Two mysterious measurements

[Max Alfy]

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I would like to find out the following information:

 

1: Focusing distance

 

If I wish to check the absolute focusing distance using a measuring tape, which distance is supposed to match the RF indication:

- object to front lens element;

- object to body flange;

- object to back of body; or

- object to where else?

 

2: Nodal point

 

Since for a prime lens there is only one nodal point useful (and mandatory) for perfect panorama stitching, it seems obvious that a helpful optical engineer would be able to compute from the lens diagrams where exactly the nodal point is situated for the main Leica lenses, and determine its distance to flange (I am interested in that information for the 24 Asph, 35 summicron asph, Noctilux and 90 summicron asph).

 

Determining the nodal point for a reflex camera is fairly easy, but it becomes a real pain to perform the same with a RF camera, even the M8!

 

Has anybody solved any of the mysteries above?

 

Thanks in advance.

 

Max

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[Shootist]

Use to be to the back of the top cover but now I think it is to the line on the top for shutter speed. Although that may be slightly forward of the true focal plane/point

I think it is 28mm from the front of the lens mount to the sensor plain. That would be the focal plane.

 

There has been a lot of talk about Nodal point and as far I can find there is no definitive mathematical formula for finding it. A good guess is where the aperture blades are on each lens.

Please explain why you think it is easy to find it on a reflex camera.

[Max Alfy]

On a tripod and using a mount which allows forward/backward camera adjustment, one seeks the camera position for which 2 landmarks, one close and one distant, remain aligned when the camera is rotated between extreme left and extreme right viewing of the landmarks through the viewfinder. When this is achieved, the nodal point is exactly above the pivot axis of the tripod. (With a zoom, this position is different for each focal length.)

 

With an RF, it doesn't work because the viewfinder is offset.

[marknorton]

It's intneresting that there's no focal plane marking on the M8 but it's 28mm behind the lens flange, or about the centre of the front-most hot shoe contact.

[adan]

1. The absolute focusing distance should be measured from the plane of the film or sensor. Cameras used to have a little symbol of a line and circle (like a profile view of Saturn and its rings) to mark this plane - but not the M8. However, the "back-focus" or lens-flange-to-image-plane distance for all M cameras is 27.8 mm, and it is fairly easy to measure back from the M8's lens flange. The image plane is thus 1.0 mm behind the back edge of the shutter speed dial - or, as Mark says - dead center in the little round contact that is front-most in the hot shoe.

 

2. For the nodal point, with a viewfinder camera like the M8, I guess one would use this technique: Finding the Nodal Point of a Lens. Presumably one could do it "in the field" by checking the images on the LCD until things align correctly

[1234]

The classic technique described by Max will work with RF lenses themselves though, which is all you need to do to make the measurement. This is another very useful application for an adapter fashioned with an M bayonet female on one end (such as the 16596/UOOND) and Focoslide ground glass with center clear spot and female bayonet to accept the vertical 5x or 30x magnifiers on the other end. You make the spacing between the flange and the glass exactly equal to the M film-to-flange distance by machining or grinding the intermediate tube to the right length then epoxy it all together. Use a depth gauge good to 0.01mm. Secure a 1/4-20 nut to the edge of the 16596, mount it atop a Linhof macro slide, and you're in business.

Harry

[marknorton]

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However, the "back-focus" or lens-flange-to-image-plane distance for all M cameras is 27.8 mm...

 

Oh, alright, what's 0.2mm between friends??

[ChrisC]

....Determining the nodal point for a reflex camera is fairly easy, but it becomes a real pain to perform the same with a RF camera, even the M8!

 

Has anybody solved any of the mysteries above?........

 

If you are asking how to set up the M8 with a specific lens for 'nodal point' photography to make panoramas; it's easy. You will, of course, need a panoramic head with sideways, and forwards/backwards shifts. In a well lit room fix your M8 in your panoramic head on a tripod with the lens nodal point region roughly over the centre of the tripod [i focused the lens for a typical landscape focusing distance but I'm uncertain whether the nodal point shifts with focusing].

 

You will need a thin line object close to the lens [i mounted a vertical screwdriver on a stand], your second target needs to be broader and a few feet away [ I used the end thickness of a white painted door].

 

Centre the 'close line' so that on your exposed screen it is exactly in the middle of the second target, and by swinging the panoramic head a set amount to the left and right for exposures you will observe the image of the close line shift off centre, adjust the position of the camera on the [say] forwards/backwards rack so the least displacement of the the close line from centre of the second target is achieved. Go through the same process with the panoramic head's sideways rack, until finally you can make exposures so the image of the close line remains centred in the second target when exposures are made after turning the panoramic head. The rig is now tuned for nodal point photography.

 

It was a hell of a lot easier to do than to describe. I advise using a focusing loupe to inspect the screen after exposures, take your time, scratch your head, and poor yourself a glass of wine. Damnit that's exactly what I'm going to do right now. Good luck.

 

.................Chris

[ho_co]

Chris--

A clear and detailed description! As often as the question has come up before, this is the first time I've seen such a full answer. Thanks!

 

Max--

Some manufacturers (Rollei for their TLRs, for example) made special tripod heads that offset the camera the proper distance so the user didn't need to calculate the nodal point at all.

 

With most SLRs, the tripod mount is directly below the optical axis, making the alignment fairly simple.

 

With the M8, despite the fact that the tripod socket is no longer at the end of the baseplate, it is still offset from the lens axis.

 

Chris's way is the only way for serious panoramists so far as I know, but for general usage there are several pieces of software that do a good job even stitching handheld images. Software is beginning to overtake traditional mechanics in this field. See for example http://www.l-camera-forum.com/leica-forum/leica-m8-forum/27196-36-shot-portrait-panorama.html.

 

(But you are probably already aware of all of that...)

 

--HC

[Philinflash]

Chris--

...

 

With most SLRs, the tripod mount is directly below the optical axis, making the alignment fairly simple.

 

With the M8, despite the fact that the tripod socket is no longer at the end of the baseplate, it is still offset from the lens axis.

...

 

 

Good point, Howard! Although the offset is obvious with a superficial physical examination of the camera, I had completely missed that. This certainly adds a complication when attempting pano shots as no pano head is going to compensate for that little "feature."

 

I suppose the tripod socket is under the center of gravity of the camera with one lens or another but not under either of the lenses I have (it would be too far back with the weight of the lens on the front). But, really, wouldn't it be more useful to have it under the center-point of the focal plane? That might be possible given the thicker width of the M8, compared to earlier film Ms.

 

I guess some after-market genius has to come up with an tripod adapter that corrects for this offset. They might sell a dozen or so.

 

Philip Kozloff

[Shootist]

Good point, Howard! Although the offset is obvious with a superficial physical examination of the camera, I had completely missed that. This certainly adds a complication when attempting pano shots as no pano head is going to compensate for that little "feature."

 

 

 

Ah I think ALL Pano head DO have adjustments for this. In fact I am sure of it.

Without that adjustment the head would be useless for pano's.

[ChrisC]

....A clear and detailed description! ....

 

Howard - Thank you; as generous as always. I was actually concerned that my description was indecipherable gibberish.

 

You may recall a few weeks ago I posted a thread enquiring where the nodal point is in the 24mm lens. I had got my knickers in a twist by convincing myself that I couldn't set my M8 accurately for nodal point photography because it lacked reflex viewing from the lens. Silly me, I should have rolled my sleeves up earlier; I would have discovered sooner that calibration for nodal point photography is, if anything, easier using the M8 screen than it is using an SLR reflex viewer. I hoped to spare the OP of this thread [or anyone else wanting to calibrate for nodal point photography] unnecessary agonising.

 

.................Chris

[jlm]

i use the really right stuff brackets which allow for sideways adjustment between the camera body and the pano slide axis. you also need their pano head which allows rotation above the pano slide buisiness

[Max Alfy]

Thank you all for the elaborate information concerning the two mysteries!

 

However, concerning the nodal point, it be so easy if a Leica engineer would be kind enough to look at current lens schematics, and tell us its distance to flange +/- some mm!

It would save a lot of hassle with the pano heads.

 

Max

[Peter Branch]

Alternatively you just look on the Leica web site. For each of the current lenses Leica kindly provide the precise distance, to within 0.1mm, of the distance from the front element to the entrance pupil - it's that easy.