Broad DOF vs image softening due to high f-stop

[Andrew]

Fellow Photoscanners,

I am curious as to your findings about sweet spot for optimum quality of photos, both for building geometry and generating textures. Specifically, while scanning large (human) subjects, higher f-stop helps keep everything in perfect focus, at the price of per-pixel sharpness. On a typical Canon (crop) 55mm lenght (EF-S 18-55 kit lens), would you go f16 and higher? Is broader DOF worthy of softening images overall?

Cheers,

Andrew

[RalfH]

It also depends on how much light you have and how close your camera is to the subject. There is no point in using f16 if this means that ISO has to go up to 400 or even higher. I would use a depth of field calculator to find the minimum f-stop for the range of distances that are to be expected in the photographs, and from there see if I could make the f-stop a bit higher without compromising ISO 80 or 100 under the given lighting conditions. In any case, the EF-S 18-55 is not a great lens if you're striving for optimum image quality.

[Andrew]

Thanks, RalfH! I realize the EF-S 18-55 kit lens is not the best piece of optical glass in existence, but if I want to extend my setup to more cameras, better grade lens will quickly kill my budget

With this lens, when shooting head shots, I am at about 80cm from the subject, shooting at 55mm f16 1/200s ISO100. This gives 12cm of focus (5.6 cm in front of focus point and 6.5 cm behind focus point), so almost entire head in perfect focus, for the price of slightly softer image overall.

To reiterate the point in my question: does Photoscan work better with slightly softer photos with wide DOF, or is it fine with slight blur in out of focus areas, for the price of crisp pixel detail in focused parts, or maybe something in between?

Cheers,
Andrew

[RalfH]

I had one project in which (for an unknown reason) one photograph was not focussed (i.e. very soft). As it was an image in a row without which the whole scene would have broken apart, I used it anyway - no problem for PhotoScan, but of course shape and texture resolution were relatively poor in that section of the model. For 3D reconstruction it is important that your pictures are sharp enough to retain sufficient texture for PhotoScan to match points. Faces can be quite smooth, and hair is a poor target for SfM. You should definitely make sure that the photos are really sharp in those parts that don't have much texture. Having said all hat, I think your numbers look OK, but do some testing anyway.

[Wishgranter]

Search for lens that you want to use, there is explanation on how sharp can lens shoot..... If can not use/understand it let me know......

http://www.dxomark.com/index.php/Lenses/Camera-Lens-Database/Canon/EF-S-18-55mm-f-3.5-5.6-IS-II/(camera)/619

see what you need to watch http://dl.dropbox.com/u/15047343/lenstest.jpg

[Andrew]

I just tried f11 on a rather smooth face and results seem better than f16, even though some parts of image escaped slightly out of focus. Per-pixel sharpness on low contrast areas seems to be really important. Thanks to the lens data from provided link I can easily see where my lens goes soft

Many thanks!

[David Cockey]

"Depth of field" does not mean everything within the depth of field is in focus and everything outside the depth of field is equally out of focus.

Keep in mind that there is only one plane (actually usually a curved surface) at which objects are in focus and therefore maximum sharpness for the lens. The distance from the lens to the plane/surface is the distance the lens is focused at. Away from that plane/surface objects will be blurred, with the amount of out of focus blurring increasing the further the object is from the plane/surface.

The overall sharpness of an object away from the plane/surface depends on two factors. One is the inherent sharpness of the lens for which there is an optimum f-stop. A higher numerical f-stop will reduce this part of sharpness. The other factor is the amount of blur due to the object being out of focus. This amount of blur increases the further the object is from the "focus distance" but decreases with higher numerical f-stop. So if the object being photographed is sufficently close to planar then for maximum sharpness use the optimum f-stop for the lens (assuming there isn't blurring due to motion, etc). But if object isn't close to planar than a higher f-stop will result in improved sharpness for the portions of the object away from the focus distance while decreasing sharpness at the focus distance. But there may be a higher f-stop at which the decrease in lens sharpness is greater than the decrease in out-of-focus blurring. The more non-planar the object is the higher the f-stop to use.

[Coadey]

One is the inherent sharpness of the lens for which there is an optimum f-stop. A higher numerical f-stop will reduce this part of sharpness.

I just went through this exercise on my NX1000 kit lens, and David makes a good point; each lens has an optimal f-stop for a given focal length to achieve maximum sharpness (often about 2 stops down):

http://en.wikipedia.org/wiki/Stopping_down

I believe this is your lens:

http://www.photozone.de/canon-eos/404-canon_1855_3556is_50d?start=1

At high stops you'll have everything in focus, but with lower sharpness, more aberrations and probably more noise due to higher ISO.