Exposure Lift Colour Cast

There have been a couple of reports online of a magenta colour cast appearing when G9 raw files are exposure lifted in Adobe Lightroom.  I have to date not noticed this myself although I have to say I am not wholly convinced by Lightroom’s raw conversion of the G9 files.  It may well be that I have not noticed this reported issue since I tend to ensure that I do not significantly under-expose images and therefore tend not to have lift exposure significantly in post processing.

All that said and given some of the quite extreme examples of the colour cast that I have seen online I thought it would be useful to look into this further and perform some testing.

The following test was conducted using my PL 12-60mm f/2.8-4.0 set at f/5.6 with auto-exposure in Aperture Priority mode, sunny white balance, manual focus, electronic shutter self timer delayed, camera mounted on tripod.  The base image was captured at iso 200 at the metered exposure (0.8s), test exposures were then captured at iso 100, 200, 400, 800, 1600 & 3200 with an exposure compensation set at minus 3 stops forcing the image to be under-exposed.

All images then imported into Lightroom CC with all default settings and the camera profile left at Adobe Standard.  All test images were then exposure lifted by 3 stops and then exported to Photoshop in order to produce the comparison images shown below – no other adjustments were made to the images.

As always click on the images to see them larger.

base
Base Image – iso 200 at metered exposure
100
Left = iso 100 -3ev lifted 3 stops in LR – Right = Base Image

The iso 100 shows no discernible colour shift although it is probably 1/3 stop lighter than the Base Image.

200
Left = iso 200 -3ev lifted 3 stops in LR – Right = Base Image

The iso 200 image quite clearly does show a magenta shift.

400
Left = iso 400 -3ev lifted 3 stops in LR – Right = Base Image

The iso 400 shows no discernible colour shift although is probably 1/3 stop lighter than the Base Image.

800
Left = iso 800 -3ev lifted 3 stops in LR – Right = Base Image

The iso 800 image does show a slight magenta shift, although not as strong as the iso 200 image.

1600
Left = iso 1600 -3ev lifted 3 stops in LR – Right = Base Image

Again with 1600 iso a slight colour shift although only very slight

3200
Left = iso 3200 -3ev lifted 3 stops in LR – Right = Base Image

The iso 3200 image shows a strong colour shift not unlike the iso 200 image.

So it does appear that there is an issue here, particularly at iso 200.  The question therefore has to be asked “is this a G9 or Lightroom issue ?”

In addition to testing at -3ev I have also run a test at a somewhat more extreme -5ev – the results are interesting.

minus5
iso 100,200 & 400 -5ev lifted 5 stops in LR

At -5ev the colour cast is most extreme at iso 100 and least noticeable at iso 400, although still noticeable – so in comparison to the -3ev test the iso 100 result is completely reversed – this is very odd to say the least and does rather suggest that there is an issue with Lightroom’s raw conversion rather than an inherent issue with the G9.

Does this issue matter ?

Well – the colour shift at -5ev is probably of little consequence since it would be highly unusual to have to lift an image or indeed the shadows by such a large amount.  The shift at -3ev is however potentially somewhat more of an issue and of course does beg the question of to what extent is a shift taking place at more moderate shadow lifting and therefore to what extent is work required in post-processing to remove the cast.

To answer that here is a test at iso 200 under-exposed at -1 to -5 ev – as can be seen even at -1ev some colour shift is present, then worsening as the degree of under-exposure increases.

iso200
Top = Base Image at iso 200 – Bottom iso 200 0 to -5ev  & corresponding lift in Lightroom

More investigation of this issue is required in particular to establish if other raw convertors have this issue.

 

 

 

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