collects light during each frame exposure.
Since most film cameras have a maximum shutter angle shutter angle of 180 degrees, a shutter speed that exposes each frame for roughly half the time or less, is considered filmic. Longer shutter speeds tend to allow for a lot of motion blur, that can make the image feel softer and more like video. The phenomenon is subjective and related strongly to the frame rate at which the footage is played back.
[edit] F-Stop Pitfalls
Because 4K images are very sharp, many cinematographers that are familiar with video may encounter lens artifacts that were previously not an issue at lower resolutions. As a general rule the sharpest images are going to be created when the F-stop is between F2.8 and F8 on most lenses in a standard prime set. The sharpest position will vary depending upon the lens. F4/5.6 is a good place to be if you don't know the properties of the lenses you are working with. There are two causes of softness on the extremes of the f-stop rings.
[edit] Lens Aberations
- As the aperture opens wider the image becomes more affected by aberations in the glass. These aberations cause for example light of certain wavelengths shift more than others, the result can be a softer looking image with less defined colors. This can vary greatly with the quality of the lense. Lenses such as Zeiss Master Primes can exhibit almost no aberations at F1.3.
[edit] Diffraction
- It's a common misconception that on 35mm formats the higher the F-stop the sharper the image will be. While its true that the image will have deeper depth of field, a high F-stop image will suffer from a phenomenon call diffraction. As the waves of light pass through a tiny aperture they are bent outward causing the entire image to appear softer, regardless of where the focal distance is set to. The ideal f-stop for minimizing diffraction and maximizing depth of field varies depending on the lense, focal-length, and format size.
[edit] Working with ND Filters
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ND filters must be used to achieve an F-stop that is close to ideal. Since these filters may also diffuse the image slightly, it's important to obtain high quality filter glass to ensure overall sharpness.
There seems to be an advantage to using glass that is formed over molten metal, as this may lead to smoother surfaces and more reliable geometry. Some high end filters also have coatings similar to lense coatings that prevent internal reflections.
