Different F stops when using coloured filters?

[Albert2]

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I have the first generation Leica monochrome and set it up on a tripod in Aperture priority to F5 .6.

 

I took one photograph without any filters, and one with a yellow filter and lastly an orange filter without moving the camera. Exposures were taken from the same area and the aperture remained the same on the Summicron crown 35 mm F2 ASPH lens.

 

When importing the DNG files into light room 2017 I noted from the histogram when in the library mode that none of the F stops were in fact at F5 .6. They were F4 .8, F5 .7, and F8 respectively. The ISO remained at 320 whilst as expected the shutter speeds varied.

 

As this is a mechanical lens set to F5 .6 my question is why have the F stops changed when there is no automatic setting on this camera and the camera remained in aperture priority at F .5 .6 whilst all three images were taken.

 

I would be grateful to know the answer please

[UliWer]

The f-stops didn't change - when you didn't change them.

 

The f-stop value in the EXIF, which is seen in Lightroom,  is only an estimation. With different filters and different shutter times the "estimator thinks" that the aperture has changed. 

 

This sort of phantasy was perhaps a reason to abandon the f-stop estimation for the M10.

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[elgenper]

I have the first generation Leica monochrome and set it up on a tripod in Aperture priority to F5 .6.

 

I took one photograph without any filters, and one with a yellow filter and lastly an orange filter without moving the camera. Exposures were taken from the same area and the aperture remained the same on the Summicron crown 35 mm F2 ASPH lens.

 

When importing the DNG files into light room 2017 I noted from the histogram when in the library mode that none of the F stops were in fact at F5 .6. They were F4 .8, F5 .7, and F8 respectively. The ISO remained at 320 whilst as expected the shutter speeds varied.

 

As this is a mechanical lens set to F5 .6 my question is why have the F stops changed when there is no automatic setting on this camera and the camera remained in aperture priority at F .5 .6 whilst all three images were taken.

 

I would be grateful to know the answer please

 

There is no mechanism in an M lens that tells the camera what aperture is set. All digital M bodies measure the light through the (stopped down) lens to set the exposure time.  To get an estimate of the aperture (for the EXIF info, and for some lens corrections) it also meters the external light falling directly on the camera front (the sensor is the small, round hole on the top front of the camera, between the red dot and the shutter setting dial.  The light on that sensor is obviously unaffected by the aperture on the lens, and so a comparison between the two measured light levels allows an approximate assessment of the f/stop the lens is set at (the wide open value is given by the 6-bit coding).  Now, if you put a filter with, say, 2 steps filter factor on the lens, the effect on the internal light metering is the same as if the lens had been stopped down two more steps instead, and that is what the EXIF block will say.

Edited April 11, 2017 by elgenper