Optics: focal length versus flange focal distance ?

[Ken Lee]

This question is not about telephoto or retrofocus designs, but about "standard" Large Format lenses.

According to the this brochure (http://www.rodenstock-photo.com/mediabase/original/e_Rodenstock_Analog_Lenses_27-42__8226.pdf), the 210mm Rodenstock APO Sironar S has a "flange focal distance" of 202mm when focused to infinity. Does that mean it's actually a 202mm lens ?

According to this brochure (http://www.prograf.ru/rodenstock/largeformat_en.html), the 210mm Rodenstock APO Sironar N has an "optical register" of 200mm when focused to infinity. Does that mean it's actually a 200mm lens ?

According to this brochure (http://www.kenleegallery.com/pdf/Nikkor_LargeFormatLenses.pdf), the 200mm Nikkor M has an flange focal distance of 194mm when focused to infinity. Does that mean it's actually a 194mm lens ? Similarly, is the 300mm Nikkor M really a 290mm lens ?

I know the differences are fairly minor, but I am just wondering and appreciate the help.

[Leigh]

Hi Ken,

Flange Focal Distance (FFD) and Optical Focal Length (OFL) are not synonymous, and only roughly related.

A lens with a long focal length will probably have an FFD shorter than its OFL, while
a lens with a short focal length will probably have an FFD longer than its OFL.

The OFL is always the distance from the rear node to the film along the lens axis when focused at infinity.
Some optical designs shift the nodes dramatically, like hundreds of mm in front of the actual lens for telephotos.

These are all just accommodations for bellows length.

The FFD can be anywhere. It normally corresponds to the location of the diaphragm.

------

You will often see slight differences between the real OFL and the value engraved on the lens.
This is a marketing decision, since even numbers (e.g. 300) sell better than odd numbers (e.g. 305).

When you look at the datasheet for a 300mm lens, you may find it's real OFL is 302.6mm or some such.

- Leigh

[Dan Fromm]

Ken, as Leigh said mechanical design and optical design aren't tightly coupled. Before you worry too much more, go here http://www.cameraeccentric.com/html/info/zeiss_4.html, read the discussion of "objective mounts" and look at the pictures.

I added this to Leigh's perfectly adequate discussion because it has concrete examples.

Re what Leigh said about focal lengths, there are at least three: marketing focal length, as engraved on the lens and mentioned in marketing propaganda; design focal length, the lens' focal length if the lens conforms perfectly to the design; and actual focal length, which means what it says. One of my treasures (absolutely not an LF lens) is a 38/4.5 Biogon. Its marketing focal length is 38 mm. Its design focal length (go look for Zeiss propaganda) is 38.5 mm. I've had 20 of the these lenses, all extracted from fixed focus aerial cameras. Most were marked with focal length as measured by the camera manufacturer and the thickness of the shim needed to collimate the lens to the camera body. Measured focal lengths ranged from 35.3 mm to 38.8 mm.

I have Boyer fiches techniques that give marketing and design focal lengths for Boyer's product slate, also 240/9 and 300/9 Apo-Saphirs with QC slips that give actual measured focal lengths. 240 mm; 240.2 mm; 239.6 +/- 0.2. 300 mm; 300.3 mm; 299.1 +/- 0.2. Very close, probably no cigar.

Cheers,

Dan

[Leigh]

I'm rather amazed that they manage to hold the true focal length that close, considering
all the individual tolerances that accumulate.

I expect they're binning all the elements. For any particular element in the lens, they measure
the real focal length and other relevant parameters like centering. They put all the nominal ones
in one bin, all the +1 in another bin, all the -2 in another, etc.

When they assemble a particular lens group they choose individual elements with complementary
characteristics (one error canceling another) so the group works as it should.

- Leigh

[Dan Fromm]

Leigh, I don't know whether Boyer binned elements. Its certainly possible. I do know that they worked to fairly tight tolerances and were subcontractors to E. Leitz, for whom they made lens elements.

Eric once told me a story about Boyer's standards. Not long before the firm failed their prices were much higher than most competitors. Even so a projector manufacturer asked them for a bid on a batch of projection lenses. Boyer's last owner balked, saying that he knew very well he couldn't compete with Japanese lens makers on price. He was told to put in a proposal anyway. It seems that such a high proportion of the Japanese lenses bought had failed acceptance testing, and such a low proportion of Boyer's, that the prices of accepted lenses from Boyer was more than competitive.

[Leigh]

Hi Dan,

I don't know as fact whether any lens manufacturer uses/d binning.

It's a common practice in all industries that assemble groups of tight-tolerance parts.
It's much cheaper than tightening the manufacturing tolerances on the individual components.

The trick is to know which parameters can be compensated this way and which cannot.

- Leigh

[Ken Lee]

Ken, as Leigh said mechanical design and optical design aren't tightly coupled. Before you worry too much more, go here http://www.cameraeccentric.com/html/info/zeiss_4.html, read the discussion of "objective mounts" and look at the pictures.

http://www.kenleegallery.com/images/forum/mounts.jpg

Thank you.

I guess the flange focal distance is merely the distance from the mounting flange to the film, which depends on how the lens is mounted. The location of the flange can vary with different configurations as shown in the 1907 Zeiss catalog.

Looking again at the first Rodenstock brochure cited, I now notice they say "with Copal shutter".

Well, that's reassuring ! :cool:

[Delfi_r]

By design Flange Focal Distance it's very important in bellows cameras: short distance it's not always attainable as the bellows completely folded has a fixed length and long distance is limited by the bellows/bed extension.

This is the reason why the designers traditionally move from pure symmetrical design to allow shorter focal lenght or longer focal length lenses mounted in fixed bellows cameras. In long focal lenght we have tele-foto designs and in short focal length we see inverted tele designs. In large format lenses the move it's not so important (symmetrical designs have their merits)

[ic-racer]

Some bellows cameras with rangefinders require quite exacting flange focal length. Especially in cases where two different focal length lenses have to share the same infinity stop.

[Emmanuel BIGLER]

When Dan says: marketing focal length

I have in mind:
"This week, our special rate for the apo-sironar S, including lens caps, @ 199.9 mm"