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The Exposure "Stops" Here

An understanding of exposure concepts is central to producing good photography on a consistent basis. But exposure concepts involve math.

I've written previously about exposure and what a stop is, but to review, let's start with the basic concept of a stop: the doubling or halving of any of the variables dealing with exposure. This could be the shutter speed, aperture or film speed. Remember that in the case of aperture, the number series used varies proportionally to the diameter not the area of the opening so they only change by the square root of two even though the size of the opening (the area) doubles and halves.

Increase your shutter speed by one stop and decrease your aperture by a stop and you get the same exposure. The change may impact depth of field and other aspects of the image, but the reciprocal relationship between the two means your exposure will stay the same. Add in the possibility of changing your ISO (easier by far when shooting digital than film) and you can begin to see that a lot is possible without affecting exposure. With three related but independent variables that can all be measured in stops, it can get a bit confusing comparing different exposures.

This is where EV and LV come in.

EV, or "Exposure Value" is a measure commonly used by light meters. While keeping ISO constant, equivalent combinations of shutter speed and aperture are all assigned the same EV number. An exposure of 1 second at f/1.0 is arbitrarily designated as EV 0. From there, you can count EV numbers by counting stops. 1 second at f/2.0 would be EV 1, at f/2.8 you would have EV 3, and so on. Equivalent combinations have equivalent EV numbers. Thus, 1/30 second at f/5.6 gives you EV 10, but so does 1/60 second at f/4.0 and 1/15 second at f/8.0. The following table shows the EV for typical ranges of shutter speed and aperture:

  f/1.0 f/1.4 f/2.0 f/2.8 f/4.0 f/5.6 f/8.0 f/11 f/16 f/22
1 sec 0 1 2 3 4 5 6 7 8 9
1/2 sec 1 2 3 4 5 6 7 8 9 10
1/4 sec 2 3 4 5 6 7 8 9 10 11
1/8 sec 3 4 5 6 7 8 9 10 11 12
1/15 sec 4 5 6 7 8 9 10 11 12 13
1/30 sec 5 6 7 8 9 10 11 12 13 14
1/60 sec 6 7 8 9 10 11 12 13 14 15
1/125 sec 7 8 9 10 11 12 13 14 15 16
1/250 sec 8 9 10 11 12 13 14 15 16 17
1/500 sec 9 10 11 12 13 14 15 16 17 18

The difference between any two consecutive EV numbers is one stop. Note that these values are independent of film speed and are also independent of the actual amount of light being reflected by our subject. They only measure the amount of light allowed to reach our film (or digital sensor) as determined by the combination of shutter speed and aperture.

If we want to talk about absolute levels of brightness we have to move from EV numbers to LV, or "Light Values."

If we are shooting at ISO 100, EV numbers are the same as LV numbers. If we set the shutter speed to 1 second and the aperture to f/1.0 (OK, just pretend for the time being that you have such a lens) with the ISO set to 100 and point the camera at something that makes the meter read dead even on zero, we are seeing a Light Value of LV 0. Switch five stops to a more reasonable f/5.6 and find something that centers the meter and we would be seeing LV 5. Since we stopped the lens down but the meter still reads zero, it must be brighter out there than it was at f/1.0. While the scale is open ended, the typical range of LV found in nature runs from about LV 1 to around LV 17 as can be seen in the table below:

LV
Brightness of typical scene
0 Very dark indeed.
1
Dark scenes at night.
2
Very dim lighting. Moon lit scene.
3 - 4
Typical candle light, Night time shot by street lights.
5 - 6
Typical indoor lighting.
7 - 8
Bright indoor lighting.
9 - 10
Outdoor light just before sunrise or just after sunset.
11 - 12
Daylight scene under heavy cloud cover..
13
Daylight scene under average cloud cover. Sun not visible.
14
Side lit medium-toned subject under average daylight conditions.
15
Front lit medium-toned sibject under average daylight conditions.
16
Bright sunlight reflecting off of water or snow.
17 - 20
Only the brightest reflections from shiny objects outdoors in full sun.

At first, this may seem somewhat counter-intuitive (a lot of things in photography are like that aren't they?). If you are shooting ISO 100 at 1/60 second and stop down from f/8 to f/11 your viewfinder will get darker. If LV = EV at ISO 100, we just went from LV 12 to LV 13 (consult the EV table to check me on this). But based on the LV table shown here LV 13 is brighter than LV 12. How come the viewfinder gets darker? Easy: reality didn't get brighter to compensate as it should have. The LV scale deals with how bright things actually are, not with how much light reaches our film or CCD. In order to keep the meter reading the same, we have to point our camera at something one stop brighter. Hence, LV 13 really is brighter than LV 12.

A typical front-lit daylight subject will be around LV 15. Thus, any shutter speed and aperture combination that gives us EV 15 when using ISO 100 will work for a "medium toned" exposure (perhaps a gray card). From the section on EV, we see that the common "Sunny 16" exposure of 1/125 second at f/16 would give us EV 15 so things hopefully are making some sense here. Equivalent exposures such as 1/500 second at f/8 or 1/60 second at f/22 will also give us EV 15. Since these combinations are equivalent the light needed to keep the meter at zero would still be the same: LV 15.

If we increase to ISO 200, the same amount of light would force us to lessen either shutter speed or aperture by one stop. Thus, for ISO settings other than 100, EV does not equal LV. Indeed, it is off by the number of stops our film speed differs from ISO 100. For example:

ISO Difference from LV EV for "Sunny 16"
50 LV - 1 14
100 LV 15
200 LV + 1 16
400 LV + 2 17

Remember, both EV and LV work in stops, so once you understand what each is, translating between them should be fairly straightforward. Both are quite useful tools when thinking about equivalent exposures and for comparing between exposure combinations. You can also use LV as a starting point for estimating exposure if your meter is broken or you want to double check it. Or of course if you're just looking for a fun way to test your math skills.

As a side note, the origins of the term "stop" are somewhat lost in mystery today, but leading theories tie it to the method used in early photography for changing the size of the aperture. The opening was changed by turning a board having varying sized holes, each differing from the previous one by a factor of two. By selecting a specific hole, the correct amount of light was allowed to reach the film. Each successive hole existed at a possible "stop" of the rotating wheel. Of course today we can actually vary a single hole by means of the interleaved aperture blades to any size we need rather than having to choose between a number of fixed holes. The unit of measure remains nonetheless, even though we can now pick values between stops and there's no need to actually stop at a stop.


Date posted: October 31, 2004

 

Copyright © 2004 Bob Johnson, Earthbound Light - all rights reserved.
Permanent link for this article
 

Previous tip: The Rule of Thirds Return to archives menu Next tip: Matrix Metering with Digital

Related articles:
F/stop Isn't Really an "F" Word
Making Better Images With The Photographic Triangle
Some Thoughts on Exposure in the Era of Digital Photography
Manual Metering Doesn't Have to Mean Using a Gray Card
Why Shutter Speed and Aperture Numbers are Upside Down
Don't Forget the ISO
 

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