Exposure in 7 Shots

Got 2 minutes?  ...like literally, 120 seconds?  Sweet.  Grab your camera, we're going to take 7 quick shots and demystify this exposure setting stuff once and for all.

Shot #1:

Pick up your camera and walk outside onto your front steps, breezeway, or whatever.  Just get outside.  Set your camera's Exposure Mode to Aperture Priority (for most cameras, this means turning the dial on top to "A").  Now, set your aperture to f/5.6.  Find something nearby to focus on, about 3 or 4 feet away.  I used the broken flag bracket on my front porch.  Make sure that whatever's in the background is decently far away, like your neighbors house or a tree across the street.

Next, look at your shutter speed.  To avoid screwing with more settings later, we want it to be about 125 right now.  If its lower than that, gradually increase the ISO setting on your camera until you end up somewhere between 100 - 160 on shutter speed.  If your shutter speed is higher than that, gradually step down your ISO until you get in that range.  Just to get you in the ballpark, if its bright and sunny outside your ISO will probably be 100 or 200.  If its cloudy or moving on towards dusk, it might be 400, 800 or maybe even 1600.

Reframe your shot, focus on the nearby object you chose, and pull the trigger.

Shot #2:

Change your aperture to f/16.  Reframe, focus on the same nearby object, shoot again.  That's it!

Now compare Shot #1 vs. Shot #2.  Notice that the nearby object (my flag bracket) is equally in focus in both images.  But look at the difference for the far away objects in the background.  They are blurry in the first shot, taken at f/5.6 (shallow depth-of-field).  They are much sharper in the second shot, taken at f/16 (deep depth-of-field).  Congrats, you now understand aperture's effect on depth of field!

Shot #3:

Turn around, open your door, and take a shot into your house.  Its much darker inside, so you should hear your shutter take its sweet time opening and closing.  Look at the picture and zoom in.  See how blurry everything is?  This is because you are hand-holding the camera at a very slow shutter speed.  What you're seeing is the dreaded "camera shake"!

Shot #4:

Walk back outside.  Set your camera's Exposure Mode to Shutter Priority ("S").  Set your shutter speed to 500 (1/500th of a second).  Hold out your free hand and focus on it.  Now pull your hand back, and swing it slowly through the frame.  Shoot when you see your hand in the middle of the viewfinder.  Check your shot.  See how your hand is frozen in place?  (If it's not, you were pretending to pitch a fastball, so slow down and try again.)

Shot #5

Set your shutter speed to 125 (1/125th of a second) and do that thing with your arm again.  Swing it the same speed, shoot when it hits the middle.  See how your hand is all blurry?  Now you understand how shutter speed effects moving objects.  Fast shutter speed = frozen in place.  Slow shutter speed = movement looks blurry.  Two more to go!

Shot #6

Set your camera's exposure mode to Program Mode ("P").  Change your ISO to 1600.  Shoot something, subject isn't as important this time.

Shot #7

Dial your ISO down to 100.  Reshoot the exact same shot.

Compare shots #6 and #7.  Zoom in allll the way on the same area in both shots.  See the grainy speckles in shot #6 (ISO 1600)?  That's noise.  See how much smoother everything is in shot #7 (ISO 100)?  Now you understand the affect of ISO on image quality!

DONE!

ISO & Noise

Besides affecting exposure, ISO also determines how much "noise" (digital term) or "grain" (film term) will be present in an image.

Back in one of the early posts on exposure, I mentioned that only aperture and shutter speed actually control how much light makes it into the camera, while sensitivity (a.k.a. ISO or film speed) determines how the camera responds to that amount of light. This is an important fact we'll need to remember as we discuss noise and grain.

Let's start by talking about noise in a digital image. In order to get a decent explanation, we'll need to say a little something about how the image sensor inside your digital camera actually works. Metaphorically speaking, the image sensor is like a bunch of tiny light-catching measuring cups arranged side by side (just image a field full of Pyrex). These metaphorical measuring cups, called "photosites", each measure the amount of light they receive and convert this into one pixel in the final image. If one measuring cup received a lot of light and is mostly "full", this translates into a bright pixel. If another measuring cup received only a little light and is mostly empty, this translates into a dark pixel. Ok, with me so far?

So, these measuring cups ("photosites") have a certain capacity, that is, it takes a specific amount of light to fill them up half-way, or all-the-way, etc. This is like the "native ISO" of the camera, i.e. the default sensitivity of the image sensor. This is normally the lowest ISO setting available on your camera and is also where you will get the highest image quality. The image below was taken at ISO 100, the "native ISO" of my camera. This is only a very small part of a picture of my dining room, enlarged to show the smooth texture and lack of noise provided by a low ISO setting.

So what happens when you increase the ISO? We haven't actually increased the amount of light coming into the camera (only aperture and shutter speed control that) so how does a digital camera cope?

Let's go back to the measuring cup metaphor. If only a small amount of light is coming in the camera, the measuring cups (photosites) are barely getting full at all. At a low ISO setting, the camera would look at these mostly "empty" photosites and render the overall image too dark. When you increase the ISO, the camera tries to amplify the amount of light recorded by each photosite to make the image brighter.

As an analogy, imagine each of your measuring cups is a 2-cup size and they all have a very small amount of water (light) inside, let's say between 1 - 4 teaspoons. If you wanted to increase the amount of water in each cup, but do it equally so they are all still the same in proportion to each other, you might try to double the amount of water in each cup.

The problem is, it's hard to accurately measure small amounts of water in a big cup, that's not really where they are at their best. Is it 1 teaspoon in this measuring cup? 1.5? 2? You could guess the first time, and be pretty accurate. However, this level of error gets multiplied if you are trying to really increase the amount of water, say 4 times, or 16 times. This is the same problem (metaphorically) that your camera runs into at high ISO, and the mistakes it makes appear as noise.

The shot below was taken at ISO 3200. The graininess and colorful speckles are noise.

Ok, sorry this post is so long, but a quick word about film. For film, sensitivity is normally called "film speed" and is an inherent property of the film that cannot be changed in-camera. Also, film works differently than digital and is made up of chemical crystals instead of electrical sensors. When enough light hits the chemical crystals, they begin bursting and create dark areas on the negative, which will translate into bright areas in the final image.

For films with low film speeds, these crystals are small and hard to burst, resulting in a fine-grained film that has high image quality but low sensitivity to light. For films with high film speeds, the crystals are large, easy to pop, but create a coarse grain that degrades the overall quality of the image. Some people find the look of grainy film appealing, and use this affect for artistic purposes (thus the "film grain" filter in Photoshop). I am not one of those people, but I thought I'd mention it just in case you are.

ISO = done! Next up, exposure mode!

Exposure Part 3 - Sensitivity

Sensitivity is the last setting that controls exposure.

Sensitivity is also called "ISO" or "film speed", which are much more commonly used, but are somewhat less descriptive. There is also the now-antiquated "ASA" just in case you needed more jargon rattling around in your brain.

Just to set the record straight, "ISO" refers to the International Standards Organization. Its not a unit per se, but just an agreed upon convention about how sensitive a photographic medium is (such as film or a digital image sensor). ASA is the old school American Standards Organization, which actually uses the exact same scale as ISO, it's just different letters. So ignore that crap.

As far as film speed goes, when you are dealing with film, its sensitivity is a fixed property engineered into the film. You can't change it, you can just change which film you are using. Since most of us are shooting digital, at least most of the time, it's important to know that you CAN change the sensitivity of a digital sensor with just the press of a button.

So what is sensitivity? Do we have to watch LifeTime movies now? Bravo? No, fortunately we don't.

Sensitivity is how easily a digital sensor (or film) reacts to light. If the sensitivity is high, it takes less light to get a correct exposure. If the sensitivity is low, it takes more light. As mentioned above, sensitivity is most commonly given as an ISO setting. Here's a progression of ISO settings, all one full stop apart.

100, 200, 400, 800, 1600, 3200

Notice something familiar? The number doubles or half when moving from one setting to the next. Just like aperture and shutter speed, sensitivity operates in "stops", but this time we aren't changing the amount of light allowed in the camera, just how the image sensor reacts to it.

Let's go back to the ever-exciting water analogy. If aperture is the width of the hose, and shutter speed is how long its turned on, sensitivity is the size of the bucket we are trying to fill. ISO 100 is a kiddie pool, ISO 1600 is a coffee cup.

Another familiar example of sensitivity is sun burn. Some people are fair-skinned and burn to a crisp in 30 minutes. They are ISO 1600. Other people are dark as night and take all day to get burned. They are ISO 100. Get it?

ISO, just like aperture and shutter speed, has other important affects on your photography, especially regarding image quality and noise. We'll circle around for that shortly when we get into the creative uses of ISO, but first we'll start applying what we've learned about exposure.