winterrose Loc: Kyneton, Victoria, Australia

It appears that there is a degree of misunderstanding of what a histogram actually is and exactly what information it displays.

I will reference this to jpeg images for the sake of this thread and for all those ready to correct me on unhelpful technical details please allow me some licence in order to keep this as understandable for as many people as possible.

A histogram is an x,y representation shown as a series of columns representing black on the left brightness with a relative luminance value of 0 and white at the extreme right with its luminance value of 255 while the space between is filled with the remaining 254 columns representing the luminance values of all the shades graduating from black through to white.

The 18% much talked about is “middle grey” which, of course, falls in the centre of the histogram and holds the luminance value of 127.

The important thing to remember here is that any histogram is not an absolute. Any given histogram is tightly related to a particular parametric circumstance. What that means is that a histogram display in a camera displays the range of luminousities within the limits of the dynamic range of that particular camera.

If one camera has a dynamic range of twelve f/stops and another is capable of six and both were used to photograph a tonal sweep, say a surface with an luminance range of fifteen equivalent f/stops in sunlight with exposure set for an 18% grey card then both histograms would look much the same even though camera (a) recorded six stops either side of the middle whilst camera (b) only managed three.

The histogram on both cameras would show that values were recorded in all the columns 0-255 because in these cases the information they display is limited to the actual dynamic range of each sensor.

The 6 f/stop sensor in this case would record as black a luminance value which the 12 f/stop sensor would record as 9% grey.

Likewise the 6 f/stopper would record a shade as being white whereas the 12 would only record the same luminance value as being around 60% grey.

In other words the 12 stop sensor would in one shot record the same range as the 6 stop sensor could if shot as a +3,0,-3 f/stop HDR.

The 12 stopper would have twice as much shadow and highlight detail even though the histograms would probably look much the same.

That’s probably enough for now, I’ll let this sink in and settle for a bit before I continue. Rob.

FilmFanatic Loc: Waikato, New Zealand

Middle grey is actually 13%

winterrose Loc: Kyneton, Victoria, Australia

I stated "unhelpful technical details please allow me some licence in order to keep this as understandable for as many people as possible."

A gray card is a flat object of a neutral gray color that derives from a flat reflectance spectrum. A typical example is the "Kodak R-27" set, which contains cards which have 18% reflectance across the visible spectrum.

Get your facts straight thanks before you put your foot in it again. Regards, Rob.

FilmFanatic Loc: Waikato, New Zealand

My facts are perfectly straight. 18% is not correct.

Edit: See this thread if you don't believe meL http://photo.net/bboard/q-and-a-fetch-msg?msg_id=000eWN

winterrose Loc: Kyneton, Victoria, Australia

State your sources

FilmFanatic Loc: Waikato, New Zealand

I also used to believe 18% was correct, Henry from B&H set me straight on that.

FilmFanatic Loc: Waikato, New Zealand

Here is another person talking about it:

"The 18% gray card was popularized by Kodak and was intended for use by the large printing press operators. It gave them a "middle gray" for adjusting their presses. Photographers soon began using the card since gave them a way to verify color balance (in the old days, we had to use colored filters to adjust the color balance when shooting slide film) as well as exposure. Since the card reflects 18% of the light falling on it and since the "average" scene reflects 12 to 13 percent of the light, you can't use an 18% gray card to set exposure directly"


http://www.dpreview.com/forums/thread/2957840

FilmFanatic Loc: Waikato, New Zealand

P.S. I am not trying to be a prick about it, I am trying to be helpful here

winterrose Loc: Kyneton, Victoria, Australia

Pleeeease.... I knew this would happen!!!

I asked that people give some latitude so we don't all get bogged down in the detail. This subject is confusing enough for so many so let's just please stick to the subject CONCEPTUALLY and we can all build on people's knowledge with a minimum of confusion.
The "13%" to which you refer is actually 12% and is the level set as middle grey as the median sensitivity of the sensor to compensate for the non-linear way the sensor reacts to a linear progression in light levels so that relative brightness levels recorded by the sensor more accurately mimic the reaction to light of human sight.
I used 18% not because I do not know what I am talking about but because I was referring to the reflectance characteristics of standard objects which is not the same as parameters selected by designers for sensor calibration.
I hope this clears that up. Regards, Rob.

Crichmond Loc: Loveland, CO

Thanks FilmFanatc!

I did not know that.

rmalarz Loc: Tempe, Arizona

A histogram is a graphical tool used to represent tabulated statistical data distribution. The data limits for our purposes in digital photography are 0 to 255, a total of 256 values. That being said, I agree with the basic premise of what you wrote.

Expanding a sensor's ability to capture more in the range of f-stops results in that extended range, still, being mapped to a histogram of 256 values.

Reliable, good quality light meters will provide an exposure based on "middle gray". I've placed a Kodak black card, "middle gray" card, and white card under the same lighting conditions. In monochromatic mode, I've spot metered each and took the exposure indicated. They all looked "middle gray" in the final result. That was pretty much expected.

When photographing a scene, I let the scene and my pre-visualization of the final image guide me through the exposure setting and planning the post processing. I rarely look at the histogram to determine whether the image I took was "correctly exposed".

All too often people new to photography, especially digital photography, are too quick to let themselves be led into schools of thought that will do more to restrict them from, rather than assist them in, making dramatic images.

EstherP

I did a search on Google and found next to nothing about 12 or 13% gray cards, but lots about 18% gray cards.
Because 18% is what is mostly talked about (till I read the above info, I didn't even know about 12 or 13% gray cards), what newbies likely hear about first, and to keep things simple, I would very much prefer to keep referring to 18% grey - whether that be right or wrong!
BTW, I also checked with a local camera shop, and the only grey card they sell states it is 18%.
EstherP

winterrose Loc: Kyneton, Victoria, Australia

Yes Esther, thank you, that is exactly why I referred to the mid point as being 18% grey. I intend this thread, and what I might follow up with, as not to sprout my knowledge but to provide an easily understood basis of knowledge of exposure for newbies, people who have been carrying around sacred cows based on bad information, or simply those who never really saw the need. Rob.

ziggykor Loc: East Texas

I often am amazed that when anyone posts something here that provides an explanation to people, some idiot has to come along and attempt to make the original poster out as being horribly incorrect. I guess that's why I've stopped sharing as what works for me is so obviously wrong to all the experts out there!

winterrose Loc: Kyneton, Victoria, Australia

I agree, ziggykor, I have tried before with a thread about lumination/illumination and other subjects but people seemed only willing to make fun or just ignore. Ah well, their loss.