Some technical help for a university project

Some technical help for a university project

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722Adam

Original Poster:

2,152 posts

212 months

Wednesday 25th February 2015
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For my final year project I'm looking into water droplet behaviour on vehicle surfaces, specifically measuring contact angles from images of droplets. The measuring will all be done automatically in MATLAB but for this to work, I require an image with a razor sharp droplet profile edge.

This is what I'm working with at the moment.



Essentially, MATLAB struggles to trace the droplet boundary, so I need it to be as sharp as possible in the raw image. I've had a play today working out which aperture my current lens works best at, using mirror lock up etc. and I already use a tripod and remote shutter, but my supervisor likes the idea of getting the droplet to fill more of the image.

The image was shot with a Canon 18-55mm IS kit lens, set at 55mm and positioned as close as it will focus (250mm IIRC), so by my understanding I can't get the droplet to fill more of the image with this lens. I possess an 80-210mm but with a minimum focus distance of 1.5m, by my reckoning it won't be any better.

Does this mean I need a new lens?

Been looking at these two, as they both quote 1.0x magnification:
Canon EF-S 60mm f2.8 Macro USM
Canon EF 100mm f2.8 Macro USM

Anything I bought would be attached to a Canon 550D.

Am I on the right track by thinking these will help me do what I want? (i.e. be able to stand close up and get an image with a nice big droplet in it)

Any other tips greatly welcomed.

GetCarter

29,358 posts

278 months

Wednesday 25th February 2015
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You should check out this thread and PM the ones best suited to help.

http://www.pistonheads.com/gassing/topic.asp?h=0&a...

K12beano

20,854 posts

274 months

Wednesday 25th February 2015
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Wot 'e sed! ^^^

Plus you might start with lower cost options:

A close up lens that screws into the filter thread on the front of your existing lens

An (set of) extension tube(s) that fits between body and lens to extend close focus distance

A reversing ring - you "put your lens on backwards" which also allows close focus

DibblyDobbler

11,257 posts

196 months

Wednesday 25th February 2015
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Cheapest option would be a 50mm + extension tubes - 60mm or 100mm are both good though just a bit more dosh...

LongQ

13,864 posts

232 months

Wednesday 25th February 2015
quotequote all
DibblyDobbler said:
Cheapest option would be a 50mm + extension tubes - 60mm or 100mm are both good though just a bit more dosh...
"Cheapest option"?

Are you feeling OK DD?

This IS PH is it not? Or did you think you were logged in somewhere else?

MP-E65 surely?

Rental if poverty is a concern.

Mind you I could foresee some problems getting really close to the subject in a vertical side profile.

DavidY

4,458 posts

283 months

Wednesday 25th February 2015
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Adam

I work in industrial machine vision, and so have quite a bit of experience in this kind of thing. Normally in machine vision we don't work with anywhere the resolution of a DSLR, and we rarely undertake measurement applications using a colour sensor, as the Bayer filter on the sensor automatically reduces the spatial resolution from a measurement point of view. There are also machine vision software packages out there that will trace the contour of an object and can provide you with a datacloud of points that you can then analyse (in MATLAB).

If you want to, please feel free to PM me, and I can point you at some other resources that you might find useful.

I won't be online during the day tomorrow, so don't expect an instant response.

David

DibblyDobbler

11,257 posts

196 months

Wednesday 25th February 2015
quotequote all
LongQ said:
DibblyDobbler said:
Cheapest option would be a 50mm + extension tubes - 60mm or 100mm are both good though just a bit more dosh...
"Cheapest option"?

Are you feeling OK DD?

This IS PH is it not? Or did you think you were logged in somewhere else?

MP-E65 surely?

Rental if poverty is a concern.

Mind you I could foresee some problems getting really close to the subject in a vertical side profile.
You're quite right LQ - I need to have a lie down and get my head straight silly

Simpo Two

85,149 posts

264 months

Wednesday 25th February 2015
quotequote all
If the budget allows I'd go for a proper macro lens as you suggest, as all camera functions will be supported. 60mm and 100mm will give the same magnification but the longer lens gives more working distance, which may or may not be useful to you. On a crop sensor you'll actually get more than 1:1 magnification, so a postage stamp will fill the frame.

You can buy one s/h from eBay if you prefer.

The downside of high magnification is shallow DOF, so bear that in mind.

Moonhawk

10,730 posts

218 months

Wednesday 25th February 2015
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Simpo Two said:
The downside of high magnification is shallow DOF, so bear that in mind.
That can be easily death with in this scenario though.

If the camera is mounted on a tripod - just stop the aperture down to extend the DOF. Trigger the camera using a cable release or using a 2 second timer - the exposure can then be as long as you need.

DavidY

4,458 posts

283 months

Wednesday 25th February 2015
quotequote all
Alternatively use a telecentric lens http://en.wikipedia.org/wiki/Telecentric_lens

The biggest challenge in any tight measurement application is illuminating the item so that the edges are sharp.

LongQ

13,864 posts

232 months

Wednesday 25th February 2015
quotequote all
DavidY said:
Alternatively use a telecentric lens http://en.wikipedia.org/wiki/Telecentric_lens

The biggest challenge in any tight measurement application is illuminating the item so that the edges are sharp.
With something like a droplet and thinkng about edge lighting ... would being sure you have the edge lit be a challenge in itself? Or, to put it another way from a camera perspective, you may have lit something but is it the edge that you want to measure?

Simpo Two

85,149 posts

264 months

Wednesday 25th February 2015
quotequote all
DavidY said:
Alternatively use a telecentric lens http://en.wikipedia.org/wiki/Telecentric_lens
Well I've read it and frankly have no idea what on earth it means. 'The simplest way to make a lens telecentric is to put the aperture stop at one of the lens's focal points.' How do you do that, and will it fit on a Canon DSLR?

DavidY

4,458 posts

283 months

Wednesday 25th February 2015
quotequote all
LongQ said:
DavidY said:
Alternatively use a telecentric lens http://en.wikipedia.org/wiki/Telecentric_lens

The biggest challenge in any tight measurement application is illuminating the item so that the edges are sharp.
With something like a droplet and thinking about edge lighting ... would being sure you have the edge lit be a challenge in itself? Or, to put it another way from a camera perspective, you may have lit something but is it the edge that you want to measure?
Well that would depend on which dimensions you were interested in measuring, length/width or height?? or both. The wavelength of the light used also effects the accuracy of the measurement. Then there is back illumination, front illumination (darkfield, brightfield, DOAL, Dome, etc, etc), and corresponding differences in potential sub-pixel interpolation. It all depends how accurate the OP wants to go, or is the shape more important??

173 pages on the effects of light and water drops!! http://www.ri.cmu.edu/publication_view.html?pub_id...



DavidY

4,458 posts

283 months

Wednesday 25th February 2015
quotequote all
Simpo Two said:
DavidY said:
Alternatively use a telecentric lens http://en.wikipedia.org/wiki/Telecentric_lens
Well I've read it and frankly have no idea what on earth it means. 'The simplest way to make a lens telecentric is to put the aperture stop at one of the lens's focal points.' How do you do that, and will it fit on a Canon DSLR?
Its virtually impossible to create the perfect telecentric lens, but effectively they are a macro lens with a larger DOF. I'm working on a project now where the camera has a 12Mp mono sensor (APS-C sized), and we have a FOV approx twice the size of the sensor (magnification 0.5) at approx 600mm working distance, but we have a DOF of 18mm. They allow us to get a short exposure time (think moving parts on a conveyor line), unlike one of the suggestions above to have long exposures (which possibly would introduce noise and reduce the measurement accuracy)

And within the working DOF the magnification is the same (important when measuring objects that might not be placed exactly in the same place each time)

You can certainly get M42 Telecentric lenses, and therefore could fit to the Canon with an adapter, but as I said before does the OP really need that much resolution??


Edited by DavidY on Wednesday 25th February 22:43

LongQ

13,864 posts

232 months

Wednesday 25th February 2015
quotequote all
DavidY said:
Simpo Two said:
DavidY said:
Alternatively use a telecentric lens http://en.wikipedia.org/wiki/Telecentric_lens
Well I've read it and frankly have no idea what on earth it means. 'The simplest way to make a lens telecentric is to put the aperture stop at one of the lens's focal points.' How do you do that, and will it fit on a Canon DSLR?
Its virtually impossible to create the perfect telecentric lens, but effectively they are a macro lens with a larger DOF. I'm working on a project now where the camera has a 12Mp mono sensor (APS-C sized), and we have a FOV approx twice the size of the sensor (magnification 0.5) at approx 600mm working distance, but we have a DOF of 18mm. They allow us to get a short exposure time (think moving parts on a conveyor line), unlike one of the suggestions above to have long exposures (which possibly would introduce noise and reduce the measurement accuracy)

And within the working DOF the magnification is the same (important when measuring objects that might not be placed exactly in the same place each time)

You can certainly get M42 Telecentric lenses, and therefore could fit to the Canon with an adapter, but as I said before does the OP really need that much resolution??


Edited by DavidY on Wednesday 25th February 22:43
The Wiki link suggests a 4:3 camera so perhaps Olympus?



Simpo Two

85,149 posts

264 months

Wednesday 25th February 2015
quotequote all
DavidY said:
Its virtually impossible to create the perfect telecentric lens, but effectively they are a macro lens with a larger DOF. I'm working on a project now where the camera has a 12Mp mono sensor (APS-C sized), and we have a FOV approx twice the size of the sensor (magnification 0.5) at approx 600mm working distance, but we have a DOF of 18mm. They allow us to get a short exposure time (think moving parts on a conveyor line), unlike one of the suggestions above to have long exposures (which possibly would introduce noise and reduce the measurement accuracy)
I doubt noise would be an issue with a 2 second exposure but it's an interesting concept. Thanks for the explanation.

DavidY

4,458 posts

283 months

Wednesday 25th February 2015
quotequote all
Personally I think that an SLR is the wrong approach, I think that the OP would be better off borrowing a high resolution 2/3in sensor C-mount mono camera (say 1600x1200) resolution. Then using something like either a Computar MLM-3XMP macro zoom lens (a cracking piece of kit) or using the Computar Tec-M55 pseudo telecentric (may need some spacer rings to get the right FOV), then he will be looking at a FOV of say 12mm or even 16mm (to make the maths easy!!), all depending on the size of his water drops.

He will then have a monochrome image and with the correct illumination will have sharp edges, which can then be processed using a machine vision software (with sub-pixel interpolation), giving a resultant cloud of points that can be thrown at MATLAB.

That should be more than enough resolution (spatially is the same as 3200x2400 colour sensor with a bayer filter) to prove the concept for a university project.

Now given that his university has departments that include computer vision. robot vision, etc, I'd be really surprised if the kit required isn't on his doorstep. But if he gets in touch with me, I'm more than happy to point him in the directions of companies that may help him out.

Edited by DavidY on Wednesday 25th February 23:07

PorscheGT4

21,146 posts

264 months

Wednesday 25th February 2015
quotequote all
just set the camera up and had a go, crappy SLR ,click on it for full res.



Edited by PorscheGT4 on Wednesday 25th February 23:39

C&C

3,281 posts

220 months

Thursday 26th February 2015
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As has already been suggested, a set of extension tubes is a good way of getting very close.

I'd not bother shelling out for the Canon ones which IMHO are overpriced and optically identical to cheaper alternatives (as there is no glass involved - just air).

Decent ones which fit and work well are made by Kenko (I have a set and am very happy with them).

Amazon link here

722Adam

Original Poster:

2,152 posts

212 months

Thursday 26th February 2015
quotequote all
Thank you all for your suggestions so far, didn't think this would attract as much interest so I've got a lot to go on!

DavidY, at this stage of the project I think it is a little too late to go down a different route to DSLR, plus I am building off the back of a project from last year which started out that way, so makes sense to continue. I will however mention these other methods to my supervisor, as it may be a possible avenue to explore next year if it is considered at the start of the project.

I'm meeting my supervisor in half an hour or so, I'll discuss the options regarding lenses and extension tubes, see what the scope is for getting equipment in, if it's not hideously expensive, they could be convinced to buy it.