ISO need some advice

[Imacer]

I have a Canon rebel XT. I would like to know the higher the ISO the more light allowed in? True or not. I do know the higher the iso more grainy photos are. [img]/forums/images/emoticons/confused-smiley-017.gif[/img]

[Norm in Fujino]

Imacer wrote:

I have a Canon rebel XT. I would like to know the higher the ISO the more light allowed in? True or not. I do know the higher the iso more grainy photos are.[img]/forums/images/emoticons/confused-smiley-017.gif[/img]

A bit of the wrong concept, actually. The shutter/aperture combination controls how much light is allowed in, per se. The ISO setting merely controls the sensitivity of the "film" (in digital terms, the sensor). Setting the ISO higher raises the gain of the sensor, which allows the sensor to respond to lower light levels, but at the expense of higher levels of noise (the grainy appearance).
Back in film days, high-ISO films used emulsions with larger silver halide grains. The larger grains meant that the emulsion could react more readily to lower levels of light, producing an image under darker conditions but, again, at the expense of the "grainier" appearance of the picture.

[Imacer]

Norm tonight I was taken some phot shots and I was using 100 ISO came out dark, than I switch the iso to 400 my photos came out brighter. I was in P mode.

[rjseeney]

As Norm mentioned, ISO does not refer to how much light the camera "lets in". ISO refers to sensitivity to light. Higher ISO's require shorter shutter speeds (and/or smaller aperatures) to get the proper exposure. Higher ISO's allow you to capture an image in lower light or freeze action better (at the expense of higher noise levels). I would recommend getting a book on the basics of photography. THis way you could learn how things like ISO, aperature, and shutter speed affect your image. Even though you have an entry level DSLR, there is a lot you need to know in order to use it effectively. Getting a handle on the basics will allow for more enjoyment and less frustration.

While your at the learning stage, I would also recommend learning about post processing. DSLR's produce superior images, but rarely are those images superior out of camera.

Good luck!!!

[JohnG]

Imacer,

Shutter Speed, Aperture and ISO all work together to determine exposure.

Aperture controls how MUCH light get's in

Shutter SPeed controls how LONG the light gets in

ISO controls how sensitive the sensor is to that light

from a purely EXPOSURE standpoint, you can adjust any one of the above 3 and get the same exposure. The following settings would all produce the same exposure:

ISO 100, Aperture f4.0, shutter speed 1/500

ISO 200, Aperture f5.6, shutter speed 1/500

ISO 100, Aperture f2.8, shutter speed 1/1000

So, in a low light situation you can either increase ISO, increase aperture (smaller f-stop) or decrease shutter speed to obtain a 'lighter' exposure.

The trick is understanding what increasing / decreasing each of those elements will do to the photograph. High ISO as you mentioned adds noise or grain. Increasing aperture reduces the depth of field (DOF) which reduces how far in front / behind your focus point objects will be in focus. Reduced shutter speed means less ability to freeze action and more possibility of camera shake entering the picture. The trick is in determining for a specific photograph which of the 3 to adjust based usually on the DOF / shutter speed trade-off. With noise reduction software, high ISOs on your camera are still very usable.

That's the extremely high level view. I think the suggestion about getting a book is excellent - then practice adjusting all 3 to see how it affects different types of photographs.

[Carrots]

In the "weekly challenge" forums there is a section on reading material. There are a lot of links to sites you can check out.

The best of the lot are (my opinion):







http://www.morguefile.com/archive/classroom.php







http://luminous-landscape.com/







http://www.all-things-photography.com/