Is it normal for RAW files to have lens distortion? How best to deal with it?

Question

I noticed it for the first time when I took an image with the camera set for RAW+JPG mode. Because, I could see the 2 images side by side. The distortion is low enough that if I just looked at the RAW image, I would not notice it. But looking at it next to the JPEG , it is clear something is off.

• Is this normal for RAW images? Does this happen just because RAW does not correct the image in any way while the JPEG does?

• What is the best way to deal with it? Do I just have to eyeball it to figure out how much lens correction I have to use in lightroom?

• Will the amount of distortion be exactly the same for every RAW image I will take with that camera + lens combo? Or does it depend on focal length, exposure parameters and other factors that might differ per image?

I am hoping the distortion is the same for a camera+lens combo as then it would be so much easier to correct for it. I could then figure out how much / what kind of correction to use once and apply the same correction to every image.

EDIT : I use Nikon D5600, with nikon AF-P DX nikkor 18-55mm 3.5:5.6G VR lens. I use Adobe DNG converter to convert the NEF files into DNG format which i then use in Lightroom

Answer 1

Is this normal for RAW images?

Yes, it is normal.

Based on my experience with Canon cameras with different lenses, the RAW image remains uncorrected w.r.t. to the corresponding JPEG.
This means that the following corrections set in camera are not applied (and can be applied separately in a RAW conversion program):

• Colour corrections with picture profiles.
• White balance corrections.
• Noise "corrections", i.e. denoising.
• Lens corrections

Sometimes RAW conversion programs of camera manufacturers (e.g. Nikon ViewNX or Canon Digital Photo Professional) read from the RAW photo file what corrections were set in the camera and apply these corrections (non-destructively) on the RAW file. This often results in an image that matches, or closely approximates, the camera JPEG.

Note that you never see the RAW image, it's always an interpretation of the RAW image by the conversion software (Lightroom in your case). Here is an excellent question about unprocessed RAW photos.

How to best deal with the lack of correction in RAW images?

Use the Lightroom Lens Corrections panel.

You don't need to eyeball it if you used a lens that is supported by Lightroom's lens correction module.

Assuming the lens is supported, you can select all photos taken with that lens and apply the appropriate lens correction profile:

1. In the Lens Corrections panel of the Develop module, click Profile and select Enable Profile Corrections.
2. To change the profile, select a different Make, Model, or Profile.

There are additional steps to the process for more advanced tweaking, you can view them on the linked Lightroom help page.

Is the required correction the same for all RAW images of a camera + lens combo?

No, they required correction also depends on the used focal length, the aperture, focus distance and your personal taste.

Lightroom uses the correction profile to derive from the lens settings (aperture, focal length) what kind of correction it needs to apply. The correction is therefore not exactly the same for every RAW image, but is exactly the same for the same lens, aperture and focal length.

Of course, you have the (artistic) freedom to apply another amount of lens correction than LR suggest, depending on the scene or the look you're going for. However, I find the default setting great to apply on all the photos (per lens) as a starting point and then tweak it from there.

Answer 2

Yes, this is normal. The raw file contains just the raw data from the sensor, with no processing of any sort done (plus metadata, previews, etc). The embedded previews and the JPG produced when you are recording raw+JPG will have had certain types of processing done, based on your camera settings, but the raw sensor data is just raw sensor data. To deal with it you typically use software (Lightroom, DarkTable, RawTherapee, .....) that either has profile information for your specific camera/sensor and lens, or that has free-form corrections that you can apply until it looks "right" to you. But that software is what you would use for all other post-processing, as well - lens correction is just one of the many things you can do.

Edit: regarding the last question.... If you are using a prime lens, then the different aberrations should be similar in each picture, but probably not identical, because the focus distance can affect some of them. If you are using a zoom lens, then the focal length also has a significant effect (e.g. there is much more perspective "distortion" with an 11-24mm lens at 11mm than there is at 24mm....).

Answer 3

Is it normal for RAW files to have lens distortion?

No. It's normal for lenses to have lens distortion and the image they distort is what is recorded in the raw file.

How best to deal with it?

Use a raw convertor that allows you to apply lens correction for distortion. For best quality, apply the lens correction before exporting from raw to any other image format, such as TIFF or JPEG.

Does this happen just because RAW does not correct the image in any way while the JPEG does?

Nothing is corrected in the raw image data. What you see on your screen when you open a file is not "THE raw image", though. It's a processed interpretation of the raw image data. The information in a raw image file is monochromatic and linearly responsive to the amount of light that struck each photosite (a/k/a sensel or pixel well).

If the raw convertor is set to apply lens correction by default, you'll see a preview image on your screen that has lens correction applied, though you can then turn it off if you wish.

If the raw convertor is set to not apply lens correction by default, you'll see an image that has not had lens correction applied, though you can then turn it on if you wish.

A lot of folks have this misinformed assumption that the first thing they see when they open a raw image file is "THE straight-out-of-camera raw file." There's no such thing.¹ All one has to do is open the same raw file with several different raw convertors to see how different the initial interpretation of the data in a raw image file can be.

Note:¹ With some raw convertors that allow total control over the imaging pipeline you could open a raw image file with each of the monochromatic luminance values displayed with linear brightness "curves" (straight line slopes instead of a gamma curve) and no demosaicing to get color information. What you'd see is something like this.

Applying gamma curves results in this:

Here's an extreme closeup of a portion of the image above.

For a more in-depth look at what a truly "unprocessed" raw file looks like, you can see the full answer these samples came from at What does an unprocessed RAW file look like?

Answer 4

The D5600, and other Nikon bodies going back at least 7 years, has a feature called Auto Distortion Control. This can be found on page 232 & 287 in the D5600 Reference Manual that came with your camera. If you don't have the manual, download it from Nikon's website. If enabled, the image will be corrected for lens distortion for known lenses in the JPEG file, not the raw file. The raw file is unprocessed image information and it is up to you to supply distortion correction in post processing. Adobe LightRoom, et al, has lens distortion correction functionality and will correct for any lens in its catalog (in camera will only correct for certain Nikon lenses and manually for unknown lenses). LR also has manual lens distortion correction.

Answer 5

Is this normal for RAW images?

Yes.

A RAW image is just that: raw. It is as close to the exact output of the sensor as you can get out of a given body. Critically, in most cases it will not even have been debayered, which gives you the opportunity to perform this step yourself (which can sometimes result in better image recovery compared to the on-body algorithm, which is almost always going to make some quality-for-speed trade-off), and better post-processing applications will allow you to experiment with different algorithms.

Consequently, of course, this means that the pixels will start with a one-to-one correspondence to sensor elements, which means no distortion correction. (Note that this is technically more accurate to film, which also would not have distortion correction.) As you probably know, the camera body does considerable post-processing of this RAW image in order to produce a JPEG, which may include operations such as noise reduction, correction for chromatic aberration, and distortion correction.

What is the best way to deal with it? Do I just have to eyeball it to figure out how much lens correction I have to use in lightroom?

Like debayering, better software will have options for correcting (or applying!) distortion. As a firm believer in Free(dom) Software I'm most familiar with lensfun (which is incorporated into e.g. GIMP and darktable). Such tools typically operate using a database of known lenses (I believe the only relevant property of the body is the sensor size). Some may include mechanisms to calibrate an unknown lens, which typically requires "reference images" containing straight lines (e.g. photographing a test chart). Basically, if you're using a "known" lens, this should just work "automagically", but (at least if you use Free software) if you have an "unknown" lens, you still have options if you're willing to get your hands dirty.

Short version: I can't speak to Lightroom as I morally avoid proprietary software, but darktable+lensfun should apply distortion correction automatically for known lenses. (There might be a box you need to tick, but you shouldn't need to futz with the distortion parameters.)

If you have an unknown lens and you don't want to go to the effort of actually calibrating it, another option would be to take a photograph of a test chart using the same lens and focal length, and use that to "eyeball" the necessary correction. You can then apply the same correction to your "real" image.

Will the amount of distortion be exactly the same for every RAW image I will take with that camera + lens combo? Or does it depend on focal length, exposure parameters and other factors that might differ per image?

AFAIK, only focal length matters. (Focal distance and aperture might, but exposure certainly does not. At any rate, most correction algorithms only care about focal length. And of course what lens you are using, but you asked about "for a particular lens". In fact, using a different body shouldn't matter as long as it uses the same sensor and mount, but a full-frame vs. DX sensor will matter. Hopefully, any body will have the lens' principle point centered on the sensor and the sensor parallel to the lens' focal plane, i.e. the mount ought to be universal.)

Keep in mind, distortion is a property of the camera optics and how they affect the path that light takes between the scene and the sensor. The shape of the lens assembly will obviously affect this, which includes focal length for variable zoom lenses. How long you expose the sensor obviously won't affect this. Other factors, such as aperture, focal distance and the distance between the camera and the subject (i.e. what you're shooting) may have an impact, but I believe it is significantly less than the "general" distortion and is typically ignored.

(One other note: software almost always assumes that the principle point is the center of the image. This is partly because this is almost always a safe assumption, but keep in mind it means you have to apply distortion correction before cropping an image.)

Answer 6

RAW Files are contain just the output from the sensor. They are produced the same way regardless of the lens. Lenses produce optical distortion (some more mild than others) but that is what you see in RAW files. So, yes, it is normal if you are using a lens which exhibits distortion.

The best way to deal with it is to disable distortion correction for JPEG and HEIF (if available) so that all your files match geometrically. Distortion correction affects composition, so you cannot properly compose for the uncorrected lens if the camera is showing the corrected JPEG/HEIF preview that is often missing corners (for barrel distortion correction) or edges (for pincushion distortion).

When you look at your images, the apparent distortion depends on the subjects and you will find that most natural subjects note make distortion very visible but architecture and things with perfectly straight lines will really show even if there is a very small amount of distortion. All software correction is diminishes (even only slightly image quality) so this way you can decide when distortion correction is needed.

If the number of photos requiring distortion compensation is low, it is best to do it visually until the output looks good. You may find some images where fully undistorting the image causes you to lose an important corner or cut off something in an awkward position. This way you can back off a little as necessary.

If you have many, then ideally you use profile-based distortion correction tools that characterize a typical copy of the lens you have. This is the amount and shape of distortion at various focal-length and aperture combination which software uses to interpolate correction for the combination of focal-length and aperture used for each particular image (note the camera rarely affects distortion but many profiles are made for a camera and lens combination to correct for other defects). This is the approach offered by most modern photography oriented software. Should you have a lens with high variation due to low quality-control, you will have to produce a profile yourself. They are tools for that but it is not something I have done.

Optical distortion is not affected by the exposure itself. It could be affected by focus-distance but whether a profile takes that into account, probably depends on software. The cameras does not affect optical distortion but is usually included in the profile to correct color issues.