Index  •   • New posts  •   • RTAT  •   • 'Best of'  •   • Gallery  •   • Gear  •   • Reviews
New posts  •   • RTAT  •   • 'Best of'  •   • Gallery  •   • Gear  •   • Reviews

Thread started 10 Aug 2009 (Monday) 00:43

# Focal length (XX mm) question

Aug 10, 2009 00:43 |  #1

Hey all, quick question about focal lengths.
I've researched this and while I've found many technical details of focal length and angle of view and all that, I haven't found a definitive definition of what the millimeters in the focal length is actually a measurement of.

-Is it the exact mm measurement between a certain part of the lens elements and the focal plane (sensor/film)? Is it the measurement of any obvious thing viewable by your own eyes relating to the camera/lens/subject??
Does it have to do with converging/diverging of light? It must be a measurement of something obvious and simple right?

Obviously, these focal lengths in mm are very very short distances (e.g. 50mm equals just about only 2 inches).

So what is it?!

EDIT: I got this from Wiki, but I still don't understand exactly what the measurement is:
"A system with a shorter focal length has greater optical power than one with a long focal length; that is, it bends the pencil of rays more strongly, bringing them to a focus in a shorter distance."

 LIKES 0

Aug 10, 2009 01:14 |  #2

The focal length of a simple lens such as a one-piece-of-glass magnifying glass is the distance between the center of that lens and where the image is created behind it. In a more complex lens, the definition gets a lot more technical (and I'm not really qualified to answer that question without researching it).

Skip Douglas
A few cameras and over 50 years behind them .....
..... but still learning all the time.

 LIKES 0

Aug 10, 2009 02:56 |  #3

Guts311 wrote in post #8430778
-Is it the exact mm measurement between a certain part of the lens elements and the focal plane (sensor/film)?

That part is easy: the focal length of a lens is the distance from the rear nodal point to the sensor. The hard part is finding out where the rear nodal point is.

Daniel

 LIKES 0

Aug 10, 2009 05:20 |  #4

Daniel Browning wrote in post #8431291
That part is easy: the focal length of a lens is the distance from the rear nodal point to the sensor. The hard part is finding out where the rear nodal point is.

This distance only equals the focal length when the lens is focused at infinity.

Photo Gear

 LIKES 0

Aug 10, 2009 08:46 |  #5

Right, I read that that's only when it's focused to infinity too.

So no one has an easy answer for what the mm focal length is actually a measurement of?? C'mon pros, help me

 LIKES 0

Aug 10, 2009 09:01 |  #6

macroimage wrote in post #8431599
This distance only equals the focal length when the lens is focused at infinity.

For a simple lens (single element) the FL is the distance from lens optic to its focal plane. For a 'normal' design lens with complex optics (multiple elements), it is like the simple lens.

For a 'retrofocus' lens (typical WA for SLR/dSLR) the distance from lens to focal plane is far longer than the actual FL, as the rear node is placed behind the rear optic, leading to a longer distance to focal plane. This is to permit the reflex mirror sufficient space to swing upward without striking the lens.

For a 'telephoto' lens (typical long FL for SLR/dSLR) the distance from the lens to focal plane is shorter than the actual FL, as the rear node is forward of rear optic, leading to a shorter distance to focal plane. This is to permit a more compact lens than if it were a 'long focus' design.

You need to give me OK to edit your image and repost! Keep POTN alive and well with member support https://photography-on-the.net/forum/donate.p​hp
Canon dSLR system, Olympus OM 35mm system, Bronica ETRSi 645 system, Horseman LS 4x5 system, Metz flashes, Dynalite studio lighting, and too many accessories to mention

 LIKES 0

Aug 10, 2009 09:14 |  #7

Way back in the early days of lenses (circa 1920) the FL would have been the focal length from the film plane to the front element - because there wasn't much else involved except for a single element. In order for a lens to be longer, it needed a longer barrel. This is because such things as higher magnification glass and corrective glass and multi-element lenses had not yet been invented.

Once complex lens systems came out, they needed to correlate focal lengths by their equivalent in the one-element system. i.e. what FOV does this lens look like had the photo been taken with a one-element optic?

Its basically carried over ever since.

If you look back to the 50's, 60's and 70's - the early days of the SLR, you notice the Tele's and Super Tele's were big long lenses, 1000mm (that wasn't a mirror lens) was give-or-take 1000mm long. These are very simple optics... there isn't much in the way of internal magnification, its 1000mm of power because its 1000mm of length. By comparison you look at 400mm lenses today and they are incredibly compact (even more than they would have to be to compare to a 400mm of those times) - yet much sharper and of much higher quality... technology marches on.

In short, today's XXmm calculation is always an approximation, AT INFINITY, of what that XXmm would look like when compared to a lens of yesteryear of the same XXmm.

Alan "NuReality" Fronshtein
Gear List | PBase | flickr
Lots of Fun, Lots of Laughs, Happy Trigger Finger!

 LIKES 0

1,141 views & 0 likes for this thread
Focal length (XX mm) question
AAA
 x 1600 y 1600

Not a member yet?
Register to forums
Registered members may log in to forums and access all the features: full search, image upload, follow forums, own gear list and ratings, likes, more forums, private messaging, thread follow, notifications, own gallery, all settings, view hosted photos, own reviews, see more and do more... and all is free. Don't be a stranger - register now and start posting!