Just a basic question about the relationship between these variables

[sbryan]

How are shutter speed, ISO, and aperture related? The Canon XTi manual refers to ISO as shutter speed, so this further complicates the material.

For an example, I'm taking photos of my keyboard in a fairly well lit room. Settings are 1/50, F5.6, and ISO 200. It's on Manual Focus and AWB. I'm less than satisfied with the result, especially with the 7 seconds it takes to display the image -- I would expect that delay when shooting at night or in the dark, but something is amiss. I haven't come up with a combination of settings that will allow me to take a photo in two seconds, because honestly my arms aren't still enough to hold the camera for a 7 second exposure.

I think if I knew some of the science behind the settings, I would be better off.

Thanks for your help.

 

[Jeremy Ordan]

From wikipedia (because it's easier than typing)

Focus of the lens
Aperture of the lens – adjustment of the iris, measured as f-number, which controls the amount of light entering the lens. Aperture also has an effect on focus and depth of field, namely, the smaller the opening [aperture], the less light but the greater the depth of field--that is, the greater the range within which objects appear to be sharply focused.
Shutter speed – adjustment of the speed (often expressed either as fractions of seconds or as an angle, with mechanical shutters) of the shutter to control the amount of time during which the imaging medium is exposed to light for each exposure. Shutter speed may be used to control the amount of light striking the image plane; 'faster' shutter speeds (that is, those of shorter duration) decrease both the amount of light and the amount of image blurring from subject motion or camera motion.
White balance – on digital cameras, electronic compensation for the color temperature associated with a given set of lighting conditions, ensuring that white light is registered as such on the imaging chip and therefore that the colors in the frame will appear natural. On mechanical, film-based cameras, this function is served by the operator's choice of film stock. In addition to using white balance to register natural coloration of the image, photographers may employ white balance to aesthetic end, for example white balancing to a blue object in order to obtain a warm color temperature.
Metering – measurement of exposure at a midtone so that highlights and shadows are exposed according to the photographer's wishes. Many modern cameras feature this ability, though it is traditionally accomplished with the use of a separate light metering device.
ISO speed – traditionally used to set the film speed of the selected film on film cameras, ISO speeds are employed on modern digital cameras as an indication of the system's gain from light to numerical output and to control the automatic exposure system. A correct combination of ISO speed, aperture, and shutter speed leads to an image that is neither too dark nor too light.
Auto-focus point – on some cameras, the selection of a point in the imaging frame upon which the auto-focus system will attempt to focus. Many SLR cameras feature multiple auto-focus points in the viewfinder.

Think of ISO has film speed, the higher the ISO the more noise but the lower light (faster shutter) you can shoot with.

Your camera's sensor will determine how high of an ISO you can shoot and receive 'optimal' results.

With Canon's Rebel line you are fine at ISO800 whereas on my D200's I try to stay in the ISO400 range if possible and only go up if I really need to.

 

[PDX_DVX]

For low light stuff with minimal motion blur, you're going to want to shoot with the fastest shutter speed possible- this of course means boosting the ISO, and opening the Iris. Doing those two things of course effect the image in other ways, the wide open iris narrows your depth of field, and the boosted ISO could potentially introduce noise (or grain if you will) into your image. With experience you'll be able to know which settings to change to get the image that you want, and with experience you'll also realize the inherent limitations to whatever settings you can shoot with- like your 5.6 aperture. Faster glass allows for wider apertures, and therefore more light gathering ability. In the end you have a few choices- one of which might be simply adding more light, or putting the camera on a tripod to avoid the motion blur associated with long, handheld shutter openings.

 

[soeborg]

First of all I dont understand why the exposure time is 7 seconds when the camera setting is 1/50 of a second?


About your question. In short, ISO is the cameras sensitivity to light. Low ISO means you need longer exposure times, high ISO means shorter exposure times. So why not just shoot at high ISO? Well it makes the picture grainy. Regarding the aperture, that means how much light is let through the lens. Shutter speed is the same as exposure time and determines how long the camera is gathering light/taking the photo.

But even with a 2 second exposure i bet you wont get a sharp image unless its on a tripod.
So why dont you let the camera decide the settings? In my experience it will always get you the fastest exposure times, which is good for handheld/point and shoot.

Try Auto ISO, Auto WB. Shoot in P where it chooses exposure and aperture. And then just see if your pleased with the result. If not, use the exposure compensation. I couldnt tell you how that works on a canon so look it up

A lot of people spend so much time on figuring out the right combination of settings for point and shoot, but they will only get as good as the camera. So unless your going to be really creative with your camera, let the camera do all the work.