A Histogram Is A Histogram Is A Histogram; Or Is It?

Fuji X-T2 LCD with histogram sample (click to enlarge)

Olympus E-M1 Mark II LCD with histogram sample (click to enlarge)

Take a look at these two images I made of the LCDs and histograms displayed on the back of the Fuji X-T2 (top) and the Olympus E-M1 Mark II (bottom).  Quite a difference in appearance.  But does it make a difference when photographing?  Read on.

We often use the histogram in our digital cameras to check exposure.  In fact, many of us (including me) use the histogram instead of the lightmeter to set best exposure for any particular image.  Generally, when looking at the histogram, either in the electronic viewfinder (EVF) before we make an exposure or on the LCD on the back of a digital SLR after we make an exposure, we try keep both sides of the series of white bars in the graph within both edges.  Having the white cut off on the left results in losing shadow detail.  Having the white cut off on the right results in losing highlight detail.  If we keep the white part in its entirety within both sides and without running off either side, we should get a good exposure without losing any detail.  Sometimes....  Sometimes not.... Let me explain.

If you look at the Fuji histogram, what you are seeing is a luminance graph—overall brightness of various parts of your image file.   That graph represents the darkness (blackness) to the left and lightness (whiteness) to the right.  That is all.  If you look at the Olympus histograms, you get the same luminance graph, but you also get a graph that represents each color channel: red, green and blue.  Is that important?  Sometimes not but at other times it could be very important.  Let me explain.

Luminance histogram only.  Doesn't show individual color channels which can result in
loss of detail (click to enlarge)

I've experienced many, many situations where the luminance graph fits nicely between the sides of the histogram, but one of the color channel graphs goes off one edge or the other.  Mostly, to the right.  Let's use the red channel as an example.  

What happens when, say, the red channel goes off the right edge when photographing red roses?  Your image may be fine brightness-wise (white graph above), but your red rose may be oversaturated resulting in loss all detail in the red petals.  I've seen it happen many times.  It happens with blues and greens and combinations of two channels as well.

It is easy for a single channel to be "blown out" while the other two channels look fine.  In the example of the red rose, you would have to decrease your overall exposure in order to bring the red channel back within the histogram.

Example of red channel being "blown out" but green and blue channels
just fine.

With the Fuji histogram, you don't know if a particular channel is or is not blown out.  No problem, you say, you'll just look very closely at the image on the LCD.  Have you tried to see fine detail in your image on your LCD?  Sometimes we can't even determine if our images are in perfectly sharp focus on the LCD, let alone determine if we've lost fine detail in the petal of a rose.  Not easy.  Seeing fine detail can be almost impossible and it is very helpful to have other aids to give us the information we need to optimize our images.

By not knowing if one or more of your color channels has exceeded the capacity of your sensor can be ruinous.  You make your images of something you've wanted to photograph for a long time.  Something special, let's say.  Before finishing you check the luminance histogram and all looks fine.  You got it!  Hooray!  You get home, load your images into your editing software and what?  The red/blue/green detail is terribly oversaturated and just gone.  You didn't get it.  You can't go back.  Too bad.

Olympus gives you luminance and all three color channels so you can see directly if one of your color
channels is blown out resulting in over saturation and loss of detail (click to enalarge)

Olympus gives us all four histograms.  That allows us, after we make the image as we are not given the four histograms in the EVF, to see if a particular channel is blown out so we can adjust the exposure and retain that fine detail.  This is very helpful.  In the example above, you didn't lose your color detail and you made your special images.

In the illustration above, all four histograms are within the left and right edges.  But when photographing a dominant color, it is easy for the image to look fine on the LCD but one of the channels to run right off the right edge, again, being oversaturated and resulting in loss of fine detail in that color.

Reds blown out and most detail lost (click to enlarge)

Reduced exposure to bring red channel back into histogram.  Lots
of detail available. (click to enlarge)

This post is just a word for those who were not aware of this.  If you only have a luminance histogram, you may want to bracket a bit if you have a highly saturated dominant color.  Just to be sure.

I wish Fuji would adopt the Olympus philosophy for this.  I've used this feature repeatedly over the years and find it very valuable.  I'd rather have this information than all of the settings displayed, or at least the option when cycling through the various options when looking at images on the LCD to have all four histograms displayed.

Thanks for looking. Enjoy! 

Dennis A. Mook 

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