Tag Archives: ISO

The Absolute Basics Of Exposure

I have the privilege of knowing a growing number of beginning photographers. It’s always casual but sometimes folks are a little shy about asking questions. I remember being new to photography, and in a lot of ways I still am. No, really. For example at San Jose State Univeristy I nearly flunked out of Photo 101 (yes I did much better later). I loved it, but I utterly didn’t have any sense for composition, basic lighting, and oh my goodness I had a crush on the pretty girl I shared the darkroom with. Stop it, I was a complete gentleman in college. That may mean that I missed out on some of the typical college guy things, but I digress. (Note to self, maybe edit that out) (Note to self, no leave it in, it’s kind of funny) (Note to other self, stop it) (Note to other other self, OK) (All the other voices in my head: GET ON WITH IT).

So here I am writing about the absolute basics essentially for a group of friends but also for anybody else who cares to read it. I mean, OK, folks are asking me. Maybe I can help.

Photography is all about light. It’s in the word. Photo (light) graphy (writing). Light writing.

Let’s start with the basics of exposure.

Most cameras since the 1980’s have built in light meters. That means that there is a sensor that measures light reflected off of a subject and back into the lens. The earliest camera that I have that did that is my old Canon A-1. This technology is not terribly new.

My trusty old Canon A-1

How brightly an image is exposed depends on how much light is allowed through the lens, striking the camera’s sensor or film. This is intended to be all about digital photography so I won’t talk a lot about film here, but there are analogies that help occasionally.

There are 3 things that will effect how your image is exposed, and they are all related in what’s usually referred to as the “exposure triangle”. When one of those three aspects changes then the other 2 change too. It breaks down like this:

  • Shutter Speed (the amount of time the shutter is open)
  • Aperture (how wide the opening in the lens is)
  • ISO (how sensitive the sensor is to light)


A couple of friends of mine read this and correctly noted that I’ve assumed your camera is in manual mode. JC Dill  thought that manual mode deserves a little explanation. She’s right.

Below are three cameras that I use fairly often. All three have a manual mode that allows for complete control over aperture, shutter speed, and ISO. Each one has a control with “M” for manual. The other modes worth noting are (on the Canons anyway) Av (aperture priority), Tv (shutter priority), and P (program mode — aka “automatic”). The Panasonic is similar: M, A (aperture priority), S (shutter priority), and P (program mode).  For this discussion we’re not letting the camera make any decisions for us. When the camera calls the shots, you get what the camera decides what you want. It’s almost never what you really wanted.

My trusty Canon 7d Mk II
My very, very well used Canon Rebel XT
The reliable Panasonic Lumix DMC-FZ47

The rear panels of these cameras show some helpful details. Each shows a basic light meter (-3…-2…-1… | …1…2…3 ). The middle is what the camera is calling well exposed. -3 is 3 stops under exposed, 3 is 3 stops over exposed. There is also a graph displayed with red, green, and blue. This is an optional display that I rely on. This histogram tells me about the dark values (on the left side of the graph), the bright tones (at the right side), and the mid tones (in the… ok you get it). The default is one single graph for overall luminance. I have it set to display red, green, and blue so I can see if any one part is over exposed. There’s a lot of bright reds here, so I would call it a little over exposed.

The live view on the 7d Mk II. Note the meter at the bottom of the camera’s LCD: 3…2…1…|…1…2…3 .

The back of the Canon Rebel shows similar information, just in a much more basic way. This is telling me that the image is a little under exposed the way I have it setup.

The rear display of my older Canon Rebel XL has something similar: -2..1..|..1..2
Another image of the back panel of the Canon Rebel XT. The meter is centered. The camera’s light meter is calling this well exposed. This is a little more subjective than you’d think, but it’s a good starting point for discussion. Don’t look too closely, this camera is really filthy. It’s seen a lot of adventures.

This Panasonic Lumix is a nice example because it’s essentially a low end consumer camera with full manual control. The light meter at the bottom should seem familiar now. Note the graph towards the right of the LCD. It’s another histogram similar to what’s on the 7d Mk II. This histogram is displaying overall luminance instead of breaking it down in to red, green, and blue (RGB). This graph is telling me that there’s a lot of dark values and midtones, and not much that’s excessively bright. While this isn’t a terribly exciting photograph (really it just shows you how very messy my desk is), it does say that this is a reasonably well exposed image.

The rear of the Panasonic is a little different. It has a meter in the LCD much like the Rebel and the 7d Mk II : -2..-1..|..1..2 .

That’s a lot of information and I encourage you to comment and ask questions. OK.. back to our regularly scheduled program picking up with… (drumroll please…) shutter speed.

Shutter Speed:

Put simply, the longer you leave the shutter open then more light passes through the lens to the sensor. A short amount of time results in a dark image (under exposed). A long amount of time results in a bright image. A very long amount of time results in white or nearly white (over exposed). What’s “correct?” That’s really up to you and what you were looking for. Unless you’re working under some very strict guidelines this is art. You either achieved what you wanted or you didn’t. I’ll go into shutter speed in greater detail later, but I touch on it in this earlier post that you might like. : MY VIEWS ON SHOOTING BETTER SURF PHOTOGRAPHY

We’ll use one of my favorite wide angle lenses. This is great for this demo because it’s entirely manual.

Aperture (aka “f-stop”):

The typical camera lens has a roughly circular opening that can be adjusted to be larger or smaller. A larger opening allows more light in while a smaller opening allows less light in during the same amount of time. A smaller number means a more open aperture. f/4.0 is a fairly wide aperture and lets in quite a lot of light. A higher number means a smaller aperture and lets in less light for the same exposure time. For example f/8.0 is a smaller aperture than f/4.0.

Aperture: f/2.8. Wide open for this lens.

If I were to point my camera at exactly the same subject in exactly the same lighting and let it expose for exactly the same amount of time for two shots, one at f/4.0 and the second at f/8.0, you would see that the photograph shot at f/8.0 is much darker than the one shot at f/4.0. For the same image shot at f/8.0 to be the same luminance as the one shot at f/4.0 you could let it expose for more time. Exactly how much time is a little beyond the scope of this introduction to the exposure triangle but you can probably start seeing how these two sides of the triangle are related now.

The aperture is smaller now. f/5.6

Right about now you’re wondering what the heck the “f/” part is in f/4.0 and f/8.0. The aperture is measured as fractions of the length of the lens. f = focal length.

Aperture is smaller still: f/8

This is very much like the pupil in your eye. When it’s dark you see a little better when the pupil opens wider. When it’s bright your pupil becomes much smaller and allows in less light. Something that’s super cool is that your pupil will change diameter as you scan across a scene. Your typical camera doesn’t do that. Not yet anyway.

The smallest aperture for this lens: f/22

A closer look at aperture and shutter speed together:

Let’s walk through an example demonstrating how changing your aperture will change the amount of time (shutter speed) needed to keep the same exposure while the same amount of light remains constant. Notice that each picture takes twice as long to expose as the one before it.

Aperture: f/1.8, ISO: 200, Shutter speed: 1/40 second
Aperture: f/2.5m ISO: 200, Shutter speed: 1/20 second — twice as much time to expose
Aperture: f/3.5, ISO: 200, Shutter speed: 1/10 second

I’d like you to notice something else that should be apparent by now. More of the items in the photo are coming into focus. In the first  example the item in the foreground was in focus. Then bits of the item behind it. From here the clock in the background will come more into focus. I didn’t change my focus point at all. I set the focus to be on the candle and never changed it.  What has changed is the aperture. By using a smaller aperture we get a greater “depth of field” (or in English “there’s more stuff in focus”).

Aperture: f/5, ISO: 200, Shutter speed: 1/5 second

Aperture: f/7.1, ISO: 200, Shutter speed: .4 seconds
Aperture: f/10, ISO: 200, Shutter speed: .8 seconds

Now here’s something interesting. There’s not much difference between these two last pictures is there? The candle, the clock, and everything in between is in pretty much the same focus. At some point you reach a practical infinity. Different camera models will behave a bit differently. At a point beyond that your photo will actually become less sharp. That’s an effect called “lens diffraction” which is outside the scope of this quick intro.

Aperture: f/14, ISO: 200, Shutter speed: 1.6 seconds

ISO (sensor sensitivity):

Finally there’s the topic of ISO, the third side of the exposure triangle. By increasing the ISO you shorten the amount of time needed to expose the image. It becomes possible to freeze action in lower light situations. A higher ISO number means greater sensitivity to light. There is a tradeoff which you may or may not mind. A low ISO number is less sensitive to light but will usually result in a higher quality image. A higher ISO value is more sensitive but will reduce image quality by introducing a grainy look, aka “noise.”

f/1.8, 1/15 sec, ISO 800. At 1/15 second I can’t hold the camera still without a tripod.

My good friend Dave Anderson suggested that I explain the captions in the snapshots above and below. I took these snapshots hand held. You are moving no matter how hard you try not to. When you have a slow shutter speed there’s very little chance of having a sharp photo because you moved a little while the shutter was open. A good rule of thumb for a reasonably sharp picture when hand held is to keep your shutter speed at the inverse of your focal length.

f/1.8, 1/25 sec, ISO 1600. That’s still a long time for hand held so it’s not very sharp.

I can here you from here: “Whaaat??”  Let’s say I have a typical kit lens that zooms from 18mm to 55mm. If I’m shooting at the wide angle, 18mm, then I would need to keep the exposure time 1/18th second or faster. If I’m shooting at the tighter end, 55mm, then I need to keep it at 1/55th second or faster. This is oversimplified because different cameras will behave differently. For example a crop sensor camera like the Canon Rebel series, the Nikon 3000 series, should still be a little faster. Explaining why is a little outside the scope of this one post. It’s also pretty unlikely that you’re going to keep completely still for 1/18th of a second.

f/1.8, 1/80 sec, ISO 3200. This is closer to what I can shoot hand held. It’s starting to get a little noisy. Your camera may vary.
f/1.8, 1/160 sec, ISO 6400. A much faster shutter speed and easily hand held. Kitteh’s action freezes mid-yawn (I bore her). It’s very noisy now.
f/1.8, 1/320 sec, ISO 128000. Easily handheld, very noisy, and Kitteh is tired of me. Click on the the picture to see the noise details.
Are you done yet?? Yes Kitteh. Thank you. Please don’t poop in my shoe.