I post too Much on Here!!
- Jul 21, 2010
Indeed.…so many people get it wrong…
That’s technically correct, but ignores the effect of sensor size on perceived noise, i.e., the concept of equivalence also applies to ISO.To say f/2.8 on full frame is "equivalent" to f/4.5 on APS-C *might* be valid if your context is DoF, with all other factors being the same. But the amount of light the lens transmits to the sensor per unit area does not change, so in terms of exposure calculation for low-light scenarios, the lens is f/2.8 for either sensor.
In practice, that means that if using the same exposure settings for a lens on FF and APS-C, the image from the latter will have more noise. Conversely, with a FF sensor one could choose a lower ISO setting for less noise, a faster shutter speed with the same noise, or a narrower aperture with the same noise to match the deeper DoF.
Larger sensor: equivalent…but better.