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Exposure Issues and SettingsShooting the best photographs starts with composition followed by choosing the best exposure. Precise exposure is more important for those shooting in Jpeg mode. When in Jpeg mode the camera does its best to choose a compromise exposure that benefits all parts of the image. That may not be what the photographer wanted. Jpeg images can be adjusted slightly with image edting software. But not a lot.
The Exposure TriangleLow contrast scenes are the easiest for the camera to properly deal with. But low-contrast photographs are seldom the most interesting. As a general rule the best way to expose a photograph is to base the exposure on a mid-tone gray. In other words if a photograph's exposure is optimized for the most accurate representation of middle gray, the photograph as a whole is (usually) represented at its best. If no mid-gray exists in the current scene of interest landscape photographers sometimes place a gray card temporarily in front of the camera, in order to test the current lighting and exposure conditions. Birding photographers seldom have that luxury.
So what does exposure mean? The amount of light that travels through the lens and focused on the camera's photo sensor is a fluid function of three parts. The iris in the lens can be set to wide open or to a narrow pin-point, both of which effect the amount of light focused onto the photo sensor. But so does shutter speed. An iris kept open for 4 seconds bathes the photo sensor in more light than a 1/2000th of a second shutter speed. The third part of the exposure triangle is camera's current ISO setting.
ISO (International Standards Organization) is light sensitivity scale for photo sensors. The lower the ISO number any given photo sensor is set to, the better the image quality will be. Low ISO settings are often referred to as "slow" settings. The higher or faster the ISO the more sensitive the photo sensor will be to any given amount of incoming light. But the lower the image quality will be. In the early days of digital photography an ISO setting of 800 often led to grainy/noisy images. As of 2020 most digital cameras make very good photographs at ISO 1600. Acceptable photographs can be made at ISO 3200. Practice and experimentation with your camera is the only way to really know what the limits are.
Contemporary digital cameras are at their best when set to ISO 400 or lower. But most can be used at ISO 800 without any perceptible loss of visual accuracy. Many modern cameras can be used at ISO 1600 with good results but there will be some at least slightly visible increase in the graininess of the resulting photo. At really high ISO speeds like ISO 3200, star-burst like artifacts or defects may begin to appear.
Lenses usually offer a range of F-stop iris opening widths that might range from as wide-open as F-2 to a pin-point opening of F-32. The narrower the iris opening (the larger the F-stop number) the more depth of field the final image will have. Conversely the wider the iris the narrower the depth of field. Note that depth of field is way to measure how much of the final image is actually in focus. Wide depth of field means most of the image appears sharply focused. Narrow depth of field means only a small front-to-back sliver of the final image is in focus, usually with both the foreground and background blurred.
Each successive increment in an F-stop number, say from F-5.6 to F-8, roughly translates to a quantitative doubling of the total light exposure transmitted through the lens, and focused onto the light sensor.