Shooting images with wide-angle lenses and exploiting large-format camera movements often result in lens falloff and color cast effects. Another effect is the so-called centerfold effect where lines may appear in the image.
Lens Calibration is able to correct lens fall-off, color cast, and centerfold effects.
The following shots show the same image with and without lens calibration:
What is lens cast and how is it recognized? Why is lens fall-off associated with lens cast?
Lens cast appears when the light from the lens strikes the digital sensor at shallow angles causing areas of the image to have color casts ranging from green to magenta. This occurs mainly with non-retrofocus wide-angle lenses that do not have sufficient lens coverage for wide image sensors. Shifting and tilting such a lens strengthens the lens falloff.
Creating Lens Calibration in a studio
1. The things you will need: Leaf diffuser or any white diffusing sheet that will fully cover the lenses to be used.
2. Connect the camera back to the Leaf Capture software.
3. Place the camera in front of a uniform light source.
NOTE: A studio flash with a softbox is recommended to be used.
With this setup, you can now create the lens calibration file using Leaf Capture’s calibration wizard.
Creating the Lens Calibration file
1. Go to the Camera menu and select Lens Calibration -> Create Files Wizard.
2. Follow the steps in the Lens Calibration wizard.
NOTE: The warning message may or may not appear depending on the lens used.
3. Tick the Load lens calibration checkbox to load the lens calibration file to your camera back after it is created. This enables you to take shots while using the lens calibration file.
4. Click OK.
5. A small icon appears in the bottom left corner of the Leaf Capture window to remind you that a lens Calibration is loaded. The name of the Lens Calibration is displayed next to the icon.
In the example above, Leaf Capture has limited the lens falloff correction to 60%. You can reduce this number as desired.
The falloff correction is applied for this load only.
The name and path of the Lens Calibration and the % of the falloff correction used will appear in the Image Info of images shot with the Lens Calibration.
Loading the Lens Calibration file
When shooting tethered, previously created Lens Calibrations can be loaded to the back to be used for shooting.
1. Launch the Leaf Capture software.
2. Connect the back to the computer.
3. Click Camera -> Lens Calibration File -> Load File to open the Load Lens Calibration dialog box.
4. Select Use Lens Calibration File and click the browse icon to select the appropriate lens calibration file. Then click OK to proceed.
The lens calibration file is loaded and the falloff correction is applied. You can now shoot using the lens calibration file.
Shooting to CF card
A lens calibration can be loaded to the back for using when shooting to CF card.
1. Format a CF card in the Aptus II back.
2. Create a Lens Calibration for tethered use.
3. Click Camera -> Lens Calibration File -> CreateFile for CF card…
4. Browse to the Lens Calibration file from step 2.
5. Give a meaningful name to the lens calibration for CF and specify its location.
6. Copy the Lens Calibration for CF to the following folder on the CF card:
7. Insert the CF card into the back.
8. Load the Lens Calibration file. Go to Tap Camera -> Camera Settings -> Lens Calibration. Select desired lens calibration and press OK.
After shooting you can apply a Lens Calibration to images that were shot without one. You can do this in both Leaf Capture and in Capture One.
Working in Leaf Capture after shooting
1. In Leaf Capture, select the images you would like to process.
2. In the Process panel, select Leaf Mos in the Format list and click on Settings.
3. The Leaf Mos Settings dialog box opens.
4. To save the .mos file with lossless compression, tick the Save Compressed checkbox.
5. Tick the Lens Calibration Adjustment checkbox.
6. Select the Select Calibration File option and click the browse icon. Select the lens calibration file for the lens used to create the image(s).
NOTE: Click Lens Calibration Folder to access the default lens calibration files folder directly.
7. In the Falloff Correction box, type the desired correction percentage to adjust the falloff.
NOTE: There is no limitation to the amount of the falloff correction. However, the desired value will fall in the range between 0 and 100 %. If the resulting image is not satisfactory, you will need to reduce the amount of the falloff correction.
8. Click OK to proceed. Now your lens calibration settings are saved.
9. Process your images to apply the lens calibration file.
Creating a Lens Calibration in the field
1. Before shooting, prepare the Leaf diffuser or any white diffusing sheet that will fully cover the lenses to be used.
2. Set up and shoot your image(s).
3. Place the white diffuser in front of the lens so that it covers the entire surface and take a shot
NOTE: To achieve good results, do not move the camera or lens position, or change the aperture before taking the diffuse calibration shot.
4. On the imaging module, display the diffused calibration image in the Shoot view.
5. Tap histog to open the histogram.
Your overall exposure should be between -0.8 and +0.4. The exposure meter shows the overall exposure in f-stops.
If the calibration image is over or underexposed, adjust your shutter speed. Do not adjust the aperture setting.
NOTE: It is recommended that you rename the calibration shot to include the lens and exposure information in the file name.
Using Capture One after shooting
To create and apply a Lens Cast Correction (LCC), the images must meet the following condition:
The image must have been shot portably and not saved in Leaf Capture.
1. In Capture One, select the reference image that you shot through the diffuser.
2. In the Lens panel, open Lens Correction.
3. In the LCC dropdown menu, select Analyze.
4. The software will analyze the image and then create an LCC file.
5. Select the image to be corrected and then select the LCC file made in the previous step.
6. You can activate Color Cast, Light Falloff, and Dust Removal by ticking the appropriate checkbox.
7. The image will be corrected.
Using Lens Falloff correction tends to create images that seem brighter.
You can adjust brightness or develop curve to compensate for this or shoot the images at a lower exposure level.
Creating the calibration shot in low light
In some conditions, you may not be able to create an optimally exposed calibration file as described above because the required exposure time is greater than the maximum. This could happen when you are shooting using indoor lighting, under a low sun, and so on.
In such cases, take the calibration shot at the maximum permitted exposure even if it will be underexposed.
NOTE: It is important that the illumination of the calibration shot be as close as possible in color temperature to that of the scene. Therefore don’t use any additional lighting when taking the calibration shot.