FREELessons: 10Length: 57 minutes

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2.1 Lens and Perspective Corrections

In this lesson you’ll learn about the types of distortion (vertical, horizontal, spherical) and their causes, and then make the necessary corrections to our image within RawTherapee.

2.1 Lens and Perspective Corrections

In this lesson, we'll discuss the types of lens distortion and their causes. And then we'll make the necessary corrections to our image within Raw Therapee. So first, let's discuss what lens distortion is and the most common types of lens distortion and their causes. Lens distortion is caused by the optical design of your lens, or by the angle at at which you take your photo. The effect can be slight or quite noticeable, warping your image. Vertical lens distortion is when vertical lines in the image are not straight, due to your camera pointing up or down. Horizontal lens distortion happens when your camera is pointed left or right, and it causes the horizontal lines that should be parallel to the ground to appear skewed. Spherical lens distortion occurs when an image is not sharply focused in it's center and edges due to a spherical lens refracting light that enters near the edge more than the center. So let's take a look at the lens correction tools that Raw Therapee offers to us. In the far right hand module we're going to select the transform icon with the scissors and the ruler, its the fifth icon in. Take note that if you hover over the button, it will display the keyboard shortcut, so we'll go ahead and select it, and then we'll select the lens geometry drop down menu. Let's take a look at each of the options available. So first you have your auto-fill check box. With this box, it will either upscale or downscale your photo so that it fits within its boundaries, so that no empty borders are showing. You also have an auto-crop button as well. And below that you have a rotate drop down menu, which will open that briefly. This allows you to rotate your image from negative 45 to positive 45 degrees. If you want to define what line needs to be straight, you can push the select straight line button and draw the line with your mouse directly on the image. And we'll go ahead and close that back up and look at the perspective drop-down menu. This offers you sliders to help correct horizontal and vertical lens distortion. And so this horizontal slider on the top allows you to correct your image horizontally in order to center it. The vertical slider beneath it allows you to straighten the vertical lines in your image. This is most useful with photos that involve buildings or architecture. And so just to demonstrate that for you. You see how it distorts the image, and with any of these sliders, if you click the arrow in the upper right-hand corner it will reset that slider to it's default. So, we'll go ahead and do that. And here's demonstration of the horizontal slider, and so you see how that affects our image. And we'll go ahead and reset that to default as well. And we'll close that perspective menu. Next, we have the lens correction profile drop down. This is where you would upload the profile for your lens in order to automatically correct lens distortion, vignetting, and chromatic aberrations. And so you would select File System, and locate your lens correction profile. We'll go ahead and select cancel for now. And next we have the Distortion Correction drop down. This slider allows you to correct spherical lens distortion. When you slide the slider to the left, your value goes negative and corrects barrel distortion. When you take the slider to the right, it corrects pin cushion distortion. We'll go ahead and reset this to default, you also have a button that will help you auto correct your distortion. In our case, our image does not have severe lens distortion issues, and so selecting that button does not make much of a change to our image. Lets move on and look at some additional options available for correcting additional image issues. We'll go ahead and close that drop down and move on to the Chromatic Aberration Correction drop down menu. This allows you to get rid of red and blue chromatic aberration. Chromatic aberration is also known as color fringing, and happens when your lens is unable to focus all of the color wavelengths to the same plane on your image sensor. It can look like halos around the objects in your photo. In order to see your adjustments, we're going to use the image details to magnify our image to 100%. So we'll go below the image, and zoom to 100% by clicking the one to one magnifying glass. And then within our Navigator on the left hand side, we'll just click and drag and navigate to one of her eyes. And while our image does not have issues with chromatic aberration, I just want to show you the effects of the sliders. And so, this is the red slider on the top, and sliding it to the left definitely effects our image. As you can see, this is the after image. And sliding it to the right is actually adding in chromatic aberration because our image did not have any to begin with. We'll go ahead and reset that to default. And you see how the blue slider subtly affects our image as well. Sliding it to the left adds in yellow tones. And sliding it to the right adds in a subtle blue cast. So we'll make sure and reset those to default because our image does not have, again, chromatic aberration issues. And we'll go ahead and close that drop down and move on to the vignetting correction drop down. This set of sliders helps you correct vignetting, or a light fall off around the corners of the image. This is common in cheaper lenses. We'll go ahead and zoom back out. And we'll take it to about 8% again. And so first you have the amount slider. Adjusting it to a positive value lightens the edges. It's very subtle, but it does lighten the edges of our image, and taking it to a negative value darkens the edges of our image. And beneath that we have the radius slider. This makes the area of darkening or lightening from the edges bigger or smaller. And it's a very subtle difference affecting our image. Beneath that you have the Strength slider. This intensifies the adjustments of the Amount and Radius sliders. And so you see as I drag that to the right, that intensifies the effect visibly. And beneath that we have the center x lighter, this adjusts the center of the sphere of correction for the image to the left or right along the x axis. And just to demonstrate that. And the center y slider adjusts the center of the sphere of correction for the image up or down along the y axis. And we'll go ahead and reset everything back to default, and leave our image as it is for the vignetteing corrections. And that brings us to the end of your lesson. In this lesson, we chatted about what lens distortion is, how it's caused, and how to make those corrections within Raw Therapee. In our next lesson, we're going to make some corrective adjustments to the exposure and contrast of our image.

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