Fixing lens aberrations for shift lenses
Fixing lens aberrations for tilt/shift lenses
Colour and distortion problems with shifted lenses
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Whilst modern tilt/shift lenses such as Canon’s TS-E lenses from 2017 are almost bereft of any noticeable distortions, this isn’t the case with older/cheaper lenses.
These lenses may exhibit colour fringing and curvature of straight lines towards the edge of the field.
Such issues are normally easy to fix with modern image processing software, but once you add shift to a lens, it quickly defeats most correction software.
This short article looks at techniques that Keith has used over the years to ‘clean up’ images.
I’ll start off with an overview of the sorts of aberrations you see with some lenses, and then look at some of the ways I approach fixing them.
Adding tilt to a lens complicates any distortions, such that correction may become very difficult and unpredictable – for this article I’ll stick to just using shift, even if it’s a tilt/shift lens with the tilt set at zero. That’s not to say you can’t correct tilted images, just that you should expect more work and variable results.
The shift lens
Tilt/shift lenses typically have a large image circle, which is the name given to the image projected behind them on to your camera sensor (film or digital).
The most obvious problem is the way the image brightness falls off to the edges. This vignetting is found with all lenses to varying degrees.
This example (from my TS-E24mm F3.5L II review) is posterised and processed to emphasise the vignetting effect but shows how the vignetting is symmetrical for an unshifted shot.
However at wider apertures, with a fully shifted view, the lens shift mechanism introduces some additional ‘shift vignetting’.
It’s much reduced with smaller apertures, but still a factor when you shift your lens so that your sensor is recording the image closer to the edge of the circle.
With an unshifted lens any vignetting is symmetrical, so your RAW processing software can make use of the known aperture and any lens profile available to correct for the vignetting, or you can adjust manually.
Since tilt/shift lenses don’t record any details of the amount of shift in the image EXIF data. it means that most processing software has troubles with a shifted image.
The ‘shift vignetting’ can also be an issue if you want to stitch multiple shots together, so I always shoot at f/7.1 or higher if I know I’m going to be stitching multiple images.
Many lenses show colour fringing on edges, which increases as you move away from the centre of the image circle.
This photo (from my TS-E45 review) has a bit of upwards shift applied to keep the horizon low, and the image verticals true.
A detail from the side shows colour fringing, typical of chromatic aberration.
A different type of colour fringing is sometimes found at bright edges. Here’s an example photo and detail from my TS-E17 review.
The purple fringing only shows against bright areas.
The most obvious distortions are pincushion and barrel, where straight lines towards the edge of your image bend in or out.
Here are two examples, but with lines added to show the effect more clearly.
Barrel distortion is slightly more common in lenses that show problems (older/cheaper ones).
It’s pretty easy to correct such basic distortions with editing software, whether using lens profiles or a simple pincushion/barrel adjustment slider, such as in Adobe Camera RAW in my old copy of Photoshop (I still use CS6).
The problem with doing this for a shifted lens is that the distortion is no longer symmetrical about the centre of your image.
Here’s a similar view, but shifted upwards and a bit to the left. Note how the distortion is higher towards the top (and left) of the image. The centre of the lens image circle is no longer in the middle of the image.
Some lenses have more complex distortions where the amount and type of distortion changes across the image circle.
Given that your photo file’s EXIF data has no information about shift, you can see that as with fixing chromatic aberrations, you’re going to have to do a bit of manual work.
I should note that if you’re looking to fix shifted images manually, then it’s a good idea to write down the shift settings you used when taking the photo.
Of course, I rarely remember to do this so I’ll show the sorts of things I actually do…
If you look at the various aberrations, the most significant complicating factor for most is the movement of the centre of the image circle away from the centre of your image.
This is often the case even if you’ve stitched multiple images together.
Since stitching images is just adding different parts of the image circle together, I often leave fixing CA/distortion and vignetting until after I’ve stitched images together.
The one exception to this is purple fringing, which as I showed in the TS-E17 review, you can try and reduce at the RAW processing stage.
The example here was unshifted so I also checked the ‘Remove CA’ box.
The TS-E17 is pretty good in this respect, but (unshifted) it helped a little bit in other areas of the image. The purple defringing worked very well here, shifted or not.
Extending the canvas
All my fixes for images, whether single shots or stitched, rely on finding the centre of the image circle.
If you do a simple up/down/sideways shift then the shift is shown on the scale at the side of the lens.
If you know the size of your sensor, then you can work out where the centre is.
So for my full frame sensors, I know they are ~24mm x 36mm.
Here’s an example from when I first tested the TS-E 24mm F3.5L (the 1991 version) at 11mm of right shift.
I’ve extended the canvas of my image by twice the amount of shift, or 22mm
How do I know what this is in pixels?
Well, the image from my old Canon 1Ds was 4064 pixels wide. (so 4064 pixels in 36mm)
(36+22) ÷ 36 is ~1.611 which is how much I need to multiply the width by for the new width.
4064 x 1.611 is ~6548 pixels
This gives a total width for the image, with it’s grey extension, of 6548 pixels (the height is unchanged).
The centre of the new image is the centre of the image circle.
In this downshifted example, I’ve added the grey extension upwards.
You can see the distinct vignetting lower down, even though it’s also darker near the floor.
Applying the Photoshop ‘Lens Correction’ tool lets me apply some vignetting correction.
It’s applied to the whole image – grey bit and all.
The effect of this is clearly visible on the previously smooth grey extended area.
Just as I’ve applied some vignetting correction, I can apply other lens corrections.
In the example below, I saved the extended file (as a 16 bit TIFF) and then opened it in Adobe Camera Raw, to use the corrections available there.
The auto CA correction worked just fine.
Essentially, it doesn’t matter what correction tool you choose, once you’ve extended the canvas to put the centre of the image circle back to the centre of your expanded image.
This is fine for basic geometric corrections too. The Samyang 24mm T/S lens is quite good optically, but has a definite issue with barrel distortion.
This example (from my review) shows this being corrected after extending the canvas.
Move your mouse over the image to see it corrected
Note how the corrected version shows really obvious bending to the edges of the image – The straight lines of the photo itself are now straight.
You’ll need to crop back to a rectangle, so this correction loses you some coverage.
If you’ve shifted your lens diagonally, then unless you know the angle it was shifted, you’re going to have to guess the canvas extension up or down, as well as sideways.
Yes, but I forgot to note the shift…
This is a bit more tricky, since you have to estimate where the centre of the image circle is.
The horizon is a useful starting point. Other clues are a point in the image at the same height as your camera (if level).
In the example below, I’ve created a large canvas and added temporary mid point markers.
The image is actually an up/down stitch (from my Laowa 15mm macro shift lens review)
I’ve then dropped the image onto the canvas, and lined up the centre.
One reason I knew where the centre was, was because I could see my camera reflection in the window.
As you can see, the image shows quite a bit of barrel distortion.
Once I’ve the image placed correctly I can simply apply lens correction to the whole image, and then crop as needed.
What’s less obvious in the example above, taken on a crop sensor Canon 100D. is the rather more complex distortions of the lens. You can see it in this up-shifted example using the full frame Canon 5Ds.
Look carefully at the pavement edge and at the building on the right where the roofline starts to bend upwards.
This shot is a bit unfair on the Laowa 15mm, since it’s much better on crop sensors (24mm equiv. on my EOS 100D).
These notes have been based on my own techniques from over the years. Obviously they are limited to my own choice of editing software, but equivalents should be available in other packages. If you’ve any particular suggestions, please do let me know by email or in the comments?
My own best solution is to use lenses without these problems, but that comes at a price.
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