Non linear NDs?

[Keith Walters]

I just noticed

this post

by Jeff Kilgroe on Reduser:

 

"ND's can effect DR because of how they block light, they are not truly linear. They block the total amount of light that reaches your lens and lower intensity sources more so than higher intensity ones. Stopping down a lens reduces the aperture size, effectively reducing the light-gathering area of your optics, but does not add any filtering or blocking to the light that is able to pass through the aperture opening as an ND filter would".

 

Is that true? First time I've ever heard of that.

Given that photons from all points on an object are going to pass through all parts of a filter simultaneously, how could this possibly be the case?

 

(Imagine the filter is like a window in a wall. With a compact pocket camera, you can take much the same photo anywhere the lens can "see" through the filter, so this must be the case.)

[Daniel Sheehy]

Talking maybe, about a non-uniform response to different frequencies in the spectrum?

[Rob Vogt]

Nah it doesnt seem like hes talking about color, but rather intensity. hes saying that without NDs you close the apature, but with NDs you keep it open which means... He doesn't really explain his reasoning. I think he may be confusing DoF and DR.

[Chris Durham]

Well the first part of the statement is off: "ND's can effect DR..." No. Dynamic Range is a function of the capture medium. Film has a certain dynamic range, a sensor has a certain dynamic range. This is unaffected by how much light actually reaches the medium. Forgiving possible defects in manufacturing an ND filter simply reduces the amount of light passing through the filter at all points. 50 lux through an ND2 would result in 25 lux reaching the lens (I think I'm right measuring in lux here). Factor into that light loss through the lens elements/aperture, and you know how much reaches the capture plane. It should be as simple as that.

 

Of course that may mean that some light passing through is reduced to the point where it has no visible effect on the capture surface in the 1/48 of a second in which it is exposed, but that didn't effect dynamic range at all, just the overall brightness of the image: It reduced light density in a neutral way.

[Chris Keth]

No clue what he's talking about. NDs are purposefully designed to have an even (neutral, it's even in the name) effect across the color spectrum. They do nothing to dynamic range at all. Just everything in the scene is slid down the curve by x stops. Simple as that.

 

Perhaps he means that, in percentage terms, it blocks more of low intensity sources than high intensity. That would be a true statement. An ND.3 would block 1/5 of a zone 5 exposure and 1/10 of a perfect zone 10 exposure. That still has no effect on dynamic range, though.

[John Sprung]

"ND's can effect DR because of how they block light, they are not truly linear. They block the total amount of light that reaches your lens and lower intensity sources more so than higher intensity ones. Stopping down a lens reduces the aperture size, effectively reducing the light-gathering area of your optics, but does not add any filtering or blocking to the light that is able to pass through the aperture opening as an ND filter would".

 

This guy is either just plain wrong, or expressing some idea so unclearly as to be incomprehensible. ND's and aperture changes have the same effect on exposure.

 

 

 

 

 

-- J.S.