Converting images shot with red filter to b&w

Discussion in 'Digital Darkroom' started by evanbell, Apr 18, 2020.

  1. Hi all, earlier today I took some shots on my Sony a6500 with a Canon 24mm FD lens and a red filter. I shot in RAW, with the intention of converting the images to b&w in post-processing. I had the b&w creative setting on the camera turned on, so while shooting I was viewing and exposing the images for b&w. However, I'm noticing some discrepancies in the exposure between what I took in camera and what I'm getting out of LR after converting to b&w. Though I properly exposed in camera, the images are coming out overexposed after the conversion in LR. I've attached some images to illustrate this. As you can see, the histograms in camera and on LR don't match. I'm guessing that when shooting on a red filter, the b&w conversion is more complicated than simply pressing a button. I just can't seem to find a way to reproduce the images I shot. Any tips would be much appreciated! Screen Shot 2020-04-18 at 4.39.26 PM.png Screen Shot 2020-04-18 at 4.39.47 PM.png IMG_6707.jpeg
     
  2. I'd try to display the RGB channels and work on them individually in BW and merge the result (which could translate to "pick the one I like most"...) - Sorry neither an active Lightroom user nor shooting red filters on my color cameras.
     
  3. Thanks for the reply. What I ended up doing was essentially working the histograms from scratch and creating my own preset. Still, I would think there'd be an easier way to go about it, if there was only a proper Sony profile I could apply to the RAW image.
     
  4. It looks as if the B&W creative setting is modifying the histogram on the camera, so you are seeing the histogram after the conversion, rather than before. It's probably important to get the red channel correctly exposed for what you are doing, as the other channels don't contribute much, having largely been filtered out. Your red channel is overexposed on the Lightroom histogram.

    It may be best not to use the creative setting, instead use manual exposure or exposure compensation and by trial and error get the red channel histogram well exposed. You will not see the B&W results on the camera LCD, rather just a red image, but when you do post processing you should have more flexibility to achieve the desired result. This is how I worked when doing infrared with an R72 filter and unconverted camera.
    .
    I should say that I have no knowledge of your particular camera.
     
    Last edited: Apr 19, 2020
  5. Looks as if you might have applied the red filter factor as an exposure compensation, but the camera also automatically compensated because of TTL metering - result; overexposure.

    They were RAW files right? So there should be no problem with simply reducing the exposure in the RAW converter.

    Alternatively, you can adjust the tonality using the curves or levels tool. The shadows are far too bright, and that's an easy fix in curves or levels.

    BTW, I'm not sure why you're bothering with a physical red filter at all. Just shoot in colour and apply a virtual filter in post. It works just as well, and is more controllable.

    For example; this is a scan from a colour negative -
    Hay-wain.jpg
    and here's a 'straight' B&W conversion.
    IMG_20200419_103611.jpg
    Now a conversion with red filter applied -
    IMG_20200419_103629.jpg
    Note the improved sky detail.

    A more dramatic example; again from a colour neg. First 'straight' -
    IMG_20200419_102359.jpg
    And red-filtered during conversion.
    IMG_20200419_102416.jpg
    These were done using the limited editor on my smartphone! Photoshop has a far more sophisticated monochrome conversion module. Or you can apply filters of any colour and depth you want by using layers in almost any decent image editor.
     
  6. Is this more the effect you were after?
    IMG_20200419_115935.jpg
    Done using the curves tool in my Android phone editor.
     
  7. Here's what I mean about exposing the red channel for infrared work - which I feel is similar in principle to using a red filter - note that the Nikon D70 I used does not display separate channel histograms, so I had to underexpose the combined histogram to get the exposure I wanted:

    IRex1.jpg


    The Photoshop histograms on the left show the true picture, the camera histogram on the right, which is a combination of the different channels, suggests underexposure.
    Astaken.jpg

    Above is my monochrome conversion using Photoshop and Nik Silver Efex 2. Below is a shot I did with a normal red filter (not infrared) using similar exposure and post processing.

    Redex1.jpg
     
    Last edited: Apr 19, 2020
    Jochen likes this.
  8. Good inputs all. In addition to, or maybe instead of, using color filters in post-processing, I might suggest using a channel-mixer to convert from color to B&W.
     
  9. The Photoshop histograms on the left show the true picture, the camera histogram on the right, which is a combination of the different channels, suggests underexposure.

    You're shooting in raw; none of those Histograms show you anything about exposure. You need to view a raw Histogram using something like RawDigger.
     
    conrad_hoffman likes this.
  10. It's a while since I made the picture with the histograms, for a camera club talk I did on infrared photography, but I recall the Photoshop histograms were from a JPEG I processed from the D70 RAW file. The object being to try and get a decent spread of exposure on the red channel. The old D70, which I used because it is quite sensitive to infrared, can't display the three channels separately, so judging exposure by adjusting the combined histogram was the best I could do. Wit a low MP camera, making an image mostly from the information in a single channel doesn't make for great image quality which is why I'm looking for a newer IR converted camera.
     

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