Exposure question

darkchild · Sep 23, 2010

What does "exposure" mean? i hear people saying exposure a lot. like on the lightning pics everyone says "30 seconds exposure" or 1.3 exposure, what does that mean?

white · Sep 23, 2010

It means exposing the light-sensitive material for a set duration of time in order to produce an image.

LCARSx32 · Sep 23, 2010

Whenever you take a picture you are "exposing" the sensor (or film) to light. So the process of taking a picture is commonly referred to as an "exposure".

It' also is used to mean that the correct settings were used. People will say "over exposed" to mean too bright, "under exposed" to mean too dark.

*EDIT*

white said:
It means exposing the light-sensitive material for a set duration of time in order to produce an image.

Damn. You beat me to it! lol. You're fast!

DanFinePhotography · Sep 23, 2010

I would highly recommend Bryan Petersons book "understanding Exposure". you can find it on Amazon Here

Bram · Sep 23, 2010

Google
Wikipedia

??

KmH · Sep 23, 2010

Exposure is controlled by 3 adjustments:

How big the opening (aperture) is in the lens. The bigger the opening, the more light being let in. This also controls how much of the scene, from front to back, is in focus, and is called Depth-Of-Field (DOF). Lens aperture is expressed as a fraction of the lens focal length. When the lens opening is 1/2 as wide as the lens focal length it is called f/2, the f substituting for the number 1. If the lens opening is 1/4th the lens focal length, the lens aperture is set to f/4, and so on.
How long the shutter is open. The longer the shutter is open, the more time there is light to hit the image sensor (or film), for things to move and blurr because of that movement. Obviously, how long the shutter is open is not relevent for stationary objects. The shutter is usually only open for a fraction of a second in daylight, perhaps only 1/4000th of a second. Night time exposure may be for several seconds.
The last part of controlling exposure is called ISO and is a measure of how sensitive the image sensor (or film) is to light. In bright daylight, low ISO values (100) are used, at night high ISO values are used (6400).

darkchild · Sep 23, 2010

ok so what does "30 second exposure" mean?

white · Sep 23, 2010

:madmad:

uke: :banghead: :gah:

lyonsroar · Sep 23, 2010

darkchild said:
ok so what does "30 second exposure" mean?

The camera is sensitive to light (shutter is open) for 30 seconds.

KmH · Sep 23, 2010

darkchild said:
ok so what does "30 second exposure" mean?

It doesn't say anything about how sensitive to light the camera is, it just says the shutter was open for 30 seconds.

In a 30 second exposure, anything that moves while the shutter is open will be blurred. A 30 second exposure is usually only used when there is very little light getting to the image sensor (or film).

Any comment about exposure that deals with time - means they are talking about shutter speed. Comments about aperture or f-stops, are about the size of the lens opening. Comments about ISO, are about the image sensor (or film) sensitivity to light.

You may hear someone say to add or subtract a 'stop' of exposure. A doubleing of an exposure value, be it shutter speed, lens aperture, or ISO is adding a 'stop'. A halving of an exposure value, be it shutter speed, lens aperture, or ISO is subtracting a 'stop'.

In the case of lens aperture, a doubleing or halving is refering to the area of the lens opening, not the diameter. As a consequence f-stop numbers are a function of the square root of 2: √2.