Thank you xpatUSA

Started Aug 3, 2014 | Discussions thread
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jackdan Contributing Member • Posts: 896
Thank you xpatUSA

In another thread I said, "Well I know the definition of exposure is the amount of light/sq unit of area of the sensor surface, but I also know it is not incorrect to say the amount of light falling on the sensor of my camera is called the exposure of a shot."

xpatUSA tried to point out that it is not correct to use the word falling: "But when you say "falling", which is kind of a continuous thing, the reader is likely to assume that you are now talking about illuminance which is just plain lux and is not the exposure under that assumption."

At the time I did not understand what he was trying to tell me, and he lost patience with me before I understood. Eventually I came to realize that without qualification the word falling means that what is falling is falling while the word falling is being spoken or written and in this case, of course, light only falls on the sensor while the aperture (the physical one, not one of the other apertures) is open.

So what I should have said was that exposure is the amout of light falling on the sensor while the aperture is open. Note that in the original context I did point out that the definition of exposure is for the amount of light/sq unit of area. Of course for my camera the sq unit of area can be the area of my sensor. (all of which is besides the point at hand)

So xpatUSA you did accomplish what you intended and I thank you. The reason for this separate thread is in addition to giving you the recognition you deserve, to call attention to the mistake so others can avoid it. Examples of how commonplace the mistake is are:

1. "Exposure: the amount of light falling per unit area on a sensor: it is determined by the scene luminance, the f-ratio (more properly the t-ratio), and the shutter speed."

Gollywop in his article Exposure vs Brightness

2. "exposure is the amount of light per unit area (the image plane illuminance times the exposure time) reaching a photographic film, as determined by shutter speed, lens aperture and scene luminance. In digital photography "film" is substituted with "sensor"."


In this case the word is reaching, not falling, but the same reasoning applies.

3. "The exposure is the density of light falling on the sensor (light per area), not the amount of light falling on the sensor", taken from the very bottom of the following post.

Great Bustard,

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