Nikon D40 and D7000 AA filter MTF revisited

In an earlier post, I have shown some early MTF measurements taken with both the D40 and the D7000 at the centre of a Sigma 17-50 mm f/2.8 lens.
In this post, I revisit those measurements, presenting some new results for the D7000, and a model of the sensor AA filter (or OLPF).

Optical Low Pass Filters (OLPFs) in DSLRs

I have noticed previously that the measured MTF of the D40 was fairly close to a Gaussian, but that there were some small systematic discrepancies that were not accounted for. But how do you build an optical filter that has a Gaussian transfer function?

It turns out that the OLPFs in DSLRs are not really Gaussian, but something much simpler: beam splitters. A slice of Lithium Niobate crystal, cut at a specific angle, presents a different index of refraction depending on the polarization of the incident light. Let us assume that a beam of horizontally polarized light passes through straight, for the sake of the argument. Vertically polarized light, on the other hand, refracts (bends) as it enters the crystal, effectively taking a longer path through the crystal. As the vertically polarized light leaves the crystal, it refracts again to form a beam parallel to the horizontally polarized beam, but displaced sideways by a distance dependent on the thickness of the crystal.

Using a single layer of Lithium Niobate crystal, you can split a single beam into two parallel beams separated by a distance d, which is typically chosen to match the pixel pitch of the sensor. Since this happens for all beams, the image leaving the OLPF is the sum (average) of the incoming image and a shifted version of itself, translated by exactly one pixel pitch.

If you stack two of of these filters, with the second one rotated through 90 degrees, you effectively split a beam into four, forming a square with sides equal to the pixel pitch (but often slightly less than the pitch, to improve resolution). A circular polariser is usually inserted between the two Lithium Niobate layers to "reset" the polarisation of the light before it enters the second Niobate layer.

Combining pixel aperture MTF with the OLPF MTF

So how does this beam-splitter effect the desired blurring? We can compute the combined PSF by convolving the beam-splitter impulse response with the pixel aperture impulse response (a box function).

The beam splitter is represented as four impulses, i.e., infinitely thin points. Basic Fourier transform theory tells us that the Fourier transform of an impulse is just a cosine function, so the MTF of the beam splitter will be the sum of four cosines.

The "default" 4-way beam splitter impulse response filter can be denoted as the sum of four diagonally-placed impulses, i.e.,

f(x,y) = δ(x-d, y-d) + δ(x+d, y+d) + δ(x-d, y+d) + δ(x+d, y-d)

where δ(x,y) represents a 2D Dirac delta function, which is non-zero only if both x and y are zero, and d represents the OLPF split distance. More sophisticated OLPF designs are possible (e.g., 8-way splitters), but the 4-way designs appear to be popular. In my notation here, the distance between two beams would be 2d; this is to accommodate my usual notation of a pixel being defined over the area [-0.5, -0.5] to [0.5, 0.5].


The degree of blur is controlled by the d parameter, with d = 0 yielding no blurring, and d = 0.5 giving us a two-pixel-wide blur. Since d can be varied by controlling the thickness of the Lithium Niobate layers, a manufacturer can fine-tune the strength of the OLPF for a given sensor.

Convolving a square pixel aperture with four impulse functions is straightforward: just sum four copies of the box filter PSF, each shifted by d in the required direction. For d = 0.375, we obtain the following PSF:

in two dimensions, or

in three dimensions. Not exactly a smooth PSF, but then neither is the square pixel aperture.

Simulating the combined effects of diffraction, OLPF and pixel aperture

We can derive the combined PSF directly by convolving the diffraction, OLPF and pixel aperture PSFs. Note that this combined PSF is parameterized by wavelength, lens aperture, pixel pitch, and OLPF split distance. For a wavelength of 0.55 micron, an aperture of f/4, a pixel pitch of 4.73 micron and an OLPF split distance of 0.375 pixels (=2.696 micron), we obtain the following PSF:

in two dimensions, or

in three dimensions. Notice how diffraction has smoothed out the combined OLPF and pixel aperture PSF (from previous section).

Using this PSF we can generate synthetic images of a step edge. The MTF of this synthetic edge image can then be compared to a real image captured with a given sensor to see how well this model holds up.

Enter the Death Star

You can certainly measure lens or sensor sharpness (MTF) using any old chart that you printed on office paper, but if you are after resolution records, you will have to take a more rigorous approach. One of the simplest ways of obtaining a reasonably good slanted edge target is to blacken a razor blade with candle soot. This gives you a very straight edge, and very good contrast, since the soot does not reflect much light.

That leaves only two other questions: what do you use as a background against which you will capture the razor blade, and how do you illuminate your target?

Previously I have used an SB600 speedlight to illuminate my razor blade, which was mounted on a high-grade white paper backing. This produced reasonably good results, but it did not maximize contrast because the flash was lighting the scene from the front. There is also a possibility that the D7000 cycles its mirror when using a flash in live view mode, which could lead to mirror slap. So the flash had to go.

In its place I used a home made integrating sphere, which I call the "Death Star" (sorry George, please do not sue me). Here is what it looks like with the razor blade mounted over the integrating sphere's port:

Yes, I know the highlight is blown out ....

Why use an integrating sphere? Well, the integrating sphere produces perfectly uniform diffuse lighting, which is ideal for creating a uniform white backdrop for the razor blade.

In addition, my home made integrating sphere produces enough light to require a shutter speed of 1/200 s at f/4, ISO100 to prevent blowing out the highlights. This is perfect for minimizing the influence of vibration (although higher shutter speeds would be even better).

Using this set-up, I then capture a number of shots focused in live view, using a remote shutter release. It appears that with this target, the AF is really accurate, since I could not really do much better with manual focus bracketing. One day I will get a focusing rail, which will make focus bracketing much simpler.

Lastly, all MTF measurements on the razor edge were performed using MTF Mapper. The "--bayer green" option was used to measure MTF using only the green photosites, thus avoiding any problems with Bayer demosaicing. The raw files were converted using dcraw's "-D" option.

D40 MTF and OLPF model

Here is the MTF plot of a D40 razor image captured at f/4 (manually focus bracketed), Bayer green channel only:

D40 green channel MTF, Sigma 17-50 mm f/2.8 at f/4

The D40's PSF was modelled by convolving the square pixel aperture (7.8 micron wide), the 4-point beam splitter (d=0.375 pixels), and the Airy function (f/4). A synthetic image of a step edge with this PSF was generated, and measured using MTF Mapper.

Purely judging the match between the model and the measured MTF by eye, one would have to conclude that the model captures the interesting parts of the MTF rather well. The measured MTF is slightly below the model, which is most likely caused by a smidgen of defocus.

The fact that the model fits so well could also be taken to imply that the Sigma 17-50 mm f/2.8 lens is relatively aberration-free at f/4, i.e., it is diffraction limited in the centre.

MTF50 resolution came in at  0.334 cycles per pixel, or 42.84 line pairs per mm, or 671 line pairs per picture height.

D7000 MTF and OLPF model

Here is the MTF plot of a D7000 razor image captured at f/4 (AF in live view), Bayer green channel only:

D7000 green channel MTF, Sigma 17-50 mm f/2.8 at f/4

The D7000's PSF was modelled by convolving the square pixel aperture, the 4-point beam splitter (d=0.375 pixels), and the Airy function (f/4). A synthetic image of a step edge with this PSF was generated, and measured using MTF Mapper.

The measured MTF does not fit the model MTF quite as well as it did in the D40's case. Given that the physical linear resolution is 60% higher, it is correspondingly harder to obtain optimal focus. The shape of the measured MTF relative to the model MTF is consistent with defocus blur.


The actual resolution figures are impressive: MTF50 is 0.304 cycles per pixel, or about 64 lp/mm, or equivalently, 992 line pairs per picture height.

If the model is indeed accurate, it would mean that the D7000 can theoretically obtain resolution figures close to 68 lp/mm at f/4 in the green channel, provided that the lens is purely diffraction limited, and perfectly focused.

Summary

Perhaps this is not such a surprising result, but it appears that Nikon is using the same relative strength of AA filter in both the D40 and the D7000; this can be deduced from the fact that both the D40 and the D7000 OLPF models fitted best with an OLPF split distance of 0.375 pixels.


The somewhat unexpected result, for me at least, was that the MTF shape is so sensitive to perfect focus. Specifically, it seems that the first zero of the MTF curve, at around 0.6875 cycles per pixel, is not readily visible unless focus is perfect. The zero is quite clear in the D40 curve, but not quite so visible in the D7000 curve. You are extremely unlikely to achieve this kind of focus in real world photography, though.

References

1. Russ Palum, Optical Antialiasing filters, in Single-Sensor Imaging: Methods and Applications for Digital Cameras, Edited by Rastislav Lukac, CRC Press 2008