View Full Version : Aperture. Exposure. What do they do?

07-10-2004, 10:28 AM
In most digital camera reviews I read, I noticed these two terms but I am not sure what they exactly do. In general terms, I simply know that higher aperture means less purple fringing. But what exactly is going on? Could you tell me what exactly is the camera doing that makes the purple fringing go down? Is there anything else in the image affected (besides purple fringing)?

I know when the camera puts too much exposure, the bright parts come out over bright. Again, what is the camera doing that makes it so bright? I mean, does it leave the lens open more then needed which makes it so bright?

Jake Conner
07-10-2004, 11:03 AM
Aperture refers to the size of the opening of the iris, the smallest, changeable opening that light passes through in the lens. It is expressed as a fraction of the focal length, so at f4, the aperture diameter is 1/4 the focal length, i.e. a 100mm lens at f4 has an iris diameter of 25mm. This is why a smaller aperture number actually means a larger aperture.

The aperture effects the amount of light that gets through the lens, every "stop" (f1, f1.4, f2, f2.8, f4, f5.6, f8, f11, f16, f22, f32, f44, f64, f88, f128) of aperture means a doubling or halving of the amount of light that gets through the lens: if you go down the scale (to a lower number) the amount of light is doubled, if you go up the scale (to a higher number) the amount of light is halved.

Aperture also affects depth of field, the amount of the frame that is in focus: at a lower number, depth of field is shallow and not much behind or in front of your subject is in focus, at a high f number, depth of field is greater and more is in focus.

Exposure refers to how much light reaches the sensor or film, it is affected by both aperture and shutter speed (the amount of time that light is hitting the sensor/film). To keep the same exposure at a lower f number, you double the shutter speed once for each stop, to keep the exposure at a higher f number you halve the shutter shutter speed once for each stop. For example, 1/400 sec at F2.8, 1/200 at F4, 1/100 at F5.6, 1/50 at F8, and 1/25 at F11 are all the same exposure. If the camera gives a picture too much exposure, then yes, the shutter was left open too long to give good exposure at the current apeture.

That explain everything?


07-10-2004, 05:01 PM
Excellent Jake!

Jeff, If you see this, copy this exactly and put it on the FAQ page. This should go right up in the top 3 items. These are the two most basic tenants of photography, and probably the least understood.

07-10-2004, 05:38 PM
Very fine post indeed!

I hope Jeff has some sort of Glossary/Photo Article/Terms section that will come along with the redesigned site. If not, I'm sure we could try and get one somewhere in there; I think it's a fine idea! I'd be very glad to contribute and write up a few things as desired.

07-12-2004, 08:27 PM
That explain everything?

Yes, it does. Thank you. Good reply!