Photography

A 50mm lens at F2.8 on a crop sensor camera will give you exactly the same depth of field and light as a 50mm lens at F2.8 on a full frame.

The difference is that the crop sensor uses less of the image circle so appears to be a narrower lens. This is where the crop factor comes into effect. The crop factor was invented in the early days of professional digital cameras, when there were no sensors that covered the full frame at the time.

It is a concept invented so that professional photographers who had years experience using 35mm film cameras could compare the field of view on the new digital cameras and select the correct lens.

After years of using the 35mm lenses they could look at a scene and know which lens they needed, but with the new smaller sensor digital cameras this would result in to cropped an image so the crop factor was used to make it easier to work out that they needed a 25mm lens when their eye's told them a 50mm full frame lens was needed.

So now that 25mm lens gives them the field of view of the 50mm, but it's still a 25mm and as the short focal length you use the deeper the depth of field you get for a given aperture, you would still have the depth of field of a 25mm at f2.8.

This is when they realised that the crop factor would also work in the description of the depth of field. So this meant that they would say that a 25mm f2.8 is the equivalent of a full frame 50mm f5.8 – but this only equated to the depth of field and not the light gathering capabilities of the lens.

All of these are parameters of the lens, they don't affect the shutter speed so the shutter speed stays the same. It also only relates to the physical size of the sensors and does not take into account the resolutions of the sensors being compared.