Originally posted by RuiC With film cameras when you have a fixed ISO there are only two variables to fix exposure: Shutter time and Aperture, no other variables. With DSLR cameras you have also the ISO setting variable. Exposure is NEVER dependent on FL. In any camera, be it at Av or Tv or M or whatever, you're setting exposure alone. And any camera sets exposure parameters based only on these 3 Variables, said shutter time, aperture and ISO. These are BASICS from Photography. Please tell me which camera/lens combo, name it please to see if I can understand, will enter FL in exposure settings. Or are you referring to variable aperture Zooms? With these of course the exposure values change because when you zoom in or out (changing FL value ) you are varying the Aperture automatically, so either Shutter time or ISO will change when in Av mode. When in Tv mode the changes would be in Aperture or ISO.
Much appreciated.
Rui
Wow... can you read? I will try to break it down for you.
I know how exposure works. Probably more than most people, down to the bits of light and logarithmic stops - I am a theory person.
Since on a DSLR, it has the ability to
control ISO automatically, it
intelligently selects a
shutter speed based on the
focal length for hand shake purposes, varying the ISO to obtain it (if in Av mode). In Sv and Pv mode, it also selects a
shutter speed based on the
focal length. The lens
digitally communicates with the
digital SLR to tell it which
focal length the lens is at. This information is even
stored into the
EXIF data. The camera needs to know a goal of what shutter speed to select because today, there are 3 variables it can control by itself, when there only used to be 2. Since there is an extra variable, cameras now need to know what to aim at for one of the variables. This could be a shutter speed that will acceptably bypass handshaking, an aperture that has a max MTF, or an ISO that has an acceptable amount of noise.
The next part I am getting a little nit-picky about the camera communication, feel free to not read this if you do not understand how it works. No I am not referring to
variable aperture zooms, because in no way does the camera really know the true aperture. It would actually be
digitally impossible to communicate the exact aperture of a variable aperture zoom, you could only get it accurate to a certain precision. What is told, instead, is the
fraction of light that will be received compared to wide open. The f/number you see in the camera on these digital lenses is
fake and not even used to determine the final exposure. In fact, no lenses transmit their exact true f-number, just what fraction of light is expected to be received by the sensor or film. This is because aperture is a ratio of lengths that can never be exact, so they are approximate. Even if lenses could transmit the exact f-number, it would be absolutely pointless because each lens, through design or even manufacture tolerances, robs light a different amount before it gets to the sensor.
This doesn't matter to a camera because it doesn't need to know the aperture, it just needs to know the light it will receive. Thus, the 3 things that
really determine exposure is the rate of light coming in (determined as a fraction of the light currently coming in), length of light coming in (shutter), and amplification of light (ISO).
So please don't explain the basics in my thread in a condescending matter, incorrectly assuming I don't know anything about photography. This isn't the beginner's corner.