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dh003i
27-Oct-2010, 16:53
The standard formula for distance to the subject is:

S2 = 1/(1/F - 1/S1), where S2 = subject distance, F = focal length, and S2 = film distance or extension.

What about when you have a telephoto lens of say 800mm with a flange focal length of 527mm? How do you account for the flange in the formula?

aduncanson
28-Oct-2010, 08:50
For real lenses (called thick lenses) Subject Distance is measured to the Front Nodal Point and Image Distance is measured to the Rear Nodal Point. The Flange Focal Length is just an expression of the location of the Rear Nodal Point. In this case, the Rear Nodal Point is 800mm - 527mm, (273mm), in front of the flange. So measure the distance from the ground glass to the flange and add 273mm to get the actual Image Distance.

Now where is the Front Nodal Point and how do you measure the Subject Distance? You have not given us enough information to answer that. My guess is that Flange Focal Length is usually published not to facilitate these arcane computations, but so you can assure yourself that your camera has sufficient bellows extension to use the lens.

By the way, if the actual Focal Length is slightly different from the nominal 800mm focal length then you should use that actual Focal Length to come up with a number other than 273mm.

Dan Fromm
28-Oct-2010, 12:46
David, what are you trying to accomplish?

Aduncanason, at infinity the distance from the lens' rear nodal point to the film plane is the focal length. The mounting flange's position is arbitrary, bears no relationship to the nodal points' positions.

aduncanson
28-Oct-2010, 14:08
Dan,

Yes and no. While the mounting flange position is arbitrary, it is a characteristic of the mounting for a particular lens model; and by publishing a "Flange Focal Distance" as Schneider does (https://www.schneideroptics.com/info/vintage_lens_data/large_format_lenses/tele-xenar/data/5,5-360mm.htm), they both indirectly document where the Rear Nodal Point is located relative to the flange and help us to know if we have enough camera extension to use the lens for the purpose we have in mind.

Sorry, I thought I was clear.

- Alan

GPS
28-Oct-2010, 23:11
Clear you are, right not. Dan is right - the flange focal distance in itself doesn't say anything about the rear nodal point position. With the same logic you use you could say that the distance of the lens cap from the film plane speaks about the rear nodal point position. It does not.

dh003i
29-Oct-2010, 04:27
Thank you all for your replies.

I want to compare how close I could focus the 800mm f/12 Nikkor-T (ED) (http://www.bhphotovideo.com/c/product/37017-USA/Nikon_1356_Telephoto_800mm_f_12_Nikkor_T.html#specifications) (527.4mm flange focal distance) vs. a "normal" 600mm or 610mm lens, like the Apo Nikkor 610/9 or the Fujinon C 600/11.5 (not sure which one I'd be interested in; the 619/9 is significantly less expensive, than the 600/11.5, but it'd need an custom shutter-mounting job).

I'm looking at these in combination with a Wehman 8x10 field camera (with a 4x5 reducing back), which as about 30.5in or 762mm of bellows. It could get to about 1:4 with the 610. I'm curious what the Nikkor-T could get to. I'm interested in this because I've found recently that a lot of my favorite shots on my DSLR are moderately long, so very long on 4x5 & 8x10. But I don't always want to shoot things near infinity.

GPS
29-Oct-2010, 05:20
By now you probably know that the Fujinon C 600/11.5 uses bigger flange focal length than the 800/12 Nikkor. As to what magnification ratio you get with both lenses you can surely find the equations yourself to calculate it ;-)

Emmanuel BIGLER
29-Oct-2010, 06:52
In a telephoto the flange focal distance allows you to know the minimum amount of bellows draw that you need to get a sharp image at infinitty. Usually you shoudl add 10% of the focal lengths to this value to be comfortable and be able to focus to about 10 times the focal length.
From this position (infinity->focus) one can use the universal formula, valid for all thick compound lenses whatever their design might be :

ext = M . f

where "ext" is the additional bellows extension needed beyond the focal point (beyond the flange focal distance) that will allow you to focus an image with a magnification "M" if your focal length is "f".

For example, Richard Avedon's portraits in the West are made with a magnification ratio of about 1:5. (http://www.galerie-photo.com/images/ouest-americain-avedon.jpg)
Hence if we know that R.A. used a 360mm for the 8"x10" format, at least we know that his additional bellows draw beyond the focal point was 360/5 = 72 mm whatever the 360mm lens might have been.
If he had used a 360 mm Tele Arton, the flange focal distance would have been about 210 mm (http://www.schneideroptics.com/info/vintage_lens_data/large_format_lenses/tele-arton/data/5.5-360mm.htm) and the total bellows draw at 1;5 about (210+72)= 282mm.
In fact he used a Symmar-Sironar-Fuji-something of 360mm, (http://www.schneideroptics.com/info/vintage_lens_data/large_format_lenses/symmar-s/data/1,6,8-360mm.html)a quasi-symmetrical design, his flange focal distance was about 360 mm minus something (e.g. : actually = 335 mm for the Symmar-S 360) and his total bellows draw (335+72) = about 407mm.

Dan Fromm
29-Oct-2010, 08:15
David, a 610 mounted in front of a #1 shutter should cover around 300 mm at infinity, if the lens itself covers at least that much.

Last year SKGrimes made a 610/9 Apo-Nikkor-to-#1 adapter for me. Ask 'em how much they'll charge for another one, and then you'll be able to make a relatively sensible decision.

aduncanson
29-Oct-2010, 08:16
GPS - I think that you are not applying all of the information that you have.

The Flange Focal Distance, if it means anything at all, is the distance from the flange to the film when focused at infinity. Wikipedia (http://en.wikipedia.org/wiki/Flange_focal_distance)supports this interpretation although it discusses exclusively small cameras and their proprietary mounts. In the case of a large format lens mounted in a shutter (or a flange mounted barrel lens), the sensible interpretation would be that Flange Focal Distance is the distance from the film to the front of the lens board (the plane that the flange or the shutter sits on) when focused at infinity. That is my assumption. Do you have a better documented definition for Flange Focal Distance?

As Dan correctly implied, when focused at infinity, the Rear Nodal Point is one Focal Length ahead of the film. Therefore the Rear Nodal Point will be ahead of the Flange by the value (Focal Length minus Flange Focal Distance).

I own the Schneider Tele-Xenar whose data (https://www.schneideroptics.com/info/vintage_lens_data/large_format_lenses/tele-xenar/data/5,5-360mm.htm)I linked to earlier. My interpretation of Flange Focal Length supports calculations that correlate well with actual use, even though I remounted my lens from a barrel to a shutter (and my lens is not the most recent Copal 3 mounted version that is most likely the version documented by Schneider.)

I performed the following experiment: Using the published Flange Focal Distance (214mm) and true Focal Length (366mm) I inferred that the Rear Nodal Point lies 152mm ahead of the lens board. I set the bellows extension to near its maximum and measured 390mm from the ground glass to the front of lens board.) I used 542mm (=390+152) as my Image Distance and calculated a Subject Distance of 1127mm using the lens equation. To find the location of the Front Nodal Point I used my calculated 152mm distance from the Rear Nodal Point to the front of the lens board and added Schneider's published Principal Point Separation value of 68.6mm. I then calculated that the distance from the subject to the front of the lens board would be 1348mm. The actual subject to lens board distance I measured from a focused set up was 1410mm.

Okay, I was off by a little more than two inches in predicting my near focus distance. Considering the many possible sources of error in measuring lens board to ground glass distance and the uncertainty that my lens may not actually be configured as documented by Schneider, it is a pretty good estimate and is perfectly useful for determining if my lens and camera will allow me to focus close enough to set up the shot I want.

In the case of the 800mm Nikkor-T and Wehman camera that dh003i asked about, this approach of calculating the Rear Nodal Point position, adding the max bellows extension and applying the classic lens equation results in a near focus distance of 3528mm for an image magnification of 0.29325x. Which agrees exactly with the results from the equation given by Emmanuel.

dh003i
29-Oct-2010, 16:03
David, a 610 mounted in front of a #1 shutter should cover around 300 mm at infinity, if the lens itself covers at least that much.

Last year SKGrimes made a 610/9 Apo-Nikkor-to-#1 adapter for me. Ask 'em how much they'll charge for another one, and then you'll be able to make a relatively sensible decision.

Dan,

Thanks. Apparently, he 610/9 Apo-Nikkor has a large image circle and will cover a lot (http://photo.net/large-format-photography-forum/006ncH):


The Apo-Nikkors cover, conservatively, about 46 degrees or about 85% of the focal length. So the 610mm covers 518mm at f/22, at the 760mm covers an image circle of 646mm. Both will cover 7x17, neither will cover 20x24.

Although they are optimized for photographing flat objects there is nothing "flat" about the sort of images these lenses produce. Like the other dialites, such as the Apo-Ronars and Apo-Artars, these are fine lenses, and aside from their size and slight coverage, I doubt you will find anything to complain about them.

Would front-mounting one of these guys onto a #1 cut off the coverage? Also, you have one, isn't it pretty heavy? Does front-mounting cause no stress problems on the lens, shutter, or camera for you?

Dan Fromm
29-Oct-2010, 16:59
Mine is front-mounted, a Copal #1 can hold it and the mount adapter. No crutch needed, but crutches aren't hard to make. No problem for a 2x3 Cambo's standard. Mind you, I wouldn't walk with it on the camera and camera and tripod on my shoulder and I wouldn't bang on it while it was on the Copal.

I'd rather have had a 600/9 Apo-Nikkor. This is a tessar type and much lighter than the 610. Also much scarcer. A 610 came along at an irresistible price so I took the chance.

I told you, it should cover about 300 mm when set up with a #1 a short distance behind it. That's how mine is set up. Ask Adam, he keeps a mount log...

Yes, of course the shutter will vignette the image. Its maximum diameter is smaller than the lens' exit pupil. But it won't vignette so badly as to make the lens unusable on 4x5, which needs >= 150 mm, or 5x7, which needs >= 210 mm. It should just cover 8x10 with no movements possible without vignetting.

dh003i
29-Oct-2010, 18:03
Mine is front-mounted, a Copal #1 can hold it and the mount adapter. No crutch needed, but crutches aren't hard to make. No problem for a 2x3 Cambo's standard. Mind you, I wouldn't walk with it on the camera and camera and tripod on my shoulder and I wouldn't bang on it while it was on the Copal.

I'd rather have had a 600/9 Apo-Nikkor. This is a tessar type and much lighter than the 610. Also much scarcer. A 610 came along at an irresistible price so I took the chance.

I told you, it should cover about 300 mm when set up with a #1 a short distance behind it. That's how mine is set up. Ask Adam, he keeps a mount log...

Yes, of course the shutter will vignette the image. Its maximum diameter is smaller than the lens' exit pupil. But it won't vignette so badly as to make the lens unusable on 4x5, which needs >= 150 mm, or 5x7, which needs >= 210 mm. It should just cover 8x10 with no movements possible without vignetting.

Ahh, ok, thanks. I haven't been able to find the 600/9 Apo-Nikkor, but the 610 is all over the place. I've only seen the Fujinon C 600/11.5 from Mpex.