Exposure is what?

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john isaacs Veteran Member • Posts: 5,798
Exposure is what?

This has become a big issue on some dpreview threads. It is a problem due to the overloading of the term "exposure". Because the term "exposure" has many meanings, and not just in photography, there is ambiguity in the use of the term. For the most part, the intent of this use is clear from the context. But frequently, a given use is called into question as being imprecise, or not matching some percieved definition.

So let's take a moment and contemplate what these competing claims for the term entail.

First, lets start with the common or colloquial use of the term. "An exposure" is used to mean the image. With film, it is the slide or negative, sometimes even the print (although it should be clear that post-processing is required to some or more extent in the making of a print). With digital, it is the image file. It is not a precise or scientific/engineering term.

The image file can be in any format; RAW, JPG, TIFF, PNG, HEIC. Whatever the camera produces. This image file cannot be viewed without computer processing; it is not a print. The processing may be simple or complex; there really is no fundamental difference.

And along those lines, "exposure" is used to mean the brightness of the image (and thus the terms under- or over-exposure are used to indicate that the image is too dark or too bright. There is a lot of latitude in these terms as well, but generally significant crushing of blacks or blowing of highlights can indicate under- or over-exposure. But there is also the issue of whether the final product (image or print) reflects the actual brightness of the scene photographed, is an artistic statement, or is a mistake. These questions cannot be answered out of context.

Now, what we have with this concept of "exposure", is the use of a camera to produce the image. This camera typically requires certain key settings to be made before the image can be captured, and these are commonly referred to as the "exposure settings". And they are generally the shutter speed, aperture, and ISO. Other settings can affect the final image (white balance and picture mode are key, especially in the production of JPG output). And exposure compensation is a setting that affects the auto-exposure function of the camera, as well as metering mode.

So we have two colloquialisms: "an exposure" which is really an image file, and "exposure", which relates to the brightness of the image and whether it is appropriate.

Second, we have the technical definition of the term "exposure", or more precisely "photographic exposure". Because there are a number of other meanings to the term "exposure" that are applicable in other fields or endeavors. People can be exposed to disease, substances, radiation. Artists or performers can be exposed to the press, the public, the audience. It goes on and on; there are many uses for the term.

Key to precision is unambiguous use of terminology. Using the bare term "exposure" in photography is inherently ambiguous. Yet many attempt to define the term "exposure" in photography to be a measure of light density at the surface of an imaging sensor. Is that sufficient? I argue that, for two reasons, it is not. Even the term "photographic exposure" is not sufficiently precise for the purposes of calculation. For although it can be used to define the general concept of light density, it is not precise enough to assign units of measurement.

But there are standards that can be applied. And specifically, there is the SI standard for radiometry units, as described in the Widipedia article on "Exposure (photography)".


We just scroll down from the introduction (more on that part in a moment), to the section "Definitions", and we find three specific units for exposure: two SI radiometry units that are "Radiant exposure" and "Spectral exposure", and one SI photometry unit that is "Luminous exposure".

And this is the key to resolving the ambiguity of "exposure". If one wants to speak colloquially of a photographic exposure and the camera settings required to obtain the exposure (exposure settings), then one can speak freely of exposure. But, if one wants to speak technically (and unambiguously) of light density and sensor characteristics, then one should use the appropriate term in the SI standards, which for photometry is "Luminous exposure", referred to as Hv and in units of lux second. Of course, one is free to use the other units if they are applicable. Clearly, the full name of these units should be used to avoid ambiguity. In the technical world, casual use of terms should only follow a clear statement of that intent. So, for instance, the phrase "Luminous exposure (henceforth, exposure)" may be allowed, but only because the intended audience may not have sufficient technical training. Otherwise, the phrase would be "Luminous exposure (henceforth, Hv)" would be the appropriate technical usage.

In conclusion: technical jargon may be needed to communicate some concept or result, but precision in language is key to presenting technical information. So let's all use the technical term, "Luminous exposure", when we want to be technical.

PS. I did mean to get back to that Wikipedia page. The introduction to this article is a mess. The first paragraph states "Exposure is measured in lux seconds". The second paragraph states that "An "exposure" is a single shutter cycle", and then introduces the term "photometric exposure (Hv) without definition.  And then introduces the concept of a multiple exposure.  This concept may be valid for film, but for digital images, if the data is extracted from the sensor with each "exposure", then it is not clear what "accumulated" means, and what it is the same as.

Simply put, it defines the term "photometric exposure (Hv)", which directly contradicts the SI photometry unit definition. It really should be corrected.

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