About CCHS Photography


Equivalent Exposure

It's such a basic fundamental I thought I'd write up a tutorial for our budding photographers or more advanced photographers who are so bored they are reading this right now.

Photography is Greek for writing with light (phos,phot=light + graphos=writing). In a nutshell, photography boils down to writing with light. All the film or CCD captures is light reflections. Thus, it is imperative that the correct amount of light reaches the recording surface. This is called exposure. Modern cameras have built in meters that do a pretty outstanding job at recording the light entering through the lens and adjusting exposure settings accordingly. In most cameras this is called "Program". However, program is a middle-line exposure and sometimes you don't get the effect you want. For instance, you may want motion to stop or even blur some or you may want maximum depth of field. If your camera has an aperture or shutter priority setting (on Canon they call this Av and Tv, Nikon, Minolta and others usually may have "A" and "S" on their exposure mode dial, check your manual for your camera's specific settings).

By selecting aperture or shutter priority, your camera does equivalent exposure for you, eliminating the guess work. If you need to stop motion, you set your camera to shutter priority, set the speed to fast enough to stop the motion and the camera sets the aperture. Conversely, in aperture priority the opposite occurs. You tell it what aperture you want and it sets the relative exposure with the shutter speed. That's pretty neat and makes it easy, but what if your camera doesn't have aperture or shutter priority or you want to shoot in manual exposure, then all you have to remember is that as the aperture opens, the shutter speed must increase to maintain equivalent exposure.

F-stops are fractions of the opening of the diaphragm compared to the diameter of the lens, so f/2 on a 50mm lens means that the aperture will be open 25mm, 1.4 means it will be 35mm, 1.0 means it will open as wide as the lens is. F-stops are also multiples of the square root of 2 (1.4142135623730950488016887242097), but we'll use 1.414 for this exercise.

So, starting at 1, this is the full f-stop scale (rounded):

1 1.4 2 2.8 4 5.6 8 11 16 22 32 45

f/16 lets in twice the light of f/22 and half the light of f/11.

OK, so now we know some of the math behind f-stops, the other factor in the equation is shutter speed.

Shutter Speed range (typical):
15 30 60 125 250 500 1000 2000 4000

As with f-stop, each step increases or decreases the light by a factor of 2x. So 500 is half as much light as 250 but twice as much as 1000. Remember that shutter speeds are usually reciprocals of 1 second, so 500 is 1/500th of a second.

The final part of the equation is your camera or film's ISO rating. ISO (sensitivity) ratings:

25 50 100 200 400 800 1600 3200

Again, each move in ISO is a factor of 2, so ISO 200 is twice as much light sensitivity as 100 and half of 400.

Ok, now that your eyes have glazed over, a quick review: exposure (the capture of light) is related to 3 things: aperture, shutter speed and ISO settings.

Now onto the rule of equivalent exposure. And that is: as one thing changes, the other(s) must change as well. ISO may or may not need to be adjusted.

So, lets say your camera says at ISO 200 f/11 and a shutter speed of 30 your image will be properly exposed (yes, I am over simplifying, but this a conceptual lesson). However, you want the shutter speed to be faster for whatever reason, we'll say 1/125th.

So, moving from 1/30 to 1/125th is 2 steps. Thus, you have to open your aperture by 2 stops from f/11 to f/5.6 in order to maintain equivalent exposure and retain the ISO 200 sensitivity setting.

The reverse is true, if you want to go from 1/125th to 1/30 you have to close the aperature by 2 stops.

And if you open the aperature by 1 stop, you need to increase your shutter speed by 1 step (i.e. 1/60th to 1/125th) and if you close the aperature by 1 stop you need to slow down your shutter by 1 step (i.e. 1/125th to 1/60th)

If you changed the ISO on your camera to ISO 800 you will find the camera will adjust either the aperture or the shutter speed to compensate. However, the price with higher ISO is more noise and possibly loss of detail. Also note that in Program, the camera will decide what to adjust to maintain the equivalency if you increase the ISO, so keep that in mind. Normally, ISO doesn't play a large part in equivalent exposure, unless the lighting is insufficient to achieve your desired result so you have increase or decrease the ISO setting of your camera.

Just remember that for equivalent exposure as one aspect increases, the other must decrease and maybe your camera's ISO may need to be adjusted.


"You are your own best teacher"

--Jimmy Buffett

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