Let's jump into it. ISO. What is it? Why is it? Who is it? How does it taste? And so on. Perhaps I can't answer all of those but maybe I can answer some of the questions you have about ISO.
When film was the only medium to shoot on there was a rating on each different film roll called an ASA rating. Lower ASA film needed more light to be properly exposed and not be too dark. These low number film rolls were usually marketed on the box as "for sunny days," or whatever. The higher your ASA, the less light a particular roll of film needed from your scene. This meant the film was more sensitive to light and it also meant more "grain." This grain was little flecks of silver that would show up in the image. So as a general rule, lower ASA less grain good for bright days, higher ASA more grain but better for darker scenes.
But how does that relate to digital photography? We no longer use film, so what do we need to know ASA ratings for? Well you still need to record the information your camera captures onto something. Back in the day this was film, but now film has been replaced by light sensitive digital sensors. Though it isn't film, we still need some way of knowing how sensitive to light that sensor is just like with film. How do we do that?
ISO stands for International Standards Organization. To put it simply these people took ASA ratings, matched them up with different sensor sensitivities and gave the sensors ISO ratings. So in theory 100 ASA = 100 ISO. This isn't exactly true but as a very crude generalization, that's good enough. So in turn this also means that the lower the ISO, the more light the sensor needs, and the higher the ISO the less it needs. But unlike with film grain, we get something a little bit different in digital sensors called "noise" as we change our sensors sensitivity higher.
This "digital noise" happens to all electronics as you push them further and further. For the sake of simplicity I won't explain that much in detail but if you'd like to know more (and you're a nerd like me) there are tons of sites explaining all the small details of how digital noise is introduced into electronics. Digital noise doesn't quite look like film grain though because digital noise shows up as random specs of color throughout the image, mostly in darker areas of the photo. Photographers can kind of bend reality to make it look close to film grain, but in the end grain is grain and noise is noise. At the end of the day just remember that higher ISOs have a lot of noise and unlike film grain, it doesn't look as nice.
|Nikon D610 at 3200 ISO|
|Nikon D5000 at 3200 ISO cheaper/older camera|
Some other things to consider when choosing which ISO to use are color and detail. When you go up through the ISO ranges colors start to change a little. Not so much that red becomes blue, but what you thought was bright orange may not be so vibrant as you climb to higher ISOs. Again, the difference is subtle, but sometimes it makes all the difference. Detail also takes a hit at higher ISOs. If an image is cluttered with digital noise, hard edges become softer. In my opinion (as with anything on this blog) I think it is best to shoot at the lowest ISO possible for the amount of light available and the image I am trying to capture.
If you have read my other photo basics posts then you will know that aperture and shutter speed go hand in hand when properly exposing an image. Now you must also think about ISO. In a future post I will put all three together and explain how each one effects the other.
Here are some examples of what different ISO's do. Keep an eye on two things as you look through each image. One: the black box and shadow area to the right of the motorcycle helmet. Here is where you will see the digital noise the most. Two: the color of the chair. Watch as it changes ever so slightly as I climb the ISO ranges. No editing was done to these images. I merely took the RAW files straight from the camera and converted them to JPEG for this blog.
|Nikon D610 100 ISO|
|Nikon D610 200 ISO|
|Nikon D610 400 ISO|
|Nikon D610 1600 ISO|
Anyways guys I hope this helped. If you have any questions, feedback, suggestions, or anything else, please leave a comment. I read them all.