Photons Missing In Action

Started Nov 24, 2009 | Discussions thread
OP Marianne Oelund Veteran Member • Posts: 7,751
Part 3 Addendum: Spectrogram examples

As mentioned in my previous post, the lens color spectra are smooth and do not exhibit absorption bands. I've selected a few representative models from the extremes of the color cast range, plus a couple others which are likely to be of interest.

As noted before, the spectral skew is slightly dependent upon the light source, and we see a little more red skew when light sources with high long-wavelength content are used. For these spectrograms, the source was a small halogen lamp, and the spectra do show more attenuation toward the blue end of the spectrum, than we would see with a higher color-temperature source. All of the plots are level, to downward sloping towards blue.

Some of you will need to think backwards in viewing these plots, since I use a frequency-order axis which places the long wavelengths (red) at the left. I've grouped the lenses together by similar type. In each multiple plot, the vertical position is not an indicator of relative lens light transmission; it's just a convenient placement which I have chosen to separate the curves a little. The horizontal axis is labeled with wavelength in nm (nanometers). Due to very low signal levels, data at the spectrum extremes has much lower accuracy, and that is the reason for the somewhat erratic plot ends.

This first group consists of some short primes. With many light sources, the AF 35 f/2 is nearly neutral and would have an almost flat plot, but with this light source, does show a little red shift. Also included are the Zeiss ZF 35 f/2, AF-S 50 f/1.4G and the 45mm PC-E:

The next group includes some interesting normal-range zooms. The 24-70 has a strong red shift, and the 28-70 tends to be neutral to slightly blue with most light sources. The AF 35-135 f/3.5-4.5 has even stronger blue tendency in most cases, but appears essentially flat with this test source:

Finally, we have the two versions of the 70-200 VR. You can see how the spectral response has been made slightly closer to neutral on the new VR II with a modest lift in the cyan-blue range:

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