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I've read here on this very site that some compact cameras don't use aperture blades to stop down the lens and limit the amount of light hitting the sensor. Stopping down increases diffraction. Instead, these cameras use neutral density filters for this purpose.

How are these implemented? Are they dropped in the light path like drop-in filters on large telephotos, or is there a LCD panel that's activated when needed?

This question is inspired by my answer to this question.

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And, do such cameras lie in their metadata and claim a different f-stop? –  mattdm Aug 9 '11 at 20:57
@mattdm - Yes they do. The F-stop is adjusted to reflect the reduced light transmission. –  Itai Aug 10 '11 at 14:36

2 Answers 2

up vote 6 down vote accepted

They are simple ND filters which simply slide in an out of the optical path.

You will notice that those cameras only offer two (or four for double ND filters) apertures at any given focal length. That is because they have a fixed attenuation unlike polarizing filters.

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I would venture to guess that a pair of polarizing filters is probably used in concordance with some micro servos or some such. Two polarizing filters rotated in opposition to each other along a 90° arc can vary between minimal filtration (few percent opacity) to near total filtration (90% opacity or around there). Its a fairly common DIY project to create a "variable ND filter" by combining a circular polarizer lens-side with a linear polarizer scene-side when you don't want to drop the cash on a decent multi-filter/interchangeable-filter ND filtration system like Lee or Cokin.

I can't say if this is actually how its done. Such a setup would allow compact cameras to simulate aperture's effect on light volume without actually encountering severe diffraction blurring issues caused by literal pinhole-sized apertures at say f/8. It would require additional on-board logic to calculate the amount of rotation of one of the polarizing filters to achieve the amount of light reduction necessary to simulate a given aperture, and the results are probably not 100% accurate. That said, aperture stop-downs are not 100% perfect in DSLR's either, and there can be minor variation from one shot to the next of the same scene, with the same camera, and the same settings.

An important difference between using a real aperture and using some kind of ND filtration would be depth of field control. Not that you really have a whole lot of DOF control in a compact anyway given how small the apertures are and how tiny the sensors are, but with ND-filtration as a replacement for aperture would completely remove any control you have over DOF and background blur.

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I'm not aware of a P&S that has a variable ND filter. The only ones I've seen have a straight unchangeable ND filter, with apertures changing due to zooming in and out. So at each focal length, you can have two "apertures", usually selected automatically based on the light level. –  Eruditass Aug 10 '11 at 1:11
I think this theory is pretty easily testable, isn't it? Point a compact camera known to use an ND filter at an LCD monitor and rotate.... –  mattdm Aug 10 '11 at 3:44
Hmm...seems I was a bit optimistic in my assumptions... It seems a bit cheap to simply "slid in" an ND filter or two. Whats really better...some diffraction, or the complete loss of DOF control and control over a mere 2 (maybe 4) stops of light filtration? –  jrista Aug 11 '11 at 4:05

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