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syndicate1
PostPosted: Fri May 20, 2011 4:17 pm 
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Based on the comments below, what would you say is the minimalist f number needed to shoot saying somewhere like the grand canyon, machu picchu or mountains?

"When shooting a normal landscape image it is normal to attempt to keep as much of the image in focus as possible.

This means selecting a small Aperture (remember the larger the number the smaller the actual Aperture) to ensure that you end up with a large depth of field. This will ensure that parts of the image that are both close and far away from you have a good chance of being somewhat in focus.



Read more: http://www.digital-photography-school.c ... z1MuTKECUQ"

Cheers
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Bob Andersson
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PostPosted: Fri May 20, 2011 4:34 pm 
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You need to specify your camera as the pixel size makes a difference. On my 5D Mark II I aim for f/8 but f/11 is OK if I need a bit extra DoF even after going "hyperfocal".

Bob.

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syndicate1
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PostPosted: Fri May 20, 2011 5:02 pm 
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I have the canon xsi with the following lenses:

1. 18-55mm

2. 50-250mm

3. 50mm f1.8
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Bob Andersson
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PostPosted: Fri May 20, 2011 6:35 pm 
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On the assumption that your lenses can produce essentially diffraction limited resolution at the f-numbers we are considering (I haven't time to check that assumption) all you need to do is calculate your pixel area which for the XSi is, if I've got my sums right, about 27 µm². That corresponds to about the pixel size of the Nikon D2X on the interactive chart on this page. Once you get the hang of making the chart work you can see the size of the Airy disk for various f-numbers. Ideally, with D2X highlighted, somewhere between f/5.6 and f/8 is optimum while maintaining good depth of field but you could probably stretch to f/11 without noticing any softening, especially with a bit of judicious sharpening in post-processing.

Bob.

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Gitzo GT1541T, Arca-Swiss Z1 DP ball-head.
Astrophotography: TEC 140 'scope, FLI ML16803 camera, ASA DDM60 Pro mount.
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lagnificent
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PostPosted: Wed May 25, 2011 11:24 am 
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From The Digital Picture's website on DLA:
http://www.the-digital-picture.com/Revi ... eview.aspx


Quote:

Here is a specification chart of Canon EOS Digital SLR models:


Model FOVCF Sensor Pixel Size Pixels/Megapixels Viewfinder DLA*
Canon PowerShot G9 4.6x 7.6 x 5.7mm 1.9µm 4000 x 3000 12.1
Canon EOS Rebel T3i / 600D 1.6x 22.3 x 14.9mm 4.3µm 5184 x 3456 18.0 .85x 95% f/6.8
Canon EOS Rebel T2i / 550D 1.6x 22.3 x 14.9mm 4.3µm 5184 x 3456 18.0 .87x 95% f/6.8
Canon EOS Rebel T1i / 500D 1.6x 22.3 x 14.9mm 4.7µm 4752 x 3168 15.1 .87x 95% f/7.6
Canon EOS Rebel T3 / 1100D 1.6x 22.2 x 14.7mm 5.2µm 4272 x 2848 12.0 .85x 95% f/8.4
Canon EOS Rebel XSi / 450D 1.6x 22.2 x 14.8mm 5.2µm 4272 x 2848 12.2 .87x 95% f/8.4
Canon EOS Rebel XS / 1000D 1.6x 22.2 x 14.8mm 5.7µm 3888 x 2592 10.1 .81x 95% f/9.3
Canon EOS Rebel XTi / 400D 1.6x 22.2 x 14.8mm 5.7µm 3888 x 2592 10.1 .80x 95% f/9.3
Canon EOS Rebel XT / 350D 1.6x 22.2 x 14.8mm 6.4µm 3456 x 2304 8.0 .80x 95% f/10.4
Canon EOS 300D Digital Rebel 1.6x 22.7 x 15.1mm 7.4µm 3088 x 2056 6.3 .80x 95% f/11.8
Canon EOS 60D 1.6x 22.3 x 14.9mm 4.3µm 5184 x 3456 18.0 .95x 96% f/6.8
Canon EOS 50D 1.6x 22.3 x 14.9mm 4.7µm 4752 x 3168 15.1 .95x 95% f/7.6
Canon EOS 40D 1.6x 22.2 x 14.8mm 5.7µm 3888 x 2592 10.1 .95x 95% f/9.3
Canon EOS 30D 1.6x 22.5 x 15.0mm 6.4µm 3504 x 2336 8.2 .90x 95% f/10.3
Canon EOS 20D 1.6x 22.5 x 15.0mm 6.4µm 3504 x 2336 8.2 .90x 95% f/10.3
Canon EOS 10D 1.6x 22.7 x 15.1mm 7.4µm 3088 x 2056 6.3 .88x 95% f/11.8
Canon EOS 7D 1.6x 22.3 x 14.9mm 4.3µm 5184 x 3456 18.0 1.0x 100% f/6.8
Canon EOS 5D Mark II 1.0x 36.0 x 24.0mm 6.4µm 5616 x 3744 21.1 .71x 98% f/10.3
Canon EOS 5D 1.0x 35.8 x 23.9mm 8.2µm 4368 x 2912 12.8 .71x 96% f/13.2
Canon EOS 1D Mark IV 1.3x 27.9 x 18.6mm 5.7µm 4896 x 3264 16.1 .76x 100% f/9.1
Canon EOS 1D Mark III 1.3x 28.1 x 18.7mm 7.2µm 3888 x 2592 10.1 .76x 100% f/11.4
Canon EOS 1D Mark II N 1.3x 28.7 x 19.1mm 8.2µm 3520 x 2336 8.2 .72x 100% f/12.7
Canon EOS 1D Mark II 1.3x 28.7 x 19.1mm 8.2µm 3520 x 2336 8.2 .72x 100% f/12.7
Canon EOS 1DS Mark III 1.0x 36.0 x 24.0mm 6.4µm 5632 x 3750 21.1 .76x 100% f/10.3
Canon EOS 1DS Mark II 1.0x 36.0 x 24.0mm 7.2µm 4992 x 3328 16.6 .70x 100% f/11.6

* DLA (Diffraction Limited Aperture) is the result of a mathematical formula that approximates the aperture where diffraction begins to visibly affect image sharpness at the pixel level. Diffraction at the DLA is only barely visible when viewed at full-size (100%, 1 pixel = 1 pixel) on a display or output to a very large print. As sensor pixel density increases, the narrowest aperture we can use to get perfectly pixel sharp images gets wider.

DLA does not mean that narrower apertures should not be used - it is simply the point where image sharpness begins to be compromised for increased DOF and longer exposures. And, higher resolution sensors generally continue to deliver more detail well beyond the DLA than lower resolution sensors - until the "Diffraction Cutoff Frequency" is reached (a much narrower aperture). The progression from sharp the soft is not an abrupt one - and the change from immediately prior models to new models is usually not dramatic. Check out this specific diffraction comparison example using the ISO 12233 chart comparison tool. The mouseover feature will show you the degradation at f/11 compared to f/5.6.
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popo
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PostPosted: Wed May 25, 2011 11:42 am 
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That isn't quite the right question being asked or answered. You're not generally looking to get the sharpest image. What you want is for as much of the image as possible to be "sharp enough".

While diffraction limiting is contributing factor, that ONLY applies to the focal plane. Everywhere else, by definition, is less sharp as it is out of focus.

In practice, depth of field calculations are more relaxed than pixel perfect levels. Personally I'd suggest not worrying about diffraction softness unless it gets too extreme. Just use a suitable aperture for the level of depth of field you want to realise. It depends on the focal length of the lens too, so you would need smaller apertures as the focal length gets longer.

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