Calculating exposure

[forknuts]

Hi everyone.
Hopefully this is the right spot to post this - sorry if it's not!
I'm a total photgraphy novice and am trying to pick up as much info as I can from wherever I can. I've read a handful of books (DSLR Cameras and Photography for Dummies etc) and read a fair few forums, and there is one thing I can't figure out. I've seen many references to 'calculating the exposure', and have a rough idea that this is to do with the shutter speed/aperture relationship but I don't know how to 'calculate' it.

Can anyone shed some light on this for me?

Cheers,
Adam

[zona5101]

A virtual treasure trove of valuable information can be found here:
Digital Photography Tips and Tutorials

[IABoomer]

It's not so much something you calculate as it is something you measure. You can use a dedicated light meter or your camera should have a built-in meter to it.

In either case, the meter will take a reading of the light in a given scene relative to your camera's aperture, shutter, and ISO settings and show you where the exposure falls. With the camera, there are different metering methods. Some average the light of the entire scene while others meter in just one spot. Which mode you use will depend on what you want to accomplish given the light.

[wulf]

With a film camera, you don't get instant feedback so you carefully calculate the settings you need based on the reading from a light meter.

With a digital camera, you can often get decent results by relying on the camera's built in calculations. However, even in full manual mode, I think you can afford to take a shot and adjust. In other words, guess and refine rather than calculate per se.

Wulf

[RichardTaylor]

In the real early days;
(1) Before light meters were commonly available, you did calculate the exposure by comparing the scene description on the piece of paper that came with the film to the scene in front of you.
(2) Before automated flash guns you always calculated the aperture needed by using the flash guide number and the subject distance. Some flash guns had a chart on the back that helped.

--------------------

With a meter, you meter (measure the light) the scene, either with the camera's light meter or an external meter.

Depending on the metering method used, and the subject, you may want to vary the indicated exposure by "calculating" a new exposure, and use that .

Film or digital the principal is the same. The only real difference is that with digital you have instant feedback, via the histogram, of the exposure results.

An example.
Both pics are candids.
Aperture priority and evaluative metering mode.
Both pics have been PP'd from the original RAW

(1)


Camera Canon EOS 40D
Exposure 1/4000 sec
Aperture f/4.0
Focal Length 131 mm
ISO Speed 3200
Exposure Bias -1/3 EV That's me dialling in a little "calculation" to darken the blacks and kill the bright background. In hindsight I should have possibly gone to -1.
Flash Off, Did not fire

(2) 34 seconds latter.

Camera Canon EOS 40D
Exposure 0.001 sec (1/1250)
Aperture f/4.0
Focal Length 188 mm
ISO Speed 3200
Exposure Bias +1 EV Here I wanted some detail in the drivers face so I have dialled in some positive exposure compensation to increase the exposure.
Flash Off, Did not fire

[LoveDSLR]

Calculate using Exposure Value (EV)?

[NicolaB]

Exposure is basically telling your camera how much light to let in and for how long in order to create the right creative image via the sensor of your camera - and hence recording an image. You 'calculate' or measure' it differently dependant on the look you want to achieve and what you are shooting.

Very basically....

If you want 'boke' or blur, or you want depth in an image you start in AV mode (Aperture - the size of the hole). If you then leave the shutter open too long, your image will be too bright. For not long enough, insufficient light will hit the sensor and the image will be dark or under exposed.

If you are dealing with movement, you start with TV (Shutter Speed - how long the hole is left open to allow in light). If the aperture is then too wide, too much light will hit the sensor over exposing, and too small - well you get the idea.

Your ISO also comes into play (sorry to throw a third element in). If there is plenty of light the camera can stay in a low ISO say 100 for a bright sunny day. If it is dark then the camera needs to work harder to collect the light and needs a higher iso eg 800+.

Where you focus to set the exposure - well:

A fantastic book to get you off to a running start is Understanding Exposure by Byran Peterson It will help you understand both why and how to measure or calculate your exposure in different lighting situations.

It may also be worth looking over you camera manual re metering, as it will guide you as to which type of exposure metering to use in any given situation (this will make sense when you read the manual - honest )

Hope this helps

[i speak in math]

Originally Posted by NicolaB

If you want 'boke' or blur, or you want depth in an image you ...

You can have a shallow or deep depth of field. The aperture controls this. Large apertures (f/1.4, 2, 2.8...) give you a shallow depth of field; only a small bit in the focal plane. Small apertures (f/11, 16, 22...) give you a large depth of field; lots inside the focal plane.

The quality of how the lens renders the out of focus area is bokeh. Its like describing how something tastes, not the actual food.