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Author Topic: Hasselblad HTS and panorama stitching.  (Read 1374 times)
jerome_m
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« on: June 08, 2013, 12:28:34 AM »
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To stitch images with the HTS, two or more images are taken using the shift function and then assembled in photoshop. The tilt function should be precisely set to zero, because it works in the same direction as the shift function. If there is a slight tilt, it will affect the far sides of the panorama considerably. One cannot rotate the tilt and shift angles independently.

The HTS has an electronic indicator for tilt and shift. Can this indicator be trusted when set to zero or is there another way to proceed when taking panoramas?
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elf
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« Reply #1 on: June 08, 2013, 03:03:26 AM »
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Pick up a machinist's test indicator and surface plate and measure it. You could also attach a laser pointer to the lens and point it at wall 50 feet away and then see if it returns to the same spot when you set the tilt to 0.

By the way, there's no reason you can't use tilt and/or shifts when shooting panoramas.  It's probably more efficient to just rotate around the entrance pupil and then you're not limited to a couple of frames. 
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BartvanderWolf
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« Reply #2 on: June 08, 2013, 05:08:05 AM »
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To stitch images with the HTS, two or more images are taken using the shift function and then assembled in photoshop. The tilt function should be precisely set to zero, because it works in the same direction as the shift function. If there is a slight tilt, it will affect the far sides of the panorama considerably. One cannot rotate the tilt and shift angles independently.

Hi Jerome,

I'm not exactly clear on what you mean with "it will affect the far sides of the panorama considerably".

If you are referring to Photoshop's difficulty with stitching shifted images correctly unless they only require simple shifting to align them, I'd say; use a better stitching program that allows to dial in an offset parameter (per image if needed).

Quote
The HTS has an electronic indicator for tilt and shift. Can this indicator be trusted when set to zero or is there another way to proceed when taking panoramas?

I'm not exactly sure how they've implemented it, but it cannot be as accurate as calibrating it on a real scene shot at different settings. Not only will there be mechanical play involved, there is also limited accuracy of the sensors (even a digital calipers also needs calibration before use). Using a longer measurement base, a distant object, will be much more accurate. How repeatable the found settings are for other shooting scenarios, remains to be seen, and I suspect that the post-processing by Phocus will try and compensate for some of that as far as lens distortions are concerned.

Good Pano-stitching software doesn't care about the input, it will handle it well regardless, and it does so on the actual images. It does help that Hasselblad gives the entrance pupil distances measured from the image plane, as a starting point for further calibration (good stitching software will allow to additionally tweak the 'focal length' setting for even better stitches), because that remains an important input parameter for the highest quality stitches.

Cheers,
Bart
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jerome_m
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« Reply #3 on: June 08, 2013, 11:15:22 AM »
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I'm not exactly clear on what you mean with "it will affect the far sides of the panorama considerably".

When the lens is tilted, the focus is affected along an axis perpendicular to the angle of rotation of the lens and the further one is away from the center of the system, the more the focus is affected.

When the lens is shifted, the translation is along an axis perpendicular to the angle of rotation of the lens on the HTS, namely the same axis as above. Therefore, the correct way to make a panorama of a flat subject perpendicular to the camera (or a subject very far away) is not to tilt the lens at all. My problem is how to find out that the lens is not tilted at all to a very high accuracy. There is a built-in stop at zero on the HTS, but that is not accurate enough. Neither is the external scale. I was asking whether the built-in electronic scale is accurate enough.

Anyway, the proper way to assemble a panorama is probably to forget about the HTS and use a panorama head to rotate the complete camera. The HTS does not seem to be that convenient for stitched panoramas.
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rem
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« Reply #4 on: June 09, 2013, 12:53:09 AM »
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Jerome, how David Grover (ex Hasselblad) said in another forum a few time ago, the digital info is the correct one, not the mechanical. I so far had never problems with this.
rem
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gigdagefg
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« Reply #5 on: June 09, 2013, 09:42:39 PM »
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If the image to be taken is in the landscape configuration, the tilt is in the opposite direction of a lens shift on the HTS. Therefore tilt can be used and photoshop will stitch it perfectly.
However on the H T S in the portrait configuration, the tilt should be set at zero when shifting for a panorama.
The HTS does not make the Haselblad body a true technical camera, but it works pretty well when shifting and tilting in a landscape mode
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jerome_m
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« Reply #6 on: June 16, 2013, 07:38:58 AM »
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If the image to be taken is in the landscape configuration, the tilt is in the opposite direction of a lens shift on the HTS. Therefore tilt can be used and photoshop will stitch it perfectly.
However on the H T S in the portrait configuration, the tilt should be set at zero when shifting for a panorama.

Actually, the tilt and shift axes rotate together on the HTS.
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jerome_m
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« Reply #7 on: June 16, 2013, 07:40:57 AM »
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Jerome, how David Grover (ex Hasselblad) said in another forum a few time ago, the digital info is the correct one, not the mechanical. I so far had never problems with this.

Indeed, when set to zero on the digital display, the axis of the lens is orthogonal to the sensor. The system seems to be accurate enough, even if not very convenient.
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jerome_m
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« Reply #8 on: June 16, 2013, 07:43:33 AM »
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I tried to find out what is gained in view angle when stitching a panorama versus using the base lens and crop. This is the result, first the base lens (HCD 28mm) without the HTS, then a stitch of two pictures taken with maximum shift left and right.

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