You know those numbers on the side of that lens you just bought for your digital SLR? 70-200mm … 50mm … etc? Those numbers are a lot less absolute then they used to be?
The reason? Not all image sensors on digital SLRs are built the same, and the sensor you have has a large effect on exactly what focal length your lens actually is. Here’s why.
Back in the era of film cameras, SLR cameras had a standard, full-frame sensor. But with the advent of digital cameras, sensor sizes began to change. Sensors on the first generations of digital cameras were expensive to produce, so camera manufacturers simply made them smaller. They created the APS-C sensor, which has become the standard on most digital SLRs.
