Exposure Triangle

The key to properly exposing a photo is evenly balancing the three points of the exposure triangle: ISO, shutter speed, and aperture.

• Bump up the ISO and you can use a smaller the aperture or faster shutter speed.  But the downside of a faster ISO is the introduction of noise. 
• Slow down the shutter and you can use a lower ISO or smaller aperture. Again, there's a downside. Slower shutters require both the subject and the camera to be stable.
• Use a larger aperture and you can use a lower ISO or faster shutter speed. Wider apertures is dependent upon the lens. There's a direct correlation between wide apertures and cost.

Fortunately technology is making it easier to balance the triangle:

• Manufactures are constantly trying to out do each other with higher ISO capabilities combined with better noise reduction.
• Image stabilization makes it easier to handhold a camera through longer exposures.
• Well, glass is still expensive. But lenses are getting sharper and sharper with each generation.