Why the term "focal length multiplier" should NEVER be used

This is a message that I posted on one of the forums at DP Review not long ago and it is something that I decided to make a permanent part of my webpage. If you have any questions about this, please email me and I'll be glad to answer them.

Whoever came up with the term "focal length multiplier" should be shot. That's NOT what it is. It is a FIELD OF VIEW CROP. I finally "got it" myself after doing this little experiment -- I suggest that everyone do the same if possible so that you can really understand what is going on.

Get a 35mm film camera alongside your DSLR. Put the same focal length prime on both -- 50mm works great because it's "life size" -- that is, the object will be the same size in the viewfinder as it is with the naked eye. Now, aim your 35mm camera at something and note what is at the edge of the frame on the top, bottom, left, and right sides. Also note that the object is the same size that it is with the naked eye. Now pick up your DSLR and center the frame at exactly the same place as you did with the film camera and notice two things:

• 1. the object you are aiming at (well, everything in the frame for that matter) is still the SAME SIZE just like it was with the 35mm film camera
• 2. those objects that you noted at the edges of the frame -- you can't see them now because those areas have been CROPPED from your view

This proves two things:

• 1. It is NOT a focal length multiplier because the objects in the frame are exactly the same size as with the film camera. If it was multiplying the focal length by 1.6, the object would be larger through the lens of the DSLR than through the film camera
• 2. It IS a FOV crop because you can no longer see the objects that were at the edges of the 35mm frame.

The ONLY way that this can be thought of as any kind of multiplier is when you get ready to print. Let's say you were going to do the above experiment and then print both pictures at 8x10. In the resulting print from the DSLR, everything would be larger than in the 35mm print because in essence you are cropping, then UPSIZING the resulting cropped image before making the print. This is no different than cropping the 35mm negative the same way then making an 8x10 from it.

There is a great article here that explains this in more detail complete with an image that shows exactly what I'm talking about...