Extremely Short Focal Length


Today’s Question: During one of your online presentations I noticed that the metadata for a photo showed a lens focal length of 4mm. I’ve never heard of such a short focal length lens, so I was wondering if that was an error in metadata?

Tim’s Quick Answer: The lens focal length was accurate, but it isn’t as dramatic as it may seem. That extremely short focal length was simply a tiny lens on a smartphone with a tiny sensor.

More Detail: The meaning of a particular lens focal length can be rather unclear, depending on your frame of reference. For example, I think most photographers are familiar with the potential confusion of lens focal length when it comes to full frame versus cropped image sensors.

A 100mm lens on a camera with a full-frame sensor would provide the same cropping as a 160mm lens on a camera with a sensor that has a 1.6X cropping factor, and as a 200mm lens on a camera with a 2X cropping factor. Thus, just saying that you used a 100mm lens isn’t necessarily clear unless we know whether the sensor was full frame or cropped. Many photographers assume lens focal lengths relate to a full-frame sensor, which is based on the dimensions of 35mm film. But that assumption is not always correct since many photographers do not use full-frame sensors.

Things can get even more confusing when we’re talking about significantly different sensor sizes. The sensor on a smartphone is extremely small, even compared to a cropped-sensor mirrorless camera. As a result, the lens also needs to be small, such as the 4mm lens referenced in today’s question.

The focal lengths of the lenses on iPhone models I’ve used range from 1.54mm to 6.6mm. Those would indeed be wild focal lengths for a full-frame sensor. But the 4mm lens (from an iPhone X) actually equates to a 28mm lens on a full-frame sensor. So, while the number may seem outrageous to many photographers, it actually represents a very normal lens in the context of the sensor it is being used with.