[Nhcoll-l] 3D scales in 2D images

[Burkhalter, Roger J.]

We use the old-school approach of a bellows/bellows lens (Leitz aristophot or Zeiss Luminar) attached to a digital slr. The lens/bellows extension is the calibrated to a given magnification on the sensor at 600dpi (x1, x2, x3, x4 … x50). Scale bars can then be added in a photo editing program. They can also be used for measurements and are repeatable. This also works to a large degree on simple objects in 3D photogrammetry. The concept of repeatability, taking images of hundreds of specimens and knowing they are at the same magnification, is refreashing.

Roger

From: Nhcoll-l <nhcoll-l-bounces at mailman.yale.edu> On Behalf Of Peter A Rauch
Sent: Friday, June 5, 2020 2:13 PM
To: nhcoll-l at mailman.yale.edu
Subject: Re: [Nhcoll-l] 3D scales in 2D images

An cheap 'n dirty alternative to the (expensive) telecentric lens approach, and depending on "... the purpose of the scale ...", you might try taking the photos at a greater distance with a long telephoto lens. There'll still be various sorts of dimensional distortions inherent in the system (lens), but you might try your pencils/rulers-stacking test bed with a long telephoto lens shot, just to see whether it serves your "purpose of the scale" better (good enough). You might also lay down a circle and vertical scale (ruler) in the image field also, and try to center the lens axis on the circle center, and be sure that the "film" (sensor) plane is parallel to the scales' plane.

You're right --it's all about "the purpose of the scale". Getting the most from the time invested in setting up the specimen object, to take the photo, is part of the game.

Peter

On Fri, Jun 5, 2020 at 11:03 AM Adam Rountrey <arountre at umich.edu<mailto:arountre at umich.edu>> wrote:
Perhaps, a telecentric lens could be used?
-Adam
...

From: Nhcoll-l <nhcoll-l-bounces at mailman.yale.edu<mailto:nhcoll-l-bounces at mailman.yale.edu>> on behalf of Gary W Shugart <gshugart at pugetsound.edu<mailto:gshugart at pugetsound.edu>>
Sent: Friday, June 5, 2020 1:02 PM
To: nhcoll-l at mailman.yale.edu<mailto:nhcoll-l at mailman.yale.edu> <nhcoll-l at mailman.yale.edu<mailto:nhcoll-l at mailman.yale.edu>>
Subject: [Nhcoll-l] 3D scales in 2D images


External.
Hi All:  When I take a picture of a specimen I usually include a scale ruler on the stage or at the base. It didn't occur to me until recently that the scale will not apply for any part of the object not on the same plane as the base. The differences are substantial with eggs, nests, bones.  For example using a scale on a base plane compared to two scales above separated by pencils (7 mm) and the top scale is 10 mm = 11 mm at base (reference photo on Slater Museum FB page (https://tinyurl.com/ybnjkq2r<https://urldefense.proofpoint.com/v2/url?u=https-3A__tinyurl.com_ybnjkq2r&d=DwMFaQ&c=qKdtBuuu6dQK9MsRUVJ2DPXW6oayO8fu4TfEHS8sGNk&r=1jjikMiDaYjeQeMi4sQT7g&m=YqYPdFtRRzHR0PDzdQa4DmKV6by9p9hbItsr845ORSc&s=3kxs80mrturiKvABkuM7-AJwlLJ1tALX1gqz8jEfH3I&e=>).  Searching Google and this appears to be something like perspective or forced perspective (not parallax view) .  There are explanations of angular size calculation online and calculators, but you have to know the distance between the base and plane to calculate a size. I noticed this especially in the new Birds of the World (formerly Birds of North American) account with eggs and nest with a scale.  Also recall the issue occurred in egg photographs.

How to deal with this?  This depends on the purpose of the scale.  If to just give a general idea of size it doesn't matter.  But if the idea is to use the scale to set the scale in imagej or other measuring software, it is a problem.  A pain to set up and redo for each object though. Or actually deal with specimens and measure them IRL.

Gary Shugart
Collection Manager
Slater Museum
Tacoma, WA


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