Digital Camera World » camera distortion

Vignetting: quick fixes and how to avoid it entirely

jmeyer — Wed, 24 Oct 2012 10:30:58 +0000

If the corners or edges of your images sometimes appear much darker than the rest of the frame, vignetting is probably the culprit. Vignetting is aberration that occurs when less light reaches the corners of the camera’s sensor, and in the resulting image these areas are more under-exposed (or darker) than the centre.

Vignetting is common with certain lenses – especially wide-angle zooms – and when using wide apertures. It can also be caused by a physical obstruction, most typically a filter, filter holder  or ill-fitted lens hood, which prevents light from reaching the corners of the frame.

Our image before taking steps to correct vignetting

If you’re having problems with vignetting, here are some solutions.

How to correct vignetting

Reduce the aperture
Vignetting is most apparent when shooting at wide apertures such as f/4 or f/5.6, so the easiest way to reduce it is to set a smaller aperture of, say, f/8, f/11 or f/16. But be aware that the shutter speed will become slower, so you may need to increase the ISO or use a tripod to prevent camera shake.

Take care with filters
Wide-angle lenses are prone to vignetting, and it’s particularly noticeable with filters fitted. Use thin screw-in filters and remove UV filters. Remove filter holders and hold the filter flush against the lens instead. Avoid the widest focal length of wide-angle zooms.

Fix vignetting in Photoshop
If you can’t avoid vignetting as you shoot, use Photoshop or other editing software to fix the problem. Shoot in raw and process in Adobe Camera Raw, using the slider in the Lens Profile Corrections panel to remove vignetting. If vignetting is bad, you may need to crop the image too.

Lens distortion: everything every photographer must know

jmeyer — Fri, 12 Oct 2012 10:30:30 +0000

However good your lens may be, you need to know how to spot (and correct) lens distortion. In this post we will explain everything photographers need to know to get the best results from their lenses. We’ll start by answering some of the most common questions about lens distortion, then look at several examples of lens distortion and, finally, try to cut through some of the technical jargon and put some of these concepts into layman’s terms.

Are you suggesting my pricey lens isn’t perfect?

All lenses have optical defects, and create images that aren’t perfect copies of the subjects they’re pointed at. However hard manufacturers try to create flawless optics, there’s simply no way to manufacture a lens that doesn’t suffer from distortions and aberrations to some degree.

So if I pay more money for my lens, I will get a less distorted picture from it?

Cost isn’t necessarily a quality indicator. The amount of distortion your lens suffers is largely dependent on the type of lens and its construction. Price plays a role, but factors such as the focal length are just as important.

The wider angled the lens, the harder it is for straight lines not to appear curved, for instance. Zooms are more prone to distortion than primes – simply because it’s impossible to correct for aberrations at every focal length.

This is no guarantee that a prime lens is perfect – but it’s true that the greater the zoom range (such as with superzooms) the more noticeable these distortions become.

I’ve never noticed any problems with my lens…

That may well be true for many of us. The thing is that lens design has improved tremendously in recent years, and the unforgiving precision of the latest digital sensors has forced the progress in lens design to accelerate. With a good digital SLR and a reputable lens, these distortions are subtle – but they are there.

We’ve never had it so good, then?

Definitely! But something as basic as darkening at the corner of the image is still a problem with modern pictures, just as it was for the Victorian photography pioneers. It’s just that this vignetting effect is not quite so marked these days. In fact, we tend to accept that pictures will be slightly darker at the edges – so we don’t even notice it (and some make their corners darker in Photoshop to exaggerate the effect). Take a picture of an evenly lit white surface, look at it carefully on your computer, and you’ll soon see if it’s brighter in the middle. This darkening effect can be eliminated using custom settings on certain cameras, or by using standard image-editing software.

How many different types of optical distortion are there?

There are dozens of these defects, from astigmatism right the way through to coma, but there are two or three that are particularly worth paying attention to.

Start me off with the easiest to understand, then…

We’ll kick off with curvilinear distortion. This comes in several different types, but the one you’ll see most commonly is barrel distortion. This is really easy to spot when you use an ultra-wide lens, and causes straight lines at the edge of the frame to bow outwards. The effect is even more obvious on a fisheye lens, where these distortions are left uncorrected by the designers in order to get the widest possible field of view.

What other curvilinear distortions are there?

Pincushion distortion is often seen on long telephoto lenses – and causes lines to bend inwards. The effect is usually subtle, and isn’t normally noticeable unless you’re photographing rectangular subjects straight on. Some zooms can show signs of moustache distortion – where one image can show both pincushion and barrel distortion. It’s most commonly seen with wide-angle zooms, and causes straight lines to appear wavy.

What else should I watch out for?

Chromatic aberration is probably the biggest bugbear of the modern SLR photographer. As we zoom in to our pictures, the tell-tale colour fringing that this causes is far more noticeable than back in the days of film (when it would only be seen in big blow-ups).

Where am I most likely to see chromatic aberration?

It affects lenses of all focal lengths, but will be more pronounced on extreme focal lengths, and with less expensive lenses. It’s also worth looking at lens tests for this phenomenon because it affects some models more than others. You’ll see it at the edges of subjects, and towards the edges of the image. It’s easiest to see where you have a white line crossing a darker area – window frames in your picture are a good place to look.

Can I do anything about it?

Yes, it can be fixed during editing. Your camera may even come with a program that will help you correct the problem. Photoshop CS has some good tools for minimising its effect on your pictures. Elements 8 users are not so fortunate, but separate distortion-correcting utilities are available. PTLens is well-regarded, can be tried for free and costs just $25.

PAGE 1: Common questions about lens distortion
PAGE 2: Types of lens distortion
PAGE 3: Why does chromatic aberration occur?
PAGE 4: Learning the lingo

What is chromatic aberration: free cheat sheet

jmeyer — Fri, 06 Apr 2012 11:00:46 +0000

Chromatic aberration has been the bane of every photographer at one time or another, with magical moments you thought you captured ruined by an unsightly halo of colour. But what exactly causes this phenomenon?

In the latest of our photography cheat sheet series of free infographics, we’ve put together this easy guide for understanding chromatic aberration. Simply drag and drop this free chromatic aberration definition and illustrated guide on to your desktop and keep it for a handy reference!

Download More Photography Cheat Sheets

Free f-stop chart: master your aperture
Photography Basics: understanding exposure
Full frame vs crop-factor lenses

How to correct leaning buildings in Photoshop Elements

jmeyer — Thu, 08 Mar 2012 07:00:11 +0000

Before image. Note the distortion in the walls.

Do you often find leaning buildings in your architectural photos? The most likely cause of this distortion is shooting a building from too close a distance. When shooting too close, you’ll need to zoom out to a wide-angle focal length, which can result in noticeable barrel distortion, making the top, bottom and sides of a building appear to bow outwards.

You’ll also need to point the camera upwards, which accentuates perspective and makes the sides appear to lean in towards each other towards the top. The solution is to keep your distance so you can use a moderate zoom setting of around 35mm (50mm on a full frame DSLR).

For photographing tall buildings, it’s ideal if you can find a vantage point that’s higher than ground level – another building, for example. You could apply perspective correction when editing – using Photoshop Elements, for example – but you’ll lose part of the frame, so leave plenty of space around the building when shooting.

But then, we all know this isn’t always possible. For those instances when you just can’t shoot it any other way, here is how you can correct distortion on the computer using Photoshop Elements.

How to correct barrel distortion in Photoshop Elements

Step 1: Run the filter
Open your image in Photoshop Elements and make any exposure, colour and sharpening corrections. Head to the Filter pull-down menu and select the Correct Camera Distortion option.

Step 2: Remove distortion
Tick the Show Grid checkbox to make it easier to line up horizontals and verticals, then adjust the Angle control to level the horizon. Use the Remove Distortion slider to cancel out any barrel distortion.

Step 3: Perspective control
Now correct perspective errors using the Vertical Perspective or Horizontal Perspective slider. Don’t use the Edge Extension option, but crop the image afterwards as necessary.

Final image.

Correct camera distortion in Photoshop Elements

dcworld — Wed, 02 Mar 2011 12:29:02 +0000

Learn how to remove lens distortion from your photographs using Photoshop Elements

Digital Camera’s 10-part series ‘Teach Yourself Photoshop’ builds into a complete video reference library. These easy-to-follow video guides will take you on a start-to-finish journey through perfecting your photos in the digital darkroom. In this video you’ll find out how to use the Correct Camera Distortion interface to remove distortion in your photographs for more realistic perspectives and angles. Click the links to the right and build up the complete collection with each new issue of Digital Camera.