How can I avoid vignetting when shooting in raw with a Canon Powershot?

Question

Very inexperienced person here when it comes to photography and raw processing here. I'm using a Canon SX 60HS camera. No attachments — filter/hood/etc. — of any sort were attached to lens.

After switching to raw+jpeg mode, I started noticing black round corners for some of the images. I had never experienced this while using camera in automatic mode.

After researching, I found out this is called as vignetting and is a limitation or function of lens and some other parameters at the time of taking the snap.

Why does this not happen when taking the picture in auto mode? Even the jpeg images generated in raw+jpeg mode do not show vignetting. Only raw images show it.

If it matters, I'm using Darktable 2.4.1 on Ubuntu. Since I don't have access to Windows PC, I can not test on Canon software.

I understand vignetting can happen. However camera image preview does not show vignetting. Is there any way to detect vignetting in the camera while I'm at scene so that I can adjust parameters and take the picture again. What sort of information I should check for possible vignetting. I tried to find a common pattern, but so far no luck.

• Raw Image
• Jpeg saved by camera in raw+jpeg mode

Answer 1

Many modern digital cameras are designed with the expectation that lens compromises will be... de-compromised... in RAW processing. That appears to be the case here.

The preview image is based on the JPEG rendering, which includes this processing. The RAW file, however, does not.

The amount of apparent vignetting will not be dependent on the scene, but on the lens zoom and aperture at which the picture was taken — that's why you're seeing differences in different situations.

The answer here is to use the lens correction module in darktable to apply this correction to your images as you process them. There's no other way around it — it is simply how the camera was designed.

Answer 2

For raw images, the preview you see on the back of the camera is based on the jpeg preview created and attached to each raw image. For jpeg images it is often also a scaled down version of the jpeg created in-camera. In either case, the camera seems to be automatically applying in-camera lens correction to the processing it does in-camera to produce the jpeg and/or jpeg preview.

Lens correction typically includes correction for things such as geometric distortion, chromatic aberration, and peripheral illumination drop off. Although everyone uses the term to describe peripheral illumination drop off as vignetting, it's actually kind of a misuse of the term. Vignetting more properly means darkening due to a physical obstruction. If the obstruction is in a very defocused area, such as just in front of the lens, the resulting shadow will be very soft.

When you open a raw file in an image editing application, such lens correction is not usually automatically applied. You need to turn it on in your raw editor and ensure that your raw editor has a lens profile for the lens with which you took the picture.

Is there any way to detect vignetting in the camera while I'm at scene so that I can adjust parameters and take the picture again. What sort of information I should check for possible vignetting. I tried to find a common pattern, but so far no luck.

If your camera model offers the option, you can turn off 'Peripheral Illumination Correction' in your camera's menu so that it will not be applied to the jpeg/jpeg previews generated in-camera.

The more or less general pattern is wider apertures and shorter (wider angle) focal lengths tend to demonstrate more peripheral illumination fall off than narrower apertures and longer (narrower angle) focal lengths do.

Answer 3

Most lenses exhibit some distortion, barrel or pincushion. Some modern cameras, like yours, automatically correct for this in-camera when creating the JPEG image.

In turn, camera producers skimp on their lens's optics to produce a smaller image circle, since the corners won't be seen on the JPEG anyway. The RAW image, though, is uncorrected. (Try to take a picture of a brick wall or anything with straight lines)

If you compensate for the distortion in the developer software on your computer, the dark corners will get cropped, too.

Answer 4

Vignetting is simply an optical property of the lens. The light falls off out toward the frame corners (further away from central axis). It is worse at widest apertures, and at shortest focal lengths, and on wider full frame sensors. Stopping down, like to f/5.6 or f/8, will help significantly to reduce it. Nothing else you can do to prevent it, but there are corrective tools available.

Look at some lens tests here: http://www.opticallimits.com/

Pick any lens, it reports measured vignetting for all of them (at analysis section). Pick some wider lens (f/1.8 or f/2.8) to see worst cases. And pick some short and longer lenses to compare. That's just how life works. :)

Adobe Camera Raw has a Lens Profile tool that knows extremely many lenses, and will instantly correct and fix this with one click.