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Forum: Troubleshooting and Beginner Help
07-17-2012, 08:13 PM
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Well, creampuff, I now I know what side of the physics and engineering line you stand on. Thanks for the clarification.
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Forum: Troubleshooting and Beginner Help
07-16-2012, 06:43 PM
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Good point, and there are actually a couple things to consider.
"Ghosting", would be present for both IR and visible if the design is prone to it. Ghosting is when reflected light inside an optical system converges, forming a ghosted spot of light. This would occur from light coming into the system from the scene being photographed. So, if you have visible light ghosting in a lens, you'll most likely get IR ghosting too. Since IR conversion involves removing a filter from the detector, then you have effectively altered the design, and you might get ghosting that wasn't present before the conversion.
Also, when the detector runs continuously and when you use the LCD a lot, heat builds up around the detector. Since the converted camera is more sensitive to IR, this heat will cause fogging inside the camera cavity. This would not likely cause ghosting since rays are not directed and are bouncing around in all directions, but it would definitely cause a reduction in contrast and likely lower the sharpness too. Plus, the sensor noise increases, which compounds the problem.
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Forum: Troubleshooting and Beginner Help
07-16-2012, 06:11 PM
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Visible light has a "thermal" signature, but since the sensors we typically use (eyes, typical cameras) see the visible, the thermal is not perceived. Lens glass does let NIR through, but you are right, many glasses have absorption in midwave and longwave. Even the atmosphere has absorption bands (H2O, CO2, for example).
And incidentally, NIR is most certainly emissive! Think of the Planck blackbody function--just because a lens does not let something 310 Kelvin through the system does not mean there is no heat being transmitted! ;)
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Forum: Troubleshooting and Beginner Help
07-15-2012, 06:09 PM
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Flash is usually around 5000 to 5500K, and simulates the most neutral sun light periods of the day (ie, mid day). So, the results with flash should be reasonably good if that is the only light source.
Edit to add: I don't actually know how far into the IR the Pentax sensor sensitivity is. I just want to add a caution that people emit heat, which is basically what IR is. So, the mixing of reflected visible light with the emitted IR light from people's bodies will definitely not look as sharp as a "normal" visible spectrum photo.
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Forum: Troubleshooting and Beginner Help
07-15-2012, 05:59 PM
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The color temperature of sun light changes throughout the day. When the spectrum is shifted more toward the blue end of the visible spectrum, detail will be better than when the spectrum is shifted more toward the red. See my post above with respect to the Reyleigh Criterion.
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Forum: Troubleshooting and Beginner Help
07-15-2012, 01:39 PM
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Aside from equipment issues, IR is far less physically capable of reproducing fine details. As wavelength increases, the finest reproducible detail becomes larger. Search Rayleigh Criterion on the internet. The physical definition of the Rayleigh Criterion is wavelength dependent.
Related to resolution, with IR, you reach the diffraction limit of the optical system much sooner than with visible light.
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Forum: Pentax K-5 & K-5 II
07-06-2012, 07:42 AM
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Thanks to the OP for bringing this up--I also would love focus peaking on the K-5. Perhaps Pentax can be convinced if they consider that having more focusing options would encourage people to invest more thoroughly in Pentax glass! ;)
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Forum: Troubleshooting and Beginner Help
07-05-2012, 05:55 PM
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Yes, CA is definitely not limited to digital. For B&W film, it obviously wouldn't be noticeable per se, but would increase the total amount of aberration, adding to whatever "monochromatic" aberrations are present as part of the geometry. For color film, the dye layers added a slight built in correction since each layer was at a different focal plane. Same is true for the Foveon detector.
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Forum: Troubleshooting and Beginner Help
07-05-2012, 06:15 AM
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Refractive lenses always have chromatic aberration. As mentioned above, it is possible to minimize it with careful glass selection. However, it never totally goes away--it just becomes more or less acceptable. The only way to truly (and easily) eliminate CA is to use mirrors instead of glass.
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Forum: Photographic Technique
07-03-2012, 05:48 AM
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Awesome--thanks for reporting the outcome!
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Forum: Photographic Technique
07-02-2012, 06:30 PM
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Ghosting from the sensor can be also affected by a filter on the lens. bobpur's assessment of a flat surface is correct--add another couple of flat surfaces with a filter and you can multiply the affect. You didn't mention whether you are using a filter--unless you have something with an anti-reflection coating, a filter could be making the problem worse.
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Forum: Photographic Technique
07-02-2012, 06:18 AM
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Ghosting can look like a sphere, and is a form of stray light. You may be right that you are pushing the limits of the lens with bright lights and white background. Does the spot also change size as you change the aperture stop?
ETA: Are you using a filter on the lens, and if so, have you tried taking it off? Have you tried just one light? Have you tried moving the lights far outside the lens FOV, eg, behind the camera? Also, have you tried turning you ISO way down? ISO 100 in very bright lighting will allow more open apertures.
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Forum: Photographic Technique
06-24-2012, 05:44 PM
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They will transmit light in the wider field of view angles, but there would be a chromatic shift at the edges, not unlike chromatic aberration, although physically different in nature.
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Forum: Photographic Technique
06-23-2012, 10:00 AM
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I have the same issue with reds, oranges and to some extent yellow. Silicon detectors are red and IR sensitive. There is an IR filter incorporated at the focal plane for some cameras, but even so, the red sensitivity is still present.
I tried the method mentioned here and on other threads--underexpose, shoot in raw, correct in post-processing. That seems fairly good for getting decent results most of the time.
I would like to see if an IR cut filter would help--the band-pass on those filters is fairly flat in the visible range, so it might tame some of the red peaks. Unfortunately, the IR cut designs are multi-layer dielectric coatings, and therefore will only work as advertised in the normal to narrow field of view range.
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Forum: Pentax SLR Lens Discussion
06-21-2012, 08:07 PM
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It really depends on how large the pieces are and how far away you can get and still control the light...but generally speaking, a normal to slightly long lens with very low barrel or pincushion distortion should work well in most situations. If you can get something with minimal vignetting and chromatic aberration, that would be a plus. If the pieces are smallish, indeed you would need either macro or close focus capability (or extension tubes, etc).
Aside from other comments, I also recommend you use a lens hood to control flare. Flat objects like paintings can cause quite a bit of stray light...
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Forum: Photographic Technique
06-21-2012, 07:53 PM
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I've had this problem many times. Polarizing both the light and the camera is the best way to get rid of all unwanted reflections without creating a special setup or using special lenses/cameras. However, the issue with polarization is that it dramatically cuts down on the amount of light coming into the camera. This can be troublesome, especially if this is a case where you need good depth of field (as when you are shooting artwork or other flat things). I have found something along the lines of shooting through black seamless (as suggested by smigol) to be the best option. Low tech and low cost option can be had by using black foam core from the art supply and/or black bed sheets.
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Forum: Photographic Technique
06-17-2012, 09:05 PM
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Brady's civil war PHOTOS may not have been altered, but the SCENES were staged. Not saying there weren't war casualties, just saying that it is well documented that Brady moved the corpses around to get a "better" photograph.
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Forum: Pentax K-5 & K-5 II
06-17-2012, 08:46 PM
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Well, converting to B&W from RGB quires computer processing too and ultimately is not the same as B&W film. So, obviously there's nothing to lose by trying the cheap filters (it is fun and you can learn a lot), but you might get more mileage out of increasing your skills with your photo software.
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Forum: Pentax K-30 & K-50
06-16-2012, 02:03 PM
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...maybe they wanted something patentable...just a little jaded ;)
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Forum: Pentax K-30 & K-50
06-16-2012, 11:51 AM
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White light is a combination of many different "colors" of monochromatic light. The basic premise of chromatic aberration is that the dispersion of the glass or plastic will cause some components of white light to be refracted differently through the optical system. It is fundamentally the same thing as when you see a rainbow coming out of a prism.
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Forum: Pentax K-30 & K-50
06-14-2012, 08:42 PM
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Great photo...thanks Ray!
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Forum: Pentax K-30 & K-50
06-14-2012, 07:08 PM
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Anvh, I don't want to speak for Class A, but I think he already answered your question. "White Light" which is basically any typical lighting situation you'll run into (outdoors in the sun or indoors with artificial light) contains many "colors". So, while a system may be optimized for ONE of those colors (green was the example), then colors other than green within the white light spectrum will exhibit chromatic aberration in a refractive optical system.
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Forum: Pentax K-30 & K-50
06-14-2012, 06:30 PM
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I would think that they don't really care about the CA of each individual lens since they can more easily create an algorithm to subtract the AF path response from the metering path response. Since both paths have input from the camera lens, the effect mathematically cancels out.
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Forum: Pentax K-30 & K-50
06-14-2012, 07:25 AM
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I would hesitate to call spectral effects a mirror, beamsplitter or coatings as chromatic aberration because it wouldn't behave quite the same as getting astigmatism in the blue, for example. However, in the final analysis, yes, it can have effects on different wavelengths, it is just not a dispersion effect per se.
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Forum: Pentax K-30 & K-50
06-14-2012, 06:00 AM
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Anvh, great top pic--it makes the three paths much more clear to me than the model on the bottom (which had already been posted). As I was saying, the common optics would only account for a portion of the CA. However, thanks to your pic, I should point out that the main mirror is also common, except that I see it is a beam splitter rather than a mirror. Side note, if the "mirrors" have dielectric coatings, there could be wavelength shift at greater angles of incidence. Interesting tangential thought.
Class A's comments are in line with what I said above...that if the camera knows something about the behavior of the AF portion and the metering portion as separate optical systems, they can put an algorithm in the computer to make a correction.
ETA: To your question about CA always happening...classic CA (Seidell aberration) mainly occurs in refractive optics. If the mirrors are "nominal" mirrors, they should be nearly no CA, but there could be other spectral effects due to the coatings.
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