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A Field Guide To Time-Varying Light Sources

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Photographing time-varying light sources is extremely rewarding. Not only can you capture images which allow you to analyze what a light is doing, but you can also produce amazing, artistic pictures too.

These videos demonstrate how to photograph time-varying light sources for documentary and artistic purposes, and how to optimize and enhance your photographs on a computer. The Field Guide has a chapter on photography, which provides additional details.


Photographing time-varying light sources

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Photography video transcript

Hello, welcome to this video about photographing time-varying light sources. I’m Dan Bennett, and this is a tutorial video that should get you started on the very rewarding area of photographing time-varying light sources. You can go to my web-site, which is to see more information, or indeed you can purchase the book – gotta plug the book! – which is “A Field Guide To Time-Varying Light Sources”, available on Amazon and elsewhere.

When it comes to photographing time-varying lights, there are two real approaches that you can take. One of them is what I call a documentary approach, which is where you’re trying to capture an image that allows you to understand what the light source is doing. That’s a documentary image. The other one, which some of you may find even more rewarding, is art, where you want to make an image that is eye-catching, that is evocative and that is an image that’s really hard to create by any other means; but you’ll find, from this video, that using a camera in what is probably an unorthodox way will actually give you great results.

Let’s start with creating a documentary image. So for a documentary image you want something that is simple, straight-forward, but clearly shows what the light was doing during the time that you took the photograph. Remember, these are lights which are changing their color, their brightness or their shape very, very rapidly; so rapidly that the human eye can’t see it, but a camera can; as indeed can the human eye if you take certain steps, and you can look at some of my other videos to see how you can observe these lights with the naked eye. But for this video, we’re talking about cameras.

I recommend that the first thing you should do when taking a documentary picture is to take what I call a reference image. Now a reference image is simply a conventional photograph of the scene that you’re photographing, so that when you look at the time-varying light source swept images that you’re going to be taking, you can look at the reference image and say, “Ah. I can see what each of the things in the scene are.” It’s another way of helping you communicate: if you have a reference image, and then you show your very best time-varying swept image, side-by-side, then people can get a better idea in their heads of what it is you photographed.

So, a reference image – how do you take that? Well, to take a reference image you simply operate the camera in the normal mode; and the easiest way to do that is to set it on a mode like “P” or “auto”, which means that the camera will adjust the settings accordingly. Now remember, you’re taking these pictures at night, so it’ll be fairly dark; so you may want to increase the ISO, which is the setting that allows the sensor to be a little more sensitive; so you may want a slightly higher ISO, say 800 or a thousand, somewhere in that range. Don’t push it too high or the image will become noisy. So set it to “P”, set it to, say, ISO of about 1000, and then you’re ready to take a reference image. But – here is a specific trick, and that is, you’re not trying to take a picture of the night-time scene, you’re trying to take a picture of the lights in the night-time scene – so how do you do that? The answer is, you under-expose. And the way to under-expose is to use exposure compensation, and a digital SLR like this will allow you to do that very easily, to adjust the exposure compensation down, by maybe one stop, a stop and a half or even two stops. Take some pictures, and you should get a good reference scene. You may find some camera shake because of the length of the exposure, so a tripod may be useful or an image stabilization lens.

OK, so that’s the reference picture taken and recorded in your camera. Now the fun starts. Here’s my recommended method for taking a photograph of a time-varying light source to give a documentary type of image. Set the camera to “manual” mode. Manual mode is where the camera will no longer make any control inside the camera over the two main components of exposure, which are: shutter speed, or exposure time, and aperture. You’re going to control those manually. Secondly, set the exposure time to be one tenth of a second. I have found that one tenth of a second is ideal for documentary pictures. It’s also really nice because with one tenth of a second it’s very easy to calculate how fast a time-varying light source is actually oscillating through its cycle, because you can count the number of pulses or number of colors in the trace, to give you a calculation. You know, if there are twelve pulses in the trace, then at a tenth of a second that means that the thing is flashing one hundred and twenty times a second. Very easy calculation. I have also found that a tenth of a second is great because the trace doesn’t get too long and complex. We’ll save that for later. Stay tuned on the video.

Now that we have the camera set to a tenth of a second, what do you set the aperture to? The answer is, you don’t really know, until you have taken some shots and then you can change the aperture to get the optimum results. So now we come to the big question: how do you actually take a photograph of a time-varying light source to give the kind of results that you see on my web-site or in the book? The answer is, you move the camera while you are taking the shot. Definitely no tripod needed in this case; you’ll be hand-holding, and you need to develop a certain action.

Imagine that I have a time-varying light source over there. I don’t; but for the purpose of this video, imagine that I do. What I will do, is I will focus on my light source – and with a camera like this you focus by depressing the button half-way; and then I look above the light source and shoot while bringing the camera down. Focus – shoot. Focus – shoot. If you get tired of focusing every time, then you can focus once and then switch the lens to manual focus, in which case you can just do shoot – shoot – shoot – shoot.

In those cases what I was doing was sweeping the camera downwards, across the light source, while exposing for a tenth of a second. You’ll find that it’s well worth doing multiple photographs like that; because it’s hand-held, not every photograph will come out the same, and by taking multiple photographs you will get the action, you will be able to see whether you’re getting the trace on the frame or not, and you’ll be able to get the very best picture. Furthermore, if you take, say, 15 or 20 photographs of a light source, you look at it on your computer, you’ll find one or two shots which are really the best, and those are the ones that you can work with. So that’s how to take a documentary picture: you set the exposure time to a tenth of a second, and you move the camera while taking the exposure.

One thing to note: there are some light sources which are vertical. If you had for example a neon tube that runs down the side of a building, then sweeping vertically is not going to help you, because you’re going to wipe the image of that tube on top of itself all the way along. So the answer is simple: you’re going to sweep horizontally while shooting. So it’s up to you to judge which direction to sweep to give you the best results for the light source that you have in front of you. That’s how to take documentary pictures

The other area of time-varying light source photography, which is extremely rewarding is art. The technique for taking art pictures is a little more interesting – and a little more fun – than it is for documentary pictures. Remember, for documentary pictures you’re trying to get an image that allows you to interpret what the light source is doing, so that you can understand its technology, understand its behavior. That’s not the same for an art picture. For an art picture, you’re trying to create a “wow” image, something like some of the images that you can see here on the video right now.

With the camera still in manual mode, I recommend that you set the exposure time to something longer than a tenth of a second. It could be a quarter of a second, all the way up to a whole second if you want. I might have, say, six tenths of a second, or half a second; something in that range. That gives you time to move the camera in a more creative way when you’re taking the shot than you could in a tenth of a second. It does mean that your image is going to be more, shall we say, congested; there’s going to be more data, more light in the image. But that’s great – that’s what you’re trying to achieve. This is art – a visual, eye-popping image, rather than a simple documentary image with very little distraction.

So, set the camera for a bit longer. In this case I’m going to set it to, let’s say, eight tenths of a second. And then, let’s say I have the light source there, the idea is instead of doing a simple sweep of the camera, I’m going to do a flourish; and by “flourish”, I mean move the camera interestingly while taking the shot. Let me aim at the video camera here and show you what I mean. Let’s say I want a kind of wavy look. I would start on one side of the light source, do a wave across to the other side. And that gives you images that look like this.

So an art picture really is done by doing a flourish which creates the shape that you want. And there are many shapes you can use. You do an up-and-down zig-zag, like that. You can do something wild and crazy if you want. Try it. See what you get, and it could be an eye-popping image. Here’s an example of exactly that.

Of course, aside from doing a flourish or some other complex motion of the camera during the shot, there are other things that you can do which can also add real flair to an art image. One of them is to zoom the camera, and of course that requires a zoom lens. You can’t do it with a fixed focal length prime; but a zoom lens allows you to change the focal length while you’re taking the picture. So what you would do in this case is you would set the exposure to maybe half a second. Let me do that… half a second. And zoom the camera while taking the shot. Let me do that again… zoom. And the result of that might be an image that looks something like this.

Another thing that you can do while taking the image is to rotate the camera. Why not? So let’s set up this shot at kind of 45° this way and rotate the camera while taking the shot. Let me do that again. That gives an image that might look like this.

It turns out that it’s possible to zoom the lens and rotate the camera at the same time. Now, that sounds like it would be really complicated, like patting your head and rubbing your tummy at the same time, and chewing gum, you know; but it’s not. It’s actually really, really simple. All you need to do is to take hold of the zoom ring and rotate the camera while holding the zoom ring still. The result is that your camera rotates while you’re zooming, and so you would get a shot that might look like this.

And then finally – and this might be the walking while chewing gum thing – you can zoom, rotate and do a flourish all at the same time. I don’t think I’ll demonstrate that right now; I might give myself an injury. But try it, see what you get, and you’re going to get some totally amazing images.

Now of course, you might be thinking, “Well it’s all very well for you, Dan, you’ve got this fancy schmancy digital SLR, and not everybody has that. Is there any hope for me? Can I take time-varying light source images even when I don’t have a camera that’s that sophisticated?” The answer is, yes, you may, but the results will not be quite as good quality. Here’s a cellphone. This one is an LG G2 cellphone, and it has a camera on it, and I have taken time-varying light source pictures with this phone; and they are OK, they’re not stellar but they’re OK. How would you do it? Well, I would do as follows. I would start the camera app and then I would set the ISO and the exposure down, a lot. And the reason for that is that the camera is typically one that will increase the brightness, increase the gain to make an image stand out. But if you’re taking pictures of lights, you don’t want that to happen. You want the gain to be down a long way. Now thankfully my camera app here allows me to adjust the ISO, which is the sensitivity, all the way down in the settings. In addition to that I can turn the brightness of the image down in the settings. And that gives me a good platform for taking time-varying light source pictures.

How do you take them? Well, it’s the same kind of technique, but you don’t really have control over aperture and shutter speed; you’ve just got to kind of guess. So you put your thumb over the button, and then you sweep like this. And you hope that it takes pictures while you’re sweeping the cellphone. It does work; I’ve tried it, and here’s an example of a picture that resulted.

Well, that just about wraps up the video on techniques for taking photographs of time-varying light sources. I hope it was useful; and there is more information on techniques and details in the book, so go ahead and be sure you buy that, and see what else there is to read on the topic.

I do have another video on my web-site, which is on the topic of post-production. In other words, now that you’ve got those photographs out of your camera, onto your computer, what can you do with them to make them really, really amazing? Go and check that video out, and thanks for watching.

end faq


Optimizing photographs of time-varying lights

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Optimization video transcript

Hello. Welcome to this video from I’m Dan Bennett. In this video we’re going to take a look at how you can optimize and enhance your images, using an application like Adobe Photoshop, which is what I’m using here. There are other applications that will do as good a job, for example, The GIMP, and Paint.Net, both of which are free and downloadable from the internet.

This image that you see here is very cool. It’s actually taken from a plasma TV. I used a computer to display an image to the screen, which was black with a white bar across the middle part of it; and to the human eye the white bar looks like it’s constantly white. But as you can see, it’s very clear, by taking a swept photograph of it, that the white bar is anything but constantly white. We have some white, and some yellow, and some dark green, and even some blue; and this shows that the plasma TV’s time-varying behavior is actually very complex, and only revealed when you do vision jiggling or sweep a camera like this: shoot, shoot, shoot, shoot.

So the question is, is this image as good as it can be or is there room for improvement? Well, there’s a very useful tool in Photoshop and similar applications which will help us figure that out. One of the key areas that we’re interested in is dynamic range. In other words, we’d like the black part of the image to be truly black, and the white part of the image to be at or nearly truly white. Is it, in this case? It looks like it might be, but let’s verify it. Photoshop and similar applications have a tool called the histogram. So I’m going to the Window menu, I click on “Histogram”, and here is the histogram.

For photography, the histogram is a very, very useful tool. Photographers use it all the time for judging whether the photograph they just took was captured over-exposed or under-exposed or with a color cast, or so on. In this case, what it’s showing us is the distribution of the brightnesses of the pixels in this image. The left-hand end corresponds to black; the right-hand end corresponds to white. You’ll notice here we have a very strong peak at the left-hand end. Well, that’s because we have a lot of black pixels in this image: the whole background here is black. This mountain range in the middle corresponds to the rectangular area here of pixels that are lighter than black, and you can see that there’s several different brightnesses represented here, as you can see from the way the rectangle varies in brightness.

But there’s one thing that’s really apparent from this histogram, and that is that the brightest pixel is right here. In other words, there is nothing brighter in this image than two thirds of the available brightness for this image. Everything out here is kind of wasted space, it’s wasted dynamic range headroom. So one thing we can do, is we can increase the brightness of this image without destroying detail, quite effectively; that’s what the histogram tells us.

I’m going to dismiss the histogram, and I’m going to use a tool called the “levels” tool, and on Photoshop you access that by hitting Control-L on Windows, or Command-L on a Mac. This is the levels tool, and you can see our histogram is reproduced here once again, very similar to what we saw before. But I’m going to make use of these little slider widgets that are underneath the histogram, because that lets me control how the histogram gets mapped to the output image. In particular, because we have a big empty area over here, I’m going to grab this right-hand slider and pull it towards the left.

As I do so, look what happens to the image: you see how it’s getting brighter and stronger? Take it back out again; and back in again. And you can see that by doing this I am strengthening the punch, the visual impact, of the image. So what I have done here is I have now mapped the lightest pixels in the image to white, a true white that uses the full dynamic range of the image; and will print better or will look more engaging on a display. Now most tools in Photoshop have a preview button, so if I uncheck “preview”, we get to see the image as we started with it, before applying any adjustments in the levels tool. If I click “preview”, then we can see where we ended up; and you can see that I have retained all the detail, but I have made the image much stronger and much punchier.

Here’s another image. In this case this is a dot-matrix LED sign that is by the roadside. It’s outdoors, it’s at night, in a city and there are street lights that are illuminating the scene. And the result of that is that the whole background has this orange wash to it; it’s not particularly dark. So what we’re going to do here is to use the levels tool is to enhance the darkness of the image to make the red LEDs really stand out. So, once again, Control-L or Command-L to bring up the levels tool. Here’s our histogram. We can see that there’s this very large area of mostly dark pixels, and that corresponds to what we’re seeing here in the background. So the first thing I’m going to do is to reach for this left-hand slider and pull it towards the right, and you can see immediately that by pulling to about here, I can make most of the background disappear down to black while preserving the bright traces of the LEDs in the middle of the screen. So that, at one stroke, has completely enhanced the picture. If I toggle the preview button, this is what we started with, and this is where we took it to, and it was very, very easy to do. So you’ll find a lot of images, especially in city scenes at night, may benefit from pulling the black level up into the histogram a little way.

So here I’ve loaded another image into Photoshop. These four red lines actually come from four red beacons at the end of the runway of an airfield, and the small dark lines that you see running across these traces actually come from the wires in a diamond-pattern wire fence that runs around the edge of the airfield. One thing that you might want to do with an image like this is to crop it, because sometimes when you sweep the camera, the composition of the image that you get is not necessarily ideal; and as long as your camera has a fairly good number of megapixels, then cropping should not destroy the quality of the image by any significant amount.

So to improve the impact of this image, I’m going to crop it. So in Photoshop, I click on the “crop” tool, and here are the crop boundaries. Now just for consistency with other images, I’m going to crop at the original ratio. I could crop “unconstrained”, which would mean that I can change it to any shape I like, but here I’m going to keep the ratio constant. So I’m going to grab the corner here, and resize it there and then pull the image down into the middle until I get the composition I want, which is probably something like that. Maybe make it a little tighter… And then I click the check-mark at the top to say, “OK, we’re done.” And if I now fill the screen with this image, my new image here has, as you can see, a much better composition where the traces of interest fill the image much more satisfactorily than before.

The final thing I want to show you in Photoshop is a cool trick for generating some very unusual images that are artistic and captivating. This image here is a scene from Las Vegas; you can see the word “VEGAS” down here – that’s kind of a clue; and all I did was I swept the camera downwards while shooting the photograph. There are a lot of time-varying light sources in this image, especially apparent in the top left, this street light here, neon here, and this blue stripy thing down here. What I’m going to do is I’m going to invert this image; or put it another way, I’m going to turn it into a negative. In Photoshop, the easiest way to do it is to press Control-i for “invert”, or if you’re on a Mac, it would be Command-i. And in an instant we have a rather different-looking image. It’s got some subtle pink in it here, the brightest things end up black, the dark background ends up white, and all the colors are inverted.

Well, it’s pretty good as it stands, and I like image, but what I’m going to do is strengthen it a bit, give it a bit more visual body. So what I’m going to do, is I’m going to pull up the levels tool as before, but to optimize the tonal balance of this image I’m going to pull the middle slider to the right a bit. What it does is it strengthens the darkness and the saturation of the colored elements in the scene. If you do it too far, everything gets rather dark and the detail gets lost and you end up with some noise. If you pull towards the left, everything gets washed out, colors are lost and it becomes a much more pastel image. So pull it to the right, to something like that, and that to me looks good. If I remove “preview”, that’s what we started with; this is where we ended up. Before; after. Click “OK”, and this image is now the way I like it.

So hopefully this video has given you some ideas of things you can do to your photographs after you’ve captured them and you’ve brought them to your computer. Thanks for watching.

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