SEEING THE LIGHT
IDENTIFYING THE SCENE TYPE. © LUIS A GUEVARA
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To understand , and therefore ,make good use of the Histogram you have to understand that it represents the distribution of tones present on the original scene , but placed over the Exposure Graph according to our exposure decisions.
And to understand what proper exposure decision means , we have to discuss the different Tonal Distributions found in Nature .
- In Nature the most commonly found Scene is one that has an equal amount and intensity of all Tones , a Forest is a Good example . Such distribution can be visualized by a Histogram that has a flat distribution that covers all the range of exposures of the Histogram , from the deep shadows to the highlights:
A less common distribution is when one group of tones dominate .
When the Highlights are more present and intense than the rest of tones we have what is called a High Key Scene; Snow and Beach Scenes are good examples and so are Clouds:
- When the lower tones dominate, we have a Low Key Scene , which are very appealing because of their Dramatic Expressive Power.
Some Scenes have just a short range of Mid Tones and no Deep Shadows or Extreme Highlights , These are Soft , Low Contrast Scenes, usually the product of Fog , Smoke or deep Forests with dense canopies that only let in Diffuse light.
Here comes the important part :
Proper exposure decisions are those taken during the capture to correctly place a reading taken of an 18% Reflectance Gray Card , of which a weathered piece of Wood is a good example ,exactly at the Mid Tone Point of the Histogram , at Binary Value of 128,( the midpoint on an 8 Bit scale that goes from 0 to 255). This point brightness ,represents the relative brightness in Nature of a Middle gray and you want to reproduce it as such.
A hand held Exposure Meter reading of such a card will do just that , because its calibration is based on this principles , but a camera built in meter is very different because it breaks the Scene in Zones and attempts to identify a value that will put the exposure as far up the Histogram , without Over saturating the Sensor . In a Typical six stop Dynamic Range Scene , this will work out pretty much the same and a Mid tone Gray will be placed somewhat close to where it belongs.
- Unfortunately when there is More or Less Dynamic range it will not. That is when You will have to Participate.
By looking at the Scene and recognizing what kind of Scene we are dealing with we will be enabled to make the proper decisions needed to correctly place the Mid tone within the Histogram so that the entire distribution will look like the proper one.
Low key scenes should have Low key Histograms and placing their tonal values higher up in the exposure curve will not reproduce the Scene that you saw, it will , in effect be overexposed in reference to Reality.
- This is an example of a normal scene of adecuate contrast ,correctly exposed image showing a well centered a "good" histogram.
- The smooth curve upper tones feathers downwards ending in 255 shows that the subtle highlight detail is preserved.
also, the shadow area starts at 0 and builds up gradually.
- This Histogram indicates ONLY MIDTONES . This could be because the scene was like that , a Foggy Day , for example , and in this case the well centered Histogram will render the Midlle Gray properly at the center with a value of B 128
However That Histogram could also be the wrong capture of a Scene consisting ONLY OF SHADOWS , by gross overexposure, placing the shadow values too high, and it will require adjusting the exposure down , during conversion.
If you try (Wrongly!!) to increase its Global contrast by adjusting the Clipping points inward in Levels, , because there are insufficient tones to begin with , the stretching will create separation of tones called banding. The gaps show colors or tones that were lost and will not be present in the converted image.
In the Histogram that will show like this:
Image with too much contrast
This image has both clipped shadows and highlights. The Dynamic Range of the scene is larger than the typical Six to 10 stops dynamic range of the camera.
Thi histogram indicates that there are a lot of pixels with value 255 or close to 255, which is an indication of "clipped highlights" , this can happen while photographing a High Dynamic Range Scene , your efforts to correctly place the Gray tone where it belongs , could exceed the Saturation Point of the Sensor , the outcome will be a Burned out section of the image that has only pure white (Binary 255).
The histogram indicates there are a lot of pixels with value 0 or close to 0, which is an indication of "clipped shadows". Shadow detail that fell to the left of 0 is lost forever . If these was a Low Key Scene ,it needs to be placed higher up in the curve by applying an Exposure correction of Plus one or plus two.
Image with too much contrast
The histogram indicates there are a lot of pixels with value 0 and a lot with value 255,with a clear presence of "Spikes" , which is an indication of "clipping". Subtle highlight and Shadow detail are being lost. There are also very few pixels in the shadow area. This situation exceeds the capabilities of your camera and you will have to choose which end to sacrifice and then compensate the exposure to suit your decision , OR use multiple exposure of the scene at different compensations , plus and minus , and combine them later in Software to accommodate all the Tones , but one single exposure would not be enough , although by using Linear Processing of the RAW file , within limits , you could rescue the upper Tones .