Thursday, March 19, 2009

Tip Tuesday #5 - Exposure Porridge

There are a few basic things that need to be understood before very many other questions can be answered. So this post is going to be a very basic explanation of exposure. To a lot of people, it sounds scary to say that we're going to talk about exposure, but I promise to make it REALLY easy to understand.

We use the term 'exposure' because we are literally referring to the amount of light to which we expose the film or digital sensor. If you exposed it to too much light, your image is overexposed. If you exposed it to too little light, it's underexposed. If you got it just right like the perfect porridge, you'll have a proper exposure.

So when we talk about exposure, we're talking about how much light gets to the sensor, and how to get that to be just the right amount of light to create the image we want.

Tools of the Trade
There are four things that affect how much light our film/sensor gets exposed to.
1) ISO
2) Aperture
3) Shutter Speed
4) Ambient light

ISO – Most of us are fairly familiar with ISO because with film we had to choose an ISO grade, even for our point and shoots. ISO refers basically to how quickly your film will change in reaction to the light. Or to how quickly your digital sensor will record the light. The higher the number, the faster the film will react to the light, or with digital, the faster the sensor will record the light. For that reason, the setting is called the ISO Speed.

Aperture – an aperture is a hole. In photography, the aperture is the hole in the lens that light passes through. It’s adjustable, and the aperture setting is how big the hole is.

It’s expressed as fractions of the focal distance (f) – so f/2, f/4, f/16, f/22, etc. We all know that one-sixteenth is less than one-half. So it follows that a hole that is f/16 is smaller than a hole that is f/2. Usually, however, these numbers are just expressed as the denominator (bottom number of the fraction) – 2,4,16,22, etc. or sometimes with the f but without the slash – f2, f4, f16, f22, etc.

So bigger numbers equal smaller holes equal less light coming through the lens.

Shutter Speed – the shutter, as we recall from the last tip, is a set of plates in front of the film or digital sensor that keep light from getting to the film or digital sensor. Shutter speed refers to how long the shutter is open and letting light get to the film or digital sensor.

We’re usually talking about fractions of a second, and like aperture settings, shutter speeds are quite often expressed using just the denominator. So a shutter speed of 1/250th of a second will quite often be displayed as 250 on your camera. If you see a " symbol being used, it is in place of a decimal. So if your camera reads 4 – that’s one quarter of a second. If your camera says 0"5, that’s 0.5 or one half of a second. 15" is fifteen seconds. There is also bulb (or just B), which means that it will stay open for as long as you hold the shutter button down. I’m sure there’s a good reason it’s called bulb, but I don’t know what it is. Feel free to enlighten me if you do.

Ambient Light - This is the one that you might have forgotten to think about because it’s the one you can’t control. But to my mind, that makes it the one you have to think about first, longest, and most often. Great photos begin with great light, which is partly about having enough light for a good exposure at the settings you want to use. This could also of course include flash, studio lights, your car headlights – it’s not necessarily all daylight and super-high-efficiency fluorescent bulbs, but I guess I forget about the other things a lot because I’m mostly a natural light gal.

THE PORRDIGE - So you’re probably realizing that in order to get that Golidlocks exposure, we need to BALANCE these things. And for now, that’s what we’re after is just to understand the relationship between these things, and how they all have to work together to achieve the right settings.

If our image is overexposed, we could
A) use less light or find an area with less light
B) speed up our shutter
C) set our aperture narrower
D) use a lower ISO setting
E) a combination of any two or more of the above

If our image is overexposed, well.. the opposite. LIkewise, if we had our image perfectly exposed, then decided we want to use a faster shutter speed, then we have to compensate by
A) adding more light
B) opening our aperture up wider
C) using a higer ISO seetting

Does that make sense? Does any of this make any sense at all? Or is my explanation completely confusing and useless, suggesting that my dreams of becoming a teacher are probably ill-fated?

If you have any questions about this post, please don't be afraid to email me or call me, or post them here. I'd love to help. If you're like me, though, reading about something is the WORST way for you to learn, so go play!

I feel like this bit of foundation of knowledge will let me answer a few questions people have sent me, so that will start either Friday or Saturday.


Anonymous said...

ahhhh! I was so excited to have a new tip tuesday.. and it was a dud. A teaser. Can't wait for the tip :)

Carmen W. said...

Wow, I don't remember how I ended up here, but it looks like this is exactlly what I been looking for. Go ahead and get your teacher's certificate.