ISO!

ISO is the your sensors ability to gather light. The higher the ISO, the more sensitive it is to light and the more light it will gather. The lower your ISO, the lower your sensors sensitivity to light, the less light gathering power it has. The only downside is noise. The higher the ISO, the more noise your image has, the lower the less!

When it comes to flash photography your ISO is going to affect 3 things in your image:

Your Ambient Exposure

Your Flash Exposure

The Noise (let's not worry about this one right now)

Read on to find out more...

ISO works in stops, just like aperture, shutter speed and flash power! They all work together to get you the image you want. 

The stops of ISO are as follows:

Each one of these steps is One stop of light! When we are dealing with ISO in flash photography it is going to control both our AMBIENT AND FLASH EXPOSURE!

What do I mean by this, well let's take a  moment and think about it.

Our ISO is the sensitivity to light. The higher the ISO the more light gathering power your sensor has, the lower the ISO the less. Your sensor does not know the difference between flash and ambient light. It's all the same light. Since the sensor can not make the distinction it is going to affect both of them equally!

For example let's say I am shooting at f/5.6, 1/60th at ISO 200. This is my perfect exposure and I was to increase my ISO by 1 stop to ISO 400. Since I increased my sensors ability to gather light by one stop, my aperture is ACTING as if I just opened it up to f/4 AND my shutter speed is acting like I just lowered it to 1/30th, even though I didn't. Since my ISO is up one stop, I now have 1 stop extra of light gathering power at my original settings.

Was that confusing?

Just know that when you increase your ISO, you increase your ability to capture ALL LIGHT! When you raise your ISO by one stop, you also raised your Ambient and Flash Exposure by one stop! If you lower your ISO by one stop, you lowered your Ambient and Flash Exposure by one stop!

Raising or lowering your ISO is crucial when it comes to flash photography, when the ambient light is starting to dwindle, you will have no other choice but to raise your ISO! I would much rather have a noisy image, than a blurry one!

Let's take this example of Catalina:

Look at this example, it was shot around 7PM and it was dark here are the settings:

Let's analyze it to see how raising my ISO helped me get the shot. First of all I want to clear it up that it was dark, my camera had trouble focusing and I had to use a flashlight. But notice how in the image you can not tell it's that dark. This is because of the light gathering increase I get by raising my ISO.

So let's work this out. My aperture is at it's maximum of f/2.8. I can not let any more light in by opening up my aperture, my shutter speed is at 1/30th of a second, that is as low as I want to go because if I go any lower I might get camera shake. My ISO was set to 800 to give me the light gathering power needed. Since I had such a great ability to gather light, my Flash Power was low because my ISO was high.

This shot would not have been possible without raising my ISO. If I was to have shot it at that wonderful ISO 100, that would mean 3 STOPS of LESS LIGHT GATHERING POWER!

So my settings would look something like this:

My aperture remains the same because I can't open it up any more. My shutter speed would need to drop 3 stops from 1/30th wich is all the way down to 1/4 of a second, WAY TOO SLOW FOR MY HANDS! My flash power would have to go Up to 1/8 power because of the less ability of the sensor to gather light! Yes, my image would have less noise, BUT at 1/4 of a second shutter speed it would most probably have MOTION BLUR!

Just keep in Mind, whenever you raise your ISO you are affecting your sensors ability to gather light, the ISO does not know the difference between flash and ambient light. It increases BOTH! The opposite holds true when you lower your ISO, it doesn't know what is what, so it will lower your light for both ambient and flash.

ISO is your light gathering power! You raise it, you raise your entire images exposure!

Ok, any questions, just leave a comment! The easiest way to learn is to experiement. Go practice and post your results!

Oh, and don't forget to tell everyone about lightenupandshoot.com!