We can simply add more light, though that’s an expensive approach that might be a big ask of a production that’s already paying for a big-chip camera. Higher sensitivity on a digital camera just means gain, which increases noise, compromising exactly the things we were trying to achieve with a bigger sensor to begin with. Stop down, though, and we’ve darkened the picture, so we might have to select a higher sensitivity. So, we might have to stop down to get to the same depth of field, to make accurate focus achievable. Longer lenses magnify things more, so out of focus areas look more out of focus, which is where we get the idea that longer lenses reduce depth of field.

What does that mean? Well, make a bigger-chip camera for lower noise and we end up using longer lenses to achieve the same field of view. HDR: Part 7 - LEDs - For Better For Worse