Wednesday, May 21, 2014

H is for Histogram

Today in the ABCs of photography: 

H is for Histogram

Histogram (ˈhɪstəˌɡræm): aimage histogram is a type of histogram that acts as a graphical representation of the tonal distribution in a digital image. It plots the number of pixels for each tonal value (1)

As we mentioned in E is for Exposure, the "correct" exposure is one that has a wide range of tonal values, from your shadows to your highlights. However, whether through underexposure or overexposure, "clipping" can occur in your image. Clipping occurs when the darkest points in your images are completely black, or the brightest parts of your image are completely white. While this isn't necessarily always a bad thing (it all depends on what you want your image to look like) it's generally safer to keep your shadows and highlights away from black and white. Why is it safer? When your shadows and highlights are not totally black or white, there is still detail that can be seen, or at least adjusted, but when you've gone black or white, that detail is lost, and you cannot come back from it. 

So, let's check our histograms. I generally process my images in Photoshop, so that's what I'll be using, though other programs, like Camera Raw or Lightroom, have the ability to show your histogram as well (as do most DSLRs). Open your image in PS, and if you do not already have Histogram open in your workspace, go to Window and select Histogram; your histogram will pop up on your screen. From there, you can move the histogram window where you'd like, I tend to keep it with the rest of my tabs on the right side of the screen. 

Now, you can view your histogram in a few different ways: 

Here, my histogram has been "expanded" to show all color channels. To choose this setting, go to the little symbol with three small lines in the very top right of your histogram tab and choose All Channels View. You can also view it as Compact or Extended. Compact shows all of the colors together on one graph, Extended gives you some stats, and Channels shows you each color individually. 

You can also choose to view different histograms, including RBG, Red, Green, Blue, Luminosity and Colors. Choosing a certain histogram allows you to see the tonal differences in each color channel and find where clipping is occurring. The Histogram also shows me that majority of my image is lights; not a whole lot of midtones or shadows. I like it the way it is, but you may want to darken some areas for
a fuller tonal range.

Notice the Triangle with the little ! in the middle, that means that there is clipping in my image. Here is where I want to show you that clipping isn't always a bad thing. The whites are just small patches of sea form, while the blacks are the shadows in the rocks. I don't mind that detail is lost in either area, because those areas are not the focus. Plus, the bright white and dark black add more drama to the lighting of the image in general. 

Here is where clipping isn't a good thing. Given, this photo was shot at night, but the darkness on the left side of the image is pretty far gone. And, because of the bright sign on top of the hotel, my histogram is kind of extreme looking.

If you're looking to adjust the tones in your image, try using Levels and/or Curves in Photoshop. Stay tuned for the rest of our ABCs of Photography for more tips and tricks. 

(1) Source