Monochrom and ND filters?

[thompsonkirk]

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I'm puzzled to find that when I use a 3-stop B+W ND filter on my Monochrom, I consistently get slightly underexposed files with more contrast and less shadow detail.  In theory, this seems impossible – but I have no difficulty repeating the impossible, until I increase exposure compensation.    

 

Is there any explanation for this besides witchcraft? 

 

Kirk

[Lenshacker]

Are you comparing the images with the same F-Stop and only altering the shutter speed? Is the camera tripod mounted?

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How do you measure your exposure with and without the filter?

[thompsonkirk]

Yes, f stop consistently 1.4 or 1.8, with varying shutter speeds.  Metering ordinarily by autoexposure, chimping a test shot to check highlights & using manual shutter if necessary/desirable.

 

K

[Lenshacker]

Longer shutter speeds means that the "Dark Signal" will be higher, basically the number of electrons that creep into the image. Think of this as background noise, the longer the CCD is collecting the image, the higher the proportion of Dark Signal. The DNG file has a "TAG" to set the level for "Black". It is usually set to get rid of the background dark signal. I would need to have the DNG files to look at them, or you could try "RawDigger" to see if it reports the level for "Black" in use. Setting this value higher would increase contrast as it reduces the number of levels used to represent the image.

 

Just a wild guess.

[thompsonkirk]

Thank you – a much better explanation than witchcraft!

 

Kirk