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Author Topic: 35mm for head scans  (Read 14667 times)

Mfranquelo

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35mm for head scans
« on: February 25, 2014, 02:49:02 PM »
Do you think that 35mm will introduce geometric distortions on head scans ? does photoscan correct the lens distortion successfully on this kind of focal distance ?

http://www.amazon.es/gp/product/B0050I7AGO/ref=ox_sc_act_title_1?ie=UTF8&psc=1&smid=A1AT7YVPFBWXBL

I was thinking on changing my head scanning set up to 35mm to make it smaller...
« Last Edit: February 25, 2014, 07:48:22 PM by Mfranquelo »

Infinite

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Re: 30mm for head scans
« Reply #1 on: February 25, 2014, 03:21:15 PM »
Anything below 35mm and you will get distortion in the point clouds.

Photoscan can only compensate so much. I've tested with 28mm and 20mm, Primes. Not only was the image quality terrible but Photoscan produced a huge amount of distortion. Very unreliable.

35mm, 40mm, 50mm + are the way to go.

But 35mm primes can produce poor image quality, like the 40mm pancakes. Chromatic aberration, vignetting, some distortion, softness but they're not too bad.

50mm primes are king IMHO.
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David Cockey

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Re: 30mm for head scans
« Reply #2 on: February 25, 2014, 04:20:57 PM »
My experience is very different than Infinite's. I have had excellent results in PhotoScan with images taken using lenses with focal lengths as short as 10mm (16mm 35mm full frame equivalent) and as long as 60mm (96mm equivalent). Most subjects have been boats and ship models from less than 1 meter to over 25 meters in length.

The focal length relative to the size of the sensor is what matters for perspective. A 35mm lens on a "crop frame" DSLR (22.3mm x 14.9mm sensor) results in an image which will have same perspective as a 56mm lens on a "full frame" DSLR (36mm x 24mm sensor) and a 10mm lens on a compact camera with a 1/2.3 sensor (6.2mm x 4.5mm) if the images are taken from the same location.

It is very important to differentiate between true lens distortion which results in straight lines being curved in images, sometimes referred to as barrel or pincushion distortion, and geometric perspective "distortion" which results in parallel straight lines appearing to converge in an image but still being straight. Lenses of the same focal length can have very different amount of barrel/pincushion distortion. Shorter focal length lenses will have greater perspective "distortion" if the location the image is taken from is adjusted so that the subject will be the same size in the image.

PhotoScan like all photogrammetry, the same geometry which results in perspective "distortion", to determine 3D shape. When I tested very long focal lengths (160mm equivalent and longer) I found that PhotoScan sometimes had difficulty with aligning the photos which was probably due to insufficient "perspective".

PhotoScan has corrected the true lens distortion with all the lenses I have used as long as there are a sufficient number of images and there are at least two overlapping rows of images.   

Infinite

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Re: 30mm for head scans
« Reply #3 on: February 25, 2014, 04:29:22 PM »
My experience is very different than Infinite's. I have had excellent results in PhotoScan with images taken using lenses with focal lengths as short as 10mm (16mm 35mm full frame equivalent) and as long as 60mm (96mm equivalent). Most subjects have been boats and ship models from less than 1 meter to over 25 meters in length.

The focal length relative to the size of the sensor is what matters for perspective. A 35mm lens on a "crop frame" DSLR (22.3mm x 14.9mm sensor) results in an image which will have same perspective as a 56mm lens on a "full frame" DSLR (36mm x 24mm sensor) and a 10mm lens on a compact camera with a 1/2.3 sensor (6.2mm x 4.5mm) if the images are taken from the same location.

It is very important to differentiate between true lens distortion which results in straight lines being curved in images, sometimes referred to as barrel or pincushion distortion, and geometric perspective "distortion" which results in parallel straight lines appearing to converge in an image but still being straight. Lenses of the same focal length can have very different amount of barrel/pincushion distortion. Shorter focal length lenses will have greater perspective "distortion" if the location the image is taken from is adjusted so that the subject will be the same size in the image.

PhotoScan like all photogrammetry, the same geometry which results in perspective "distortion", to determine 3D shape. When I tested very long focal lengths (160mm equivalent and longer) I found that PhotoScan sometimes had difficulty with aligning the photos which was probably due to insufficient "perspective".

PhotoScan has corrected the true lens distortion with all the lenses I have used as long as there are a sufficient number of images and there are at least two overlapping rows of images.   

Do you have any results from face or body scanning using those lenses?

For faces it might be OK but you will loose detail. Body, it wont work at all.

Ariel shots of landscapes, buildings, boats etc are quite different to syncing multiple cameras to retain as much detail as possible on organic subjects.

I've also heard there can be a mismatch in accuracy of photogrammetry data when compared to the accuracy of a Faro3D laser scan, from using those types of lenses.
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David Cockey

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Re: 30mm for head scans
« Reply #4 on: February 25, 2014, 04:41:23 PM »
A longer focal length can be advantageous for a highly curved object, such as a head, when photographs are taken at relatively large angle increments a longer focal length, such as might be used in a multi-camera rig. The reason is with the camera further from the subject the angle between the camera and parts of the surface curving away from the camera is reduced.

Mfranquelo

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Re: 35mm for head scans
« Reply #5 on: February 25, 2014, 07:48:14 PM »
Im very grateful for your suggestions,

Size is quite critical for me. Thats why i wanted to use 35mm.
Im really afraid of point cloud distortion and loosing quality, im working with 50mm lenses at the moment, but making my system almost 30% smaller is really important for me.

According to http://www.kenrockwell.com/tech/samyang/35mm-f14.htm the review seems to be pretty good, it is amazingly sharp.

Could you specify which 35mm lenses have produced bad quality in your scans Lee ?
« Last Edit: February 25, 2014, 08:06:36 PM by Mfranquelo »

chadfx

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Re: 35mm for head scans
« Reply #6 on: February 25, 2014, 08:10:14 PM »
Yes, lenses of the same focal length are definitely not equal.

Another 35mm to consider would be the Sigma 35mm, which seems to be regarded as far better than the Canon 35mm lenses.

http://www.dpreview.com/lensreviews/sigma-35mm-f1-4-dg-hsm/5

But I cannot say how they would fare in a body/head scan situation. (and they are probably 3x more expensive than the Samyang, although cheaper than Canon's 35mm L lens.)

Mfranquelo

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Re: 35mm for head scans
« Reply #7 on: February 25, 2014, 08:13:53 PM »
Thank you for your suggestion chadfx, however the price of the sigma 35mm is like you said, almost x3 times higher, for almost the same quality.

The reviews of the samyang are really positive, ill buy one and test it, i can post results next week :).

Like David said, 35mm on APS-C equals to 50mm more or less.
« Last Edit: February 25, 2014, 09:07:38 PM by Mfranquelo »

David Cockey

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Re: 35mm for head scans
« Reply #8 on: February 25, 2014, 09:12:15 PM »
Generally any lens used to take photos of three dimensional objects for PhotoScans should be used stopped down to at least f5.6 and frequently to f8 or f11 to obtain sufficient depth of field. Lens tests only look at sharpness in the plane the lens is focused to, and basic optics causes sharpness to be reduced away from that plane as the "circle of confusion" grows. The distance over which the image is acceptably sharp is referred to as depth of field, and it becomes greater as the aperture size decreases (higher f-stop number).

Sharpness and vignetting differences between different design lenses tend to be much greater at largest aperture (lowest f-stop number) than when stopped down. Lens distortion is generally independent of aperture. Chromatic aberration can vary with aperture and some lenses with large amounts of chromatic aberration at large aperture can improve at smaller aperture. Lens tests frequently concentrate on large aperture performance (lower f-stop numbers) because this is where the differences between lenses are greatest.

But for taking photos of 3D objects for PhotoScan performance at f1.4, f2.8 or even f4 is generally irrelevant because smaller apertures are used. When comparing results of objective lens tests for PhotoScan use the results between f5.6 and f11.   
« Last Edit: February 25, 2014, 09:13:56 PM by David Cockey »

Mfranquelo

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Re: 35mm for head scans
« Reply #9 on: February 26, 2014, 02:34:46 AM »
I've seen many tests of closing the aperture to f22 and carefully sharpening the "softened" image, the results are practically the same as the ones taken using the "sweet" spot at f9-f11

David Cockey

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Re: 35mm for head scans
« Reply #10 on: February 26, 2014, 07:58:46 AM »
I've seen many tests of closing the aperture to f22 and carefully sharpening the "softened" image, the results are practically the same as the ones taken using the "sweet" spot at f9-f11

Are you talking about the visual appearance of images, or results of using the images in PhotoScan? If it's for use with PhotoScan I'm interested in any information you have about the technique.

At f22 the sharpness of most lenses of a given focal length will be about the same.

Marcel

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Re: 35mm for head scans
« Reply #11 on: February 26, 2014, 12:00:34 PM »
If you are worried about diffraction I would do some tests with the lens you use. A lot of people like to bring up diffraction for some reason, but in reality it's really not such a big problem. I've never seen any problems when shooting up to F16.

And even if the sharpness is the tiniest bit lower due to diffraction, this is more than made by the fact that more parts of your image are sharp due to a bigger depth of field. I would rather have a 95% image that is in focus all over, than a 100% sharp image that has lots of blurred parts due to depth of field.

If you process the Dense Cloud at 'High' instead of 'Ultra' then you should not worry about diffraction at all (source images are downscaled to 50% on 'High', so any pixel level sharpness problems will be mostly gone).

Mfranquelo

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Re: 35mm for head scans
« Reply #12 on: February 26, 2014, 01:32:26 PM »
That is very usefull to know Marcel, thanks.

The samyang is coming tomorrow, i hope to run tests this weekend.

Infinite

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Re: 35mm for head scans
« Reply #13 on: February 26, 2014, 02:56:39 PM »
It can be hard to know what is truth and what is not in this field. The only way to find out, is to test yourself.

If your looking to get the best results out of your system, diffraction IS an issue. It's a real pain.

You can't just alter the images after the fact with sharpening, as you are in fact altering reality. It's not recommended to post process images at all, only for the colour usage. It can have a detrimental effect on the build quality if you alter the images. It's OK afterwards for texturing, just not before.

From my experience the ideal aperture setting is between 11-14. 16 if you have to.

Don't forget, if you go up over 16, you need a lot more light. Which can start to become uncomfortable for subjects. Think the Lightstage requiring their talent to wear dark contact lenses!

There is no rule book in this field. Many voices. Many methods. Your best bet is to find your way from experience.
« Last Edit: February 26, 2014, 02:59:08 PM by Infinite »
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EMULAT3D

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Re: 35mm for head scans
« Reply #14 on: February 26, 2014, 05:02:35 PM »
You can't just alter the images after the fact with sharpening, as you are in fact altering reality. It's not recommended to post process images at all, only for the colour usage. It can have a detrimental effect on the build quality if you alter the images. It's OK afterwards for texturing, just not before.

So should any post adjustments be made when converting from RAW to TIF? Programs like DXO have so many different color, sharpening and noise reductions settings built into their presets. Is it best to just convert the RAW to TIF directly?