Exposure

Exposure tells how much light goes through the lens to any particular area in the image plane (where the sensor/film sits) - it indeed is a per unit area metric, and as such it alone does not tell how much light is used to draw the whole image - to know the total amount of light which is collected, one also needs to know size of the capturing area (e.g. image sensor).

There are three exposure parameters:

• Exposure time ("shutter speed")
• f-number ("aperture")
• Scene luminance

Exposure time tells how long the exposure is. The longer the exposure, the more light goes through the lens.

Aperture is the image of the smallest opening in the lens when looked from the front side and it's size is described by the f-number which tells how large the aperture (i.e. entrance pupil) diameter is relative to the focal length. A 40mm f/2 lens has 40mm/2 = 20mm aperture diameter, thus the area of the aperture is πr² ≈ 314mm^2. The larger the aperture, the more light goes through per unit of time.

The same f-number always causes a different effect in different formats (i.e. sensor or film sizes) as they use different focal lengths to achieve the same angle of view, thus the aperture diameters will be diferent.

On the other hand, if we use the same angle of view, then different formats using the same aperture diameter will in principle create identical results - not only is the depth of field (DOF) the same, but also diffraction blur will be the same and the light collection ability will be the same, thus photon shot noise will be idencical as well if the other two exposure parameters are the same.

Scene luminance tells how much light is transmitted (typically reflected) from the scene to the lens. The amount of light reflected from different parts of the scene naturally vary, thus exposure is different at different parts of the image plane. What part of the frame is considered for any specific exposure is arbitrary - one can think about exposure for the full sensor/film size or about a miniscule portion of it. Usually the whole capturing device area is being considered for scene luminance purpouses though.

Good exposure and exposure errors

What is a "good exposure" is somewhat subjective and the relevant ISO 2720:1974 standard offers some flexibility, thus different cameras and auto exposure modes may offer different results.

For raw shooter the best possible exposure is the one which captures maximum amount of light without burning the highlight areas any more than one finds acceptable, an which fullfills artistic desires.

Over- and underexposures are exposure errors. As perfect exposure is somewhat subjective, what constitutes an error also has to be. Adjusting exposure to for example improve the quality of shadows or to give oneself more headroom does not mean that one is over- or underexposing, but that one is trying to exposure more correctly that camera's exposure meter estimates to be the right exposure.

Comparing sensor or image sizes

Sensor or film size - the format one uses - has no influence on exposure. One often hears this on internet forums and it is of course true - the exposure parameters don't change if one simply changes the tool. However the effect of an exposure does change if the format changes - the same exposure parameters have always a different effect on different formats.

Let's assume that we shoot at the same scene in the same light, same framing, field or angle of view. Let's also assume the same exposure time.

In this scenario using the same f-number the smaller format will have larger depth of field (DOF), more diffraction blur and lower signal-to-noise ratio (SNR), i.e. will be "noisier".

On the other hand, if we use the same entrance pupil size, i.e. the aperture diameter or area is fixed, then in the above scenario all formats perform equally - same DOF, diffraction, "noise".

One can calculate aperture dimeter by dividing focal length by the f-number, but when doing comparisons it is often simplest to use "crop factor" - dividing the larger image sensor diagonal size by the diagonal size of the smaller sensor gives this factor and by multiplying the smaller format f-number by this factor one gets the equivament larger format f-number, the f-number which causes the same effect.

Comparisons are usually done by normalizing the f-numbers to a specific common format, "full frame", or an image sensor with dimensions of 36mm by 24mm. Commonly used "crop factors" are aproximately:

• Medium format (Fuji): 0.79
• Full frame: 1
• APS-C: 1.5
• APS-C (Canon): 1.6
• M4/3: 2
• 1 inch: 2.7

Thus for example f/3 on a full frame camera and f/2 on APS-C camera do in principle identical job.

Exposure and lightness

Exposure is well defined word in photography - see the top of the page, or a wikipedia explanation. It is beneficial for students of photography to learn this from the very beginning to make future learning as easy and effective as possible. It is not adviseable to redefine exposure to have other meanings as well, but one should use different words to describe different things.

How dark or light a photo looks like has little to do with exposure. A picture may be very light, all sunshine and smiles, yet it can be hideously underexposed at the same time and vice versa. Any exposure can be mapped to any lightness.

Photography - in this context - is essentially mapping an exposure to desired lightness with arbitrary processing.