Cropping Factor Effect


Today’s Question: Please help me understand the difference in field of view (really image size) between an ASP-C lens on a crop frame body and a full frame lens on a full frame body.

For example, would a 11-16mm APS-C lens set to 16mm mounted on my camera body with a 1.6X crop factor have the same angle of view as a Canon 16-35mm full frame lens set to 16mm mounted on a full frame body? Would both images be the same size?

I understand about multiplying the focal length times the crop factor when switching a lens between bodies. I don’t understand the difference, if any, in image size between a full frame lens and a crop frame lens when each is mounted on its respective body and set to the same focal length.

Tim’s Quick Answer: A 16mm lens on a “cropped” sensor will not provide the same field of view as a 16mm lens mounted on a “full frame” sensor. The field of view of the actual lens remains the same in both cases. However, because the smaller sensor is capturing only a portion of the image circle projected by the lens, in this case the cropped sensor camera will produce a field of view equivalent to a 25.6mm lens (16mm X 1.6) on a full-frame camera, assuming a 16mm focal length lens in both cases.

More Detail: I think most photographers understand the basic concept of a “cropped” sensor. Because the 35mm film capture format became such a popular format, it is used as the basis for a great deal of photographic equipment. So we have “full frame” digital cameras that have an image sensor that is essentially the same size as a single frame of 35mm film. As such, with a “full frame” camera your lenses will provide the same field of view you may have come to expect when using a given focal length lens with a 35mm film camera in the past.

A camera with a “cropped” sensor is simply capturing a smaller area of the image circle projected by the lens. So while you aren’t actually getting any extra “zoom” factor from your lens, you are getting a smaller field of view that matches what a longer focal length lens would achieve with a full frame camera. So you’re getting a zoom effect without an actual optical zoom.

For example, on a camera with a 1.6X “cropping factor”, a 100mm lens would produce the same field of view as a 160mm lens on a full frame camera. You aren’t truly zooming, at least from an optical standpoint, but you are achieving a smaller field of view equivalent to that of a longer focal length lens.

An additional point of confusion can result from lenses that are specifically designed for “cropped” sensors. However, the only real difference here relates to the size of the image circle being projected by the lens. A lens designed for a cropped sensor camera generally can’t be used on a camera with a full-frame sensor, because the image circle projected by such a lens will not cover the full area of the image sensor on a full-frame camera.

The bottom line is that there is a relationship between focal length and field of view. However, the specific field of view achieved with a lens of a given focal length depends upon the size of the image sensor being used to capture the scene. Ultimately the real challenge here is in describing the behavior of a lens while talking about the focal length of the lens.

A great solution in my opinion would be for lenses to be described not on focal length but instead based on field of view. But again, that field of view depends upon the size of the image sensor used to capture an image, and many lenses can be used with cameras of varying sensor sizes. In other words, there isn’t really a simple way to describe the field of view of a lens unless you also refer to a specific sensor size.

It is worth noting, by the way, that whatever field of view you’re ending up with for a given sensor and lens focal length combination, in general you are making use of the full image sensor to capture that scene. Thus, while a smaller sensor doesn’t truly provide additional zoom for a given lens, it can still provide excellent image quality for that image with a smaller field of view. In other words, we are largely talking about semantics here. Once you’re using a given camera, you can simply focus on the field of view you want to obtain when making a decision about which lens to use.