> I need some help on this one as I don't understand what you were trying to
> illustrate with the use of a red filter on a positive image. If you were
> using a red filter on B&W negative film one would expect the blue sky to be
> darkened. But, since you're working on a positive image that doesn't
> happen. All that happens is that most of the blue and green are filtered
> out. If you then attempt to convert this image to B&W you get little more
> than the tonality that was in the red channel. Using the color channel
> sliders is ineffective since there's very little blue and green left in the
> image and that's only in the shadow areas. Perhaps that's what you intended
> but, if so, to what purpose?
A red filter does not ALWAYS result in an ink-black sky. As to the
rest of your question, I'm not following. The first set of images
(P103072) was processed as a straight digital image with no lens
filter. You see the in-camera BW JPEG (which is essentially just a
desaturation), a raw-converted B&W image with red filteration applied
(standard contrast trick in post and is a reasonable facsimile of
using a lens filter) and a straight raw image with no adjustments so
you can see how it was shot.
Compared to P103073, the results of the red-channel vs the red lens
filter is very close. I applied auto contrast to these images, which
did result in the sky going bright, but since I did the same for all
B&W images, by using auto white/black points, the results can be
compared.
I really wanted to point out P103073. This was the image taken with
the red lens filter. As you look at the in Camera B&W, you can see
that the image is horribly underexposed. This was shot with the same
exposure as the unfiltered P103072. Theoretically, the red sensels
received the full tonal spectrum, but the blue and green channels are
almost zero. The in-camera JPEG shows what I was talking about in
regards to the three/four pixel merge for determining luminance value
and then the color gets applied afterwards.
As we ALL know, we're supposed to increase exposure when using lens
filters. P103074 is +1 stop exposure compensation and P103075 is +2
stop exposure compensation. As you can see, the in-camera JPEG is more
correct in regards to exposure, you see a significant loss of
contrast. When looking at the raw converted files, you can see that
we've actually over exposed the image.
BTW, this test sequence is illustrative of what happens with the other
color filters too. It is absolutely vital to exposure the digital
camera with absolutely no filter-factor exposure compensation offset.
Do NOT increase exposure to compensate for the filter. This also means
that the in-camera metering is going to be very wrong. Determine you
exposure as with no filter and lock it in.
I have a complete set of exposure tests with different color lens
filters taken in three different locations on three different days. In
a nutshell, I do find that using a yellow filter or a green filter
actually does give decent digital monochrome conversions, red
certainly doesn't.
Ken
>> http://zone-10.com/tope2/main.php?g2_itemId=13348
>>
>> P103072 - No lens filter
>> In Camera B&W
>> Raw Converted to monochrome with red filter applied and auto contrast
>> Raw Converted with no adjustments
>>
>> P103073 - Red #25 lens filter, no exposure compensation
>> In Camera B&W
>> Raw Converted to monochrome and auto contrast
>> Raw Converted with no adjustments
>>
>> P103074 - Red #25 lens filter, +1 exposure compensation
>> In Camera B&W
>> Raw Converted to monochrome and auto contrast
>> Raw Converted with no adjustments
>>
>> P103075 - Red #25 lens filter, +2 exposure compensation
>> In Camera B&W
>> Raw Converted to monochrome and auto contrast
>> Raw Converted with no adjustments
>>
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--
Ken Norton
ken@xxxxxxxxxxx
http://www.zone-10.com
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