photography iso settings
Technical photography question using ISO to control exposure?
Ok, lets say I am using a little point and shoot digital camera…it has mainly automatic settings… but for some reason, even in landscape mode, where I would assume it would pic a low shutter speed and higher fstop number, very frequently it will pick something like 1/500 f2.8 instead of 1/60 f8…(as an example)
Here is my question…everything else being equal, if i take a picture
and the setting says 1/500 f2.8 and I am using iso 100, if i increase
the iso, say to 200, will i accomplish my goal of forcing the camera
to pick maybe something like a 1/250 f 3.5, and if I further increase the ISO, will the camera be forced to select something like 1/125 f5.6???? IE, force the camera to select a lower shutter, higherfstop number by increasing the ISO…
If I am correct, WHY would increasing the ISO cause this to happen…I am missing a basic fundamental of the relationship of how ISO can control exposure….
All cameras are just dumb boxes. Any assumptions you make about what a camera that only has Programmed Auto Exposure will or will not do in a certain situation or Scene Mode are invalid. The camera’s programming is for a correct exposure. It is incapable of reasoning and deciding “I’m taking pictures of a landscape so I’d better use f16 and a slow shutter speed.” All it can do is find a combination of f-stop and shutter speed that will produce a correct exposure.
First, a camera that uses Programmed Auto Exposure will tend to select a higher shutter speed to help minimize the effects of camera movement.
Second, increasing the ISO will result in more digital noise which will give you poor image quality, especially with a digicam.
Correct exposure is a triangle composed of f-stop, ISO and shutter speed.
ISO is a measurement of the sensitivity to light of a light sensitive surface, whether film or digital sensor. A low ISO (50, 100) is very insensitive and requires a lot of light. It also gives you the best picture quality. A high ISO (400, 800) is more sensitive and requires less light. It also causes picture quality to suffer as digital noise increases.
The aperture is the opening in the diaphragm inside the lens and controls how much light is admitted. A large f-stop (f1.4, f2) admits all the light. A small f-stop (f11, f16) admits very little light. Our shutter speed is determined by the ISO we set and the f-stop we choose. The aperture also helps determine our Depth of Field (DOF) in conjunction with the focal length of the lens being used. DOF is defined as that area of acceptable focus in front of and behind our main subject.
The shutter speed determines how long the light admitted by the f-stop we chose is allowed to expose the film or sensor, based on the ISO we chose. If we use ISO 50 and f16 our shutter speed will be much slower than will be if we use ISO 400 and f2.8. It also determines whether motion in our scene will be stopped (fast shutter speed) or blurred (slow shutter speed).
One of the best ways to understand the exposure triangle is to apply the venerable “Sunny 16 Rule.” It was developed when cameras were 100% manual and often did not have a built-in light meter. Here is what it says: “On a sunny day, set your aperture to f16 and your shutter speed to 1/ISO.” So on a sunny day using ISO 50 we would shoot at f16 @ 1/60 sec. (Remember, shutters were mechanical back then and your only choices were 1, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000. Today’s cameras have electronically controlled shutters which are essentially “stepless” and will more closely match the 1/ISO component of the Rule). A further limitation was most cameras had a maximum shutter speed of 1/1000.
ISO 50
f16 @ 1/60
f11 @ 1/125
f8 @ 1/250
f5.6 @ 1/500
f4 @ 1/1000
ISO 100
f16 @ 1/125
f11 @ 1/250
f8 @ 1/500
f5.6 @ 1/1000
ISO 400
f16 @ 1/500
f11 @ 1/1000
As you can see, using a camera that does not allow you to choose Aperture Preferred or Shutter Preferred is very limiting. You are at the mercy of the camera’s programming and, as previously stated, in Programmed Auto Exposure the camera will always choose a higher shutter speed over a smaller f-stop simply to minimize the effects of camera shake.
Nightclub Photography TV Ep. 6 (Part 1): ISO Settings
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