Saturday, August 4, 2012

Exposure part I: Proper Exposure

Today I will try to answer the following questions:

What is 'exposure'?

How to determine what a proper exposure is?

What does my camera's light meter do and why should I care about it?

What is the difference between SLR and pocket cameras when it comes to determining exposure?

and why do I need P,S,A and M modes, really?

For expert photographers these questions are trivial, but for people trying to get into the field these concepts always seem to be scary. I aim this post at people who want to know more, and I will try to explain the how-to's for SLR owners that are just starting up, and to pocket camera enthusiasts who want to get more out of pictures.


This post will be more technical than the last 'how to make the most of your camera' section I put up a few weeks ago. I hope the examples will be more useful than what theoretical texts offer online these days.

The next part in the series will be about avoiding camera shake, out of focus or grainy photos, by understanding what each camera setting does (shutter speed, aperture and ISO) and what the limits on each camera type are. If you already know what shutter/aperture is, but still don't know how to choose the right combination, the next post is for you.

The third part in the series will be about choosing the right exposure for creative purposes, in places where 'proper exposure' is not necessarily the best choice. Hopefully I will have useful tips even for people that already know what depth of field is...

What is exposure?

Exposure is the amount of light a sensor (in the old days, film) sees when you take a picture. The same exact picture, under the same lighting conditions, will be brighter or darker depending on the camera settings.

The relevant settings are Shutter Speed (sometimes abbreviated 'S'), Aperture size (abbreviated 'A') and ISO settings.

In film days the ISO was chosen by what kind of film was in the camera, so it didn't play such a crucial role (you couldn't change it from frame to frame). Today the ISO is hidden deeper in the menu for most cameras, or set to automatic.

I want to say this about ISO, just to get it out of the way for a while: use the default (usually the lowest or near lowest) setting for ISO unless you really don't have enough light. Usually this means ISO100 or 200 (for newer DSLRs). At higher ISOs the picture gets grainy, and you lose quality quickly.

In the previous millennium these cylinders were used as primitive, disposable memory cards

So, assuming we are shooting at ISO100, what level of brightness will I get from setting shutter speed and aperture to this value or that?

The simple answer is: the camera does that for you, so don't worry.

If you are in P mode, the camera meters the light, decides how much light the picture needs (this is the Exposure Value, or EV) and selects S and A by itself.

In A mode, you will select the aperture and the camera sets the shutter speed to get the EV it wants. In S mode the same happens only you select the shutter speed and the camera selects the aperture.

When you click (or dial) the aperture wider, with values towards smaller numbers (f/2.8 is for wide aperture and f/18 is for small aperture) you get a shutter speed that gets shorter.

In A mode, change from f/2.8 (the widest) to f/4 (a little bit more closed aperture), the shutter speed will change from 1/400 second to 1/200 second.

The value 1/200 means one part in 200 of a second (pretty fast). This exposure corresponds to afternoon light. The camera, in A mode, knows how much light you reduce when going from f/2.8 to f/4 (exactly one half the amount of light) and compensates by letting the shutter stay open twice as long.

The camera does this for you, you don't need to calculate anything.

So, bigger aperture, shorter exposure. Another example:

Set the camera to S, determine you want a shutter speed of 1/800, on a sunny day the camera wants f/5.6. If you change to a speed of 1/400 the aperture will automatically go to f/8.

In both cases you choose one value and the camera choses the other to make a proper exposure. In both examples the change of settings was a full stop of light.

Stops are when we increase or decrease the amount of light in a photo by multiplying or dividing it by two. Raising one stop of aperture (opening it) will mean the shutter needs to be twice as short (to reduce one stop). Two stops is four times the light, three stops is eight times the light and so on.

This is what you see in the menu when you set EV +1 or EV -1. The + or - one is add or reduce a stop of light, from what the camera thinks is the right exposure.

Useful button

Usually it goes in 1/3rd of a stop every click, so you have to click three times to get from 1/200 to 1/400, going through 1/250 and 1/320 on the way. Once more, this stuff is built in, you don't need to remember any of it, just understand what happens in the background.

Ok, so you can raise one value and the other goes down. How does the camera know what initial values to pick anyway?

Answer: it has a light meter and microchip that decide what the proper exposure is.

The camera does not always know what looks best.

How do you change what the camera thinks is best?

  • Aim at something brighter/darker, press the shutter release half way down, and point it back at your original frame (on most cameras this tells the camera to measure light at the first point while allowing you to take the picture at another point)
  • Use EV compensation (usually a +/- sign). Tell the camera you want more or less brightness. The same scene will be taken with the appropriate shutter speed or aperture increased or decreased. The camera will change what ever setting it can. In A it can only set the shutter and in S it has freedom to change the aperture. 
  • Use M and decide everything yourself.

The last option brings me to the tricky part. In M mode you determine the aperture and shutter speed, and there is nothing to stop you from overexposing or under exposing your images.

The effect of over/under exposing. EV=0 is 'proper exposure'

There is some assistance, though (in ancient times, people carried light meters and did calculations, and before that they just knew what exposure to set by hunch and experience, and only true professionals could hope to get something worth while from the developed film). In DSLRs there is a light meter inside the viewfinder. It tells you if you are over or under and by how much (If you are way over or way under it will just be at the edge of the ruler).

Highlighted: the light meter on a D90, indicating EV +1

While in M mode, change any setting (aperture or shutter speed) until the light meters shows you are at zero. If you want more exposure, change things until you reach +1 (you can have smaller changes, such as +0.3 or +0.7 as well). For darker images (less exposure) change the settings to get the negative side of the light meter.

If you have more than +2 or less than -2 in the meter you are probably not going to get much detail in the pictures.

In most pocket cameras there will be some indication of what the exposure is, in M mode. It could be a light meter ruler (similar to what is shown above) or just a number popping up when you press the shutter release half way.

The real bonus for Non-SLR cameras is that you can see what the picture will look like on the digital display, even before you take a picture. Try it: press the button half way and if it is too dark, open the aperture or increase the shutter time. If it is too bright do the opposite. This is a great way to understand exposure, by taking pictures in M under different lighting situations and seeing what values give a nice exposure.

My first camera was a Minolta Z6, I took almost everything in M, and just looked at the screen to see if the brightness was good.

Old picture of a great camera. Non SLR, M mode available. Lost in South America 2007

When using SLR you need to trust the light meter a little more, but you can always take a picture and see if it is too bright or too dark. Taking the light meter as just a suggestion can free up your creativity, which is what I will talk about in the third (and last) section of this series.

Airport photo. Underexposed EV -2. Camera wanted 1/100 at f/2.8, it looked much better with 1/400.

One more comment: cameras usually have different metering modes: central, spot and matrix/evaluative (which just means looking at the whole frame).

Three types of light metering modes. 
Example camera: Sony H7

Each mode just tells the camera what part of the frame should be used to decide the 'proper exposure'.

The spot is to measure just the spot in the middle (sometimes you can move the spot, which is pretty much the same as pointing, pressing half way, and returning to the original frame technique I outlined before).

The central mode (or center weighed) is like spot only looks at a large area in the middle of the frame.

Full sensor/matrix/evaluative is usually the most useful, it allows the (by now pretty clever) computer on-camera to see the whole picture and determine what is best. For special purposes this sometimes fails miserably (again, these special cases will be discussed in the third part) but even then it is usually easier to change EV up or down or by going to M, which lets you get the shot through trial and error.

Grace, my current camera, with Nikkor 105 micro used for the above images (dusty filter not included)

That pretty much covers the basic concepts.

If you are familiar with them but still need tips on how to choose the best exposure (why use bigger aperture and shorter shutter time or the reverse?), I will address these decisions in the next two parts.

next post: Exposure, part II: Know Your Limits.

In this part I will discuss (and show examples) of when shutter, aperture, and ISO chosen beyond reasonable values, can seriously mess up the quality of a photograph. I will talk about how to avoid making such mistakes (and how to learn from them when you make them). I will compare the limitations on several classes of cameras (DSLR vs. Super Zoom vs. Pocket) and show examples of badly exposed images.

next next post: Exposure, part III: Being Creative

In this part I will give tips for getting the most out of shutter and aperture. I will talk about when it is important to ignore the camera's light meter, and say a few words about the built-in flash.

For those who already know all this, I apologize. Hopefully I will have some less pedagogical stufff coming soon...

Comments are welcome, as always.

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