When I am shooting pictures, I will often get asked by aspiring photographers about what settings they should be using. While there are some basic rules in photography, it is as much an art as a science and as such, you should set up your camera to achieve the results you want. Photography is all about trade-offs.
Good photographs are all well-exposed, focused accurately and framed well.
First, some definitions. It helps when we start with a common understanding.
Aperture: The size of the hole in your lens that determines, in part, how much light is going to hit your sensor or film (is anyone still using it?). While apertures are commonly referred to as a number like 2.8 or 5.6, it is really a fraction and really are 1/2.8 or 1/5.6. I think of it this way. 1/2.8 is a bigger number than 1/5.6 (roughly speaking 1/2 of a pizza is bigger than 1/5 of the pizza.) A 2.8 lens is said to be faster than a 5.6 lens as it can let in more light and can get you a faster shutter speed than the same camera with a 5.6 lens on it. Aperture (in conjunction with lens length) will determine your Depth of Field of Focus. The larger the aperture (2.8 is larger than 5.6), the smaller the amount of the image that will be in focus. See Depth of Field of Focus below.
ISO: How sensitive the sensor or film is to light. An ISO of 200 is less sensitive to light than an ISO of 1600. The trade off here is the higher the ISO, the more grain or digital noise the image will contain. Most good DSLRs have a usable ISO range of about 100 to 6400. Beyone 6400 gets a bit dicey...too much grain/noise for most folks.
Shutter speed: Fairly self-explanatory. This is how fast the shutter opens and closes, allowing light to hit the sensor while it is open. The faster the shutter speed, the less light hits the sensor. Slower shutter speeds let in more light. Fast shutters speed can stop or “freeze” the action. Slow shutter speeds may result in blurry images, either from camera shake or movement in the subject. Generally, to avoid camera shake blurriness, your shutter speed should be at least as fast as 1/focal length. Thus, if you are shooting with a 300mm lens on a Canon 50d, your shutter speed should be at least about 1/500 or so. Although you are only using a 300mm lens, the smaller APS-C sensor on the 50d adds a magnification factor to the lens of about 1.6, so that the 300mm lens has the effective reach of about 480mm. Therefore, your shutter speed should be about 1/500 to compensate for camera movement. Unless of course, it is on a tripod.
To avoid blurriness caused by subject movement, you need to set a shutter speed fast enough to freeze the subject. Generally, this will depend on how fast the subject is moving. A professional soccer player usually runs a lot faster than a elementary school level soccer player, and therefore requires a faster shutter speed. Some times photographers will purposely shoot with slow shutter speeds to get an artistic motion blur. This can be overdone. First, learn how to freeze the action, then you can learn how to get artistic with motion blur.
How to get the proper exposure
The first goal in photography is to get a properly exposed image or at a minimum, to properly expose the part of the image that you want. An over-exposed image can be as bad as an under-exposed image as both result in the loss of data. Underexposed images loose data in the shadows and over-exposed images lose detail in the highlights. The bad thing about losing data is that it can not be recovered if it is lost. If you over-exposed your image by just a little bit, but did "blow" the highlights, then you can most likely recover them in your digital darkroom. If you have "blown" your highlights, then the data is gone and there is nothing to recover.
To get the exposure correct, you can adjust three variables; ISO, Shutter Speed (SS) and Aperture (AV). These 3 variables work in concert. If you increase your shutter speed, you will need to either adjust your ISO, your AV or both your ISO and AV to compensate. If you close down your aperture from 2.8 to 5.6, you are letting less light hit the sensor, so you must make adjustments to your ISO or SS, or some combination of both ISO and SS, to let more light hit the sensor. So, you could decrease your shutter speed or increase your ISO.
Let's take, for example, shooting your child's soccer game, out doors, on a sunny day. We'll start with some nice conditions that will make your life easier. Remember, we have 3 settings to adjust. First, let's start with ISO. Since it is a nice sunny day, we can start with a fairly low ISO, of say 400.
Next, we want to freeze the action. We do this by choosing an appropriately fast shutter speed. I recommend at least 1/500 of second or faster to stop fast action movement in sports. Generally, I'd like to be even faster, say 1/1000 or so. As the athletes get bigger, stronger and faster, you will generally need a faster shutter speed to "freeze" the action. A little league pitcher's fast ball might be "frozen" at 1/1000 of a second, but a high school pitcher might require a shutter speed of 1/2500 or faster to freeze the ball in mid-flight.
Lastly, we can adjust the aperture. The aperture controls two things. One, it controls how much light can pass through the lens and secondly, it determines Depth of Field of Focus (DOF). Actually, the aperture, in conjunction with the length of the lens and the distance to the subject, will determine how much of the image is in focus (I will use the DOF abbreviation).
For example, let's say we have a really nice Canon 300 f4 lens on our 50d camera. Using a DOF calculator, easily found online, (like this one: http://www.dofmaster.com/dofjs.html), one can calculate the DOF for any given distance. So, if I am shooting my daughter playing soccer with my Canon 50d and 300 f4 lens and I know she is about 50 feet away, I can use the calculator and find that the DOF is about 1.25 feet. That is to say that if I focus directly on her, approximately .6 feet in front of her and .6 feet behind her will be in focus and everything beyond that in both directions will get progressivley less sharp or more out of focus.
This is a fairly narrow DOF in that if your focus is not spot on, your child may not be in focus. So, you can close down your aperture a bit, to say 5.6 and now your DOF goes from 1.2 to about 1.8 feet, thus a little more margin for error. The closer you get to your subject, the smaller the DOF will get and conversely, the farther you get, the bigger the DOF you will get.
Many photographers will use DOF to their advantage. When taking a picture of your child playing soccer, you really only want them and the ball in focus. You probably don't want the Porta-Potty and parking lot in the background in focus too. So, in this case, the DOF is helping you blur out all of the bad stuff.
Now, let's move on to a more challenging situation. Shooting your son's basketball game in a poorly lit elementary school gym. In this example, we will use the Canon 50d and an 85 1.8 lens. Now, since I know I want to freeze the action, I select a shutter speed of 1/500. Now, I only have to set the ISO and Aperture to get the correct exposure. This is really pretty simple. In a poorly lit gym, I will generally set the ISO to 3200 or in some cases 6400, and then using the camera’s meter, determine what aperture I can use. Sometimes it is so dark, I’ll be at ISO 6400, 1/500 and an aperture of 1.8. Since I’ve maxed out my ISO and my lenses aperture, the only variable I can change is the shutter speed. I can continue to lower that until I get a properly exposed image. So, I may have to drop the SS from 1/500 to 1/400. I’d rather have a bit of motion blur than an underexposed image. This is even more important at high ISOs like 6400 because an underexposed image at ISO 6400 looks really bad but a properly exposed image at 6400 can still look really nice.
In the above example, I am shooting at an aperture of 1.8. At close distances, my 85 1.8 on my 50d will yield a really small DOF. Maybe just a few inches. This means that if I focused on the basketball player’s waist, his face is more than likely going to be out of focus. So, in this example, I may want to lower the shutter speed from 1/500 to 1/400 and close the aperture from 1.8 to 2.2 or something like that to get more of the player in focus.
This is what I was talking about when I said photography is all about trade-offs. In the above example, I could decrease the shutter speed and risk some motion blur, or keep the aperture wide open at 1.8 but have less of the player in focus.
No comments:
Post a Comment