No matter how much you spend on a lens, it is bound to have optical imperfections. Problems like distortion, vignetting, and color fringing are present—and distracting—in all images to varying degrees. Fortunately, when we want to create a technically excellent photograph free from distortion, software can correct most lens flaws.
In this tutorial you'll learn more about lens distortion and how to correct images with PTLens, an inexpensive application for Windows and Mac. PTLnes is one of those single-purpose tools that does one thing and does it very well. It's probably not a program that you'll use every day but when you do need it, it's a great tool to have in your kit.
What to Correct
As Jeffrey Opp said: "I strongly suggest you choose better glass first because the lens forms the image on the sensor and if that image is not at its
best, no sensor can make it better."
But what if you've already taken the picture? Or what if upgrading your lenses isn't an option? As it turns out, there are indeed a few things you can do to get the absolute most out of a less-than-ideal lens.
Before we dive into correcting an image, however, let's take a look at the types of corrections that your pictures may need.
Distortion is visible in an image when a real-life straight line becomes curved or wavy in a photo. This is the result of the way light passes through the elements in your lens, and it is especially noticeable when straight lines are prominent, as in architectural images.
Distortion comes in two main varieties: pincushion distortion and barrel distortion. Barrel distortion is commonly seen in wide-angle lenses, and looks it like the middle of an image is bulging outward. Pincushion distortion is present when the center of an image presses inward slightly.
When the corners of an image are noticeably darker than the rest of an image, you're witnessing vignetting. Because light travels in a wave, the further it goes the less light you have (at a the rate of one quarter the light for each doubling of the distance). As light travels through the lens it has to further travel to reach the corner of your sensor than the middle, so you get vignetting. Lens correction software can offset the darkening of those edges so that the frame is consistently exposed.
A vignette is actually sometimes desirable and added in post-production. Subtle vignetting can help draw the viewer's eye into the center of the frame and create natural focus on the subject. However, not all vignetting is created equal; a solid blue sky should be consistent across the frame, for example.
Aberrations and Fringe
Aberrations in a lens create color fringing effects. Photography Life has an excellent, scientific write-up on why this occurs as light hits a lens. Aberration creates color fringing in images, particularly in ones that are backlit. The effects are often visible in the form of "color fringing," or unusual colors at an object's edge.
Now that we understand these common lens aberrations, let's look at how to use PTLens to correct them.
I found PTLens while trying to correct the distortion from an inexpensive Tamron ultra-wide-angle lens. It's a lightweight, all-plastic lens that's great to travel with. Unfortunately, it has some complicated barrel distortion that really threw my images off.
Normally, I would apply lens correction in Adobe Lightroom. However, Lightroom didn't have a profile for my trusty Tamron. I started searching and found PTLens, which offered easy correction and included a preset fix for my lens.
PTLens was created by Tom Niemann of ePaperPress and is available on Windows and Mac. You can test up to 10 images, and license is just $25. It can be used as a standalone application, or as a plugin with the Adobe Photoshop, PhotoLine, or PaintShopPro raster image editors. Read on to learn how to use it.
1. Add Images to PTLens
After you install PTLens and launch it for the first time, you need to show it where your images are stored. PTLens doesn't support working with raw image files, so I suggest you make lens correction the first step after your raw-processing workflow. That is, export a TIFF file out of Adobe Lightroom or your raw-processor of choice, then use PTLens on it before you dig into the final touches in your raster image editor.
To add images to PTLens, click on the Directory button that's at the top right of the application. In the new window that opens, browse to the folder that contains the images to correct. Let's get started on correcting an image.
2. Profile Your Images
Now select a file from the far right panel of the application. The image preview will appear in the viewer on the left side of the app. PTLens renders the preview with the corrections included. If you click on the preview of the image, you'll see the "before view" of the uncorrected image.
PTLens uses lens profiles to correct an image. Inexpensive kit lenses are built to suit the widest range of needs, so they tend to have the most problems, but every lens has its faults. The developer has built a big database of how hundreds of lenses behave. PTLens will determine the lens used to capture the image, and apply the appropriate corrections automatically.
The lens profile is shown at the bottom of the application. You'll see the make, model, and lens used. Sometimes, PTLens will ask that you manually select the lens you used from one of the dropdown menus.
You can also manually adjust the focal length in the bottom box shown in the screenshot. Lenses show different distortion at different focal lengths, so getting this right is important for correction.
What if PTLens profiles the lens incorrectly? Or, what if our image is a film scan, and doesn't have a lens included in the metadata at all? We can manually select the camera and lens data by tweaking the Preferences. Go to the PTLens > Preferences menu to adjust the preferences.
On the Preferences menu, several options are checked by default that we can turn off. Uncheck Detect Camera, Detect Lens, and Detect focal length to manually select all of these for your corrections.
With all of these options turned off, you can manually select the settings at the bottom of PTLens. Choose the camera, then lens, then the focal length to choose the profile.
3. Refine Lens Corrections
While distortion correction is automated, we can tweak several other factors for fine-tuning the final image. Let's look at several controls to refine lens correction.
Let's correct an image's vignette, or dark edges in the corner of the image. With an image selected, find the Vignetting tab to the right of the image. Pulling the Amount tab to the right will reduce the natural vignette of an image. If we wanted to darken the corners of the image for an artistic effect, pulling the slider to the left will do so.
Perspective correction is essential when you capture an image at an extreme angle. Next to the Vignetting option is the Perspective button. Switch to this tab to correct for perspective.
The Vertical slider is used to correct vertical perspective issues, when we've angled our camera sharply upward or downward. The Horizontal slider corrects for sharp shifts of the camera along the x-axis. With some testing, these sliders will begin to make perfect sense.
In the example below, you'll see what I mean. I shot an image of a building with a wide angle lens, angled upwards. This causes the building to appear as if it's leaning backwards. Perspective correction can help compensate for this effect.
Finally, the chromatic aberration tab allows for color fringing correction. The red-cyan slider can be pulled to correct the red and blue fringing you see in an image, while the blue-yellow slider is for correcting the corresponding colors. Use the zoom slider to get a closer perspective of your color fringe corrections..
4. Save the Finished File
When you are finished correcting an image, it's time to save the file. Let's look at how to tweak the preferences so that our finished file is exactly what we need.
The PTLens preferences are on the PTLens > Preferences menu on Mac. There are two key settings for exporting on this window:
- Suffix: the suffix is the set of characters (text and numbers) added to the end of the finished file. By default, it's set to "_pt", but you could changed this to "corrected," for example. The key is that the finished file will have this text at the end of the file name.
JPEG Quality: we can also set the quality of a finished image from the Preferences. I keep the quality pretty high, as the image has already been through one JPEG conversion already. A quality setting of 85-90 seems to be the sweet spot for file size and image quality.
With our preferences set, we can save the finished file. In the lower right corner of PTLens, press Apply to save a corrected image. You'll notice that the app will create a second file with the suffix at the end of the file. This is the corrected image.
That's it! We've learned how to correct an image with PTLens with just a few clicks. Now, let's move on to using PTLens inside of Photoshop.
Use PTLens with Adobe Photoshop
Although PTLens works great as a standalone application, it can also be used as an Adobe Photoshop plugin. In the PTLens download on Mac, there is a PTLensPhotoshop.plugin file. Move this file into your Applications/Adobe Photoshop/Plug-ins folder.
On Windows, PTLens includes an installer that automatically adds the plugin to Photoshop. The ePaperPress website has additional instructions for installation on Mac in case you need them.
If you've installed the plugin correctly, you can launch it from the Filter > ePaperPress > PTLens menu option. This will launch your current image in the same familiar PTLens interface, but inside of Photoshop. Press "ok" when finished, and you'll return to the main Photoshop window with a corrected image.
Recap and Keep Learning
You might not apply lens correction to every image in your collection. But when you need a straight line for a stunning architectural image for example, lens correction becomes essential. PTLens is the most powerful, easy to use solution I've tested for perfecting an image.
How do you approach the process of lens correction? Is it part of your normal image workflow, or do you use it on specific images? Let me know what you think in the comments.
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