Where the Far Range Begins (In Focal Lengths)

[Rob-F]

(I thought of this thread title based on the old Rin Tin Tin movie, "Where the North Begins.")

In 2012 I started a thread on the distance to infinity. That one got a lot of replies, so I want to see what everyone has to say about the distance to the "far range."

I had better define my terms. The Macro range seems well defined, as 1:1 life-size (on the film) to 1:10. Ten focal lengths of distance from the lens's principal plane, to the subject. Now, let's divide the camera-to-subject distance into three major zones: macro (or near range); middle range; and far range. It is safe to say that the macro range ends, and therefore the middle range begins, at a distance of ten focal lengths.

What does not seem defined is where the middle range ends and the far range begins. In fact, distances beyond close-up range don't even seem to have names. Why does this matter? Some of my macro lenses still give outstanding sharpness at a distance somewhat beyond the macro range. And some lenses known to be optimized for infinity give perfectly sharp images at some distance short of the point marked as infinity on the focusing scale.

So: At what distance, in focal lengths, does the middle end, and the far range begin?

Note: this is not about rangefinder accuracy, nor about using the moon for an infinity target, nor about infinity divided by two, nor hyperfocal distance, nor DOF nor blur circles.

Thanks for your thoughts!

 

[mfogiel]

I recall a discussion on the net a few years ago, about the distance at which the Hasselblad Planar 100/3.5 would have been outperformed by the Makro Planar 120/4, which is notoriously far less sharp at infinity than the 100/3.5. A Zeiss expert replied: the Makro Planar will be sharper when you fill the frame with objects smaller than an average window, conversely, the 100/3.5 will be sharper for bigger objects. To calculate the distance, if I recall well, you need to divide the FL by negative height and multiply by the object height. Personally, anything which is farther than 7 meters away is long range to me, when I have a Leica with a lens within 50mm, but this can change substantially if you shoot bigger formats.

 

[Rob-F]

Yes, that one was on Photo-Net Medium Format Forum and the Zeiss expert was Dr. Kornelius Fleischer. He said that the Makro Planar should be used when the subject is the size of your computer screen or smaller; and another lens (I think he mentioned the 150mm Sonnar) when the subject is the size of a window or larger.

You are right, it was the 100mm planar he was comparing to the 120mm Makro-Planar. Here's the link:

http://photo.net/medium-format-photography-forum/000PxL

 

[David Hughes]

Once upon a time, macro meant 1:1 I blame the PR people.

Regards, David

 

[Rob-F]

I have some resolution test targets provided by the National bureau of Standards, or NBS (now the National Institute of Standards and Technology) that are meant to be photographed at 25 focal lengths from the lens's principal plane (26 focal lengths from the film plane). NBS's choice of 25 focal lengths might be an indication of where they felt the far field begins.

Or it could have to do with powers of ten: 1 to 10 focal lengths for the near field; 10 to 100 for the middle; and over 100 for the far field. Let's see: for a 50mm lens, that would be near field, 2 to 20 inches; middle, 20 to 200 inches, or 16.7 feet; far field, beyond 16.7 feet.

Someone with some optical theory knowledge?

 

[michaelwj]

I have some resolution test targets provided by the National bureau of Standards, or NBS (now the National Institute of Standards and Technology) that are meant to be photographed at 25 focal lengths from the lens's principal plane (26 focal lengths from the film plane). NBS's choice of 25 focal lengths might be an indication of where they felt the far field begins.

Or it could have to do with powers of ten: 1 to 10 focal lengths for the near field; 10 to 100 for the middle; and over 100 for the far field. Let's see: for a 50mm lens, that would be near field, 2 to 20 inches; middle, 20 to 200 inches, or 16.7 feet; far field, beyond 16.7 feet.

Someone with some optical theory knowledge?

I would have thought that far range would be when the incident light is coming more or less parallel, defined as when tan(theta) ~= theta, with theta in radians being the angle of divergence from normal. So for example, tan(0.1) = 0.1003. Of course I have no idea how this would relate to focal length (I normally deal only with NA and lensless 'optical' systems in my work).

I trying to remember and not really doing so well at it.

 

[Rob-F]

Agreed but at what point may we decide that tan theta is acceptably close to theta? Whether stated as a trig function or in focal lengths, is there some standard?

 

[michaelwj]

Agreed but at what point may we decide that tan theta is acceptably close to theta? Whether stated as a trig function or in focal lengths, is there some standard?

I've never worked in terms of focal lengths, but in terms of NA. For me, far field begins at the transition from Fresnel to Fraunhofer diffraction, which are both in terms of aperture and in involve arbitrary parameters such as << 1, much like tan(theta) = theta. Where does 'much less than one' start?

I think the NBS/NIST value of greater than 25f seems as good as any. Hopefully someone with some more relevant optical knowledge will chime in.

 

[Rob-F]

I've never worked in terms of focal lengths, but in terms of NA. For me, far field begins at the transition from Fresnel to Fraunhofer diffraction, which are both in terms of aperture and in involve arbitrary parameters such as << 1, much like tan(theta) = theta. Where does 'much less than one' start?

I think the NBS/NIST value of greater than 25f seems as good as any. Hopefully someone with some more relevant optical knowledge will chime in.

I didn't know about these concepts. I knew what diffraction is, but Fresnel and Fraunhofer diffraction were new to me. Still you make it clear that the transition point is somewhat arbitrary. As to NBS, I don't know if they felt that 25f is the start of the far field, or just a reasonable testing distance. Could 25f be acceptable as the start of the far field, rather than my guess of 100f?

I'm not sure if I should be thinking in terms of a middle field. For all I know, maybe near field and far field are all that is needed. Though some lenses with a manual Close Range Correction (CRC) control do have three distance settings. That's probably where I got the idea of near, middle, and far fields.

 

[wes loder]

Once upon a time, macro meant 1:1 I blame the PR people.

Regards, David

Absolutely agreed. 'Macro" refers to big. i.e. bigger than life-sized. Any reduction is merely close focusing. The only manufacturer that ever got it right, as far as I know, was Nikon. Its "Macro Nikkors" are only designed for greater than life-size photography. 'Micro Nikkors' were designed—originally—for repro ranges of 1:10-13: the proper reduction for microfilming. WES