3 steps to fix a blue sky using the Skin Tone tool
Capture One Pro 9 has a number of tools designed for working with images captured using a Technical Camera System. The Skin Tone tool in the Color Editor, however, is not one of them. This Tool is actually designed for improving the render of skin tones, ensuring that the skin is clean, smooth and pleasing. Surprisingly this tool also works extremely well for fixing variations in tone of a blue sky captured with a Technical Camera System.
The above image has been captured using an ALPA Technical Camera equipped with a Rodenstock ALPA HR Alpar 4.0/35mm lens and a Phase One IQ3 digital back. To avoid converging vertical lines in the building and to preserve all the details the camera system provides, I have shifted the lens up 8 mm while maintaining the camera is horizontally straight and perfectly level.
This lens may not be specifically designed for movements when paired with a Full Frame 6×45 image sensor, as you find in the IQ3 digital back. However when the lens is stopped down to f/11 you find the best compromise between depth of field and, you can then get away with movements of approximately 8-9 mm without losing your image in the corners.
Image on the left is a capture directly out of the camera. It shows large color variations in the sky caused by the wide-angle technical camera lens. The image on the right is the same image processed using Capture One Pro 9 and applying the tools for technical cameras as well as the Skin Tone Tool.
To achieve perfect color render in images like the above, taken on a technical camera, the three steps below are required in addition to capturing the image:
1) Capture a Lens Cast Correction (LCC) reference image
2) Correction for Lens Cast (LCC)
3) Fixing minor color issues
1. Creating a LCC reference file
For Capture One Pro 9 to compensate for the strong color variations seen in the sky, a separate image of a translucent white reference plate needs to be captured.
Without changing the movements of the camera or the aperture, hold the translucent plate directly infront of the lens and make a capture. Typically you’ll need to change the exposure time equivalent to 2 f-stops to get a good exposure.
2) Correcting for Lens Cast
Lenses for Technical Cameras Systems take advantage of the fact that there’s no need for the mirror box which facilitates a normal camera’s prism, and therefore the lens can get much closer to the sensor. This is is a great advantage when designing a wide-angle lens as it allows them to be smaller and optically superior. However, with the lens so close to the sensor there is a significant challenge with the angle of incident light reaching a pixel. The angle of incident light can be quite extreme and exceed the normal range that the micro-lenses of each pixel are designed to accommodate.
In Capture One Pro 9 select the LCC reference file of the translucent white plate. From within the LCC tool in the Lens Tool tab, click “Create LCC”.
Straight out of the camera the images of the translucent white plate show color variations as well as light fall-off.
When you create the LCC make sure to tick the box for Dust Removal Information, as this will allow you to automatically fix minor dust spots in your final image.
Also tick the box for “Wide Angle Lens with Movements” as this will initiate additional calibration data needed for images taken with wide-angle lenses that include movements.
After the LCC calibration has been created, it is automatically applied to the selected image in order to verify the correction actually will be able to correct the image.
With all boxes checked the LCC reference image now shows a perfect grey image without color cast, dust spots or light fall-off.
To apply the LCC calibration information to the desired image, you simply select both the LCC reference image and the image of the building. Once selected, right-click on your desired file and from the menu options select “Apply LCC”.
The task of creating and applying LCC files can be done on multiple LCC files, just select all the translucent white reference files and select “Create LCC” from either the LCC Tool or from the right-click menu in the thumbnail browser. If all your images are organized by a number of exposures followed by a LCC reference image, followed by exposures and then LCC shots (and so on), you can properly batch-apply the LCC calibration information. Simply select all the images including the LCC reference image and select “Apply LCC” from the right-click menu in the Thumbnail browser.
With the LCC calibration information applied to the image, it immediately looks much better. The calibration has reduced most of the colorcast in the image but as I have applied movements to the lens, causing an steeper angle for the incident light on the sensor, the LCC calibration doesn’t fix the color variations perfectly.
3) Fixing the remaining color variation using the Skin Tone tool
To fix the blue sky I will be using the Skin Tone tool in the Color Editor. I could work on the whole image but as there are quite a bit of blue tones in the building I would rather work in a Local Adjustment mask, created from a color selection in the Color Editor. Doing so, I can modify the mask to only include the sky.
Now when I know I have the right color range selected I can convert this selection to a Local Adjustments Mask. I do so by clicking on the three dots (tool options) in the Color Editor Menu line and select “Create Masked Layer from Selection”.
In the Local Adjustments Tool tab I can finally move into the Skin Tone tab in the Color Editor.
This tool is designed for improving skin tones using the Uniformity sliders; these are designed to even out differences in a subject’s skin. It’s a very specialized tool for specific use, however these sliders also work perfectly on the blue sky of this image. In this example I adjusted the Uniformity of the Hue to 100%, Saturation to 67% and Lightness just a tiny bit. This adjustment totally eliminates any color variation in the sky.
With the Adjustment sliders I can further tweak the color tone to my desired blue tone.
All the best,
May 12 2016
By The Image Quality ProfessorCategory: Professor Tips Tags: color editor Capture One Pro 9 Skin Tone tool IQ3 Digital Back