Exposure Basics, lesson three?

Started Mar 18, 2013 | Discussions thread
texinwien Veteran Member • Posts: 3,326
Nice, but missing an important point

First, thanks for taking the time to share your thoughts on the subject, and to give some positive suggestions, since the easiest thing in the world is to be a critic without offering solutions. This is well-written, but I can see at least one problem that, I believe, is insurmountable.

Najinsky wrote:

A few smart people take objection to including ISO, but they are wrong (in this context) and are not used to being wrong and not handling it too well.

I disagree, simply because it seems that so many people have a difficult time unlearning incorrect information they've learned previously (especially in the case of ISO as it applies to image brightness).

On the other hand, I'm not sure how to teach exposure, image brightness and ISO to beginners who just want to start taking pictures, and have no interest in the theory.

The films were sold in rolls that denoted the number of exposures, 24 exposure, 36 exposure, and so forth. Put a 24 exposure film into the camera and you have, for regular shooting, 24 exposures. And most cameras showed a little number that was called 'Exposures remaining indicator' that would keep track of how much film was used and how much is remaining. However, fail to put a film in the camera and how many exposures do you have? Zero. You can still set the Aperture, and release the shutter, but the camera is not capable of making an exposure in the photographic sense. Again, the act of recording is inherent in the term Exposure.

On a 24-exposure roll of film that you have already shot, your 24 exposures are recorded on the actual, physical film. If you never develop that film, you will still have 24 exposures, but you will have no final images. Those exposures might even be 'correctly' exposed, but we won't be able to say anything about their brightness.

And this is the crux of your definition problem, as I see it. ASA isn't just a factor the photographer uses to adjust one or all of the three exposure variables (shutter speed, t-stop and scene luminance) in order to record a 'correct' exposure. ASA also tells the lab that develops the film how to develop it to achieve the correct brightness.

And, if the photographer has his own dark room, or works with a lab that will follow special instructions during processing, the photographer is free to expose a roll of 400 ASA film based on an 800 ASA factor (underexpose by 1EV) and, subsequently, 'overprocess' the exposures as if the film had an 800 ASA rating.

This is exactly what some 800 ASA films were - they were rebranded 400 ASA films with 800 ASA stamped on them. This 800 ASA stamp told photographers to expose one EV less than they would have with the same film, had it had a 400 ASA stamp on it. It also told the labs to process with one 'EV' of brightness more than they would have with the same film, had it had a 400 ASA stamp on it.

See the difference? Brightness is actually about processing, whether it's film in a lab or data in your camera or in a RAW developer like LightRoom. For an extreme example, you could record a film or digital exposure without removing your lens cap and process that exposure into several output images of varying brightness, ranging from pure black through pure white. The exposure had nothing to do with the brightness of your output images - it is totally a function of your processing choices.

For film/JPEG/TIFF shooters, the term 'Exposure Triangle' has to include the three components of Aperture, Shutter and ASA/ISO in order that the photographer can control the appearance of their recordings to their liking. This is the correct foundation for beginners where the overwhelming majority of cameras and photographers shoot JPEG.

The recording has no appearance and no brightness. Only the output, a result of processing the recording, has brightness, which is why we should talk about exposure as being something different than brightness, and why it should be made clear that ISO plays no role in exposure, while it does play a role in brightness.

In the days when film was the only choice, and few had their own dark rooms, it may have (arguably) been OK to teach amateurs that ISO is a part of exposure. It may have been safe to assume that anyone who built his own darkroom would probably be interested enough in the theory to correctly relearn what 'exposure' means, had he been taught a simplified and incorrect version, earlier. Now that everyone with a PC can have his own dark room for free (i.e. dcra, RAW Therapee, other free RAW developers), the practice of teaching incorrectly that ISO is a part of exposure has much less merit.


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