Exposure Basics

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Exposure is simply the collection of light, to much collected - over exposed, to little - under exposed.  This is how it has always been in photography. However, there is one major difference between digital photography and old fashion film photography.

In the film days you purchased film with a certain sensitivity to light. In today’s digital age you can dial in this sensitivity level, it is called the ISO as opposed to the film’s ASA or DIN. Now for the similarities, when you purchased film that was more sensitive you had more grain in the images and they appeared to be not quite as sharp. That is true today only it is not called grain but noise. It is like a stereo when you crank up the volume to high, you hear noise from the speakers even though no music may be playing. In a photo this would be an area that does not have enough information collected about it to make a sharp image.

As technology changes camera sensors have gotten better and better at dealing with noise. However, regardless of how good they are one thing that will always be true is the proper exposure will always produce the best image.

Ideally you want to have the proper exposure for the base line of the camera as this will produce the least amount of noise. Usually cameras have a base line of around ISO 100 (some offer a lower ISO but the baseline is still 100 - you have to research your model). Think of it like this, when you increase the ISO it is similar to turning up the volume on a stereo. The more you turn it up the more static/noise you will hear along with the music.   The more your turn up the ISO the same will happen, the more noise in your photo.

However, if there is not enough light to get a good shot at the base line it may be necessary to change the ISO. You should test your camera model to see what you find acceptable for an ISO. For instance for bug shots you may be happy with ISO 200 and for snap shots of the family perhaps ISO 1600 works when you are indoors.

The proper exposure always consists of collecting the proper amount of light for whatever the ISO is set at. There are only two things that can be changed to collect this amount, the time (shutter speed) it is collected and the volume that is being collected (aperture). So there are three parts to exposure, ISO, Shutter Speed and Aperture (sometimes called the exposure triangle).

The key is in balancing these three items. Many times you will see it explained as filling a bucket with water.


ISO Bucket Size Photo

In this analogy the bucket size would be the ISO.  You would think a big number would be a large amount of light but it is not. So a small ISO number needs more information for the sensor to create the image. The more information collected the better the photo can be.

Filling the exposure bucket

To get the bucket filled there are two values that can be changed, the time the faucet stays on and how much is going through the pipe (pipe size). These would relate to the shutter speed and aperture.

To make things easy one change in any value requires an equal change in another value. So, if you were at ISO 100, f8 at 125th of a second and you changed to ISO 200, you would have to change to f11 and stay at 125th of a second or stay at f8 and go to 1/250th of a second. These would be full stop changes, you can refine them to 1/3 stop changes with most cameras. Although they are not exact changes they are close.

An easy way to remember the f stops is to only remember 2 of them, 1 and 1.4. Then you just keep doubling them and rounding them off. So you have 1,  2, 4, 8, 16 and f32 along with 1.4, 2.8, 5.6, 11, 22 etc.


Understand flash duration.

Most people do not really understand flash.  It can be used two ways you can use it to supplement your current light or use it to entirely light your subject.

When you use it to completely light your subject your flash duration (time flash is on) will be your new shutter speed.

People see it as follows, I set my aperture, my shutter speed and my flash.  I looked at the histogram and the image was dark so I turned up the power of my flash and now I have a perfect exposure.  What most people don't understand is that the flash intensity did not increase it is not like turning up a dimmer on a light switch, what increased was the flash duration (the amount of time it was on).  In order to get good consistent macro images you need a very short flash duration.  To get a short duration you will need to have the flash head very close to your subject.



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