A Focal Length is a Focal Length is a Focal Length...

Mingus Mill, Great Smoky Mountain National Park (click to enlarge)

Pick a focal length.  Any focal length.  Now attach a camera, any camera to that focal length lens.  Guess what?  There is no difference in image size.  A lot of photographers don't understand that concept.  No matter what camera you have and no matter what size sensor or film you use, the focal length of any given lens produces the same sized image.

Let's suppose we have a lens with a 90mm focal length that was made for a 4" X 5" camera.  It will create an image circle that is at least 6.5" or 165mm (the diagonal of the 4X5 film) in order to fully cover the film.  It will need more coverage if you use swings and tilts, but that is a story for another day.  Back to the story.

That 90mm lens will produce an image of a certain size to cover the full 4" X 5" film surface.  What happens if you put that same lens on a medium format camera or 35mm film camera?  The lens still produces the same sized image BUT, the film now only records the center portion of that same image circle.  The parts of the image outside the film area goes unrecorded.  Looking through the viewfinder of the medium format or 35mm flim camera, it appears the image is magnified but, in reality, it is not.  It is the same sized image.  It is an optical illusion due to the reduced viewfinder coverage. The same goes for digital imaging.

The wide-angle 90mm lens on a 4" X 5" camera has the same approximate field of view as a 28mm lens on a 35mm film camera or what we commonly term a "full frame" digital SLR.  On a digital SLR, since we are only seeing a small center portion of the image it produces, it appears to be a "telephoto" lens, which in our minds, magnifies the image.  But, in reality, it does not.  It is the same sized image as would be projected onto a 4" X 5" piece of film.  The 4" X 5" film just shows more of the same sized image.

When you have that same 90mm lens on a "full frame" digital camera, the same size image is recorded.  You can only see the center of the image because that is the only part the sensor will "see" and record.  Put the same image on an APS-C sized sensor and you show even less of the same image.  As the film or sensor gets smaller, the image size doesn't change, only the part you see through the camera's viewfinder is changed. Thus, you think that you are looking at a longer focal length lens.  It is an optical illusion–like looking through a tunnel.  We commonly call that a "crop" factor.  The only thing that changes is the field of view you can see, not the image size itself.  Erroneously, many state that, say, a 50mm lens on a "full frame" camera will act like a 75mm lens on a "crop sensor" camera.  Not true.  The image created by the 50mm lens is the same size in both cameras, but in the "crop sensor" camera, you can only see the center of that image.  It doesn't get any larger and is not magnified.

No matter what focal length you choose, for any given focal length, the lens will produce the same sized image.  It just appears different to us based upon how much of it we can see.

Thanks for looking.  Enjoy!

Dennis Mook

Many of my images can be found at www.dennismook.com.  Please pay it a visit.  I add new images regularly.  Thank you.


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