Photographers: Explain EV to me

Seems like a pretty simple concept, no? Using an easily tabulated or calculated number to express the correlation between aperture and exposure time. It’s also frequently associated with the film speed of ISO 100 in the literature to give consumers an idea of the range in light values a particular camera can handle.

Or maybe not.

For example, let’s look at some camera specs – how 'bout, let’s say…the Yashica T4 Zoom and the Nikon One Touch Zoom 90s? The Yashica is capable of an exposure range from 5 EV to 17 EV, calculated at ISO 100. At the low-light end, the camera has a maximum aperture of f/4.5 and shutter speed of 2 sec. on autoexposure. Plugging the aperture (using f/4 as an approximation), exposure time, and ISO numbers into the calculator linked to above, we get a result of 3 EV. That’s 2 whole stops less than what the camera spec says. And for high-brightness scenes, the camera has a minimum aperture of f/8 and shutter speed of 1/250 sec. (at 70 mm) – and the calculator returns a result of 14 EV, which is 3 stops less than the camera manufacturer’s number.

Similarly for the Nikon, it is capable of an exposure range from 6 EV to 18 EV with ISO 100 film and the lens at 90 mm. Taking the smallest aperture (f/10.5) and fastest shutter speed (1/250 sec.), the calculator returns 15 EV – also a 3 stop difference from the manufacturer’s specs. At wide-open (f/4.8, 2.6 sec.), the calculator gives us an approximate result of 3 EV, which, once again, is way off from the manufacturer’s number.

So, why the difference? I just might accept the explanation that reciprocity failure causes unpredictable results with long exposures, but numbers such as 1/250 sec. at f/8 or f/11 seem about as “normal” of an exposure setting as one might expect, no? Did I mess up the calculations, or what concepts have I missed?

Remember that EV (for a given film speed rating) is a measure of light level, formulated in such a way to make calculating camera settings straightforward. It is not a measure of those settings.

When manufacturers quote an EV range they are specifying the range of light levels that the built-in light meter can cope with, not the range of possible exposure settings which the lens/shutter combination can make.

Just to illustrate, I had a Hasselblad camera with a Synchro-Compur shutter built in to the lens, which was graduated in EV numbers, as well as the standard selection of shutter speeds and apertures.

Using either the accessory light meter, that only gives EV settings, or a Lunasix, that also gives EV settings, it was a very simple step to select the EV number and then choose either the shutter speed or aperture, depending on reigning priorities, with the shutter and aperture being synchronised to the EV scale the camera would select the other setting.

It very often happened, when using fast film in bright conditions, or conversely, slow film in reduced lighting, that there was a very limited range of shutter speeds and apertures available, but this would have been the case anyway. The Synchro-Compur system just brought it home much more graphically.

More modern cameras with a programmed exposure control just do the same thing rather more elegantly and unobtrusively.