Feiertag Loc: British Columbia, Canada

Great explanation, Todd.

The 18% gray factor has always puzzled me. Why isn't it 0 or 1% metering? White, not gray!

Notorious T.O.D. Loc: Harrisburg, North Carolina

I would say that it is 18 percent gray because the the meter is trying to find the midtone. The 18 percent refers to the light that is reflected by the surface. Some people will say that digital cameras are actually between 12 and 18 percent but 18 percent is the standard that most people will using in discussing it. You may want to look into the Zone System to better understand middle gray and how the dark and highlight areas related to the middle gray.

Keep in mind that cameras are using a reflectance meter that can only measure the light being reflected of the subject. A handheld meter operates differently in that it measures the light falling on a subject. That is fine as long as you are in the same light or near the subject. If you are in different light or far away from the subject you can get handheld meters that also have spot meters that measure reflected light. The Sekonic 758 and 858 have the spot meter built into the meter. It is a 1 degree spot meter. Some less expensive ones are 5 degree spot meters. There are good videos on YouTube by Joe Brady/Sekonic and Mark Wallace that explain metering with both cameras and handheld meters for both portrait and landscape shooting. Most people will never get a handheld meter and will instead rely on the histogram on their camera and its reflectance meter. Good results can be achieved in that way but you have to know what the scene lighting, metering mode on the camera and histogram are telling you. That comes with experience.

A handheld meter is much more important in studio shooting as there are often multiple lights being used and ratios of lighting that need to be set and maintained between the light sources. Your camera really can't measure this type of lighting as well or a quickly as a handheld meter can. And the camera will only measure reflected light not the light falling on the subject, reflectance vs incident light. The handheld meter can also measure the strobe output and the percentage of ambient vs strobe or flash light if shooting portraits outdoors. TTL metering systems are getting better all the time and you can do ratios with Speedlights or some more sophisticated monolights. There is a lot of information on YouTube if you have a desire to learn more about lighting and metering techniques.

Best,
Todd Ferguson

Feiertag Loc: British Columbia, Canada

Thank you, Todd.

Psergel Loc: New Mexico

The luminance histogram is a graphic representation of the number of pixels that are exposed at different brightness levels from pure black to pure white.
If most of the graph is "bunched up" to the left it means that most of the image is very dark. (possibly underexposed). If most of the graph is "bunched up" to the right it means that the image is very light (possibly over exposed).

"Ideally" there should be a distribution throughout the range of the graph. Not too many black (shadows with little or no detail) and very little if any right up against the right side of the graph. These would be pure white with no detail at all (blown out). Generally speaking this is glaringly unpleasant to see in an image.
We can accept deep shadow more than we can accept blazing white.

This is all a generalization because......if you are taking a picture of a white cat in a field of snow you would expect to see most of the pixels to be near white and close to the right side of the histogram.
Conversely, if you were taking a picture of a black cat in a lava field you would expect to see most of the pixels to be near black and closer to the left side of the histogram.

I think most of us use it as a quick check to see if, in an "average" scene we have a reasonable spread of levels across the graph and that we don't have any tight up against the right side of the graph.

There is also an RGB histogram which breaks the luminance values into red, blue and green. I find it useful when photographing something that is predominantly one color (such as flowers). If I'm taking a picture of a red rose I check the red histogram to make sure that the red highlights are not "blown out" and lacking detail.

CPR Loc: Nature Coast of Florida

Some cameras and PP software, like Photoshop, will give you Histograms by color also.
Bunching at one end is a good indicator of the overall brightness of the photo, the problem I look for is if the spikes go off the chart and that tells me it's blown out at that point.

Psergel Loc: New Mexico

If I remember correctly, someone (maybe Kodak) determined that 18% gray was the average tonal value of the “average scene”

older beginner Loc: Holland, Michigan

Thanks everyone for their input. All your reply's were a great help.

canon Lee

Thanks Steve for you additional info. I wouldn't rely on the highlight alert as it is not as accurate as the histogram, which will show how the light is spread out.

canon Lee

The mid tone is what the gray factor is all about. The camera adjusts ( except in Manual) for mid tones, so that the entire light is evenly balanced from black to white.

wdross Loc: Castle Rock, Colorado

The simplest explanation is that it is a graph of the light hitting the pixels. Usually the bottom edge of the graph goes from 0 (black or no light) to 256 (white or max light) in light intensity. The graph's vertical side indicates how many pixels are at that particular level of light. Many camreas break the histogram into three graphs, one for each primary color (red, green, and blue). Ideally one wants few pixels at 0 or 256 and most if not all pixels in between 0 and 256.

burkphoto Loc: High Point, NC

It is a RELATIVE graph of how many tones of each value there are in an image, from 0 to 255 (the 256 possible values of an 8-bit per channel image file). You can have a "gray scale" histogram that sums all three (red, green, blue) channels together, or you can have separate histograms for each color channel. By itself, a histogram just displays how many of each tonal value you have in your image. It DOES NOT indicate proper exposure, unless you reference a known target!

0, the "left wall" of the histogram, is detail-less black. 255, the "right wall," is detail-less white. Monitors might be able to display tones 0 to 18 and 236 to 255, but silver halide photo paper cannot. On conventional photo paper, you will perceive black from 0 to around 12-18, and you will perceive white from around 236 or 242 up to 255 (under MOST print viewing conditions).

The only time a histogram can give you a truly accurate idea of exposure is when you photograph some sort of known-value reference target. If you fill the frame and photograph a Delta-1 gray card, evenly lit, with no shadows, the JPEG exposure will be correct when the histogram has a "spike" in it at the mid point of the graph. A One Shot Digital Calibration Target will show you a perfect JPEG exposure when the three stripes are centered, with equal space to the left of the left stripe, and to the right of the right stripe. Both targets also can be used to perform a custom white balance that gives you very neutral white balance (color balance).

Such targets are used to meter light and to perform custom white balance when you need to make JPEGs in the camera that need to be correctly exposed and white balanced. This is especially useful when you are photographing low key, high key, and monotonal color scenes. Any of those would fool your camera's reflective light meter and automatic white balance circuits. They are EXTREMELY useful for photographing large numbers of subjects under controlled, consistent, unchanging lighting conditions.

A histogram of a low key scene may not have *any* whites in it. A histogram of a high key scene may not have *any* blacks in it. So for high key and low key scenes, trust a reflected light meter pointed at a proper exposure target, or trust an incident light meter held in the light falling on the subject, but do not trust a reflected light meter pointed at the subject *by itself,* and do not trust a histogram of such a light reading!

In very high dynamic range scenes containing glass and chrome or other silvery bright, polished metal objects, properly exposed specular highlights (mirror-like reflections of light sources) should always be pure, detail-less white. So if your histogram contains a lot of tones against the right wall, it may, OR MAY NOT BE correctly exposed. You don't really know without reference to a target. But if such a scene has NO tones against the right wall, you know for sure that the JPEG is underexposed!

Bear in mind that the histogram is *always* based on the camera's JPEG processing. It is showing you the relative distribution of tones created by the current exposure and menu settings, as applied to the raw image and saved by the camera as a processed JPEG file. If you record raw images, there is considerably more tonal range available to edit. If the JPEG is correctly exposed, there may be one to two stops of additional highlight and shadow detail you can play with in post-processing. Remember, too, that if your camera is set to save ONLY raw images, you are STILL processing a JPEG and stuffing it inside the file wrapper of the raw image, for display on the camera's OLED or LCD screen, the EVF on a mirrorless camera, and your computer's operating system.

burkphoto Loc: High Point, NC

It's actually closer to 13%, and meters are calibrated to that, but Kodak made slide films. They did not want people to overexpose them. So they built in about a third of a stop of underexposure (by reading an 18% card with a meter calibrated for 13% reflectance).

Many folks used to OVER-expose color negative films and black-and-white negative films by 1/3 to 1 full f/stop, and alter development, to preserve shadow details and add some snap in the highlights. Film has a non-linear response to light, which creates an 'S' shaped response curve. So negative films can stand some overexposure and retain highlights. Digital sensor response is nearly linear, so if you want an 'S' curve response, you have to add it in processing (which is what camera Picture Styles and post-processing software are for).

The same thing is true today, because JPEGs processed by a camera have about the same narrow latitude as slide films. If you look at dxomark.com tests of camera sensors, the typical sensor tests around 1/3 stop slower than the rated ISO, across its normal ISO range. This phenomenon is uncannily consistent from brand to brand to brand... It has to be a deliberate "over rating" of the sensor by the manufacturer, used to build in some highlight latitude (burnout protection!).

Notorious T.O.D. Loc: Harrisburg, North Carolina

Steve Perry Loc: Sylvania, Ohio

I did a video on it awhile back that you might find helpful:

https://www.youtube.com/watch?v=XtdQz6piFpI

Paulie Loc: NW IL

very well explained, thank you.