Is there a way to measure the "true" focal length of a lens?

So we all know that sometimes lenses aren't exactly what they say. Between focus breathing and just straight up false advertising, our focal lengths aren't always what is listed on the side of the lens. Is there an accurate way to measure the true focal length of a lens? Here is an image for comparison. First image was taken with my 24-70 f2.8L II at 70mm. The second image was taken with my 70-200 2.8 IS II at 70mm. I didn't do any post on these images. Only thing I did was move the 24-70 slightly forward so the last element of the lens matched up to where the last element of the 70-200 was. I couldn't decide if that was a more true test or if having the sensor in the same spot was a more true test, but I ultimately moved the sensor slightly forward to match the ends of the lenses up.

I did the very same test years ago, only with the 24-70mm mark I. And got the same results. Canon does publish the angle of view of all theirs lenses. So I guess you can take that and pop it into a formula somewhere and get the effective focal length.

frugivore wrote in post #18249699
I did the very same test years ago, only with the 24-70mm mark I. And got the same results. Canon does publish the angle of view of all theirs lenses. So I guess you can take that and pop it into a formula somewhere and get the effective focal length.

Canon lies, just like the rest. I recall when I first purchased my 20D, I had a 70-200mm zoom and a 17-85mm zoom, and the 70mm indication on the two lenses did not match the FOV!

With the 70-200mm mounted on 5D, at a distance of 119cm (46.85") the camera should see 38.4cm along the short dimension of the frame. What it sees is about 30cm, as seen here:

You might have to rethink your procedure. The true definition of focal length is the distance from the senor to the focal point when the lens is focused at infinity. So if you focused the lens at infinity and measured 70mm from the sensor plane (the circle with line through on top of the camera), that would be where the focal point should be. Mark that location and then change lenses and repeat. Probably would be best done outdoors since you have to be at infinity for the correct locations.

Jarvis Creative Studios wrote in post #18249693
Is there a way to measure the "true" focal length of a lens?

Yes. But first you would have to come to a precise working definition of what 'focal point' is. There are many definitions, and many differences between those definitions. So in order to determine the actual focal point, you would first need to determine which definition you are going to measure for.


Here are a few of the most readily available definitions for 'focal point':

Focal length (shown in red) is the distance between the center of a convex lens or a concave mirror and the focal point of the lens or mirror — the point where parallel rays of light meet, or converge

the distance from a lens to its focus

the distance between the imaging plane (e.g. the image chip in a digital camera) and a point where all light rays intersect inside the lens (the ‘optical center’). So a focal length of 20mm means that the distance from the optical center to the imaging plane is 20mm long

The focal length of a lens is the distance from the optical centre of the lens to the sensor (or film) when the lens is focused on an object at infinity.

When parallel rays of light hit a lens focused at infinity, they converge to form a focal point. The focal length of the lens is the distance from the middle of the lens to this focal point.

Hopefully the thing that you will get from all of these definitions is that the focal length of a lens is an actual physical distance between the lens element(s) itself(themselves) and the focal point of the lens, usually when the lens is focused at infinity. The focal length is not determined by field of view. That's right! Focal length is not determined by how wide the projected image is. It is determined by a measurement between the element itself (or optical centre in multi-element lenses) and the focal point.

Field of view is merely a by-product of focal length, not the essence of it.

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For a 'simple lens' one can measure from the lens center to the plane of focus, to determine focal length

For a 'normal' lens, the same principle of distance from 'rear node' to focal plane is the focal length.
But for a 'telephoto' design lens, the rear node is CLOSER to the focal plane than the FL of the lens, and
for a 'retrofocus' design lens, the rear node is FARTHER FROM the focal plane than the FL of the lens!

Wilt wrote in post #18249866
For a 'simple lens' one can measure from the lens center to the plane of focus, to determine focal length

For a 'normal' lens, the same principle of distance from 'rear node' to focal plane is the focal length.
But for a 'telephoto' design lens, the rear node is CLOSER to the focal plane than the FL of the lens, and
for a 'retrofocus' design lens, the rear node is FARTHER FROM the focal plane than the FL of the lens!

I think you have the tele/retrofocus statements off a little.in the telephoto case the rear node will be infront of the forward one, usually infront of the physical optics, i.e. focal length longer than length of lens. The retrofocus having the node again outside the physical lens but this time to the rear of it. The reason that ideal lenses are substituted so often is the complexity of modern lens designs.

Alan

BigAl007 wrote in post #18250042
I think you have the tele/retrofocus statements off a little.in the telephoto case the rear node will be infront of the forward one, usually infront of the physical optics, i.e. focal length longer than length of lens. The retrofocus having the node again outside the physical lens but this time to the rear of it. The reason that ideal lenses are substituted so often is the complexity of modern lens designs.

Alan

Alan, what I indeed wanted to say:
For the 'Telephoto' lens the Image Distance is shorter with Telephoto designs than with Normal designs. So if FL of a Telephoto lens is 500mm, the distance from the rear optic to the Focal Plane is shorter than the Rear Node distance (e.g. image distance is 200mm vs. 500m FL).

Just like describing an elephant there are multiple characteristics, all true.

• Yes, what you describe is indeed an effect of Telephoto design, that the rear nodal plane is forward of the optical center of the lens, where the rear nodal plane is one FL in front of the film plane.
  
• Another characteristic is that a telephoto lens is a a long-focus lens in which the physical length of the lens is shorter than the focal length.