Exposure (1) A balancing act

[RichardTaylor]

Exposure (1) Introduction

A balancing act.

When you take a photograph the camera collects light to turn it into an image.



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How much light it needs to collect is determined by the cameras sensitivity (ISO)
How much light it collects is determined by

(1) How long it collects light for (time) and is determined by the shutter speed (in seconds).

Plus

(2) How much light the lens will let in which is determined by the aperture (size of the hole) in the lens. Aperture is expressed in F stops (numbers) and as it is a ratio it may be a bit confusing. Low F numbers let in a lot of light, increasing the F number stops more light.

When everything is in balance, and the sensor collects the correct amount of light, the picture is "correctly" exposed, as per the example above.
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When the sensor collects too much light (either because the shutter speed was too long and/or the aperture was letting in too much light - low F number) the picture is said to be over exposed and will look too bright - washed out.

Conversely under exposure means the picture is too dark and is caused by the sensor not collecting enough light, either because the shutter speed was too short and/or the aperture was not letting in enough light - high F number)

If the picture is "blurry" it may be due to the exposure being too long and the camera was moved or the subject was moved.

Here is an example where the bird is moving too fast for the shutter speed and the camera is being moved as well, notice the streaks on the water.


Taken at 1/15 second

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When a camera is used in "Auto" or a "scene mode" it will look at the scene and determine the sensitivity (ISO) the shutter speed and the aperture to attempt to expose the picture correctly. For a lot of straight forward scenes it will be fine.

Example (I vary rarely use auto, however this will give you an idea)

Camera Canon EOS 5D
Exposure 0.008 sec (1/125)
Aperture f/14
Focal Length 80 mm
ISO Speed 200

When a camera is in P (program mode) it will set the exposure however you may vary the suggested settings and select the sensitivity (ISO)

With other modes like aperture priory/shutter priority you have even more control and in manual mode you have total control.

The main thing to remember it is a "balancing act".

ISO, shutter speed and aperture all work together and normally if you change one you change the other.

There are two other elements of exposure and that is focussing and "white balance" where the lighting type (Sunny/shade/indoor/flash/etc) is taken care of.
Keep in mind some exposure settings may be an "artistic" decision.

Part #2 : ISO and introduction to aperture is here.
Exposure (2) ISO + Intro to Aperture

Part #3 Aperture.
Exposure (3): Aperture

Part #4 Shutter speed.
Exposure 4: Shutter speed.
Future posts in this series will include more detail on;

Putting it all together:
Exposure 5: Putting it all together

White balance
Exposure Compensation

Thanks for looking.
Feel free to ask questions and feedback would be appreciated.

Richard

[OceanHorizons]

Your whole series of tutorials on exposure is really helping me understand how to use my camera - thank you! I really appreciate the sample pics and also the charts/illustrations. Thanks so much for taking the time to do this - I bookmarked it right away so I can find it again!

[RichardTaylor]

Glad I could help out.

[cray101]

I understand ISO as derived from the original film camera days where the sensitivity of the film (towards light) was standardized to some values like 100, 200, 400 etc.

But how does this actually relate to a digital camera? How does the CMOS sensor (that records the image) vary in image recording as ISO is changed??

[zona5101]

nicely done. thanks for taking the time to put this together.

[RichardTaylor]

Thanks very much guys.

@cray101
I am not an expert, however from personal experience film speed (ASA) was not standardised. It was a result of what the technology of the time was capable of and not a linear progression.
For example Kodak colour slide film I used was 25 ASA, 64ASA, 100ASA (Kodachrome) and 160ASA (High Speed Ectachrome).

With digital sensors & ISO (The ASA equivalent) the ISO is determined by how much the signal from the sensor is boosted. (elctronically).
See Wikopedia.
Film speed - Wikipedia, the free encyclopedia

[cristen]

Thank you, Richard. This is the best explanation on the correlation between those elements. It's so helpful to a newbie like me. Looking forward to more tutorials from you. Keep it up.

[RichardTaylor]

Thanks. Glad to be of help.

[maikerukun]

Thank you for this, it helps SO MUCH!

[RichardTaylor]

Thank you.