Exposure triangle explanation please.

Started Jul 9, 2013 | Questions thread
bobn2 Forum Pro • Posts: 58,594
Re: Not exactly

TTMartin wrote:

texinwien wrote:

clack wrote:

texinwien wrote:

For starters, ISO is not a variable in the exposure equation. The three variables in the exposure equation are:

  1. Scene Illuminance
  2. F-stop (actually T-stop, to be 100% correct)
  3. Shutter Speed

APEX camera exposure equation:

A: relative aperture (f-number)
T: shutter speed in seconds
B: average scene luminance ("brightness")
Sx: ASA arithmetic film speed (ISO goes here)
K: light meter calibration constant

K is a constant, B is not part of the camera settings.
This leaves the camera operator - or, camera control chip - with A, T and Sx.

Voila, the exposure triangle is part of 'the' exposure equation...

That's the wrong formula. It's an artificial formula built specifically to tell the photographer how to adjust the exposure values (aperture, shutter speed, scene luminance) to match the ASA / ISO.

Exposure (photography)

In photography, exposure is the amount of light allowed to fall on each area unit of a photographic medium (photographic film or image sensor) during the process of taking a photograph.

Note: Nothing about ASA or ISO above, and neither ASA nor ISO have any effect on the amount of light allowed to fall on each area unit of a photographic medium.

Exposure is measured in lux seconds, and can be computed from exposure value (EV) and scene luminance in a specified region.

What's EV, and how do we calculate it? Let's see:

Exposure Value

EV corresponds simply to a combination of a shutter speed and an aperture setting, independent of any ISO setting—independent even of whether there is film in the camera or any light available.


So, Exposure can be computed from scene luminance and EV (which, in turn is computed from the shutter speed and aperture setting). This leads us back to my previous post - the only 3 variables involved in the equation used to determine photometric exposure are scene luminance, shutter speed and aperture setting (or t-stop to be extra technical).

From the Exposure article linked above, this is the equation (photometric / luminous exposure) you need:

Hv is the exposure, Ev is the image plane illuminance (determined by the scene luminance and aperture setting) and t is time (aka the shutter speed).

et voila, APEX isn't the equation used to determine photometric exposure. It's simply a formula that photographers can use to adjust the three variables that make up the photometric exposure equation - shutter speed, aperture and scene luminance (where possible) - based on a given ASA or ISO.

Interesting how that Wiki article has been manipulated through the years, to imply that photographic exposure has nothing to do with the photographic medium. It has been changed from a definition of photographic exposure. To simply one of light exposure, which is incorrect.

No, it's correct. The definition now used concurs with all the classic sensitometry texts, including my old Focal (Ilford) Manual of Photography, which I had before the Web existed. the present definition is definitely not something thought up by Wikipedia, it is the correct and science based definition of exposure.

-- hide signature --


Post (hide subjects) Posted by
(unknown member)
Keyboard shortcuts:
FForum PPrevious NNext WNext unread UUpvote SSubscribe RReply QQuote BBookmark MMy threads
Color scheme? Blue / Yellow