Shutter speed, F stops, ISO, and exposure compensation.

ALL,

Before i get going into the meat of this, i wanted to share a few quick things.
I'll be posting photos soon that help explain as i go along, but to start with just some basic info.
I'm going to keep things on here simple, in basic terms. If you want to get into the science behind how some of this works, either search online, or get a hold of me personally, i do run one on one or group online photo classes. This is meant more to get the basics down for those interested in learning how to use their cameras, not an in depth photography class.

With that said, this will be the first of many posts and i'm more than happy to post topics/discussions of almost anything. If you have ideas feel free to email me at savoie.images@yahoo.com and ref the blog (and for those following along the BT you can either email me, post on there, or PM me).



When taking photographs, you have three basic exposure controls that allow you to adjust for lighting differences and also allow you creative choices.

The three are:
Shutter speed,
Lens aperture (or F stop),
and ISO (film speed for those that remember film).

The three work like this:

Shutter speed:

The shutter speed is how long the shutter of the camera is open, and exposing either the film or sensor to light. If you have a window with a curtain over it, shutter speed is the time it takes you to open and close that curtain. Leave it open longer, and more light comes through, jerk the curtain open quickly and back shut, and less light comes in.
Shutter speeds are expressed in fractions of a second ( 1/250, 1/500, etc or in full seconds or minutes for longer exposures, 3sec, 20 sec, 5 min, etc).

Aperture (f/stop):

The aperture, or F stop as its shown on the camera/lens, is the size of the opening within the lens. The higher the number Fstop, the smaller the opening (ie an f stop of f/22 is a much smaller opening than one of f/2.8). If we move back to the window analogy, this is how wide we open the curtain. open it just a foot or so, and only a small beam of light comes in, throw it all of the way open, and much more light is allowed to enter the room.

ISO (film speed):

ISO is the rated speed (sensitivity to light) of the film/image sensor.
this one is pretty simple, the lower the ISO rating, the less sensitive it is to light. Something with an ISO of 50 is MUCH less sensitive than an ISO of 800. Back to the window one last time, this time inside we have someone wearing a pair of clear, lightly tinted, and dark sunglasses.
the clear glasses let in the most light, the lightly tinted let in a bit, and the dark hardly any. A high ISO would be the clear glasses, a low one would be the dark glasses.
With digital cameras, you can change this at anytime, with film you needed to change the roll.

ISO rating, F stops, and shutter speeds all work in relation to one another to make a correct exposure. To understand how they work with one another, we quickly need to take a look at the scales for each.

Shutter speeds are easy, they halve or double in time as you go up or down, ie part of the range would look like this: 1/125th, 1/250th, 1/500th so on and so forth.

F stops work the same way, up or down, halve or double below is a link to how this looks. http://shuttermike.com/wp-content/uploads/2009/10/aperture-f-stops.jpg

ISO changes are done the same way. 50, 100, 200, 400, etc.

All of these changes up or down are expressed as stops, ie a change from 1/250th to 1/500th of a second is a one stop change, the shift from ISO 200 to 400 is a one stop change, etc.

Now how do all of these relate to exposure?

All of those 1 stop changes effect the light that the film/sensor sees in exactly the same amount. In other words a one stop change in F stop, shutter speed, or ISO, will all change the exposure in the same way. if you shift the shutter speed one stop slower, and up the ISO one stop, nothing changes exposure wise (and any combination you can come up with using f stop, shutter speed, and ISO, as long as you keep the relationship the same, nothing changes as far as light let in).

*Note that with f/stops and shutters speed, and ISO, there are now half and third stop changes as well, not just doubling. IE instead of jumping right from 1/250th to 1/500 there are now two more stops in between as well.


Now, all three of these also have a creative aspect as well as simply controlling the light let in (i'll briefly touch on them here, we'll go more in depth later).

Shutter speed:

Because this controls how long light is let in, it is also responsible for how far a subject can move across the frame while it's open. The faster the shutter speed the more action you can freeze, the slower, the more the subject can move. To freeze a race car, or bullet in the air, you may need a shutter speed of many thousandths of a second, to capture a mountain, one of many minutes would still result in a sharp image.

Aperture (f stop):

This controls how much of the image is in focus, or the depth of field (there are other ways to do this as well, we'll hit those later).
The smaller the f stop the greater any given part of an image is in focus. An f stop of f/22 (very small opening) will result in a huge depth of field like in a landscape, while a large aperture of f/2.8 will result in a very shallow depth of field like in a portrait.

ISO changes result in two main changes outside of light.
First, the lower the ISO the higher the color saturation tends to be (ie brighter more vivid colors at lower ISO), and second, the lower the visible grain in the photos will be (expressed as noise in digital images).

Within any given camera/lens setup, those are your three main elements of control, and they hold true for all cameras across the board that use visible light, from old pinholes to the newest digital cameras.