Aperture and the RGB channels - Lightroom Tutorial

- [Deke] In part four of our discussion of the elements of wide angle exposure, we're going to take a look at aperture, and it's relationship to both the foreground and background, as well as all three channel based histograms. So in other words if you need to modify the exposure of the foreground and the background all at once, aperture can be the way to go. - [Hergen] So let's look at aperture and it's relationship to the RGB histograms. So aperture, as Deke mentioned, controls both the foreground and background exposure, and it actually does that in ratio, which we'll discuss a little more later on in this movie. So aperture's also going to affect the luminance histogram, as well as all three RGB histograms. So as usual, let's go ahead and let's take a look at some real world examples. So in this image, we're starting at a fairly high aperture of f 11, and you can see it's a pretty dark image overall. Both the background and the foreground are pretty dark, as is reflected in the histograms on the left, so the luminance, the R, the G, and the B histogram, all of them pretty packed in the shadows, and a little bit bleeding over to the mid-tones. - [Deke] And so this is a classic example of not pushing the histograms to the right, and so anytime those histograms are leaning this far to the left, we're going to have an extremely dark image. - [Hergen] So in this image, we've opened up our aperture a little bit, letting in a little more light, so we're at f 8, and if you look at the histograms, both the luminance, R, G, and B, all of them are starting to show a little more information in the mid-tones, and starting to creep into the highlights, and this is, of course, also reflected in the image, cause as you can tell both the foreground and the background are both getting a little bit brighter. So here we've opened the aperture up to 6.3, and you can easily see that both the foreground and the background have gotten significantly brighter than when we started at f 11. And looking at the histograms, well that's just reinforcing it. Both the luminance, the R and the G, have started to populate the highlights. Now that blue histogram is starting to clip a little bit. - [Deke] So in the red and green channels, these are great examples of moving the histogram to the right without touching the right-hand edge, whereas in the blue channel, we've gone too far. - [Hergen] So we're going to take it one step further just to see what happens. So here we've gone down to 5.6, and looking at those histograms, the red and the green still look pretty good, but that blue is having some serious clipping happening now. So as we can see, we've pushed that blue channel a little far, and what that means is we probably got some loss of detail in our water. - [Deke] But I should say that some of that might be made up by the fact that the red and green channels are not clipping. - [Hergen] And looking at our luminance histogram, it doesn't really look like we've caused anything to go white. So here we are, really wide open at a 4.5. So we're letting in a lot of light, and you can see in both the R,G,B, and luminance histogram, we definitely have clipping in all three channels, and something has gone white, I would say most likely it's the stalks of the soft coral. - [Deke] Now as we discussed back in the fundamentals course, aperture also has an effect on depth of field. And so what we've done just by way of demonstration is we've taken more or less that same scene, and we've gone ahead and opened it up to an f stop of 2.8, and as a result you can see that the coral's very in focus, and then the background, of course, is very soft indeed, and so just to make things as clear as possible, here we're zoomed into the background, and here we're zoomed into the corals, and notice right there in the center we have some great focus, but as we go out to the right hand side, and the left as well, we're losing focus because we have such a shallow depth of field. So tell me, Hergen, given what we've seen, what is your favorite aperture setting for this specific scene? - [Hergen] It seems to me that we had the best results at about f 6.3. So if we look at our histograms, we do have some of that blue clipping, but our red histogram looks pretty spot on. - [Deke] And that luminance histogram, in the upper right corner, looks absolutely great. We're not seeing any clipping on the far right hand side. Alright so let's talk about some takeaways here. What is the moral of this story? - [Hergen] So because aperture adjusts both the foreground and the background simultaneously, and as I mentioned, does so in ratio. So if we change the aperture by one stop, it should also change the foreground exposure by a stop, and the background exposure by a stop. Now of course, this isn't hard and fast, but it's a pretty good rule of thumb to go by. So instead of adjusting both the strobe power and the shutter speed independently, if necessary you could just adjust the aperture. And so this could be important, and a good solution, if you have a subject that's both continuously changing distance and position in the water relative to the sun. So for a quick example here, let's say we had a Manta ray. He's been swimming past you. He's probably about a meter and a half away, so you've got your strobe set up for a subject that's about a meter and a half away, you've got your shutter speed all set for the nice blue background, and the Manta changes direction and starts swimming towards you. So now he's getting closer to you, so your strobes are going to over expose him, and as he's passing over the top of you, your shutter speed isn't going to be high enough to combat that sun, so you're also going to blow out your background. Instead of adjusting each one of those independently, by the time of which the Manta probably would've already passed overhead, by just increasing that aperture, you're going to take down both your strobes and your shutter speed simultaneously, and probably get a good exposure as that Manta swims overhead. - [Deke] And that is how you can use the aperture to adjust the exposure of both the foreground and the background simultaneously.