Determining Exposure for ALP
In this section, we're gonna talk about determining exposure. For Astro Landscape photography we're going to be pushing the limits of our equipment. Shutter speed is gonna be the starting point of every exposure. Focal length determines the longest usable shutter speed. Stars will begin to appear as trails rather than points after just a few seconds. The longer your focal length, the shorter amount of time it will take before stars start to move. So if you are using, say for example, you're using a 14 millimeter lens, you have more time to work with before the stars will trail. They'll register at points for a longer period of time than if you're shooting with a longer focal length lens like a 35 or 50 millimeter lens. First we wanna figure out at what shutter speed can we get it, what's the longest shutter speed we can get away with before the stars start to trail. We're gonna use a technique called the 400 rule to figure out what that longest shutter speed is going to be. And how, wh...
at's the 400 Rule, take 400 divide it by the focal length of your lens, and that will give you the number of seconds that you can use before the stars will start to move. So for example, say a 24 millimeter lens, 400 divided by 24 is 17. So you got 17 seconds before the stars will move with a 24 millimeter lens. So if you're using a 14 millimeter lens, 400 divided by 14 is 29. So you can see with a shorter focal length lens you've got a longer period of time to work with. So for APFC cameras or crop sensor cameras, use the 500 rule instead. So for example, 24 millimeter lens, 500 divided by 24 will give you, I think 21 seconds. Is that right? Yeah, I think 21 seconds. Okay so, moving on. Camera orientation affects how quickly the movement will appear in your images. So for example, if you are pointing your camera towards the North, you'll have more time to work with than if you're pointing towards the South. The further away you move from the North, the shorter period of time you have to work with because stars in the South are gonna be much longer than stars, star trails in the North. So it gives you a little bit of extra time to work with if you are shooting towards the due North. Longest usable shutter speeds are subjective, right? So how much tolerance do you have for stellar movement? Do you want them to be absolutely crisp little points with no sign of movement whatsoever? Then use a shorter shutter speed. If you're a little bit more forgiving and you don't care if the stars are slightly elongated you have a little more leeway. And also, what's the end use of your image. If you're gonna be looking at it only in low magnification on a screen, or in a small, say 8 1/2 by 11 print, you can get away with a longer shutter speed than if you're gonna be making a big print. Because the more you magnify an image, the more that stellar movement is gonna be apparent in your final images. So you have to decide how much movement is acceptable to you and base your longer usable shutter speed on your own preference for how sharp those stars need to be. And again, big prints are much less forgiving than looking at an image on Facebook or Flickr or something like that. So once you have your shutter speed based on stellar movement, next pick your aperture. And the aperture is gonna be whatever you use, is going to be determined by how much depth of field you need. Do you have nothing in the foreground, only a distant horizon and sky? In that case you can get away with shooting wide open or nearly wide open if you've got a little bit of coma in your lens. But in general, expect to be using wide apertures between f 1.4 and maybe about 4. Again, like I said, we're gonna be pushing the limits of your gear with shutter speed due to stellar movement, with aperture, and with ISO. So once you've determined what aperture you're going to use based on depth of field, next set the ISO just high enough to give you a decent histogram. All right, and there you go. That's basically how we're gonna go about figuring out what settings to use for exposure. You're probably familiar with the term ETTR or expose to the right. And the reasoning for that is the way that our sensors record information is that the histogram, the way the histogram represents our image, is that it's tonally rich in the highlights and tonally poor in the shadows. So there are relatively few tones in the darkest part of the image. And because there are fewer tonal variations, there's more space in between each tone and that space is where the noise is able to come through in the image. So if you think about a noisy image, you never see noise in the highlights, you always see it in the shadows, and that's because the sensor is tonally poor in the shadow areas and most of the information is recorded in the highlights. And that's why we, historically, push our images over to the right into the highlight area. A couple of caveats here, in certain situations like photographing in full moon, if you push your histogram way over to the right, the image is gonna look kind of like strange daylight and it'll be hard to bring that exposure way back down to the point where it looks like a night image in post processing. So you have to be kind of judicious there in your use of exposure. You wanna get enough so that you don't have to lift the exposure very much in post, but not so much that it looks like a daytime image no matter what you're gonna do. Good exposures avoid the far left part of the histogram, or you know the left one quarter of the histogram where the sensor is tonally poor. All right, so this is an example of a histogram that's too much, it's more exposure than you need and it's gonna be difficult to make an image with this histogram look like a night shot, even if you bring it down in post. This one is just blown out. You've got, it's pushed all the way over against the right hand wall of the sensor, and you've got overexposure and clipping in your highlights. Once you clip in all three channels, red, green, and blue, there's no recovery, you've lost all detail, it's just pure white. So typically, the only areas that you wanna have pure white in your image are light sources in the scene or possibly the moon if you're doing a long landscape exposure shooting into the moon. So this exposure is too cold, it's under-exposed. You're gonna have very noisy shadows and it's not gonna be usable, especially if you need to lift it or push it up in post processing. Now there are some newer cameras that give you much more flexibility and allow you to increase the exposure in post without bringing in too much noise. And that's, it's still, it's good practice not to lift the exposure in post processing if you don't have to. Okay so this is an example of a typical Astro Landscape histogram with light painting. So it's kind of a left-centric histogram with a long skinny tail that reaches out to the right. And that tail is created, that's representing the highlights and that's usually representing the light painting that you may have added. So when reading histograms it's a good idea to think about both the x, or the horizontal, or the y, or the vertical axis. Of course, the horizontal axis, the left hand side is the shadows, the right hand side is the highlights. But what we don't really think about as much is the vertical axis, or the height of the histogram, which is really not that important to us. The height is representing what percentage, or how much of the image is that particular tonality. So if you've got a big high spike in the middle of the image it means you have lots of mid-tones. If you got a big high spike on the left it's lots of shadows. And a big high spike on the right, that's lots of highlight details. And every image, you know, that you could possibly create is some combination of shadows, mid tones, and highlights. And the histogram just describes those tonalities. So it's not really important if the height of the histogram is centered on the left, the center, or the right. What we wanna do is use the entire horizontal axis of the histogram, so even if it's just a skinny little tail going over into the highlights, that's okay. It just means that the image is more dark and more shadows and mid tones than it is highlights. Okay so here's kind of an ideal histogram. This is showing using pretty much the entire tonal range of this sensor. And, you know, you can't ask for much more than that. But in Astro Landscape conditions, you're rarely gonna have this kind of ideal histogram. It's most likely going to be a bigger spike on the left and a skinny little tail on the right. The key is, make sure that it's not pushed up against that left hand wall and clipping in the shadows. So the image for this histogram has lots of detail, plenty of detail and it's gonna make a good print. And here it is, even though this is an Astro Landscape image of the milky way, we have a really nice histogram and the reason for that is all of the ambient light in the scene. You look along at the horizon, there's a lot of brightness. And that's from the lights of the town in the distance. It's also a slightly foggy night, so that low level fog down at the ground is kind of reflecting a lot of the ambient existing artificial light. So again this is kind of an ideal or great histogram. It's gonna make a print with a lot of tonality. But it's not typically what you would expect from an Astro Landscape exposure. So starting points for exposures. In a new moon environment with absolutely no moonlight, nothing but starlight, no ambient artificial light, 20 seconds, f2, 6400. And that's kind of like just about the minimum of what you're gonna be able to get away with. With a crescent moon to shave a stop off of there and do 20 seconds, f2, 3200. A gibbous moon, take another stop off of there, maybe you can go to 2.8, get a little bit more depth of field, 20 seconds, 2.8, 3200. And of course you can vary the aperture and the ISO depending on your camera. If it has good performance at high ISO you can use that 32. If you've got a lot of noise at higher ISO then maybe you wanna keep it lower and use a wider aperture. If you have a lens that has a lot of comatic aberration, then you need to stop down a little bit more, use the higher ISO. The key is, know your gear. Learn its limitations, understand where the problems are and where your equipment shines. That is really key to determining exposure in Astro Landscape photography. Okay, quarter moon, 20 seconds, f 2.8, 1600. Full moon, 10 seconds, f4, or 10 minutes, up to 10 minutes f8, 100. All right say up to 10 minutes f8, because that's gonna be a kind of a full on right biased exposure, right biased histogram. And if you're gonna be light painting you're probably gonna wanna reduce the exposure down there. So, full moon situation at low ISO, somewhere between three and 10 minutes depending on your added light. We'll talk about light painting later. All right, so atmospheric conditions. Light pollution will significantly affect the appearance of your histogram and your exposures. So if you've got a lot of fog or dust in the air, that's gonna affect how much light reaches your camera from moonlight or starlight. And if there's also a lot of ambient illumination from distant or nearby towns that's also gonna affect your exposure and the appearance of your histogram. So you have to keep these things in mind when you are making an exposure to termination. Okay, full moon night photography. Long exposures, three to 10 minutes, f8 at or near the full moon. Moonlight is typically, in these situations, gonna be your primary light source and any light that you add will be secondary to that moonlight. All right so moonlight has generally moderate to low scene dynamic range, unless you are shooting right into the moon in a back light situation, then it's very dynamic or very contrast. So full moon and star trails. All right, so full moon night photography. Use high ISO testing to determine your focus, exposure, and composition. Well exposed images or right-biased histograms in moonlight can look like daylight. And it's good to be aware of that and consider that when you're making your exposure determination. And also consider, are you adding light or are you using just moonlight? If it's just moonlight, you probably wanna push that histogram little further to the right. If you are adding light painting you can do a little bit less exposure and then bring the histogram to the right with your added light or your light painting. Remember the highlights are gonna be your light painting. Light painting is a technique that adds drama and contrast and makes your scene more dynamic. And it can also help to bring out that feeling of a nighttime image. Remember I said a full right biased histogram in moonlight conditions can look like daylight. You wanna consider the ambient and added light ratios when you're making your exposure determination and when you're doing your light painting. So for example, if you are going to be adding light, then you might consider reducing the ambient exposure a little bit. One, two, or even three stops. So if you have lots of ambient exposure relative to your light painting, your light painting is gonna be fairly subtle. And if you reduce that ambient exposure your light painting is gonna stand out more and be more dramatic, more obvious.