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Frequent Contributor
Posts: 40
Registered: ‎11-30-2012

Optical Low Pass Filters way of the past?

[ Edited ]

It seems that Nikon (D5300) and Sony (Alpha a-7R) have released some newer model cameras with no optical low pass filters installed claiming better image sharpness and overall image quality. I am wondering when Canon will follow suit and why does it seem that the industry is going this route now? Thoughts?

Thanks, Bob

"Cameras don't take pictures, photographers do."
Esteemed Contributor
Posts: 3,854
Registered: ‎06-11-2013

Re: Optical Low Pass Filters way of the past?

And... I'm COMPLETELY confused by this.  Because the physics of the situation is that the complete opposite would be true and the low pass filter is there to IMPROVE the image.

 

Perhaps I'm misunderstanding and it would help if rather than using a generic name if they would be specific as to the bandpass of the filter.

 

In the age of film, it was the film that was sensitive to light... not the camera.  Load in IR film and you've got an IR camera.  Load in regular film and you've got a visible light camera.  Some films were a bit sensitive to UV... others not so much. 

 

The problem is that the dispersive properties of a lens naturally cause it to break light into it's constituent wavelengths (like a prism) whereby each wavelength no longer focuses at the same distance from the lens because they bend a different amount.  Additional elements attempt to correct for the problem but the correction isn't perfect.  UV will bend the most, IR the least, and everything else is somewhere in the middle.  You're trying to focus for that "somewhere in the middle" range.  

 

Digital sensors, on the other hand, ARE sensitive both to UV and IR even though the human eye can't see it.  And since the amount to which these will bend is farthest from the amount that most visible light will bend, the lens will suffer from dispersion effects getting chromatic aberration and color fringing in color, but even in monochrome you get a soft fuzzy focus and can't achieve tack sharp focus.

 

By trimming away these wavelengths that your eye can't see (but your sensor can see), you can sharpen up the image.

 

Correcting for the problem is software is tricky because the camera is reducing full-spectrum light down to just three colors (when in reality there are so many more).  The Bayer mask doesn't collect IR, R, G, B UV.  It just collects RGB.  So by adjusting the blue to to get rid of IR fringing and adjusting red to get rid of IR fringing you end up shifting things you didn't want to shift.

 

If indeed adding a filter only made the image worse... then one must wonder why a manufacturer would ever include such a filter in the first place... if the only purpose it serves is to increase manufacturing costs.

 

Tim Campbell
5D III, 5D IV, 60Da
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