Open Photography Forums  

Go Back   Open Photography Forums > Digital Camera Discussion > Imaging Technology: Theory, Alternatives, Practice and Advances.

Imaging Technology: Theory, Alternatives, Practice and Advances. This is a brand independent discussion of theory, process or device. Ignore this forum unless this matters to you!

Thread Tools Display Modes
Old January 19th, 2010, 11:50 AM
Doug Kerr Doug Kerr is offline
Senior Member
Join Date: May 2006
Location: Alamogordo, New Mexico, USA
Posts: 8,625
Default Effective focal length

I'm going to talk a little about "effective focal length", and not in the way you are probably expecting.

We are all familiar with the convention in which we describe the field of view to be given by a lens of a particular focal length, used on a camera with a particular format size, by essentially answering this question:

"What focal length lens, used on a camera with a 36 mm x 24 mm format, would yield the same field of view that this lens yields on this camera?"

That number may be reasonably called the "full-frame 35-mm equivalent focal length" of the lens of interest used on the camera of interest. Like all reasonable names, it of course does not explain the whole concept - we must have some awareness of the concept before we go fooling around with it.

I'll mention in passing that "full-frame 35-mm" is a fairly accurate and complete way to describe cameras with a format size of 36 mm x 24 mm. "Full-frame" by itself doesn't do it (that term actually refers to the largest normal frame size used with a certain film size; for "8x10" film, the full frame format is very nearly 8" x 10"). "35-mm" by itself doesn't do it; there were numerous cameras using 35-mm film ("type 135" film, to be really precise) whose format size was 18 mm x 24 mm (the "half-frame 35-mm" cameras).

Now "effective focal length" (which we sometimes hear) is not a good shorthand, if for no other reason that the one I will discuss shortly.

Note again just for completeness that the "full-frame 35-mm equivalent focal length" is not in any way "a focal length" of the lens. The focal length of a lens (for zoom lenses, assuming a certain zoom setting, and in any case, perhaps assuming a certain focus setting) is a fundamental optical property of the lens. It has it regardless of what camera it might be on, if any. It has it when in your sock drawer. And it is not defined in a way that has anything to do with format size. (We sometimes hear, "The reason we need to use a focal length factor is that the focal lengths of lenses are stated in '35-mm camera' terms". Not so at all.)

But helping is to get, or remain, confused in this area is that, in techncial papers and optical textbooks, we see reference to the effective focal length of a lens. What is that?

It is just the focal length of the lens, as I have discussed above. So why the "effective"?

To understand this, we have to go back a little in the history of optics. Initially what was noticed was that if we have an "object" at infinity, its image was formed a certain distance behind the lens - that it, behind its "rear vertex", the face of its rearmost element. That distance was called the "focal distance", or even, "focal length", of the lens.

For thoroughness, some of these workers did the same thing in reverse, putting a test object "at infinity" behind the lens, and allowing an image to be formed in front.

Then, of course, to keep things straight, they began to refer to the "front focal distance" and "back focal distance", or perhaps "front and back focal length".

Now, as the understanding of optical systems grew, the workers realized that, for any lens (even a "thick" one, or a compound lens composed of several elements), there was a critical parameter that affected such things as the magnification of the lens in a certain situation and how the object and image distance were related. In the classical case, of a test object in front of the lens at infinity, forming an image behind the lens, that was the distance to the image, but not measured from the rear vertex, but rather from some "abstract" point inside the lens.

Since it was this "length" that was a parameter of all the emerging equations, and governed many optical properties of the lens "in place", it was decided to call it the "effective focal length". And that notation is in use today in formal optical textbooks and papers.

But remember, this is just the "focal length" of which we normally speak.
Reply With Quote
Old January 19th, 2010, 12:06 PM
StuartRae StuartRae is offline
Senior Member
Join Date: Jul 2006
Location: Wiltshire, UK
Posts: 1,025

.......if we have an "object" at infinity.........
That reminds me of our old physics teacher at school. Someone once asked him "Sir, where is infinity?", to which he replied "Gentlemen, for the purpose of this experiment infinity is at the end of my desk".


C&C and edits always welcome
Reply With Quote


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are Off
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Effective focal length vs full-frame 35-mm equivalent focal length Cem_Usakligil Imaging Technology: Theory, Alternatives, Practice and Advances. 7 January 9th, 2010 01:41 PM
Equivalent focal length - New tutorial article Doug Kerr Lenses: DSLR and Rangefinder, MF adaptions to 35mm such Zoerk 0 March 25th, 2009 04:59 PM
Canon focal length converters - communication Doug Kerr Lenses: DSLR and Rangefinder, MF adaptions to 35mm such Zoerk 2 March 16th, 2009 10:36 AM
Perspective and focal length David McKinny Entry Digital Photography 5 November 19th, 2008 01:01 AM
Canon EF lenses - focal length reporting Doug Kerr Lenses: DSLR and Rangefinder, MF adaptions to 35mm such Zoerk 6 January 18th, 2008 07:48 AM

All times are GMT -7. The time now is 01:21 AM.

Posting images or text grants license to OPF, yet of such remain with its creator. Still, all assembled discussion 2006-2017 Asher Kelman (all rights reserved) Posts with new theme or unusual image might be moved/copied to a new thread!