Exposure and ISO

tag:blogger.com,1999:blog-14039217.post-1122595714462039392005-07-28T17:05:00.000-07:002005-07-28T17:09:13.736-07:00Exposure and ISOSteve on the 350d mailing list asked me "When/why would you change the ISO No.?" I thought the answer I gave him will be useful for more people, so putting it here: ISO number is the third dimention in controlling exposure (first two being aperture and shutter speed). Consider the scenario: 1. You want a loooooooong depth of field since you are shooting a lush green field. So, you have set your aperture to, say f22. With f22 (which is a very small aperture), you have very little light entering the camera. So, you have t have a loooong shutter speed. BUT!!!! you realize you have forgotten your tripod at the hotel. So, there is no way you can hand-hold the camera at that slow a shutter speed and avoid camera shake. So, you nudge up your iso to , say 1600, which will allow you to have faster shutter speed at the same aperture level while keeping the same exposure. 2. You want a very short depth of field (and a larger aperture), WHILE keeping a slower shutter speed (to get the sense of motion, maybe?). However, there is a lot of light and you run the risk of over-exposing your photo. So, you nudge the ISO sensitivity lower, so that you can use slower shutter speed as well as larger aperture. If you look at those charts (ev charts, in my post <a style="color: rgb(255, 204, 153);" href="http://rebelxt.blogspot.com/2005/06/ev-exposure-value-lv-light-value-table.html">HERE</a> ) , they are for ISO 100 speed. You'll have to chuck them down onfe fstop for each ISO level increase. Also, remember, whith low light, if you use really high ISO settings (say 1600), you have a chance of getting high noise in the picture. So, I'd recommend using upto ISO800 for critical shots. Hope it helps someone!!Indiangeeknoreply@blogger.com