In-camera histograms - Photoshop Tutorial

- [Instructor] In this chapter we'll take a look at a few elements of exposure as they specifically pertain to underwater macro photography. Starting off in this movie with a review of in-camera histograms. So as many of you know you have the option of viewing histograms on the back of your camera as you're shooting. And these histograms are bar graphs, or more accurately, column graphs of the distribution of tones from black on the left to white on the right. And so what your camera is doing is it's capturing grayscale images that are filtered with primary colors. Starting with the red channel, and then we have the green channel, and finally we have the blue. And then these channels are merged together in order to create the full color composite image, which has its own luminance histogram. - [Hergen] And it's important to remember that each pixel on your sensor is capturing each of these channels, so each pixel is capturing red, green, and blue. - [Instructor] Now on land these histograms serve a couple of different purposes. You can use them in order to preview an image that you're about to shoot, as well as review an image that you have shot. But underwater it's different. - [Hergen] Underwater we're relying heavily on our strobes and unfortunately the camera can't see what the final exposure's going to look like until those strobes have fired, so all we can use those histograms for underwater is for review purposes. - [Instructor] And I want you to note that these image review histograms are based on JPEG renderings, not on the raw data. And so our assumption here is that you're shooting raw underwater, that's going to give you way more flexibility when it's time to develop the images either inside Lightroom or in Photoshop. - [Hergen] So use these histograms as more of a guide to make your on the fly adjustments. - [Instructor] Now you may wonder what these tones really look like. Well, essentially we're capturing a gradient of luminance levels, starting with black here on the left and ending with white on the right. Now rather than naming every single one of the gray values in between we clump them into groups. Starting with the darkest colors, which are known as the shadows, and ending over here on the right with the brightest colors, which are known as the highlights. All those grays in between are generally known as midtones. And all these luminance levels are captured on a channel by channel basis. So in the red channel we start with black and end with bright red. In the green channel we start with black and end with a very bright green. And then in the blue channel we start with black and end with a deep blue. And because we're working with the primaries of light as you mix these colors together they become brighter. And so green and blue mix to form a bright cyan, red and blue mix to form a bright magenta, and red and green mix to form yellow. So tell me Hergen, how do we use these histograms to measure proper exposure? - [Hergen] So the purpose of these histograms underwater is for us to help judge proper exposure of our image. And what we mean by proper exposure is that we're capturing the full tonal range available to us in the image. So again, here we have our red channel, our green channel, and our blue channel. And you can see that we've captured a fair amount of the tonal range available in these histograms. Again, our highlights are going to appear on the right, our shadows on the left, and our midtones in the center. And if you look at each of these histograms we are capturing shadows on the left, midtones in the center, and highlights on the right. Now remember though that this histogram is a graph, it's not a map, so it reveals tone populations, not locations. - [Instructor] So, for example, let's take a look at the red channel here. You can see that we have a few shadows over on the right hand side, we've got a fair number of midtones there in the center, and we've got a lot of highlights. So let's say, for the sake of discussion, that that white column, the one that just came up on screen, is telling us that there are 400,000 pixels, we don't know where they are, they're just some place inside the image, 400,000 pixels that are that particular shade of red, then the tallest column is telling us that there are 600,000 pixels that are that shade of red. So again, it's just a population graph and nothing more. - [Hergen] And so the shape of the histogram is going to change based on the actual tones in the image. - [Instructor] And just in case you're curious, these histograms amount to this particular photograph, although you might find a ton of photos that have similar histogram shapes. Alright, now let's talk about color clipping. - [Hergen] So color clipping occurs when a single channel becomes over-exposed, and in that case the affected pixels become infused with that particular primary color. So here in this case we have the red channel and if you look over to the right you can see we have this red bar running up against the edge of the frame. And what that's telling us is that we have some pixels in this image that have become pure red. Same thing over here in the green channel, but obviously to a lesser extent. And down here in the blue channel we've got some pretty extreme blue clipping. And this tells us that there are several pixels in our image that have become infused with pure blue. - [Instructor] And by several we mean several hundred thousand. Bearing in mind, of course, that your image contains millions upon millions of pixels. But that doesn't mean however that just because we're seeing a lot of color clipping in the red channel, a little bit in the green channel, and a ton in the blue channel, that that clipping is all occurring on the same pixels. It could be totally different pixels, in which case you're okay. However, it becomes a problem if it's happening on the same pixels. - [Hergen] And if that clipping is all occurring on the same pixel we get what's called highlight clipping. So when all those channels clip right on the same pixels those pixels go completely white. At that point we've lost tonal variation in that area of the image. - [Instructor] And when we're saying clipping right we mean that we're seeing that clipping on the right hand edge. So notice here we're seeing big spikes in the red channel, the green channel, and the blue channel on the right side of those histograms. - [Hergen] But until we look at the luminance histogram we're not sure whether that clipping is all occurring on the same pixel. But here when we look at the luminance, which is a composite of that red, green, and blue, we can see by looking at the right side of that histogram where we've got a pretty big spike, that means that we have parts of the image, certain pixels that have gone pure white, and we've lost tonal variation on those pixels. So when we see this highlight clipping that's telling us that we've over-exposed the image and we need to adjust our exposure accordingly. - [Instructor] Now the same thing can happen with the shadows. And so when all channels clip left, on the left side of the histograms, on the very same pixels, those pixels have gone black. And again, we loose tonal variation in that particular portion of the image. So here we have some very dark histograms in which we have little or nothing in the way of highlights, very little in the way of midtones, and then tons and tons of shadows, all of which are running up against that left hand side. Now that might be okay if all that clipping is being distributed across the image, so that it's not collecting in a single group of pixels. However, when we look at the luminance histogram we can see that a very large group of pixels is clipping on the far left hand side, and that tells us that those pixels have gone black. - [Hergen] So again, we would use this as a tool to tell us that we've under-exposed our image and we should adjust our exposure accordingly to capture more of that tonal range. - [Instructor] Alright, so let's take a look at a few histogram takeaways. - [Hergen] We want to make sure that we're capturing as much tonal range as we can in each channel, because all that information is going to be information that we can use later on when we're fine-tuning our image in post. In order to capture that full tonal range we want to make sure that we're exposing to the right. So we're pushing that histogram as much to the right as we can without touching that right edge and getting clipping. - [Instructor] And so in other words we're erring on a side of highlights as apposed to shadows. - [Hergen] It's important to pay attention to those histograms as you shoot, because if we're not capturing that full tonal range and we have to adjust the exposure in post we're going to end up with something called posterization or stair-step tones where the program that we're using is actually having to generate false tonal data to make up for the gaps. And finally, make sure you're observing your histogram, but don't obsess over it. Perfect exposure's going to mean nothing if it comes at the expense of great lighting and composition. - [Instructor] And that's our look at the red, green, blue, and luminance histograms offered by your camera. In the next movie we'll see the effect of strobes, specifically on the red channel.