Thursday, May 3, 2012

What are ISO numbers? Lesson 6



A Definition:

The ISO number = how sensitive the camera is to light

“The lower the number the lower the sensitivity of the film 
and the finer the grain in the shots you’re taking.”  
- Darren Rowse

There are 3 basic variables that are part of the process of taking a picture. We talked about the first two variables in Lesson 4 and Lesson 5 – the camera’s aperture and shutter speed. The last variable we will talk about is the ISO.

Can you remember waaay back to the film days before basically everything went digital? (I wasn’t even taking pictures back then!) Anyway, when people bought their film, they had to choose what light sensitivity they wanted – what ISO number to choose. The higher the number, the more sensitive the film was to the light. People chose a higher speed film such as ISO 800 for use indoors and bought a lower speed film like ISO 200 to use outside.

Of course, nowadays most people have digital cameras and therefore don’t use film, but changing the ISO according to your circumstances is still important. There’s no reason not to! It’s so much easier than changing film!

Q: Why not just use ISO 800 (or another high ISO number) all the time? After all, it’s more sensitive to light!

A: The higher the ISO number, the grainier, “noisier” and lower the quality of the resulting image. The lower the ISO speed, the better the quality and the less grainy or noisy the picture is.

 



This is the view at 100% of part of a picture that was taken using a high ISO number. You can see the grain and noise, can't you?




Here is a view zoomed in at 100% of part of a picture taken at a low ISO number. The picture details are much cleaner and clearer in this shot.









Here is the total picture. The place where the 100% crop was taken from is circled in red.
The more you spend to buy a camera, the better it will perform at higher ISO numbers. (Point and shoot cameras are especially prone to noise and grain, even when the ISO number is only sort of high!)




The camera’s aperture, shutter speed, and ISO all word together. You can change any one of these and the exposure will change (the picture will turn out lighter or darker) if the camera is set to M (manual mode). For the other camera modes, the camera compensates for the changes you make to any of these settings.

Look at the three variables - aperture number, shutter speed, and ISO number - as if they are 3 crucial ingredients in a cake. If you change one of them the picture will turn out differently.


In the photo on the left, I used an aperture of f/3.5, and a shutter speed of 1/250 sec. at ISO 200.

In the photo on the right, I used an aperture number of f/3.5 and a shutter speed of 1/1500 sec. at ISO 800. By having the ISO number at 800 instead of 200, I could raise the shutter speed. (not that I needed to in this instance!)
Even though the settings are different, the picture can still turn out basically the same. The only difference is the lower quality of the one that was taken at ISO 800 (When an 8x10 print is made of each image, the difference in quality is much more apparent.)

 





For this picture, I changed the aperture number to f/19. This meant that the shutter speed or ISO number had to change also. Since I didn't want to raise the ISO number any higher than 800, I used a longer shutter speed - 1/45 second - to let in more light and compensate for aperture number I chose.








Do you see in these pictures that in order to get the same exposure (same brightness of the image) if I change the aperture, the shutter speed changes. If I change the shutter speed, the aperture changes too. If I change the ISO, either the shutter speed or the aperture (or both) have to change also.

Q: How do I know what camera mode to use and what settings to adjust?

A: This depends on what you want the final picture to look like. Let me explain.
  • If the background in the image needs to be out of focus, set the camera to Av (aperture priority mode) and set the aperture to a small aperture number for a small amount of the picture to be sharp. The camera will adjust the shutter speed for you.


  • If you are after the dreamy, soft feel of a slow shutter speed, set the camera to Tv (time value) and select a slow shutter speed. The camera will then adjust the aperture to get the correct exposure – not too bright and not too dark.


  • If you are in a dim environment and need the camera be more sensitive to the light, know that the picture quality will suffer somewhat when you boost the ISO number, but boosting the ISO number will enable you to take a picture that otherwise might have been impossible to capture.


A Review:

  1. If you used to use a film camera, bumping up the ISO is the equivalent of putting in a higher speed of film
  2. Low light – in situations where the light levels are very dim and you can’t use a lower shutter speed or bigger aperture to let in more light, boost your ISO
  3. A higher ISO number makes the camera more sensitive to light
  4. A high ISO number does degrade the image quality –  it makes the picture more noisy or grainy
  5. Don’t pick a higher ISO number than is needed.
  6. The lower the ISO, the better, but make sure not to get a blurry picture!


“Whoso offereth praise glorifieth me: 
and to him that ordereth his conversation aright will I shew the salvation of God.”

Psalm 50:23

2 comments:

  1. Thank you, Laura!
    What camera do you use?
    My family has a Nikon. Can you still do a lot of this stuff with a Nikon?
    I tried looking for some of the settings that you mentioned, but didn't find a lot of them.

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  2. Hi Kaylee!
    I have a 2 SLR cameras and a point and shoot - all are Canon, but the same things apply to a Nikon. (To be specific, I have a Canon 7D, a Canon Rebel T1i and a Canon SX 100 IS.)I can do all of the things referred to in these lessons with any of the 3 cameras.
    Which settings were you looking for? I might be able to help you.
    What specific Nikon camera do you have?
    We'll get this figured out!
    Laura

    ReplyDelete