The Exposure

(Base Knowledge VI)

Exposure is a compensation caused by combination between shutter-speed and aperture –according to the selective ISO number. Can also be said as brightness or darkness level on one a photograph.

- Exposure Value (Ev)

Exposure Value is got from calculation result between speed, aperture, and ISO. Follow the formula :

EV = log2(aperture2 x (1/shutter speed) x (ISO sensitivity/100))

So, Ev is quantity of the light that need when taking one-time exposure. Follow the table Ev below (note = in ISO 100) :

For example :
When inside a Room-A, we get speed at 1/125s and F/5.6 with ISO 100. That’s mean (according to the table above) Ev = 12. And in Room-B, we get F/4, So to get same Ev as in Room-A, we best use speed at 1/250. Because Exposure value at “1/125; 5.6” is the same as Ev at “1/250; 4” –with the result : “12”.

- Exposure Value (Ev) base on Light-meter

Light meter can help us in measuring light intensity –which bounces back from our object and creating our base exposure. Exposure value can be measured by light-meter, inside of a digital camera. Which it can be set as we need, generally at range : -3, -2, -1, 0, +1, +2, +3. “Plus (+)”-number indicate the picture result is more lighter, and “minus (-)”-number indicate the picture result more darker, of course “0”-number indicate normal (relative) result. This brightness level can be measure in a few technique (metering method). So we can achieve better exposure showed on our photograph.

Basically there's only one measurement method from our camera, that is reflected-light meter, measures reflected light from our subject / object –in order to later determine combination between shutter speed and aperture.

- Relationship between Exposure and Metering

Metering is the brains behind how our camera determines the shutter speed and aperture, based on lighting conditions and ISO speed. Metering options often include partial, evaluative zone or matrix, center-weighted and spot metering. Each of these have subject lighting conditions for which they excel and for which they fail. Understanding these can improve one's photographic intuition for how a camera measures light.

As we known before, All in-camera light meters have a fundamental flaw : they can only measure reflected light. This means the best they can do is guess how much light is actually hitting the subject.

If all objects reflected the same percentage of incident light, this would work just fine, however real-world subjects vary greatly in their reflectance. For this reason, in-camera metering is standardized based on the luminance of light which would be reflected from an object appearing as middle gray. If the camera is aimed directly at any object lighter or darker than middle gray, the camera's light meter will incorrectly calculate under or overexposure, respectively. A hand-held light meter would calculate the same exposure for any object under the same incident lighting.

In order to accurately expose a greater range of subject lighting and reflectance combinations, most cameras feature several metering options. Each option works by assigning a weighting to different light regions; those with a higher weighting are considered more reliable, and thus contribute more to the final exposure calculation.

The whitest regions are those which contribute most towards the exposure calculation, whereas black areas are ignored. Each of the above metering diagrams may also be located off-center, depending on the metering options and autofocus point used.

(furthermore see this articles)