bananas13 wrote in post #10438573
You have no idea how much that helps me =).
You asked about where I am in my curriculum, I have taken only one class so far (a film based intro class). I got a Rebel in October 2008 while I was majoring in something else. I loved the quality I got out of a DSLR and I found myself wanting to learn more and more... I decided this last year to change my major to photography. So yes, I am new to a lot of this and I can't wait to take more classes this Fall semester!
Ah, okay. That does explain some. I'm a little surprised they didn't cover some of this in a bit more detail in your intro class. Or perhaps they did and it wasn't until later that you discovered how cool this stuff is, at which time you started paying more attention.
I really appreciate your reply... the things I've already figured out are:
- Lower f-stop = wider aperture = lower depth of field = faster shutter (and vise-versa)
- Higher ISO = faster shutter = more image noise / Lower ISO = slower shutter = less noise
I know it's not much, but it's a start right?
It's definitely a start. You can go a long way just by understanding the basics. However, please note that it's also a little imprecise. Allow me to add a bit of precision, if I may...
- Lower f-stop = wider aperture -> shallower depth of field and more light coming in.
- Higher ISO = more sensitive sensor -> lower overall exposure required to yield same tonal values -> faster shutter and/or narrower aperture to retain same tonality. Also means more noise.
- Faster shutter = less time for light to come in -> smaller interval of time captured -> stopping motion. Also means less total light coming in.
The basic characteristics of a shot result from a combination of three things: shutter speed, aperture, and ISO. Each of those things has a separate set of implications for your shot.
Shutter speed controls how much time is captured. The longer its open, the more motion blur you'll see. Leave it open long enough and you'll start to see "camera shake" as well (which is the result of the fact that no person can hold anything absolutely steady -- we all move around involuntarily to some degree). The general rule is that you want your shutter speed to be at least 1/<focal length> in order to eliminate camera shake (assuming you're not using an image stabilized lens). At least, that's what works for full-frame cameras. For crop cameras, because the sensor captures a smaller part of the image circle, the general rule is 1/(1.5 * <focal length>
. And these days, the sensors are so densely packed that if you want to avoid visible camera shake when viewing the images at 100%, you really should use 1/(2 * <focal length>
. Your 40D isn't super-densely packed like the 7D is, so you can use the 1.5 version. An example would be, suppose you're using your 85mm lens. To avoid camera shake, you'll want your shutter speed to be no slower than 1/(1.5 * 85) = 1/125th of a second.
Aperture controls how much light is allowed through per unit time and also affects the apparent depth of field. The wider it is, the more light can come through and
the shallower the depth of field. The aperture value itself is the ratio of the focal length to the diameter of the aperture (the "f" in the aperture designation is "focal length", so f/1.4 means an aperture size of whatever the focal length is divided by 1.4). An f/1.0 lens literally has an aperture with the same diameter as the focal length of the lens. There exist some 50 f/1.0 lenses out there, and their apertures, when wide open, are 50mm wide!
ISO controls how sensitive the sensor is to light. The more sensitive you make it, the less light it takes to achieve the same tonality in the shot. However, and this is oversimplifying quite a bit, if you amplify the signal generated by the light hitting the sensor, you also amplify the noise in the sensor and other electronics. Since you're bumping the sensitivity in order to be able to use less light to achieve the same tonal results, this results in a lower signal to noise ratio (since the "signal", or amount of light, is lower but the amount of noise remains more or less constant), and as a result the noise winds up comprising a larger portion of the tonal values you see than it would at lower ISOs (which would force you to record more light). And since noise is generally random, the result is a "graininess" in the tones. The noise has always been there, of course, but at low ISOs it comprises a small enough portion of the total signal that it's more or less invisible. That graininess isn't all bad -- sometimes it's desirable to make the shot look "film-like", and using a higher ISO can help achieve that (but then, so can adding noise after the fact).
A "stop" is a power-of-two difference between two values. When we use it in conjunction with photography, we're generally talking about the effect on exposure (more precisely, the effect on the resulting tonal values in the image). If we say we're opening the aperture by two stops, what we really mean is that we're opening it enough to cause the resulting exposure to be increased by two stops, which results in everything in the image being brighter by two stops, all other things being equal.
A shutter speed that is half that of another is one stop faster. A shutter speed that is 1/4 that of another is two stops faster. 4 is, of course, 2*2, which is also 2 raised to the power of 2 (or, written differently, 2^2). A shutter speed that is 1/8 that of another is 3 stops faster. 3 is 2*2*2, which is another way of saying that it's 2 raised to the power of 3, a.k.a. 2^3. You can see where this is going -- take 2 and raise it to the number of stops of change you want, and the resulting number is the factor you have to apply...
ISO is directly proportional to the sensitivity of the sensor. So if you want to change the ISO by a stop, you double (or halve) the number. ISO 200 is twice as sensitive as ISO 100 and thus requires that you cut your exposure by a stop in order to achieve the same tonality in your image. That means cutting your shutter speed in half (making it twice as fast) or narrowing your aperture by a stop (which means multiplying the f/number by 1.4).
Apertures are a little different with respect to the numbers. Remember that we're interested in the values that result in a change in exposure (in stops). Well, remember that the aperture value is the focal length divided by the diameter of the aperture. We're controlling the amount of light coming through the aperture by adjusting its size. But the amount of light coming through isn't proportional to the diameter of the aperture, it's proportional to its area
. And the area is proportional to the square
of the diameter. Which means that the amount of light coming through is proportional to the square of the aperture diameter. So if you want to double the amount of light coming through, you have to increase the diameter not by a factor of two (that would quadruple
the amount of light coming through, because you'd take that factor of two and square it), but by a factor of the square root
And so, the "next stop" of aperture is the result of taking the previous aperture value and multiplying it by the square root of two (or 1.4 for short). Which means the aperture progression looks like this:
f/1.0, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/44
See how each successive number is gotten by multiplying the previous one by the square root of two?
An easier way to think of it is to remember that you double (or halve) the number every two
stops. If you can remember f/1.0 and f/1.4, you can derive the rest (the number after f/1.4 is f/1.0 times two, the number after that is f/1.4 times two, etc.).
I understand focus more thanks to your reply. I'm truly grateful for people like you who are willing to actually help instead of make judgmental remarks.
You're welcome. I just hope it helps!