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The difference between exposure, brightness, contrast, ISO &

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The difference between exposure, brightness, contrast, ISO &

Postby Peru » Fri Aug 21, 2015 6:57 pm

I thought this was interesting to read:
https://www.reddit.com/r/explainlikeimf ... rightness/
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Re: The difference between exposure, brightness, contrast, I

Postby Bob » Sat Aug 22, 2015 2:03 am

Strictly speaking, "exposure" is the amount of light per unit area reaching the film or sensor. It's a function of the scene illumination, the lens aperture, and shutter speed. "Brightness" is a fuzzy subjective term relating to how the eye perceives the recorded image -- is it light or dark. Brightness is a function of "exposure" and film/sensor sensitivity.

With film, sensitivity was fixed for a given film type with standard development. With digital sensors, sensitivity is not fixed and can be adjusted within limits. In either case, the more sensitivity, the more grain/noise that would be present in the final image. For film, the sensitivity was designated by the ASA rating. The International Organization for Standards (ISO) now defines these ratings and the newer ISO number has subsumed the older ASA number.

For a given sensitivity, there is a range of exposures that will be recorded. Too low of an exposure will not record or be lost in the inherent noise. Too high an exposure will saturate the medium and the detail will be clipped. This is referred to as the dynamic range of the film/sensor. An optimum exposure is one that, for a particular sensitivity, results in the recorded values being within the dynamic range of the medium and resulting in a desired brightness. The photographer, of course, has the say as to what that means for a particular photograph.

"Contrast" is also a fuzzy term and there are many ways to define it. In general, contrast is a relationship between the tonal values in a image that make the objects in the image distinguishable. Contrast can be the result of luminance or color differences. If you plot the response curve of the film or sensor, the central portion will be nearly linear on a logarithmic scale. The slope of this portion is called the gamma. The steeper the slope, the greater the difference that will result from an increase in exposure and the greater the contrast. This portion of the graph contains the midtones. If you use a levels adjustment, the center adjustment control is used to change the midtone contrast and is sometimes called the gamma slider for this reason.
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Re: The difference between exposure, brightness, contrast, I

Postby Steve Grisetti » Sat Aug 22, 2015 7:28 am

Great breakdown as usual, Bob!
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Re: The difference between exposure, brightness, contrast, I

Postby Chuck Engels » Sat Aug 22, 2015 9:59 am

There I go, learning something new again ::C When is it going to stop !!! ](*,) Thanks to Muvipix probably never :yh:

Thanks Bob :TU:
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Re: The difference between exposure, brightness, contrast, I

Postby _Paz_ » Thu Aug 27, 2015 11:16 am

Learning again


Me too! A new word: subsume

As for the technical distinctions, I think I'm going to have to read and re-read them a half dozen times or so.

In my recent quest to discover the results of using a variable neutral density filter with video vs with film I learned that for video purposes the gain is related to aperture settings. Perhaps Bob can put that into better words than I !
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Re: The difference between exposure, brightness, contrast, I

Postby sidd finch » Mon Aug 31, 2015 6:13 pm

Life always needs a good visual.

Image

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Re: The difference between exposure, brightness, contrast, I

Postby Chuck Engels » Tue Sep 01, 2015 9:01 am

Love that but it needs to be part of a live demo :)
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Re: The difference between exposure, brightness, contrast, I

Postby momoffduty » Tue Sep 01, 2015 10:10 am

Nice graphic! :TU:
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Re: The difference between exposure, brightness, contrast, I

Postby Bob » Tue Sep 01, 2015 1:15 pm

The text for "ISO" bugs me. Instead of "Less light" and "More light", I think it should say "Less sensitive" and "More sensitive". Also, I think "More Detail" and "Less Detail" should say "Less noise" and "More noise" respectively. Other than that, it's fine.
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Re: The difference between exposure, brightness, contrast, I

Postby Chuck Engels » Wed Sep 02, 2015 10:45 am

Very good Bob :)
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Re: The difference between exposure, brightness, contrast, I

Postby sidd finch » Wed Sep 02, 2015 11:10 am

The text for "ISO" bugs me. Instead of "Less light" and "More light", I think it should say "Less sensitive" and "More sensitive". Also, I think "More Detail" and "Less Detail" should say "Less noise" and "More noise" respectively.


This is most likely the grade school version, you were expecting the graduate level. :ha:

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Re: The difference between exposure, brightness, contrast, I

Postby Bob » Wed Sep 02, 2015 5:18 pm

I was trying to keep it at the grade school level. Even in grade school you should teach the correct concepts.

Every sensor has an innate ISO speed where it's considered to have the best dynamic range and least noise. When you change the ISO setting in your camera, you are not changing how the sensor responds to light. Instead, you are amplifying the signal produced by the sensor. It's similar to listening to an analog recording. If the recorded signal is too low, you'll have trouble hearing it. Turning up the volume will make it louder, but it also amplifies the noise and you'll hear that better too. Turn the volume up all the way and you may find the noise is so loud that it's unpleasant and, at the same time, the loudest sounds are clipped and distorted. The same thing happens with the photo sensor. Amplifying the signal amplifies the noise too. And, values amplified to greater than the maximum value that can be recorded will be clipped.

As for detail, noise is present even where there is no detail. And, because changing the ISO only amplifies the existing sensor signal, detail that is lost in the noise will be lost at any ISO setting. In other words, changing ISO does not affect detail.

So, sensitivity and noise are the appropriate labels -- not light and detail.

If you are interested, here's a "high school/college undergrad" level article about digital ISO you may find interesting.
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Re: The difference between exposure, brightness, contrast, I

Postby _Paz_ » Thu Sep 03, 2015 9:01 am

Every sensor has an innate ISO speed where it's considered to have the best dynamic range and least noise.


Yes? Wow. Is there a way to figure out what the optimum ISO is for each individual sensor, be it in a DSLR or a camcorder? That would be a great thing to know.

Bob, thanks for your detailed and informative thoughts.
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