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Author Topic: Best F-stop?  (Read 7490 times)

MrBuffaloe

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Best F-stop?
« on: February 15, 2015, 09:27:44 PM »
Hi all!

I just started using Photoscan for creating videogame and 3D assets, and I'm getting some really good results so far, but I was wondering what the ideal fstop is for the images I'm taking?

The manual indicates a fixed lens is preferable, so I'm using a Nikon 1.4d 50mm, usually my preferred video lens. Currently I'm usually taking pictures at F8, but I'd like to go further down if possible. Any advice on this?

Thanks in advance!
« Last Edit: February 15, 2015, 10:01:49 PM by MrBuffaloe »

igor73

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Re: Best F-stop?
« Reply #1 on: February 15, 2015, 09:40:29 PM »
Excelent lens for photogrammetry. .  You need to maximize DOF so f11-F16 depending on lens.  Above F16 refraction usually occures.  I would start at F14-F16.  Check some lens test for your lens. 

MrBuffaloe

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Re: Best F-stop?
« Reply #2 on: February 15, 2015, 09:59:13 PM »
thank you igor! i'm planning on scanning a head this week, i'll get some additional lighting and try F14/16 and see how that comes out!

igor73

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Re: Best F-stop?
« Reply #3 on: February 16, 2015, 01:37:28 AM »
Sorry i made a typo.  I meant to say you may lose sharpness at F16 and above due to diffraction.  Not refraction. 
Here is an explanation.  of the term.  http://luminous-landscape.com/understanding-lens-diffraction/

driftertravel

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Re: Best F-stop?
« Reply #4 on: February 16, 2015, 01:44:37 AM »
Yeah, if you have the light and a tripod then go max DOF before refraction. You can look up this data for your lens.

igor73

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Re: Best F-stop?
« Reply #5 on: February 16, 2015, 01:45:11 AM »
Found this review on your lens.  The author notes the lens loses  sharpness due to diffraction after F11.  It is hard to say if DOF is more important than the little loss of sharpness due to the diffraction.  i would guess its better to trade a little sharpness for DOF so i would use F14-F16.  F21 is probably to much though. 

http://www.olegnovikov.com/technical/50mmf14d/50mmf14d.shtml

igor73

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Re: Best F-stop?
« Reply #6 on: February 16, 2015, 01:49:22 AM »
And as drifter travel mentioned you also have to factor in you will need 2x more light for each F stop.  Shoot f16 instead of F11 and you need double the light or compensate with half the shutter speed or double the ISO.  Then you have to factor in camera shake and noise from higher ISO swell.  So its more complicated than this. 

Usually the recommendation is to shoot at no lower shutter speed than your lens focal length.  In your case 50mm so no slower than 1/50 sec.  To be safe i would not shoot slower than 1/100 though.  So ii depends on the lighting situation. 

MrBuffaloe

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Re: Best F-stop?
« Reply #7 on: February 16, 2015, 11:53:10 AM »
that should all not be a problem, i'm shooting from tripod with remote control to ensure max stability at even low shutter speeds; the lighting is the only issue, but it should be no problem to get a few umbrellas in if need be

igor73

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Re: Best F-stop?
« Reply #8 on: February 16, 2015, 11:55:08 AM »
Then you are all set for some great results! 

Marcel

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Re: Best F-stop?
« Reply #9 on: February 16, 2015, 09:27:25 PM »
A Depth of Field calculator can be very useful to get an idea how big your 'sharp' zone is:

http://www.cambridgeincolour.com/tutorials/dof-calculator.htm

For example with a full frame camera:

50mm lens @F11 and focussed at 2 meter distance = 1 meter depth of field
35mm lens @F11 and focussed at 2 meter distance = 3 meter depth of field
200mm lens @F11 and focussed at 2 meter distance = 0.06 meter  depth of field

Try downloading a DOF calculator app on your phone (there are several available for free), it will give you a good idea on what to expect when you are photographing a certain subject.

Diffraction degrades image quality from F8.0 and higher, but Igor is correct that the extra sharpness you gain from a deeper depth of field is usually more important than the sharpness lost due to diffraction. Diffraction is very subtle, and blur from depth of field is usually not. I would try to keep the F-stop at around F11 unless you really need the extra depth of field.

For perfect quality scans, be extra super picky about image quality. And I mean extra SUPER picky, because even the slightest blur can cause noise in your scans. Examine your shots at 400% and if you even have the slightest doubt that it's blurred then dump the shot.

igor73

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Re: Best F-stop?
« Reply #10 on: February 19, 2015, 08:58:11 PM »
I often use 35mm focal length because of the greater DOF.  So depending on the subject 35mm might be a better choice than 50mm. You also need far less shots. 

David Cockey

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Re: Best F-stop?
« Reply #11 on: February 20, 2015, 09:09:05 PM »
Adjust camera to subject distance so that the size of the subject in the image is the same, and the depth of field will be about the same for different focal lengths if the f-stop is the same.

Integr8d

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Re: Best F-stop?
« Reply #12 on: February 25, 2015, 05:23:57 AM »
I caught a review, a long time ago, that compared a bunch of standard primes. The nikon 1.4 and 1.8, the zeiss 1.(whatever) manual, I think a higher-end Sigma and some others. It actually turned out that the nikon 1.8 had the sharpest images...

To OP, 2nd on the DoF calculator. If you want to capture a Coca-cola can, you can figure on needing 3-4" of DoF. Figure on 12" to be safe (unless you use a measuring tape from the soda can to the sensor plane). And decide on achieving that DoF with either the aperture or focal distance. Personally, I'd put more pixels on it (and thereby stop down the aperture to whatever it needs to be; calculator will tell you). And since you're using a prime lens, I wouldn't be too concerned with sharpness. But that's just me. Stick that bad boy on a tripod, use a 2-second shutter delay (and mirror lock-up, if you have it) and let it rip!