# Measuring Lighting with two values or one

Discussion in 'Beginner Questions' started by russell_spears, Jun 25, 2009.

1. ### russell_spears

Why do some photographers only give the f stop when discussing lighting, while others give both a shutter speed and aperture. Only giving both makes since to me, unless they are just giving a relative measure that is good for whatever EV others may be working with?
How do I know how they shot if they do not give both settings?

2. ### sam_mahmoud

If you saw this in the context of studio lighting or flash, it does make sense to only give an aperture, because the shutter speed turns out to be almost irrelevant.
As an example, let's suppose you keep your shutter open for a long time, like 5 seconds. Some time during that 5 seconds, the studio strobes (or your flash) emit a huge bright pulse of light. Let's just arbitrarily say that the pulse of strobe light lasts 1/1000 of a second. You really get all your exposure during that 1/1000 of a second. The rest of the 5 seconds your shutter is open, there's a tiny bit more exposure accumulating from the relatively dim ambient light in the room, but that exposure is so little compared to the exposure during the strobe that it doesn't matter.
As long as you are working under these kinds of conditions, that shutter speed of 5 seconds could have been quite a lot longer or shorter and it would make no difference.
Obviously the shutter speed is not 100% irrelevant, just "irrelevant within reason." It must not be so fast that you don't capture the whole strobe pulse, it must not be so fast that it exceeds your camera's rated flash sync speed, and it must not be so slow that the ambient-light exposure starts to be comparable to your strobe-light exposure.

3. ### pete_harlan|1

Depends on the imge if shutter speed is relevent information.
Photographing a grove of trees on a sunny afternoon for instance..who cares what the shutter speed is? f/stop will give the novice an idea why the DOF is what it is.
On the other hand, a photo of a bumble bee with wings frozen in flight makes the SS more relevent.

4. ### William MichaelModeratorStaff Member

My guess, too, is you have read text or commentary pertaining to studio Flash lighting - as Sam described. Flash Studios are usually dark (comparable to the intensity of the Flash), making the Tv (shutter speed) mostly irrelevant. A reference to the exact text you are citing would be useful to make more precise comment, rather than a guess.

"How do I know how they shot if they do not give both settings?"

To know what is really going on it is usually good to know all three exposure parameters, being: Aperture / Shutter Speed / Sensitivity (ISO).
And when commenting on Studio Flash, often DISTANCE from the Flash Head to the subject is mentioned as a factor of the computation. Another factor mentioned might be the intensity of the Flash, usually as "POWER" (eg Full / Half / Quarter / etc)

WW