photoing_blues.txt 091001 0100 ========================== ========================== =========================== Exposure:: Most tutorials on beginning photography seem to be mostly about exposure. There are whole books on choosing the appropriate shutter speed and aperture to get the photo that you want. There are also a myriad of techniques for deciding how much light you want to hit the sensor (or film) of your camera: expose to the right, grey card, incident light, the zone system. Lest this discussion become yet another text on basic exposure, I will assume that the reader has at least a passing familiarity with how to manually set a cameras exposure, understands the histogram and "blinkies" on a digital camera, and can at least do a web search on cocepts such as "expose to the right". On a digital camera there are three values that you can set to determine your exposure: ISO, shutter speed and aperture. In theory, you will want to balance ISO with noise, aperture/speed with depth of field, and shutter speed with motion blur. In practice, you will usually be running with your ISO at the highest value that you can get using just the in camera electronics, and your aperture wide open. You then increase the shutter speed to fastest value that will expose the sensor with enough light to capture a decent photo. Note that I said "enough light", not "correctly expose". You may often find it advantageous to trade off correct exposure for some other characteristic, such as subject blur. ---------------------- ISO: Film sensitivity used to be measured in ASA, but at some point, the initials ISO began to be used. When digital sensors came about, the same metric was used to measure the sensitivity of the sensor. The sunny 16 rule states that in direct sunlight, at f/16 the correct exposure will be (1/film speed) seconds. In short with ISO 100, it'll be 1/100 of a second, ISO 400 will be 1/400 and so forth. But what are ASA and ISO acronyms for? American Standards Association and International Standards Organization so someone sufficiently pedantic will note that refering to ISO is rather nonsensical. It is somewhat like referring to the speed limit as the DOT. Even so, as nonsensical as it may be, that is the term people tend to use, rather than sensor sensitivity, so that's the term that I'll use. Besides, it's easier to spell. http://en.wikipedia.org/wiki/Film_speed http://en.wikipedia.org/wiki/International_Organization_for_Standardization http://en.wikipedia.org/wiki/American_Standards_Association ---------------------- Noise: Without going into boring (to many) technical detail of why, the darker an area in a photo is, the more noise that will be visible. Likewise, the higher the ISO, the more noise that will be visible. There's a technique called "Expose to the right" that helps minimize noise. The premise is that you expose the brightest part of the image (that you don't want to lose detail in, so you may accept clipping if there is a light, or specular highlight in the image) at a level that is just short of clipping. That raises the "signal" in all parts of the image as high as possible above the "noise". The technique of exposing to the right does require that you be able to view, and read, the exposure histogram. Do be aware though, that the histogram is based on the camera's conversion of the data to JPEG, and not the raw data, so it is not a perfect representation of the raw data. In post processing, you can then adjust the final exposure for the value that you want. This is one of the reasons to shoot RAW rather than JPEG. Note that you may not be have enough light to expose to the right, and still have sufficient shutter speed to sufficiently stop the motion. ---------------------- Shooting subjects under colored lights: Many venues enhance the mood by the use of colored lights. They may have gels over spots, or they may use colored rope lights. The first thing to note is effectively impossible to get good color balance in this situation. Another thing to note is that your digital sensor is effectively three interwoven sensors, one for red, one for green and one for blue light. Some cameras only have a "white light" histogram, which averages the three, while others have both a "white" and three separate histograms for each color channel. When shooting saturated colors, such as flowers, or people (dancers and musicians) under colored lights, it's very possible for the white histogram to show that the brightest part of the image is well below clipping, while in fact one color is past the point of clipping, and the other two are woefully underexposed. ---------------------- Trading dynamic range for shutter speed: Most DSLRs have 12 bits of dynamic range. That means that any sensor site can record a value between 0 and 4095. As an aside, the larger sensor sites on some full frame cameras allow them to have 14 bits of DR, which gives them 2 stops more range, or values from 0 to 16383). On top of that, at high ISOs, there's going to be a certain amount of noise, for the sake of argument, call it 4 bits, so you really only have the ten bits from 16 to 4095, that you're really able to use. If, for example, the nominal exposure is 1/5 second with ISO maxed and your aperture wide open, that makes it very tough to get clear shots without the dancers blurred, even if you manage to hold the camera perfectly steady for 1/5 second. If you are willing to trade dynamic range for shutter speed, you could under expose by a stop or two. Underexposing by a stop would mean that, in this camera, you would have the 9 bits between 16 and 2047, which when you "fix the exposure" in post processing would effectively double the noise so that the usable image was only the values of 32 to 4095. But even so, a noisy image where you can see the dancers may be better than a less noisy image of a couple of blurs. Different cameras react to underexposure in different ways. With my K100, it works better to underexpose at ISO 800, than to "correctly" expose at ISO 3200. With the K20, it definitely works better to shoot at ISO 3200. This is a case where you need to experiment with your camera to see what works. ---------------------- Shoot the subject's best lit side: It sounds obvious to say "Shoot subjects that are in the light", but the human eye works surprisingly well in low light situations, and its an easy detail to forget, especially when it seems that the best angle, or where you happen to be standing, is not looking at the lit side of the dancers. If you can, stand with your back to the lights, or at least facing the same direction as the lights. This has two benefits, it means that you'll almost certainly be photographing the subject's lit side. It also means that you're less likely to be shooting into a light, which can cause areas of the photo to clip, or worse, cause glare. -------------- ---------------------- Manual exposure:: A lot of the same problems with Autofocus, also apply to auto exposure. If there is a bright light in the frame, the camera will very likely adjust its exposure so that the light doesn't overexpose. Unfortunately, this tends to happen when you don't care if the light (which you may crop out anyways) is overexposed, and you do care if the dancer is underexposed. On Pentax, in manual exposure mode (or hypermanual as they call it) you can press "the green button" and the camera will automatically set the exposure to what it thinks is correct. Take a couple of test images, and look at the histogram, and the blinkies, and adjust your exposure accordingly. Unless you move your position, or they change the lighting, chances are that the correct exposure won't change much. At the very least, you should get a bunch of shots that are all fairly correctly exposed, rather than letting the camera choose and getting some that are right on, and some that are way off. ---------------------- Chimping can help, but it's not perfect: There are a some photographers that seem to take a picture, then spend the next minute looking at it on the display, going "Ooh, Ooh, Ooh" like a chimpanzee. This is why looking at the playback photos on your camera is called "chimping". One of the beauties of digital photography is the instant feedback, especially in difficult lighting situations. You can take a test shot, look at the histograms, or the blinkies, and adjust your exposure to something that should be pretty close to perfect. If you're worried about shutter speed, focus, or noise, you can zoom in, and see how blurry or sharp the picture is. We've all taken shots that look just fine on the quick review, but are out of focus, or blurred when we look at them on the computer monitor. This review shot can tell you a lot about the photo, but bear in mind, that it isn't based on the raw data, it's based on the camera's conversion of that shot to a fairly low resolution jpeg. You won't really know how it turned out until you look at it on your computer monitor. ---------------------- Shoot for the raw data, not for the image on the display: There are a lot of reasons that the display on your camera can lie to you. Not the least of which is that pictures on it will look darker if you look at them in daylight, and much brighter if you look at them in a dark room. The histograms, on the other hand, only fib a little bit. They will say the same thing whether you look at the picture in bright light, or pitch black. You may want a section of the photo to be pitch black in the final copy. If you set the exposure at 1/100 Sec so that the bright areas are acceptable and the dark area is black, when you're done, the black area will be black, except for the noise. If, instead, you set the exposure for 1/50, or depending on the highlights, 1/25 Sec, so that the bright areas are just shy of clipping (we're back to Expose to the Right again), if you print it straight, the bright areas will be brighter than you like, and the dark will have about the same amount of noise, as well as showing details that you don't want. However, if you then process the final exposure so that the dark areas are completely black, then the noise in the dark areas will be reduced by a factor of two, or four, and the light areas will be brought down to what you want them to be. You can always throw away data in post processing, but you can never regain data you lost at the time of exposure. ---------------------- Color balance:: Blues dancing is usually done in dimly lit rooms. The dim light is generally achieved by dimming incandescent (tungsten) lights, though it is sometimes achieved by the use of colored rope lights. In any case, the nominal source of the light, is usually a tungsten filament. You can usually get fairly close to as good as you are going to get by setting your camera's white balance to "tungsten". ---------------------- Setting the color balance in the camera: I am constantly talking about how you should shoot for the raw data, and correct everything in post processing. If that is the case, then setting the color balance at the time you shoot should not make any difference. On the other hand, it doesn't hurt. It can make post processing easier, if the pictures start out closer to correct, and I've read about some people doing experiments with some cameras that do alter the data stored in the raw file based on the color correction. ---------------------- Some cameras allow you to set a custom white balance by taking a picture of something color neutral and telling the camera that that is to be used as the reference for "white". When you do this, be sure to do it in the light that the dancers will be in when you photgraph them. If you're standing in a doorway next to a well lit hallway, but the dancers you're photographing are in the corner under a 10 Watt light bulb, take your greycard over to where they are standing (if you can), and set your white balance with the grey card in that light. You should also, if you get the chance, take a photo of the grey card, so that you can fine tune the white balance in post processing. For that matter, for the best results, you should go around the room, and photograph the grey card in every lighting situation presented, because the lighting is never even, and different incandescent bulbs, when turned way down, will be running at different color temperatures. That's what you should do, but that's not what you will do. Chances are, you'll set your color balance to tungsten, shoot with your camera set like that, and when you're post processing you'll look for color neutral objects in the photos that you can use to set your white balance. ---------------------- If it's white balance, why use a grey card rather than a white one? One reason is that many grey cards are specially printed to not only be 18% grey, but to be color neutral. Things that are meant to look white, might have extra blue in them so that they look white, even under yellowish light, or they may have yellowed and discolored with age. Your eyes are so good at correcting for the different color balances you get between sunlight, shade, electric lights, candle light, that you may not notice the subtle discoloration. Until you're trying to process your photographs, that is. Another reason is that something white will reflect a lot of light, often more than anything else in the frame. That means that one, or more, colors are at best very close to clipping at a perfect exposure, and are very likely to exceed the dynamic range of the camera, so rather than the channels reading R: 4113 G: 4098 B: 3897 the data will be stored as: R: 4095 G: 4095 B: 3987 so the red channel will be way off, the green a little bit off, and the blue will be nominal, tricking the post processing software into making the picture a lot bluer, and a little bit greener than it should be. ---------------------- Leaving the grey card at home: Greycards do a great job. The problem is that the 8.5x11" greycards are big and awkward to carry. There are credit card sized ones that are much easier to carry, and even easier, it seems, to leave them home, in the car, or someplace equally inconvenient. One trick that I use is that I have a watch where part of the face is nearly a perfect neutral grey. As such, as long as I'm wearing that watch, I've got a handy white balance reference, I just need to get a photo with it in the frame.