tenbach Loc: SW Utah

What is the difference between exposure and brightness in general and how does that apply to the histogram??


How do the dark, shadow, highlights, and white sliders relate to the tone curve? They must me overlap some how. Moving the sliders in LR does not effect the visual tone curve. So the follow up is, do the tone curve adjustments effect the histogram.

I understand the 256 'bin' limitation on the horizontal axis, but what is a tone??

Enough for now.. Being a retired engineer I can not just accept qualitative mumbo jumbo. Obviously there are algorithms for each of these terms!!

Apaflo Loc: Anchorage, Alaska

Ouch! That's distinction causes anxiety in certain quarters, and leads to fist fight, food fights, and cuss fights.

Exposure is how many photons are captured. It is simply the aperture multiplied by the shutter time. The only way to change exposure for a given scene is to change the lens aperture or change the shutter time. ISO does not change exposure (it changes which exposure is useful). And there is no way to adjust exposure with any kind of data processing, either in the camera or with post processing.

All that said, several raw converters and editing programs have labeled adjustment sliders as "Exposure". Technically they are adjusting brightness, not exposure.

Hence how pedantic one wants to be about terminology is what draws the line between "exposure" and "brightness" in the venacular.


All of these depend on which program, and the specifics of exactly what each does is probably not available to users.


Each brightness level is a "tone". So each of the 256 values for the horizontal axis is a single tone value.

ronwande Loc: Hendersonville NC

A histogram is just a bar graph of how many pixels there are at each brightness level. At the left edge the brightness value is zero which of course indicates black. At the right edge of the histogram the brightness is 255, which is pure white. If the histogram for an image has a "spike" at the left end, some of the shadows are underexposed. If it has a spike at the right end, some of the highlights are "blown out", that is, they are pure white. This sort of histogram indicates that the brightness range of the scene was more than the sensor could accommodate. If there is a spike only at the left end of the histogram, the image was underexposed. If there is a spike only at the right end of the histogram, the image was overexposed.

If the histogram shows a "hump" in the middle with values tapering off to zero or near zero at either or both ends, this indicates that the scene was not contrasty and that the brightness values have been captured completely. Depending on what you want to do with the image, you can move the leftmost slider to the right to get a true black in the image. The leftmost slider indicates the level that will be adjusted to be black. Similarly at the right end of the histogram, the right slider can be adjusted inward (to the left) until there is a true white in the image. You are essentially increasing contrast.

Ansel Adams always strove for at least one area of pure black and on area of pure white in his photographs. You can accomplish this with the slider adjustments.

The center slider adjusts the brightness of the image without changing the black or the white. Move it to the left to make the image brighter or to the right to make the image darker.

If you are using Photoshop, make these adjustment with the LEVELS control, using the histogram that it provides. Now go back and look at the histogram provided by photoshop and you will see that it has changed.

In addition to these controls, Photoshop or Elements has a control called Shadow Highlight. Frequently when there is little detail in the highlights of an image, this control can recover a great deal of the detail. The shadow control can recover detail from the shadows, but it will also increase the noise in the shadows.

Hope this helps.

tiger1640 Loc: Michigan

Try this
http://www.zoner.com/en/tutorials/histogram?utm_source=newsletter&utm_medium=mail&utm_content=non-paid&utm_campaign=tutorial-histogram

abc1234 Loc: Elk Grove Village, Illinois

I would say that intensity is how many photons are captured. The units would be photons per area. Exposure or brightness is how the detector perceives it. The same number of photons will appear differently depending upon the detector. For example, the detail you see in the sky at night is different than what a film or CCD sensor sees through a telescope.

Intensity is not "simply" the product of those two variables. Rather, it is simply proportional to it.

Apaflo Loc: Anchorage, Alaska

You are saying the exact same thing, just using more words and confusing people unnecessarily... except for the incorrect use of "exposure" to mean "brightness". Two distinctly different things.

Keep in mind the OP's question is the difference as it applies to a histogram.

Exposure is indeed the amount of light per unit area. But that, for any given camera, equates precisely to the number of photons captured.

Exposure is determined by the aperture and the shutter time for any given scene.

Brightness is what the sensor and the following data processing chain does with the sensor data collected. The ADC, for example, can change image brightness but it cannot change image exposure.

abc1234 Loc: Elk Grove Village, Illinois

Apaflo, to talk about photons and photons per area is what is confusing. People could care less about those concepts. All they want to know is whether the shot looks right. They understandably want simple answers. That is why histograms do not have labeled axes. Who cares about the absolute values? A quick glance is all you need to evaluate a histogram. Other posts address the practicality of using a histogram.

abc1234 Loc: Elk Grove Village, Illinois

In LR, I can change those intermediate sliders and the black and white warnings can turn on.

Apaflo Loc: Anchorage, Alaska

Which is why your article, attempting to go into detail about photons and their significance, was confusing.

The basic issue still comes down to just answering the OP's question about the difference between exposure and brightness as it applies to histograms. Exposure is set by aperture and shutter speed. Brightness is how the captured data is processed.

abc1234 Loc: Elk Grove Village, Illinois

Thank you for your response. However, it does not shine any light on the subject.

agilmore Loc: Baltimore

Ha, good one.

So, does his explanation need more exposure or more brughtness?