Exposing exposure

Introduction

Ok, so that's the first hard part out of the way (the catchy name for this little bit). A bit about this entry in the blog, it's purpose and it's intended audience.

I often get asked by folks, in various ways, for help understanding exposure. Not in the technical sense of what's over and under exposed, but rather in the sense of what/how do my changes of f-stop, shutter speed and ISO interrelate in maintaining or changing exposure.

So where to start with this, is the part I always struggle with. Since I don't know the technical level of those of you who will be reading this, I'm going to make some assumptions. You know what shutter speed is (I'm going to call it Tv from here on out because it's far shorter and easier to type). You know that a slow Tv is bad for moving things, and so on and so forth.

I'm also going to assume you know what the aperture (f-stop, hereinafter Av, for the same keystroke saving reasons as Tv above) is and roughly how it works. You'll know that a wider (aka faster) aperture will result in a shallower depth of field, and you'll know that a slower (aka narrower) aperture will result in a larger depth of field.

If any of the above is beyond you, don't worry, I'll be sure to write a few little snippets explaining the above when I have time. I might even be a really nice guy and include examples.

So that's two of the three variables that give people grief. The last is ISO. Luckily it is probably the easiest of the bunch to understand as it pretty much does what it says on the tin. I may do an article on the finer points of ISO (specifically as it relates to digital cameras) at some point down the road, but for now let's assume all ISOs are created equal from an image quality perspective.

Ok, so let's see, we have Tv, Av and ISO. These three things all combine to control how much light makes it into your camera and how sensitive it is to that light. I bolded that because that's the important thing a lot of folks have a problem fundamentally understanding. All of this comes down to light. All of photography is light. If you can remember that your life will invariably be easier :)

Stops

Before we go on there is one more thing I want to introduce here, and it's the commonly used term of 'a stop'. Everything in photography is measured in these stops. If you can get this concept then the rest is going to be really easy, so I'm going to try and explain (and perhaps over explain) 'a stop'.

In simple English 'a stop' (ok I'm going to stop using the single quotes now) refers to a doubling (or halving) of light. If something is a stop faster, it means that twice the light will make it into the camera. If something is a stop slower, it means that half the light will make it into the camera. 'A stop' in this sense should not be confused with f-stop or aperture values. (For the sake of clarity I'm not going into the origin of the term... once you are comfortable with the content of this blog entry you can Google it on up.)

Changes in exposure can also be measured in stops. For example, a picture that is half as bright as it should be is underexposed by a stop. This means that half the light was captured by the camera that should have been for a 'perfect' exposure. If something is twice as bright as it should it is overexposed by a stop. Most cameras will display their light meter with a little graph that shows 'EV'. They usually look something like this:

-3==-2==-1==0==+1==+2==+3

in your viewfinder. What do these numbers mean you ask? Well assuming you are operating your camera in an appropriate mode (let's go with 'M' mode here), the camera will show you whether you are underexposed (not enough light) or overexposed (too much light) with your settings. But you knew that much already. So, what do the numbers mean? Well each of those numbers represents a stop. So -3 means you are 3 stops under exposed. It means you have a half, of a half, of a half of the light you need to take the picture. Similarly if the value was 2, you have twice as much of twice as much light as you need (that's 4 times as much light for those of you keeping score at home). In other words you are 2 stops overexposed.

So why am I telling you all this you are asking.. I just wanted to know how Tv, Av and ISO work together. Well the reason is simple. I'm hoping you'll understand the concept of a stop as it relates to exposure and light. Once you do the rest is REALLY easy.

Easy one first, ISO

I'm going to start with ISO first because it's the easiest. If you look at ISO values you'll normally see 100, 200, 400, 800 and so on. ISO is simple because the numbers have a very boring predictable nature to them. Each change in ISO is a stop worth of change in sensitive of your sensor (or film if you still use it.. and some of us do ;) ). That is to say, ISO 200 will record twice as much (1 stop more) light as ISO 100. ISO 400 is a stop faster (twice as much light) as ISO 200 and so on. ISO is easy because you can simply divide one value by the other and see how much more light you are recording.

Example Time: So let's say you are shooting a scene and you are at some Tv (doesn't matter what) and some Av (again for this example it doesn't matter). You are also at ISO 400. You look in your handy dandy viewfinder and see it shows you are at (+1) on our little chart above. This means that you are one stop over exposed. Going back to what we read above, that means you have twice as much light as you need. So flipping that around, it's the same as saying you need to record half the light you are currently getting. So what do you do? Well like I said, ISO is simple, the numbers are linear in scale. If you need half the light you need half the ISO. So if you change from ISO 400 to ISO 200 you'll be set. This is because you will record half as much light now.

Shutter Speed, the other easy one

Shutter speed (Tv) is just as easy as ISO. It is again linear. But some of the numbers are a little oddball and you don't always end up with perfect doublings'. Why this is goes back to the old mechanical timing days, but, for our purposes let's pretend that if it's almost double, that it is. Tv has values like 1/30, 1/60, 1/125 (there's that weirdness), 1/250, 1/500, 1/1000 etc. What Tv represents is how long the shutter will stay open to take the picture. 1/30 means the shutter will stay open for one thirtieth of a second (.033333 seconds for you decimal people). 1/60 means one sixtieth, and so on (this is why it's called Tv, Time value).

What this means is that.. well let's just visit an example again.

Example Time Again, yay!

Let's take our scene above, where your viewfinder said you were at (+1). This time we're going to pretend you are shooting with film and you can't just change your ISO. So you are at Tv 1/400, Av (doesn't matter) and your film is ISO 400. Now, you have twice as much light being captured as you need. What do you do? Well again, the answer is fairly straight forward; You need to let in half as much light. You do this by leaving the shutter open for half the time. So changing your shutter speed to 1/800 will mean your shutter will be open for half the time. This means half the light will enter the camera.

Conversely, let's say your meter read (-1) with the above settings (Tv: 1/400, Av: x, ISO 400). Now you have half the light you need. To correct this you need to leave the shutter open twice as long. Well twice as long as 1/400 is 2/400. 2/400 reduces to 1/200. Changing your shutter speed to 1/200 would result in a correct exposure.

Hopefully all the above isn't clear as mud. Bottom line, as your shutter speed slows down (1/60 > 1/30 > 1/15) you end up letting in twice as much light. As your shutter speed increased (1/60 > 1/125 > 1/250) you end up letting in half the light. Each step 1/60 to 1/30 is a one stop change in the amount of light entering and subsequently being recorded by your camera.

The Ugly One - Aperture (Av)

Ok, before we get into this, I'm going to throw this out there. I can never spell aperture right on the first pass. I always throw extra letters in there. I'm sorry, it's my personal cross to bear.

So why is Av and ugly one to deal with... well because it's hard to explain (concisely) what all those weird numbers mean. I mean 1.4, 2.0, 2.8, 4.0 who comes up with this stuff right? (Here's a hint, some sadistic math nerd turned photographer, that's who). Ok let's start out with what full stop Av values are (I'll explain why they are what they are in a minute).

"Full" f-stop values are as follows: 1.0, 1.4, 2.0, 2.8, 4.0, 5.6, 8.0, 11, 16, 22... the list goes on indefinitely, but you will rarely leave that range, so let's pretend that's where it ends.

Now, if you want to just accept the above as gospel, then please skip to the next 'example' section. If you want to know why we have those god awful numbers, I'll do my best here to explain it.

Av, f-stop, f-number, aperture value value represents the ratio of the focal length of the lens to the diameter of the entrance iris of said lens (I warned you this was ugly stuff.. really skip to the ... fine I'll go on). Each of the f-stop numbers above (1, 1.4, 2.0, etc.) represent an opening that is progressively half as small. Because we are dealing with area we need to use the square root of two. (1.4 is roughly the square root of two). So, an f-stop of 1.0 would mean that the internal iris of a 50mm lens is 50mm in diameter. At f/1.4 it would be 35.71mm in diameter, at f/2.0 it would be 25mm and so on. What's important to remember (and then forget for your own good) is that you're talking about diameter here, because of this changing the diameter by the aforementioned amounts has the effect of halving the light allowed in each step of the way.

Okay, so that was ugly, hard to understand and really you don't even need to. What you do need to understand is simple (I'm going to put the 'example' section here so folks that skipped this don't miss the next bit):

Wheeeee Example Time!

Ok folks I lied, it's not quite example time yet. I want to say one thing first. Each change along the list I gave you above represented a change of one stop of light. That is to say, f/1.0 lets in twice as much light as f/1.4. f/1.4 lets in twice as much light as f/2.0. So on and so forth.

You've probably noticed a trend here, that's right, everything in photograph is based around stops of light. Everything is based around '2'. Either twice as much or half as much light.

Soooooo, onto the example.

This time, I'm leaving you with a film camera. Loaded with some choice ISO 100 film (this is good stuff man, don't waste it!). You've also got a longer lens on, let's say it's a gorgeous 400mm prime lens (if you don't know what this means don't worry). This means you aren't going to want your shutter speed to drop below 1/400 (which is where you happen to be already). That fantastic uber lens is also really fast it will go as wide as f/2.8 (if you have one of these lens and you're reading this please just send me the lens, I'll give it back to you in a year or two when you're ready for it :P ), but you are set to f/4.0.

So to recap: Tv: 1/400 (you don't want to change this), ISO 100 (film baby), and an Av of f/4.0. Your meter shows (-1). This means (again) you have half as much light as you need. How do you solve this. Well if you look at the list of 'full stop' values (way up there ^^) you will see that f/2.8 will let in twice as much light as f/4.0. So if you change your Av to f/2.8 you will solve your problem.

Similarly let's say you are at a Tv of 1/1000 (that's as fast as your camera will go), ISO 100, and an Av of f/2.8 and your meter is showing (+2). This means you have four times (twice, twice) as much light as you need. You can't go to 1/4000 for Tv because your camera won't do that (yes, lots of old film cameras stopped at 1/1000). Again you can't change the ISO because you've already loaded your film. Again looking at the handy chart above you will see that you need to set you Av to f/5.6. The logic is as follows; changing to f/4.0 will halve your light. This will move the meter to (+1). Changing again to f/5.6 will halve your light again moving the meter to (0). You have effectively changed your light by 1/2 * 1/2 which equals 1/4 the light you had when you started (and just happens to be exactly what's needed).

Odd bits and conclusion

Ok.. so some of that may be confusing, and even hard to follow. If it is I apologize (send me a note on what you have issues with and I'll try and review to clarify). The first thing I would recommend is you practice... then practice some more, and finally practice a third time.

There are some other things I didn't cover above to keep things simple.

1) Most modern cameras will deal in 1/3 stop increments for Av and Tv (and some for ISO). This means you'll see f-stop values of 3.2, 3.5, 4.0, 4.5, 5.0, 5.6 etc. Each of those is one third of a stop. It also usually corresponds to the little ticks on the meter in your viewfinder. For now I would just consider these 'fine tuning'. Get as close as you can using the full stop math on this page and then tweak from there. It will eventually become second nature. (Similarly you'll see shutter speeds like 1/320, 1/400, etc. Apply the same math).

2) Don't get too worried about any of this, just remember your basic full stops:

ISO: 6400, 3200, 1600, 800, 400, 200, 100
Shutter Speed: 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000, 1/2000, 1/4000
F-stops: 1.0, 1.4, 2.0, 2.8, 4.0, 5.6, 8.0, 11.0, 16.0, 22.0, 32.0, 45.0, 64.0 (most modern lenses won't go past 32.0)

If your meter says you are (+1) work right from the above chart for any one value at a time (this is why I reversed ISO). If your meter says (-1) work left. You're welcome to copy that down, print it out, keep it handy until you've memorized them. Believe it or not you will really quickly.

3) When you have the time to do it, throw your camera into M mode and experiment until this becomes second nature. It will happen a lot faster than you think it will. However, if it's something important and you aren't comfortable with this use a mode other than M. There is no need today to lose an important moment or brilliant shot to the above. Let the camera manage it for you for the important stuff until you are comfortable. Flowers won't go anywhere, but first steps only happen once. Not capturing a magical moment is a far greater shame than letting your camera handle things for you.

Anyway, that's it, that's all. Sorry it's so long. Apologies for typos, I'll proof it over in the next couple of days, but I have to call it a night soon.