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Author Topic: How to take photos  (Read 7499 times)

mrafiei

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How to take photos
« on: December 16, 2015, 10:10:17 PM »
Hi
I have a question about close range imagery (like 3D modeling of a building).
please tell me when I want to take photo which parameters (focus, f ,...)of my camera must be constant?!!
Is it important to fix the focal length?
what about the focus?

thank you very much
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Kiesel

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Re: How to take photos
« Reply #1 on: December 17, 2015, 10:00:29 AM »
The more robust your lens and your camera the better! Any lens moving inside your lens changes the inner orientation of your lens (camera calibration parameters).
In general prime lenses are better than zoom lenses and a sturdy camera with metal body is better than a plastic one. So fix at least the focal lenght (tape it) and avoid focusing (better choose a higher aparture for that but not the highest=>diffraction).
On the other side you can also use groups with different focal lenghts in PhotoScan.

Karsten

Artisan S

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Re: How to take photos
« Reply #2 on: December 17, 2015, 11:11:39 PM »
Sorry Karsten,

A: Close range......what is close range when your shooting buildings. Meters, Centimeters......

B: Agisoft doesn't mind focussing.....you can shoot up close at point blank range for a detail and from 20 meters away for a overview of the same building AS LONG AS YOU KEEP YOUR FOCAL LENGTH FIXED, that is from experience and that stands.

C: I shoot in concentric rings, and try to get every corner of building with at least three shots and I try to get every plane from th normal at least once, better twice.

D: Think scanning......lots of photo's with basically the same content clog up the workflow without contributing to much to the mesh quality. So a few total schots of each side of the building and each corner are great....then home in on the details, doors, stairs, hallways, lamps, fences and rails, if you use the camera as a scanner and keep in mind the overlap between all adjacent shots should be 66.6 procent (more or less :-)), so each point is on at least 3 photo's, you will start to see the details you like.

E: Beware of shrubs, shrubs are dangerous, if they obstruct the view of a wall, photogrammetric scanning can eather create a whole (the honest way) or create a textured blob (the dishonest way) but you rarely will see a shrub and a wall behind it, unless you scan behind the shrub.

Scanning buildings is difficult and it take a lot of experience (I noticed) and if you are new to Agisoft it might be better to get your feet wet with simpler objects, statues, rocks, trees,

Greets, Ed.

P.S. avoid modern buildings with lots of glass, or use a double polaroid filter to stop reflections as much as you can so glass won't show up......but better to leave them be.

Kiesel

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Re: How to take photos
« Reply #3 on: December 18, 2015, 12:28:36 AM »
Sorry for not to be clear... "ideally you should...".;)
Yes you can do all that Ed and more. Additionally even change the focal length in groups (shot one sequence with focal length xx  another with focal length yy and ....).

Cheers Karsten
« Last Edit: December 18, 2015, 07:42:29 AM by Kiesel »

mrb

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Re: How to take photos
« Reply #4 on: December 18, 2015, 07:26:43 AM »

B: Agisoft doesn't mind focussing.....you can shoot up close at point blank range for a detail and from 20 meters away for a overview of the same building AS LONG AS YOU KEEP YOUR FOCAL LENGTH FIXED, that is from experience and that stands.

I often change focal lengths and combine photos from different lens and the results come out fine without any trouble aligning, so I would say to worry more about making sure the photos are technically good (proper exposure, in focus, etc..) before worrying about focal length.

stihl

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Re: How to take photos
« Reply #5 on: December 18, 2015, 08:02:21 PM »
Just because the output you get looks good doesn't mean it is good nor does it mean it's as good as it can be.
From a theoretical standpoint on photogrammetry a fixed focal length and preferably a fixed aperture parameter is always better.

mrb

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Re: How to take photos
« Reply #6 on: December 20, 2015, 10:19:11 PM »
Just because the output you get looks good doesn't mean it is good nor does it mean it's as good as it can be.
From a theoretical standpoint on photogrammetry a fixed focal length and preferably a fixed aperture parameter is always better.

That goes without saying. The amount of care and meticulousness you take in capture is in direct relation to what you intend to use the end result for.  Hope to take millimeter-accurate measurements from the model? Then of course you'd better be as absolutely careful in shooting as possible. Just want a quick and dirty digitization of something? Shoot with your cell phone.  I suspect most people are aiming somewhere in between, and that's the group my comment was aimed at.

Packy_GIS

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Re: How to take photos
« Reply #7 on: December 21, 2015, 12:52:07 AM »
I have tested images taken from an aircraft (with built-in gimbal) shot with fixed focal length and A) auto focus B) fixed focus.
The data were compared to RTK surveyed data and to data from "ordinary" photogrammetry. The difference in the A) case was 1-8 cm from the "ethalon".  The B) case was a little bit better.
On the other hand - the differences could be caused by the fact, that the images were taken within 2 hour interval.
So I wouldn't care much about the auto-focus. And if you do - there is "split" function in the camera calibration dialog and each autofocused image is calibrated as separate camera.
Packy

Artisan S

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Re: How to take photos
« Reply #8 on: December 21, 2015, 11:49:32 AM »
Sure I shoot at apperture 8 or 11, in order:

a) to hit the sweetspot of the lens

b) to have enough DOF (I use and APS-C 550D).....

Greets, Ed.