There are 3 basic components you need to master when photographing in manual mode: Shutter speed, aperture (f/stop) and ISO. I’m going to talk a little about ISO.

WHAT IS ISO?  
ISO stands for International Standards Organization. That doesn’t really help us a lot here, but just so you know… that’s what it stands for. ISO is basically telling your camera how sensitive it should be to light. When setting up a shot, I always set my ISO first (which isn’t necessarily the “right” way… but it’s what I usually do first). Let’s say your camera has a minimum ISO of 100 and a maximum of 3200. If you’re outside on a bright sunny day you would have plenty of light. You can use a LOW ISO (say, ISO100). If you’re photographing your son in his bedroom in the evening, it’s going to be much darker so you will need a HIGH ISO (say, ISO3200).

-As you can see here, in the first image I was outside on a bright sunny day. In the second image it was night time and there was almost no light at all.-

Cool, so why don’t I just always shoot on ISO3200 then? That way I have lots of light coming into my camera!

 

Not so fast.

 

The higher your ISO the more noise/grain you will get in the image. In traditional film photography, “noise” was described as “film grain.” Let me show you an example.

Again, this image was shot on a bright day and I chose ISO100.

When cropped in 100% you can see the image is smooth and crisp at ISO100. The next image was taken in a very dimly lit room. I had to use a much higher ISO to get the image exposed properly. Because I used ISO 3200, we can expect to see some noise. 

When cropped in 100% you can definitely see the noise in the image. 

Can you see the difference? Can you see how much noise & grain we have in the ISO 3200 image?

In a perfect world, you want to keep your ISO at it’s base level (ISO100 for most cameras) for the best quality photo.

 

I hope this helps you have a better basic understanding of ISO and how it can help you photograph in manual.