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The Histogram explained | Photography Props

The Histogram is a graph shown in your camera and in Lightroom, Photoshop, etc that shows the brightness of the pixels in any given photo. Reading a Histogram is quite interesting and useful, once you get the hang of it. Histograms look very intimidating at first but they are actually quite straight forward.

There are 2 directions in which to 'read' a Histogram: Bottom to Top and Left to Right. Bottom to Top represents the Number/Amount of Pixels and Left to Right represents the Brightness of Pixels. 

In the image below, the Pixels are the grey mountain looking object in the Histogram. In this particular Histogram most of the pixels are in the Midtone and Highlight range (the two 'peaks' in the 'mountain'). There are also some pixels in the Shadow and Black range, and some pixels in the White range. 

Notice how 18% Grey is smack bang in the centre (Left to Right direction) of the Histogram? And remember how the zero on an In-Camera Light Meter is exactly 18% Grey? See how the Histogram and In-Camera Light Meter correlate to each other? -3 on the In-Camera Light Meter is the same as Black on the Histogram. +3 on the In-Camera Light Meter is the same as White on the Histogram. -2 on the In-Camera Light Meter is the same as Shadows on the Histogram and +2 on the In-Camera Light Meter is the same as Highlights on the Histogram. Midtones on the Histogram are anywhere between -1 to +1 on an In-Camera Light Meter. 

 

histogram

 

Let's take the above knowledge and use it to study the Histogram of a photo I took of a photography magazine cover, that was correctly exposed. 

 

histogram explained

 

The Histogram of the magazine cover photo is shaped very differently to the first Histogram we looked at, isn't it? Are there lots of light highlight pixels, dark shadow pixels or medium midtones pixels? Are there any black or white pixels in this photo? Answer: Read notes in the image above to see where the black, dark, medium, light and white pixels are in the magazine cover. 

Let's look at another photo and it's Histogram.

 

strawberry histogram

 

This photo of my poor dying strawberries is *just* slightly under exposed. The pixels in this image are sitting in the middle to left hand side of the Histogram because the tones in this scene are medium to dark, NOT because the image is *drastically* under exposed! There are no really light or white areas in this scene so there are no pixels sitting in the lighter ranges on the histogram. 

Adjusting/correcting the (slight) under exposure in Lightroom, takes the Histogram from this:

 

before adjusting exposure

 

To this:

 

after adjusting exposure

 

I increased the exposure of this image by +0.36 in Lightroom. As such, the photo is lighter and the pixels in the Histogram have shifted to the right. The photo now looks correctly exposed to me.

You may have heard the term 'blowing out the Whites' or 'loosing detail in the Shadows'. What this means is the highlights are over exposed or the shadows under exposed. Over expose the highlights and you will 'blow out the Whites'. Under expose the shadows and you will 'loose detail in the Shadows'. When you loose detail in or blow out the pixels, it means there is no information there in that part of the photo and you can't correct the exposure in post processing.

In RGB (Red, Green, Blue Channel) terms loosing detail in the shadows will mean the RGB value is 0. Blowing out the whites will result in an RGB value of 255. You can check the RGB values yourself by using the Eye Dropper Tool and hovering it over an area in the image. 

On the Histogram, the pixels will touch the left most side of the Histogram if you've lost detail in the shadows and touch the right most side of the Histogram if you have blown out the whites, as such:

lost shadow detail

blown highlights 

Sometimes, blowing out the Whites or loosing detail in the Shadows is unavoidable. It's a creative decision that has to be made by the photographer at the time. For example, if the photographer has a subject/person standing in front of strong backlight (like the sun), and they want to expose their skin correctly, then they will make a decision to blow out the sky in order to expose the subject's skin how they want it. The only alternative to this is to try to brighten the subject's skin using a Reflector or to use a Flash to illuminate the subject (whilst the camera exposes the background). 

Well, this has been a lengthy blog post but I hope I have made the Histogram a little easier to understand. 

In my next article I will talk about what it means to 'Expose To The Right' (ETTR) and 'Expose To The Left' (ETTL) and why some photographers choose to do either of these. 

Love and {Great} Light,
Penny
SHOOT BABY!


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