• Photos

  • Level

  • Score

BetterPhoto Q&A
Category: New Answers

Photography Question 

Brian Kreutzer 		 

Shutter speed, exposure & appeture


I know shutter speed refers to how long the shutter stays open and appeture is the actual size of the lens opening and that shutter speed, appeture and exposure all combine to give us our picture, but what exactly is exposure? I thought exposure referred to how much light was allowed into the camera - but all these things do the same? How does exposure differ from shutter speed and appeture?


To love this question, log in above
January 11, 2006

 

doug Nelson 		  Here's part of an article for students that I wrote on the subject. Lemme know if you'd like the whole article:

Exposure is the process of delivering light to the film (or CCD/CMOS in digital terms).

When you set the ISO/ASA knob on an older film camera (or, on newer cameras, let the DX code system do it for you) you are setting your light meter to the film maker's recommended sensitivity.

Your light meter will read, depending on the exposure system your camera has, the whole frame, the whole frame with emphasis on the center, a portion of the frame, or some razzle-dazzle matrix. It makes NO difference, as long as YOU know what part of the frame in the viewfinder is being read.

Now, the easy part that everyone tries to make SO complicated:

There are two ways to adjust the amount of light getting to the film, only two! The aperture is a hole in the middle of the lens between the glass elements that is adjustable. The maximum aperture of your 50mm lens is probably f1.8 or 1.4. That’s as far as it will open. The smallest aperture is f16 or f22. The metal aperture blades close down to a tiny hole. Note that there are several aperture sizes between the largest and the smallest.

The other control is the shutter, a door with many possible durations of opening. 1/60th of a second is pretty fast; 1/500th is REALLY fast.

The film needs X amount of light to be considered a proper exposure. The film manufacturer decides what X is, and tells you on the film box. (X can vary, but we'll talk later about that.)The film does not care whether it gets the right amount of light from light passing through a huge aperture, and through a very brief shutter opening (think of it as a door) OR
whether the light is dribbled through a tiny aperture with a fairly SLOOOW shutter. It’s the SAME amount of light. For any film, there are a number of aperture and shutter combinations that deliver the right amount of light.

It's a 1 to 1 relationship between the shutter and aperture. A shutter speed of 1/125 sec at f8 delivers exactly the same amount of light as a 1/60 shutter at f11. Because I slowed the shutter down a step, I had to make a 1 step adjustment in the aperture.


To love this comment, log in above
January 11, 2006

 

David M 		  whoah.... how about this:

Exposure is the RESULT of your shutter/apeture/ISO setting.


To love this comment, log in above
January 11, 2006

 

Michael H. Cothran 		  Brian,
You're on the right track. The f-stop (aperture) is the size 'hole' in your lens that determines what volume of light is allowed into the camera, and the shutter speed determines for how long that 'hole' will be open.

Here's a silly, but accurate and (hopefully) easy-to-understand analogy -

Pretend you have an empty bucket. This is your unexposed film. Filling your bucket to the top with water represents proper light exposure on your film/digital sensor. Now, you have a choice of 4 different hoses - a 1/4", 1/2", 3/4", and 1" hose (relate to your f-stops/aperture settings). The amount of water that can come through each is the volume of water - the bigger the hose (aperture), the more water/light can get in at one time.
Lastly, once you decide on a hose size, you would turn on the water. The amount of time you have to have the water running in order to fill up your bucket is akin your shutter speeds. The bigger the hose (think aperture), the less time (think shutter speed)it would need to run in order to fill your bucket.

Example - a 1/2" hose running for 10 seconds would provide the same amount of water (film exposure) as a 1" hose running for 5 seconds.
Hope this helps.
Michael H. Cothran


To love this comment, log in above
January 11, 2006

 

William H. Grover 		 
 
 
As you probobly know, when you bump up your ISO, it is more sensitive to light. If your camera has a built in light meter, You can then adjust either the aperture or the shotter speed to get your meter where you need it. Now here's the deal. I like to do a lot of bird stuff, ya know, stop the action, flight captures. I set my ISO to 400, open the apurture all the way up and then control the expoture with the shutter speed, some times as high as 1/4000. That can get you a clean, hand held shot when the feathers are flying. Then as the evening starts to set in I will bump the ISO up to 800. That will in able me to continue to use a fast shutter speed. This is why the old timers used to call 400 film "High Speed" film.
Then on the flip side, if your doing tripod landscape stuff, you want to slow it all down. The only draw back to high speed exposures is a grainy picture.
Hope this helps you a little!!


To love this comment, log in above
January 11, 2006

 

Pete H 		  Hmmm?
Lots of explanations. Let's say we simplify this.

Exposure= e*t = v

e=light
t=time
v=value

If any variable equals 0, then you have no exposure.

Ex:
If e=1 and t=.000001 , then you have an exposure.
If e=5 and t=0..No exposure
ISO does not factor into this basic equation. ISO will only amplify and modify the (gain) or final (V) only.


Pete


To love this comment, log in above
January 12, 2006

 
This old forum is now archived. Use improved Forum here

Report this Thread