This subject has been covered from other angles but the following description might help make the subject clearer. I have summarized this from a PDF that I posted on my web site. The PDF and associated Excel spreadsheet in my next post.
To record a gray card as gray you need to balance ISO with exposure value (EV). EV is a combination of aperture and shutter speed so together ISO, aperture and shutter speed make up the exposure triangle.
The fundamental assumption underlying EV is that an exposure of 1 second at f/1 equates to an EV of 0.
An exposure of one second is given a value of 0. Each doubling of the shutter speed increases the value by one:
1 sec 1/2 1/4 1/8 1/15 1/30 1/60 1/125 1/250 1/500
0 1 2 3 4 5 6 7 8 9
An aperture of f/1 is given a value of 0. Each full stop of reduced aperture increases the value by one:
f/1 f/1.4 f/2 f/2.8 f/4 f/5.6 f/8 f/11 f/16 f/22
0 1 2 3 4 5 6 7 8 9
The shutter speed and aperture values are added together to get the exposure value (EV). You can see how aperture and shutter speed are combined as EV in this table from the back of a Rolleiflex TLR:
F: 2.8 3.5 4 5.6 8 11 16 22
1 sec 3 3.5 4 5 6 7 8 9
/2 4 4.5 5 6 7 8 9 10
/4 5 5.5 6 7 8 9 10 11
/8 6 6.5 7 8 9 10 11 12
/15 7 7.5 8 9 10 11 12 13
/30 8 8.5 9 10 11 12 13 14
/60 9 9.5 10 11 12 13 14 15
125 10 10.5 11 12 13 14 15 16
250 11 11,5 12 13 14 15 16 17
500 12 12.5 13 14 15 16 17 18
You can expose with an EV of 10 at several offsetting shutter speed and aperture combinations. In some film camera lenses (like Hasselblad) EV is mechanically linked so that you can change both shutter speed and aperture while keeping the EV constant.
ISO can also be expressed it as a logarithmic series:
25 50 100 200 400 800 1600 3200 6400 12800
2 1 0 -1 -2 -3 -4 -5 -6 -7
When everything is combined:
1/500 sec 9
f/16 8
EV (subtotal) 17
ISO 400 -2
Total 15
Recommended Exposure ValuesYou will often see published EV tables based on ISO 100. So long as you adjust for ISO, these can be used as a reality check or for actually getting a starting exposure in unfamiliar situations. For example:
EV Lighting Conditions*
16 Light sand or snow in full or slightly hazy sunlight (distinct shadows)
15 Typical scene in full or slightly hazy sunlight (distinct shadows)
Rainbows: Clear sky background
14 Typical scene in hazy sunlight (soft shadows)
Side lighting in full sunlight
Rainbows: Cloudy sky background
13 Typical scene, cloudy bright (no shadows)
Back lighting in full sunlight
12 Typical scene, heavy overcast, Areas in open shade, clear sunlight
Sunset
11 Subjects in deep shade
10 Neon lights at night
9 Night sports, ice shows
8 Night street scenes, window displays, circuses
7 Fairs and amusement parks at night
6 Home interiors
5 Christmas tree lights outdoors, night vehicle traffic, subjects lit by bonfires
4 Floodlit buildings, monuments and fountains
3 Fireworks
2 Distant view of lighted buildings
* Taken from the
Wikipedia article on Exposure value where you can find additional information.