BetterPhoto Q&A
Category: New Answers

FIle Sizes, and EV vs. ISO

1. On a digital camera, what is the net difference (or advantage of one over the other) between increasing the E.V. or increasing the ISO level? It seems to me that both these functions do the same thing and are interchangeable.

2. Image size vs. compression?
Hypothetical scenario.... say desired file size of 3MB. Is it better to reduce the megapixels or to increase compression to achieve this goal? Is the net result in terms of quality the same?

Thanks.

Dave

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April 28, 2003

EV = Exposure Value
The full name for it: Additive Photographic Exposure System. Units of EV are "stops" of exposure . . . be it an aperture f-stop or shutter speed stop . . . which doubles or halves the amount of light (depending on whether you increase or decrease exposure). The entire system is based on an equation with four values:
EV = BV + SV = AV + TV
BV = brightness value (how bright subject is)
SV = speed value (film speed or the digital equivalent)
AV = lens aperture value
TV = time value (shutter speed)

Basically the equation states that subject brightness in combination with film speed determine a combination of lens aperture and shutter speed for the desired exposure. Note that a change in shutter speed combined with the equivalent change in aperture to compensate for it (or vice versa) results in the same exposure. This is called the Reciprocity Principle.

A faster the film speed or the higher subject brightness give a higher EV number. It also means the faster the shutter speed and/or the narrower the lens aperture. If nothing else is changed, increasing an EV number is the same as increasing film speed (ISO level). In ISO units, doubling the film speed is the same as increasing the EV by one. Halving the film speed (in ISO units) is the same as decreasing the EV by one.

Note that the equations for finding units of EV from shutter speeds measured in seconds, lens apertures measured in f-stops, film speed measured in ISO (formerly ASA) units and subject brightness (luminance) measured in foot-Lamberts are not that simple. The relationships between all of them and EV units are logarithmic.

I'll let the digital gurus answer the image size versus compression level question in detail. Based on working with film scans, I suspect it's a trade-off in which you don't want to go too extreme with either and that a balance between them works best.

-- John

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May 02, 2003