Best F-Stop for the best Data?

[marlin]

Dear All,

During the capture process, what F-Stop do you generally capture with?

We try to stick to F-8 to F11.

We are using Canon DSLRs and canon L 'redline' lenses. Typically the, 24mm prime, 50mm Macro, and the 100mm macro. (the 24mm is not and L lens).

We are really tempted to use a higher number, small aperture, to get a larger Depth of Field - as we frequently document objects (large and small) as well as environments.

Being aware of the consequences of 'Lens Diffraction' and the trade offs between higher F-stops and softness/slightly blurry results, I'm still interested to know what your rules of the F-stop are when capturing data for photogrammetry.

Anyone care to opine on this matter?

thank you. I look forward to reading some professional opinions.

marlin.

[David Cockey]

"Best" F-Stop depends on the shape of the subject and the lens characteristics, and can be a tradeoff between sharpness at the focused distance and depth of field. Most lens tests use flat subjects for testing purposes and the reported f-stop for best sharpness applies only to flat objects.

The optimum f-stop for overall sharpness for objects with "height" variation will be smaller (numerically higher) than the f-stop for flat objects. How much smaller is optimum depends on the amount of height variation, the sharpness of the lens and the variation of the sharpness with f-stop. More variation in height means depth of field is more important.

Lens test results and calculations could be used to determine optimum f-stop but experimentation is probably simpler.

[Wishgranter]

see this image https://www.dropbox.com/s/7h52gvsotp9uzo6/lensbenchmarks.jpg

go to the http://www.dxomark.com/ and search for your lens and try read the proper information.... this image shows what sharpness can get out with the lens and the tested camera.. there is more stuff, but for some of then need to digg on the page.......

[marlin]

Dear Dave and Wishgranter!
thanks for the replies and the info. Much appreciated.

@David, reading btw the lines, it really seems as tho which Fstop to use is really case by case basis. I'm constantly trying to get as much of the subject 'in frame' AND keep everything in focus. Documenting a smaller object with great height variances presents DoF issues. I find myself backing out, away from the subject --- adjusting for a greater DoF. But then my subject gets smaller and smaller in the frame. Its a tug of war btw capturing as many pixels on the subject and getting the maximum DoF as possible.

Is there a 'silver bullet' fix for this issue (in direct relation to photoscan?)

@wishgranter - thanks for the reading material. I'll check those out.

cheers.
marlin.

[chadfx]

Hardly a silver bullet fix, but using using focus stacking is an option in some cases. Photoscan does not seem to mind the processed images as long as most of the EXIF data comes across. For macro sized objects, I haven't found a better way to keep everything sharp once you are looking down the longest axis of your object.

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

[marlin]

Focus stacking, ahhh. Brilliant. love the idea. Makes a lot of sense, esp if PhotoScan ingests this data ... thanks!

[chadfx]

I've been using Helicon Focus, which generally is well reviewed and not too expensive. It's just a drag to add yet another step into the pipeline, but it definitely gives you nice results.

If you have lots of time to capture, they also sell software that will remote control your camera to shoot the proper number of focus steps based on your front/back focus distance, camera sensor, focal length, f-stop. But it's SLOW, so a lot of the time I've just fired off the shots manually and eyeballed it. ;-)

Good luck! -C

[Magnus]

Hello!

Since you are speaking about focus stacking I thought I'd mentioned Smart Shooter which I (and I think a few others here) use for triggering multicamera rig. It also has focus stacking (don't know if it works with multiple cameras).

http://hartcw.com/smart-shooter/examples/focus-stacking/

Best, Magnus.

[chadfx]

That's good to know, Magnus...especially if you've already invested in that package. Their implementation seems a bit less polished (not much tech info displayed, requires scripting, etc)  than the one from Helicon (which also doesn't require stacking in another app).

(video of how Helicon's setup works: http://www.youtube.com/embed/3LDSGWPNe3o)

What would be really great is to have a setup that could use the extra remote features of the Helicon app to drive a turntable along with the focus stacking. It would go a long way towards making the capture process almost completely automated.

http://www.heliconsoft.com/helicon-remote-stackshot/

I bet Smart Shooter could be similarly configured. Nice to know there are options!

Cheers, -C

[Kiesel]

Another option for focus stacking is to use Magic Lantern for Canon DSLR's or CHDK/SDM for Canon's point and shoot cameras together with one of the free image stacker (Enblend, Tufuse, Picolay, ...) .

Cheers,

Karsten