Exposure

Can you please use that hose to fill the bucket to the top? Filling it to the top means you leave the facet open for a certain amount of time based on how big the hose is, how much water pressure is and how big the bucket is. That’s Exposure.

What you need to know

The size you open the facet valve to is the Aperture, the diameter of the lens. The amount of time you let the facet run is the Shutter Speed, the length of time the shutter is open. The size of the bucket is ISO, the sensitivity to light. The water pressure is the brightness of your scene. Filling the bucket up is letting the correct amount of light in for a proper exposure. Playing with Aperture, Shutter Speed and ISO is playing with exposure.

Understand: If a camera is set to get the right exposure but you want a wider aperture then you need to shorten the shutter speed.

New Term

A “stop of light” is change in light by a factor of two, doubled or halved.

To “Stop Up” means to double the light. To “Stop Down” means to divide light by two.

Measuring a stop of light for ISO, Shutter Speed

ISO and Shutter Speed are easy. ISO and Shutter speeds are halved or doubled for a stop of light.

If you camera is at ISO 200 and you want to double the sensitivity to light you change to 400. You’ve stopped up one stop.

If you’re at Shutter Speed 1/200sec and you want to let twice as much light in you let the shutter stay open 1/100sec. You’ve again stopped up one stop.

Measuring Aperture

To double light in the lens means to double the area of the opening. You can see the math if you’d like but bottom line is to change one stop you change the f/stop change by a factor of 1.4 (the square root of 2).

That’s why the numbers come out so odd.

1.0, 1.4 ,2.0, 2.8, 4.0, 5.6, 8.0, 9.6, 14.0, 18.0, 22.0

Application

Here’s the scale your camera uses for stops of light. It’s really pretty nice. Every full number is a stop of light. They are called Exposure Values (EVs).

You’re in Manual Mode and you see this.

exposure

ISO 800, Aperture f/4.0, Shutter Speed 1/60sec and EV is –1.

You want to get to EV from -1 to 0. You have to move one stop by doubling the light. Think about your three choices.

ISO

Double the ISO to 1600.

fstop

Make a bigger diameter for light to come in by one stop – f/stop 2.8.

ShutterSpeed

Make a slower Shutter Speed – 1/30sec.

All these choice end up with the same exposure. Which you decide depends on what you know about Aperture, Shutter Speed and ISO and what the story is asking for.

Wrap up

If your camera is in an auto exposure mode, Auto, P, AV(A) TV (S) it’s making all these decisions for you based on what part of the image you tell it to look at.

In AV (A for Nikon) or TV (S for Nikon) you are control that one setting and the rest are changing automatically for you. My method is to use Manual Mode with my metering mode set to Spot Metering.

This article discusses how you can use this knowledge to get your pictures in focus.

If this article helped you out then it probably will help someone you know so share is below or with email.

Ask questions below, in our community page, our Google+ community or send me an email at mark@technologyformedia.com

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