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02-25-2016, 08:06 AM   #1
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Sensor Plane and tripod socket

Is the tripod socket on the K3 aligned with the sensor plane? I am trying to set the non-parallex point of a fisheye lens (Samyang 8mm Fisheye) and I do have the manufacturer's distance from the non-parallex point to the sensor plane. Thanks for your help.

02-25-2016, 08:22 AM   #2
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Looks like it to me. Sensor plane should be marked by the white line on the left of the prism housing by the mode dial. That appears, as best I can tell to be in line with the tripod socket.
02-25-2016, 08:30 AM   #3
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QuoteOriginally posted by jatrax Quote
Looks like it to me. Sensor plane should be marked by the white line on the left of the prism housing by the mode dial. That appears, as best I can tell to be in line with the tripod socket.
Thank you. I never gave any thought to the lines on the prism housing. But looking at the camera after I read your post there are two lines, one on each side. The line on the side of the top LCD has a centerline symbol. This line is slightly different than the line on the mode dial side.

I am guessing the sensor plane is indicated by the center line symbol - the circle with a line through the center.

I never would have noticed this without your post.
02-25-2016, 08:31 AM   #4
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QuoteOriginally posted by jatrax Quote
Looks like it to me. Sensor plane should be marked by the white line on the left of the prism housing by the mode dial. That appears, as best I can tell to be in line with the tripod socket.
That white line, the one on the left and right by the mode dial, is just the indicator for the mode dial.
On the other side of the prism you will see another mark that looks like a circle with a line through it.
That mark is the sensor plane mark.

02-25-2016, 08:38 AM - 1 Like   #5
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QuoteOriginally posted by Quartermaster James Quote
That white line, the one on the left and right by the mode dial, is just the indicator for the mode dial. On the other side of the prism you will see another mark that looks like a circle with a line through it. That mark is the sensor plane mark.
Correct, no coffee yet and typed before I thought. Than you for the correction Quartermaster James.
02-25-2016, 08:41 AM   #6
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QuoteOriginally posted by Quartermaster James Quote
That white line, the one on the left and right by the mode dial, is just the indicator for the mode dial.
On the other side of the prism you will see another mark that looks like a circle with a line through it.
That mark is the sensor plane mark.
Thank you - I also feel the tripod socket is not lined up with the sensor plane. That is just an eyeball guess. I need to do some additional measurements. Added to this I have the grip which has a tripod socket further offset.
At least I now have a reference point.
02-25-2016, 10:46 AM - 1 Like   #7
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My first MX in 1978 and every Pentax I have had since then all had the circle with the center line engraved on them. In the old days, this was the film plane. Now, I suggest it is the real sensor plane.
02-25-2016, 01:14 PM   #8
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There's an article on Wikipedia that shows the Flange Focal Distance for many, many camera mounts. Pentax M42, M39 and K are all 45.46 mm. Some simple arithmetic should get you the answer you need.

Here's the link https://en.wikipedia.org/wiki/Flange_focal_distance

(By the way I have 6 Pentax camera film bodies (2 MXs, a Super A, a Super Program, a Program A and an ES) and I can't find any film plane marks on them. But, maybe I'm not looking in the right place.)

02-25-2016, 02:17 PM   #9
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If you're trying to find non-parallax point (a.k.a. nodal point) for purpose of panoramic shots, sensor plane is of no use. Drop me a message, please, if you're interested in more details.
02-26-2016, 02:10 PM   #10
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I'm sorry I was wrong - RockvilleBob has informed me, that accordingly to Samyang entrance pupil (a.k.a. 'no-parallax point') is 97mm from film plane, what makes film (sensor) plane useful. The image below shows location of entrance pupil in the camera, and in our case the distance between red dot and sensor plane is 97mm.



The location of entrance pupil can be verified (or determined) experimentally: we need a tripod and two reference points (I have used batteries). Camera should be placed horizontally, to avoid distortions - I have not done this for purpose of demonstration. We line up camera and reference points, the first one should be around 0,5m from the camera, and second one 2-3m behind the first one. These distances are not crucial, they can be changed if needed. The first step is the shot with batteries straight ahead, just to verify whether our setup is correct, batteries are aligned and in the centre of the frame and so on (please excuse 'quick and dirty' pictures).



Then we have to rotate the camera, so batteries are near edge of the frame - here we see that axis of rotation does not intersect entrance pupil, as apparent location of batteries is a little bit off, and they do not form straight line with camera any longer.






It indicates that we need to change the axis of rotation by moving it back or forth along optical axis of the lens - I found that macro rail comes handy in such situation. Tripod mount can be located in pretty strange places, not aligned with film plane, or even optical axis of lens. The drawing of Olympus compact camera shows such arrangement. Fortunately most (if not all) DSLRs have tripod mount aligned with optical axis, what makes adjustments easier.



I have used portions of 'Podstawy fotografii panoramicznej' paper published online by Zbigniew Małach.
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02-26-2016, 03:17 PM   #11
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QuoteOriginally posted by pentageek Quote
I\

It indicates that we need to change the axis of rotation by moving it back or forth along optical axis of the lens - I found that macro rail comes handy in such situation. Tripod mount can be located in pretty strange places, not aligned with film plane, or even optical axis of lens. The drawing of Olympus compact camera shows such arrangement. Fortunately most (if not all) DSLRs have tripod mount aligned with optical axis, what makes adjustments easier.



I have used portions of 'Podstawy fotografii panoramicznej' paper published online by Zbigniew Małach.
Thanks for the schematic - I find it surprising that the tripod socket is not centered on the film plan. Further complicating this for me is that I use the K-3 with a grip attached. I sent a request to Ricoh USA for the distance offsets for the camera and grip tripod sockets from the film plane. They sent a request to Pentax/Ricoh in Japan.

The K mount standard is that the film plane/sensor plane is 45.7mm from the flange. My plan is to mark the lens position 51.3 mm from the flange to mark the the theoretical no-parallex point. Then attach a string with a weight and position the rig so the string is centered on the pano-head center of rotation.

Then use the "2 point" method to test the results and adjust as needed.

A fisheye is difficult because the "2 point" method may indicate good alignment at the center of the lens but not so on the edges due to distortion.

As a final calibration I plan to take several 360x180 of a room in my home, making slight adjustments, to see what the stitching software deems to be the best point.

A lot of work but doing this should eliminate significant editing of panoramas for a project providing virtual tours to a local historical society.

Thanks again for all the help

---------- Post added 02-26-16 at 05:28 PM ----------

QuoteOriginally posted by pentageek Quote
I'm sorry I was wrong - RockvilleBob has informed me, that accordingly to Samyang entrance pupil (a.k.a. 'no-parallax point') is 97mm from film plane, what makes film (sensor) plane useful. The image below shows location of entrance pupil in the camera, and in our case the distance between red dot and sensor plane is 97mm.




I have used portions of 'Podstawy fotografii panoramicznej' paper published online by Zbigniew Małach.
Do you have a reference for the drawings/dimensions? Maybe they have one for the grip?

Never mind - my error - the schematic is for Olympus - I overlooked that initially. However the Pentax Battery Grip tripod socket is offset.

Last edited by RockvilleBob; 02-26-2016 at 03:35 PM.
02-26-2016, 03:29 PM   #12
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QuoteOriginally posted by RockvilleBob Quote
Thanks for the schematic - I find it surprising that the tripod socket is not centered on the film plan. Further complicating this for me is that I use the K-3 with a grip attached. I sent a request to Ricoh USA for the distance offsets for the camera and grip tripod sockets from the film plane. They sent a request to Pentax/Ricoh in Japan.
Determine entrance pupil on your own - with help of macro rail you'll have working rig in no time, especially when you know its (expected) location. Distortion may be problematic, but not to great extent. Good luck with your project
02-29-2016, 04:51 PM   #13
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I did get some information from Ricoh regarding the tripod socket and image plane - they are not aligned.

Here is what I received:

Thank you for contacting RICOH.

The tripod socket is forward of the image plane in the K-3.

We have contacted Japan for more specific information if it is available.


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