Originally posted by NicoleAu Argh, I get so confused reading about iso, aperture and shutter speeds
I just can't seem to get a handle on it at all and it all makes me feel rather stupid. I've done insane amounts of googling and reading on the subject but it just doesn't seem to want to sink in. I fear I shall be on auto for the rest of my life
Aperture is the opening of the diaphragm blades within your lens. It may help you to think of it this way.. If you shine a light into your eye, your pupils become very small so what you are looking at isn't so bright. When it gets darker, your pupils get larger so you can take in more light. Aperture works basically the same way except that you are controlling how much light you let into the camera for your photo. The smaller that opening, the more of your photo will be in reasonable focus (depth of field, DOF). The higher the aperture number (f11, f22, etc) the smaller the opening in the diaphragm. The smaller the number (f1.4, f2.8, etc) the larger the opening. Larger openings will result in less depth of field.
Shutter speed determines how long you let the light pass through the lens and aperture to the sensor. If you are taking a picture of a person running down the street for instance. A faster shutter speed will tend to freeze the action and allow you to get a sharper photo of the person. A slower speed (if you are not following the person with the camera) will result in a blurred runner in your photo. If your shutter speed is slow enough, you may not even See the runner in your photo.
ISO is the camera's sensitivity to light. The higher, the more sensitive. For most daytime conditions, you can set your ISO at 100 and leave it there. When to raise it depends on what you are taking a photo of and the amount of light available. Sunlight, ISO 100. A single Candle, you may want to go to ISO 1600 in order to get enough sensitivity to record something.
The way these tie together is this..
With a Fixed ISO (not automatic) you will have a range of shutter speeds and apertures (in combo) that are useful. Which one you choose to change, will depend on what you want to end up with. If you want a lot of the scene in focus, you would use a small aperture opening (higher number) and a lower shutter speed. If your intent is to freeze action, you may want to go to a more wide aperture and faster shutter speed. The time to change the ISO is when you A: Cannot open the aperture enough to get a clean shot (because the shutter speed is too slow) or B: you cannot slow the shutter enough for your chosen aperture.
ISO, Aperture, and Shutter speed can all be thought of as Stops or Steps. ISO 100 for example is step 1. A full stop faster is ISO 200. A full stop faster again is ISO 400 and so on. Where this is important to you is this. If your running person (from above) was moving too fast to get a properly exposed photo (aperture is wide open and you cannot use a fast enough shutter speed), you could raise the ISO in order to allow you to use a faster shutter speed.
Aperture steps are f1.4, f2, f2.8, f4, f5.6, f8, f11, f16, f22. Those are Full stops. Our cameras can change them in either 1/3 or 1/2 steps. Changing the aperture one stop has exactly the same effect on exposure as changing the ISO one step. That is, if you are at ISO 200 and change to ISO 100, as far as exposure is concerned, it is the same as going from f8 to f11 (with ISO and shutter speed remaining the same).
Shutter speed full steps are 1 sec, 1/2 sec, 1/4 sec, 1/8 sec, etc up to 1/8000 sec (on the K7). Below 1 sec would be 2 sec, 4, sec, 8 sec down to 30 seconds (on our pentax cameras). Changing shutter speed, and keeping aperture and ISO constant, has exactly the same effect on Exposure. That is, ISO 100, f8, 1/250 sec for example, if you want to change the exposure one step, you would go to either 1/125 or 1/500 depending on what you are dealing with.
I've used step and stop interchangeably here and for my purposes, they are one and the same. Note that exposure has nothing to do with composition, DOF (aside from getting the desired one), or any of the rest. I am only referring to the amount of light that the camera is recording.
If you sit down, put your camera in Manual mode (M), and play a little bit with each of these settings, I promise you (as much as I can half a world away) that you'll get it. Pick a stationary object, well lit, and spend some time experimenting with each of the 3 settings but keep two constant for each round of your experiment. Pay attention to what the camera meter is showing you as you adjust each one. It may be confusing at first but once it comes, it'll be one of those slap the forehead with the hand moments.
Using your camera in an auto mode, allows you to adjust at least one of these settings while it adjusts the other two (If ISO is left on Auto). The green mode and the scene modes, if they are there, will adjust all 3 for you. There is nothing wrong with that but it doesn't do much to teach you about exposure. Once you have a handle on it, you'll stress about the choices it opens up just like the rest of us
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Good luck..
Edit: For practicing, and learning what the meter does, I would suggest setting it to spot metering. You don't have to use it in practice but it will narrow in everything so that what you are pointing the center of the camera at, is what it is metering. Hint: Typically for proper exposure, on a light grayish area, you want the meter to be centered. For now, don't worry so much about that though, just learn to center the meter. It will at least demonstrate how each factor affects exposure.