Live Histogram In Optical Viewfinder

Different metering methods in modern photography have all something in common. They measure light in different ways using a sensor array. And display the average or weighed average or some sort of related indication on the optical viewfinder in the form of an indicator under a scale at the bottom of the image. So you have an intuitive indication that even tells you which way you have to correct the exposure, moving that button or ring in the other direction until you get it right. So you can turn the button until you come close to center or do it completely automated, sometimes using exposure compensation to move the indicator one step or a few permanently off center.

But here came the digital cameras with megapixel sensors and real time computer processors. They can use the main or auxiliary sensor to measure way better the light. Why? Because instead of having let's say 9 sensors that measure light in a conventional array, they have millions of pixels, each actually being a sensor and not only on broad spectrum but color specific.

But how to use this new fantastic possibility? The first way they thought of is the histogram. I looked at the definition of the histogram especially for the purpose of writing this and it does not match with my intuition on the subject.

What the "live histogram" function in my camera does is it displays in real time an animation with number of vertical bars in the left lower corner of the LCD display when in that mode, each bar giving an indication of the number of pixels of a certain intensity (for live histogram, black and white only thus an average of all colors). So actually the software in the camera counts many times a second the different intensity pixels on the sensors and displays them in an animated graphic. I don't know why it is called a histogram. To me is just a graphic with light intensity on the X axis and number of pixels on the Y.

Nobody really needs to know how many pixels of a certain intensity are on the screen. But by looking at the "histogram" one can instantly... well it depends of one's definition of instant... can realize if there are missing pixels of a certain intensity, and this happens usually at the left or right end of the graphic. Or how "thick" is the "histogram", that giving you an information of how much usable information is ready to be transferred in the flash card when you press the button. Missing bars on the right means i'm underexposing and on the left i'm over. If the last right bar touches the top of the graphic it means i have "burnt" - saturated, useless white pixels and on the left, black burnt pixels. It's that simple. I am saying by one's definition of instant, because it actually takes seconds to look at the histogram on the LCD display and interpret it even in the most intuitive way. And by doing this you lose the framing and change the histogram itself. Then you have to come back at the optical viewfinder for a better framing and this is serious time when you are trying to take instant or candid shots.

Histogram is the best and unmatched exposure measuring method before taking the picture, the next best thing to looking at the picture itself after. But even looking at the picture sometimes you cannot tell if you have burnt pixels but the histogram will tell you. Another method of visualizing the burnt pixels is one of the many display options on the camera on playback mode, when you can see them as blinking areas.

If you have a big, fat histogram with no bars touching at the ends, you will have the most incredible, fine tone, balanced, high contrast, high dynamics and colorful image. And that will not always be with the exposure indicator in the middle. Sometimes, the indicator is 3 or 4 steps off center while the histogram shows the best picture.

But then I've been thinking even further... Why a histogram anyways? What if you had an indicator like the one in the picture above that would work using the same information used to draw a histogram from a megapixel sensor (instead from the metering sensor array), through counting all the pixels of a certain intensity and instead of displaying the histogram, displaying an indication of the thickness or the white area on  the histogram, and letting you center it (or optimizing the non displayed histogram) while adjusting the exposure the same way as with the current indicators based on the old metering systems? After all it is all the information you need and use from that histogram. With keeping the histogram only as an option.

My Sony DLSR 300 has two sensors and two mirrors. One is the main sensor behind the lens and mirror like any DSLR camera. One, smaller, like for a compact camera, is up near the prism like in the images below and it works when the main mirror is closed and secondary mirror is tilted and is for the LCD display behind the camera body and not for the picture itself.

The optical viewfinder is unbeatable for framing. The histogram is unbeatable for exposure. So what if they could be combined?

Today (post date) i've been looking on the web for such a camera. Couldn't find one. I found some with an electronic viewfinder (that might include a histogram), which is better then an LCD viewfinder  because no ambient light comes on it (especially on sunny days) when you close it with your eye while watching through it but it has a poor resolution. It is a compromise with poor results.

And than i said to myself... What if the manufacturer would add a crude histogram made of let's say 10 bars on the optical viewfinder? But the problem with building a histogram while looking through the viewfinder is when the eye see the image through the optical viewfinder none of the sensors does.

I can think of two ways this can be solved, one by using the same second tilting mirror, or second, a semitransparent one, sort of like for the rear view mirror at cars in night mode and temporarily redirecting all of the light or permanently just a percentage towards the live (secondary) sensor. For the semitransparent mirror solution, you don't actually need that much of the image on that sensor since you only want to build a histogram. At the expense of the quality of the optical image in the viewfinder. Or by having a button that can (or periodically for short period of times) tilt the second mirror temporarily sort of like the main mirror opens when you take the shot, to redirect the image for short periods of times towards the live sensor just to build a histogram. My favorite is the last one but both would still be better than switching to LCD viewfinder and back to optical as i am doing right now. And probably better than the electronic viewfinder, i don't know, haven't had one yet on my hands. And you are not so interested in the quality of the image when looking at the histogram and/or indicator anyways. (Or maybe even an eye direction sensor that would tilt the mirror while you are looking down at the histogram or indicator).

September 30, 2012 at 4.25pm PDT, Near Mt.Hood, OR