Interpret camera exposure from a Photoshop point of view.

[RL355]

Hi All

wondered if anyone can help ?

I am trying to interpret camera exposure from a Photoshop point of view. Just wondered has anyone used the Filter > Blur > Average function. and to tell me whether my thinking is correct for the below.

1. I open up a photo that I have taken in Photshop CS2

2. I set my histogram to show luminosity from the drop down menu and then apply the filter. Filter > Blur > Average.


When you apply the filter. Filter > Blur > Average.. to a photographic image in Photoshop. It changes the photographic image to a grey tone photograph. Lets say with a MEAN value of 191 luminosity as shown in the luminosity histogram.

This Filter applied in Photoshop to a Photograph. Is this equivalent to how the camera metering system averages the scene to obtain a middle tone value? Once it has this mean value any pixels that falls exactly as 191 luminosity will be exposed by the camera as 18 percent middle grey tone.
Any pixels above value of 191 luminosity will be exposed as highlights
Any pixels below the value of 191 luminosity will be exposed as shadows


I hope Iam making sense. or is my thinking completely wrong :) Thanks in advance.

[tzalman]

In Evaluative and Center Weighted modes the camera measures the intensity of light in a number of discrete areas - for xxD cameras it is 35 zones. The average value is then biased to the active focus point and perhaps altered by data from the database of light distribution scenarios for Evaluative or biased to the central zones for CW. During jpg creation the resulting value is set to a grey scale value of around 112 since the camera is calibrated to 13% grey, not 18%.

[RDKirk]

This Filter applied in Photoshop to a Photograph. Is this equivalent to how the camera metering system averages the scene to obtain a middle tone value? Once it has this mean value any pixels that falls exactly as 191 luminosity will be exposed by the camera as 18 percent middle grey tone.

Once upon a time, when through-the-lens meters were first developed back in the 60s, the meter fully averaged the entire scene in the viewfinder. That was the Pentax Spotmatic, which was actually the opposite of a "spot" meter.

But pretty quickly, camera makers went to meters that were weighted toward the center (a very primative "evaluative" effort) because the average scene did not quite average out to "average" and photographers needed a bit more control.

In other words, we've been where you're trying to go and then left. One photographer has been touting for a few years the concept of the "facemask exposure" which basically presumes that the skin of the human face plus a bit of the hairline averages to middle gray. Umm, yeah, if you know in advance how much hairline you have to include to make a pale face average to middle gray and how much hairline you have to exclude to make a dark face average out to middle gray.

After all is said and done, I still find it invaluable to be able to look at any given tone and estimate whether its luminosity is middle gray or how many stops above or below middle gray it actually is ("actually is" during shooting an "should be" during editing).