I think there is a lot of


I think there is a lot of confusion about images sensors, depth of field (DoF), bokeh, low light performance, noise etc.

First off, the image sensor is just that, it encodes whatever is projected upon it into three electrical signals that eventually become RGB or some variation of color representation. If two sensors have the same number of light sensitive elements, the larger sensor will be more sensitive to light (i.e. better low light performance) because the individual elements are bigger. In light sensing semiconductors, bigger is better because the larger elements gather more photons which raises the signal to noise ratio where the signal is photon-generated current and the noise is thermally generated in the circuitry (seen as “grain”). So much for the physics.

Three sensors produce a much better picture than a single sensor for two reasons. The first is that they usually represent more and/or bigger light sensitive elements. The second is that the use of three sensors allows for better color separation filters. The down side of three sensors is the complex and expensive optical path.

So, the sensor is responsible for low light performance. Given the same number of light sensing elements, bigger sensors are better because the light sensing elements are bigger. Picture quality is better with three sensors because of better separation filtering (don’t get excited, I’m summarizing, I know there are other considerations). Notice there is no mention of DoF or bokeh.

DoF and bokeh are related to the lens and the sensor size or number has nothing to do with them. How can that be?

Depth of field is related to lens focal length and aperture. If you want less DoF, use a longer focal length lens or a wider aperture or both. The reason that some persons believe that a smaller sensor causes greater DoF is that, for a given image size, the smaller sensor requires a shorter focal length lens. Think of it as a wider angle lens is needed to “squeeze” the image onto the smaller sensor. Since for any given aperture, the shorter the focal length of the lens the greater its DoF, it appears that the smaller sensor has caused the greater DoF.

One way to decrease the DoF is to zoom to maximum telephoto and maximum aperture, setting shutter speed, gain and ND filters appropriately. Of course, the camera has to move back from the subject.

Bokeh is an elusive quality associated with the internal construction of the lens. It is the way the out-of-focus parts of the picture appear as modified by the shape of the iris. In the still world, some portrait lenses are cherished for their bokeh alone. Sensors have nothing to do with bokeh. they can produce sort of a flare when overloaded that some people seem to think is related to bokeh.

Finally, Bigger sensors are better because of their light gathering ability. A 1/3″ sensor has about 174% the area of a 1/4″ sensor. Three sensors are better than one because of more accurate color separation. DoF and bokeh are qualities related to the lens.

To really control everything, you need a camera with a large sensor and a selection of interchangeable lenses. But, then again, that’s what Panavision is for. Jaimie

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