Capturing a Coin and Merging Chunks

[Magnus]

Hello!

Just a tip that could be useful if you want to suspend things seemingly floating in the air is the threads used for levitation magic tricks, like this for example: http://www.emagicsupply.com/index.php?main_page=product_info&cPath=30_27&products_id=371

I've used something similar myself a long time ago for some other photo stuff.

Best, Magnus.

[tincansassoc]

I have never heard of anyone using "focus stacking" on photoscan before. This may introduce errors. Generally any editing done to the photos is avoided before importing into photoscan. Im interested to know if anyone has been successful with "focus stacking".

[chadfx]

Thanks for the tip, Magnus.

I kept thinking of a museum or display supply store that would have things to mount items unobtrusively, but a lot of the time their mounting methods don't have to be as seamless as magic! What a great idea.

@TinCan - I have read that people here have successfully used Helicon Focus to pre-process focus stacked images for PS. It keeps the EXIF data intact as well. I'm going to give it a try for the next round with this.

Cheers, -C

[aphextwin]

chadfx,

can you provide a sample-image of your series taken for the coin?

I was not able to accomplish this task with PS some month ago, but I was able to make a 3d-scan of a coin with a SL-scanner and a macro-lens for the projector.
I guess my problem was that I didn?t geht the coin large enough on the camerasensor. And I don?t own a macro lens with a focal length that was great enough...

[chadfx]

Here you go, Aphextwin.

I used a 60mm macro lens on an Olympus OM-D body. I was still trying to maximize DOF, so I was keeping it a bit smaller in frame to cheat it a bit. It still didn't really cut it once you are shooting the coin along its length. But for the next shoot I'm going to focus stack with Helicon Focus post processing to try and nail down the sharpness issues that keep coming up when I'm shooting small objects.

You can also use extension tubes on regular lenses that also can give you a similar amount of magnification...sort of a poor man's macro, but they usually work quite well...and much cheaper than a dedicated macro lens.

http://digital-photography-school.com/extension-tubes-close-up-photography-lesson-2

[tincansassoc]

Extension tubes will ruin your cameras EXIF data, although you may be able to use the lens calibration tool to correct for this. But I haven't tried that tool myself.

[chadfx]

It's true that some extension tubes lack electrical contacts, so the EXIF data will not be recorded for the lens. But there are a number of extension tubes that WILL allow you to retain full control and info over your lenses.

Or do you think that the lens calibration in PS will become invalid anyway because the optical characteristics of the lens has changed?

I would imagine the EXIF focal length for a 50mm lens with 25mm of extension would probably still appear as 50mm. I wonder if you could just change that in the Calibration Tool and it would work correctly? It isn't very hard to set it manually.

Has anyone tried this?

[aphextwin]

chadfx,

when you select all the information that comes with this image you'll notice that in the image you provided only about 80000 px are valid "information" but only few are used as features. The image contains 786432 px total, so only about 10% of the sensor data are actually used, the rest is noise. when the coin is in a perpendicular position, the information is even much less. Imho you need to maximize the data that comes with each foto so I'd recommend you to get the coin closer to the lens / use a bigger lens. Also I'd try to use a polarizer. You'll probably notice some light shining on the coin in certain rotations. You're shooting with ISO 200. Is this the lowest ISO of the camera? Besides there is no noise visible, taking a higher ISO will get you very little noise. When shooting still, there is no need to increase the ISO, so I'd bring it to the minimum of the camera. For PS this is a definite no go ;-)
just my 2 cents...

ps: I'd not use any macro stacks. I tried this once with the equipment in our university. Total waste of time, because the lens distortion is different with each stacked image.

[chadfx]

Hi Aphex,

Thanks again for the input. Yes, shooting something that has very little width is certainly posing a challenge with PS. I originally tried shooting the coin laying flat and shooting each side separately. But PS wasn't able to align the two sides with so little information shared between the two sides (that same narrow edge of the coin). If I had the pro version, this would probably not be an issue, as I could use markers to guide the alignment.

I purposely kept this coin farther away from the lens to try and squeeze a bit more depth of field out of the setup, so more would remain in focus along those narrow angles. It was shot with an Olympus OM-D and the 60mm macro lens. The lowest ISO of the camera actually is 200, but it's pretty much noise free. However, with the smaller 4/3 sensor, it typically becomes diffraction limited earlier than larger sensor cameras (like f/8-11 vs f/16-22). Yet it also tends to have deeper depth of field for equivalent FOVs/focal lengths, so it winds up being a bit of a wash. I've also tested to see how close/deep I can preserve detail with a Canon 5D2 and it's actually relatively similar, even though I can stop the lens down more.

Next time I am going to focus stack with Helicon Focus. People here have written that it works fine with PS, so it's my turn to find out. ;-) I'm also going to soften the lighting set up, using larger sources farther from the coin. (they were very close in this test). And I also will use a polarizing filter to try to minimize reflections. I have a large polarizing sheet on order, so I would like to try that in front of the lights as well to see if that helps further.

I'm out on a field shoot for most of this week, so I'll have to wait until the weekend to do the next round on this one. But I'll let you know how it goes!

Cheers, -Chad

[aphextwin]

I just got a crazy idea concerning our coin-issue... I'll try something tomorrow and tell you if it works... if not, I'll tell you anyways, but not this evening... If it works, alle problems that involve coin-reconstruction are solved ;-)

[chadfx]

Tincan gets a gold star!...with your encouragement I stuck with the original game plan of holding the coin in a clip and shooting two 360's with the clip in two different positions (although I will hold it a bit less aggressively next time). I also used focus stacked images from Helicon Focus and I softened the lighting setup as best I could, and I masked out the clip from all photos. I also had a polarizing filter in place, but it didn't cut out as much reflection variation as I had hoped, but it did help a bit. The pipeline in PS handled everything just fine once all of the prep was in place. Bravo!

Now I just need to refine the process a bit more, especially the focus stacking pipeline. It required more hand holding than I had expected. Still, it's better than shallow depth of field and a bad build!

Cheers, -C

[tincansassoc]

Good to hear it worked for you

[aphextwin]

Mine's not working yet, but I'm sure it will... just need more time =/

[chadfx]

Keep at it Aphex! I'm looking forward to seeing your results once you get a chance to work it out. Good luck!

-C