Link: What Is a Low-Pass Filter
- Thread starter Eduard
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That was an interesting read! 
I was under the impression that one of the D800/800e models lacked a low pass filter, yet the link states:
I'm trying to understand what is a half a low-pass filter vs. a weak low-pass filter. How can one have half a low-pass filter? Any ideas?
I was under the impression that one of the D800/800e models lacked a low pass filter, yet the link states:
I'm trying to understand what is a half a low-pass filter vs. a weak low-pass filter. How can one have half a low-pass filter? Any ideas?
BackdoorArts
Senior Member
Here's as clear a description of the difference as I can find.
What is a Low-Pass Filter?
"The full low-pass filter cannot be completely removed, because it would cause the focal plane to move; plus, the camera still needs to be able to reflect infrared light rays. Instead of making a single filter with one layer, Nikon decided to still use three layers, but with two layers canceling each other out."
It would seem that Nikon may have figured away around this since the introduction of the D800e since the D7100 and D5300 do not have one at all.
(I'm just waiting for the Marketing Hype argument...)
What is a Low-Pass Filter?
"The full low-pass filter cannot be completely removed, because it would cause the focal plane to move; plus, the camera still needs to be able to reflect infrared light rays. Instead of making a single filter with one layer, Nikon decided to still use three layers, but with two layers canceling each other out."
It would seem that Nikon may have figured away around this since the introduction of the D800e since the D7100 and D5300 do not have one at all.
(I'm just waiting for the Marketing Hype argument...)
"The full low-pass filter cannot be completely removed, because it would cause the focal plane to move; plus, the camera still needs to be able to reflect infrared light rays. Instead of making a single filter with one layer, Nikon decided to still use three layers, but with two layers canceling each other out."
That does not make sense, the part about the two filters canceling each other out. Nikon used a glass layer in place of the low-pass filter in order to maintain focus with the sensor at the same location in the camera body. The IR filter used on the sensor does not reflect Infrared, it absorbs it. A thick IR absorbing filter will blur light slightly, as it does "acts like" a weak Anti-Aliasing filter. When converting a Nikon Coolpix 950 to full-spectrum, I took out the IR absorbing glass and substituted clear glass to maintain focus.
Leica has never used an Anti-Aliasing filter in their rangefinder cameras. The thickness of the filter was a problem for handling non-retrofocus wide-angle lenses. The Leica M8 has a 0.5mm thick IR absorbing filter, the M9 has a 0.8mm absorbing IR filter. The images from the M8 are "more crisp" compared with the M9.
Reflection style "Dichroic filters" are used on very small and inexpensive sensors these days. They can also be used over the lens. They also reflect a portion of the RED spectrum. Hold one in sunlight to give the Cats something to chase around the house.
That does not make sense, the part about the two filters canceling each other out. Nikon used a glass layer in place of the low-pass filter in order to maintain focus with the sensor at the same location in the camera body. The IR filter used on the sensor does not reflect Infrared, it absorbs it. A thick IR absorbing filter will blur light slightly, as it does "acts like" a weak Anti-Aliasing filter. When converting a Nikon Coolpix 950 to full-spectrum, I took out the IR absorbing glass and substituted clear glass to maintain focus.
Leica has never used an Anti-Aliasing filter in their rangefinder cameras. The thickness of the filter was a problem for handling non-retrofocus wide-angle lenses. The Leica M8 has a 0.5mm thick IR absorbing filter, the M9 has a 0.8mm absorbing IR filter. The images from the M8 are "more crisp" compared with the M9.
Reflection style "Dichroic filters" are used on very small and inexpensive sensors these days. They can also be used over the lens. They also reflect a portion of the RED spectrum. Hold one in sunlight to give the Cats something to chase around the house.
BackdoorArts
Senior Member
The idea of two filters cancelling each other out makes perfect sense. It's like lining up two prisms, one that refracts the light and the other that joins it back together. Nikon may have done this as a first pass with the D800e and has since moved away from it, which would seem to be the case based on the original article linked, but not with the D800e. Why they decided to do it this way can be questioned, but I suspect it was a first pass at no low pass.
This article explains it: Nikon | Imaging Products | Nikon's original technology - Nikon D800/D800E OLPF is still there in the D800e, the effects are just cancelled out.
I read the Nikon link- two Horizontal Displacement Filters placed back-to-back rather than using a horizontal and a vertical.
Seems like a lot of work for nothing, if the two filters are misaligned with respect to each other, you will not get perfect canceling. Like two prisms- hard to get two that match perfectly, and get some fringing. What you get with the Nikon method is two plates with the same index of refraction. So- keep the thickness and refraction a constant. The M Monochrom left the glass layer in where the Bayer filter would be, but left the Dye out. Just like a Jack Nicholson Bacon, Lettuce, and Tomato sandwich.
Seems like a lot of work for nothing, if the two filters are misaligned with respect to each other, you will not get perfect canceling. Like two prisms- hard to get two that match perfectly, and get some fringing. What you get with the Nikon method is two plates with the same index of refraction. So- keep the thickness and refraction a constant. The M Monochrom left the glass layer in where the Bayer filter would be, but left the Dye out. Just like a Jack Nicholson Bacon, Lettuce, and Tomato sandwich.