Tilt shift lenses are by far my favourite kind of lenses. Although they are manual lenses, they offer a huge degree of flexibility and some of the best image quality. There are occasions, however, when you may want to push these lenses to their respective limits by shifting right to the edge of the lens. Doing this, however, creates vignettes in the image and due to Tilt shift lenses not having correction profiles they can be quite tricky to remove in post.
Polarising filters are another piece of equipment that is extremely useful. They can help cut glare and reflections or add some punch to the sky. The issue is that they can create dark spots in the image which may not be useful in some circumstances.
To correct these issues in a simple and effective manner you can do something called a Lens Cast Calibration. Medium format shooters may already be familiar with this technique due to colour cast issues in technical cameras and this same technique can be used on tilt shift lenses and filters. The calibration can be achieved with a specifically designed white diffuser card and once complete, you will end with an even exposure without any dark spots in your image.
Once you've taken your intended picture, without changing the composition or aperture, place the appropriately sized card onto the front of your lens.
The image below is an example of what to expect once you take the picture.
You may need to change the exposure to allow enough light through and using the shutter speed to achieve this is recommended. Changing the ISO or aperture can cause issues with the calibration.
The rest of the process needs to be completed in Capture One, as far as I'm aware Lightroom and Photoshop do not currently offer this feature. Again the process is very straight forward simply right click on the calibration image and select "Create LCC".
This can take up to about a minute and once complete you will notice a dramatic difference in the image. The exposure will become much more even without any of the vignette or dark spots from the filters. Select all of the images you wish to apply the LCC to including the calibration image and then simply right click and select 'Apply LCC'.
This method can really improve your images cleaning up any issues with exposure, dust and can also improve colour rendition. You're now able to use your tilt shift lenses to their full potential without any drawbacks.
For a more detailed explanation please check out the vide below.