I am upgrading my D200 IR830nm converted DSLR to full frame to fully utilize the focal length of the 25mm/2.8 ZF-IR and 50/1.4 ZF-IR lenses, and as a bonus bigger prints. Unfortunately I have Nikon DNA so I am stuck with a FF Nikon, which currently limits the choice to the D700, D610 or D800 models.
The D600 is excluded due to OLPF cleanliness issues.
The D700 is also suspect due to the internal IR LED used for shutter monitoring/calibration (LED wavelength is 850nm and can create issues with long exposures).
D200 10MP, 6.1 um pixel diam, 164 pixels/mm
D700 12MP, 8.5 um pixel diam, 118 pixels/mm
D610 24MP, 6.0 um pixel diam, 167 pixels/mm
D800 36MP, 4.9 um pixel diam, 204 pixels/mm
Since the relationship between the Airy disk diameter for 850/900nm IR light (rather than 550nm) and pixel size has a significant effect on f/ratio choice and subsequent DoF, it has raised some questions after reviewing several websites (including this one) discussing diffraction and f/ratio.
1) When relating Airy disk diameter to pixel size, factors from 2x to 3x the pixel size are often used to account for AA blur and Bayer matrix effects. (Bart van der Wolf utilizes 1.5x to indicate the onset of diffraction effects). An IR converted DSLR has no AA filter but still utilizes a Bayer matrix.
What would you recommend the best approach to account for this in the Airy disk relationship to pixel size?
2) In the Cambridgeincolour diffraction tutorial I am somewhat confused by the presentation of the calculated Airy disk diameter and the visual presentation of the Airy disk superimposed on the pixel dimensions. The visual presentation shows the disk to be smaller than the calculated size AND a footnote states
“Note: above airy disk will appear narrower than its specified diameter (since this is defined by where it reaches its first minimum instead of by the visible inner bright region).”
This is where I would like some clarity. Isn’t the area bounded by the first minimum representative of the, “visible inner bright region” (it certainly incorporates it) or have you chosen to eliminate that fraction of the Airy disk that also falls below 20% of the relative field strength within the first minimum?
The calculated Airy disk diameter shown in the calculator is based on the diameter of the first zero (minimum) of the diffraction pattern, which is
1.21967 lambda/(2 R) or 2.43934*N*lambda
This value represents the main “inner” core of the Airy disk and accounts for around 83.8% of the total field strength of the Airy disk. This would taper from bright in the center to black at the calculated diameter but still represents 84% of the total field strength.
The visual Airy disk representation superimposed on the pixel size appears to be calculated using
1 lambda/(2 R) or 2*N*lambda
As such, this only represents relative field strength greater than 20% within the diameter of the first minimum, hence a smaller diameter. As such, isn’t this the brighter portion of the inner bright region?
No problem with this, but
WHY is it presented this way AND
WHICH is the most important value when relating to the pixel diameter, the calculated first zero diameter or the visual representation only representing a portion of the field strength in the first zero diameter?