# Focal length definitons (EFL, BFL, FFL...

#### Susurs

##### macrumors 65816
Original poster
A bit stupid question

I have found definitions for EFL, BFL, FFL... but still have no answer of what is the exact technical definition of the value given by the manufacturer (Focal Length)?? Is it a distance from the front focal pane to the convergence point where sharp image is formed? Or is it Effective Focal Length?

"By definition, the effective focal length EFL is the distance between the rear principal point P’, and the rear focal point F’ of the lens.

The back focal length BFL is the distance between the rear vertex V’ of the lens, and the rear focal point F’.

The front focal length FFL is the distance between the front focal point F and the front vertex V."

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#### dwig

##### macrumors 6502a
...exact technical definition of the value given by the manufacturer (Focal Length)??...
... is whatever number the mfg chooses to state, no more and no less. The relation between the mfg's value and any real world value is a matter of pure whimsy.

The mfg will usually claim a number that is close to the real FL, the EFL as per the link. This number will generally be accurate +/-5% for fixed focal length lenses and +/-10% for variable focal length lenses (e.g. zooms, varifocals, ...). Also, it will be the measured value when the lens' optical system is set to infinity focus. Many lenses shift FL as they are focused closer.

A MASSIVE AMOUNT OF CAUTION should be used when using the EFL term in the link. That definition applies to the study of optical physics and is quite different from the definition of "Effective Focal Length" in the real world. There are quite a number of cameras, both stand alone and embedded (e.g. cell phone cameras, ...), where the mfg quote a FL based on the real world definition, which is: EFL = the focal length need on a camera with a 24x36mm imaging area to achieve the same angle of view, measured across the image diagonal.

Susurs

#### Susurs

##### macrumors 65816
Original poster
Thank you! Can FOV angle in degrees be different for a lens of the same marketed FL coming from differenf MFG in a case where the marketed FL can mean various properties as you said? Or there are also other physical properties which have impact on angle in degrees?

#### bunnspecial

##### macrumors 604
Thank you! Can FOV angle in degrees be different for a lens of the same marketed FL coming from differenf MFG in a case where the marketed FL can mean various properties as you said? Or there are also other physical properties which have impact on angle in degrees?
The field of view is determined by BOTH the focal length and the size of the imager(whether it be film or a detector).

For the sake of convenience, field of view is often quoted in a "35mm equivalent" or in other words the focal length for a 35mm(24x36mm) lens that would produce an equivalent field of view. This is done as a convenience since 35mm has been one of the most familiar formats for the past 50 years(and now is back in prominence with full-frame digital cameras being common) and most photographers can relate to what a 50mm or 135mm or 24mm lens will show them.

With that said, the equivalence conversion works great when you're comparing APS-C(aka crop sensor) DSLRs to 35mm film or full frame sensors as both have the same 2:3 aspect ratio. It falls apart when you start comparing to other film formats-many P&S cameras(along with cell phone cameras) use a 4:5 aspect ratio, as do the common sheet film formats. Medium format film(60mm wide) can get into a mess as you have 6x4.5(almost 2:3), 6x6(which is usually cropped to print to a 5x7 or 8x10) and 6x7, which is CLOSE to a 4x5 aspect ratio.

Folks will often compare the diagonal field of view across formats, but to me this as good as useless when different aspect ratios are in the mix. I prefer to compare the horizontal angle of view.

The "benchmark" lens for 35mm has a focal length of 50mm. This is considered a "normal" lens, although some will argue for various reasons that a true "normal lens" is somewhere around 47mm. This is roughly 40º horizontal and 27º vertical. On an APS-C sized sensor, 35mm is often considered a "normal" lens. As you move up formats, you'll find the commonly referenced "normal" focal lengths as being 75mm for 645, 75 or 80mm for 6x6, 90mm for 6x7, and 150mm for 4x5.

And, of course, for smaller sensors all bets are off.