Distortion in lenses?

Hey all,

Sick of me yet? Hope not!

I have a question regarding lens distortion. Now, I'm an avid user of Canon lenses (cos they came with the camera hehehe) but I've been told they distort quite a lot, and also have a fair bit of vignetting.

So, distortion, obviously I know what distortion is, but what does it look like? I've heard all kinds of different distortions... ISO distortion, barrel distortion, lens distortion, etc, but whats the difference and how can I tell?

Now, I've taken photos in RAW and put them in Lightroom and used "lens correction" for the lens I was using and the result was a "less curved" image. I honestly didn't see any curves before, but after using the correction I could see it.

So, vignetting is darkening towards the edges, but what is barrel distortion? I'm not expecting the lenses to be free from distortion, but I'd like to be able to learn to pick it so I can improve etc.

Generally, if someone uses the term 'distortion', they're talking about 'lens distortion', which can be 'barrel distortion' or its slightly less annoying cousin, 'pincushion distortion', both of which are the (in)ability of the lens to take photos of straight lines and keep them straight.
For almost all reviews here, Bryan's got graphs of how distorted each lens is.
For an extreme example of 'barrel distortion', look at the Samyang 14mm or the 28-300L @ 28mm. Barrel happens most on wide-angles, zooms or primes. Fisheyes do it too, but on purpose.
Pincushion is the reverse effect, lines bend the other way, it's not nearly as common or annoying (to me). Normally only happens at the long end of superzooms, check out the 28-135 @ 135mm for an example.

Vignetting is darkening of corners. As a generalisation, it's normally worse on wide-angle and or wide-aperture lenses, but it can happen anywhere. An example I'm familiar with is the wide end of the EFs 15-85. Closing the aperture down (narrower, higher f/ number) can reduce most problems. I've never heard vignetting called 'distortion' though, it's just 'vignetting' or 'corner darkening' to me.

Also, i've never heard of 'iso distortion'. Does that mean 'high iso noise' maybe?

And I wouldn't just generalise on brand, each brand has its good and bad lenses, some better than others. The EFs 15-85 has really bad vignetting and barrel distortion at 15mm. But other than that, it's in the top 2 of Canon EFs lenses in that focal range. Vignetting can be fixed in DPP ('peripheral illumination correction'), and you've seen that you can fix distortion too in the 'lens correction' bit too.

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Originally Posted by Squidy

I have a question regarding lens distortion. Now, I'm an avid user of Canon lenses (cos they came with the camera hehehe) but I've been told they distort quite a lot, and also have a fair bit of vignetting.

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Squidy are these the same folks that were telling you that your crop camera is "junk"?

I think Dr. gave you good descriptions. No doubt you will get a few more.

But really being told that "Canon Lenses" distort quite a lot, isn't an accurate statement. No doubt some lenses that Canon make that might fall in the "quite a lot" of distortion catagory. But there are "quite a lot" of lenses that Canon makes that do not.

Nah, different people. Also I'm reading a lot of different articles/reviews etc trying to learn about F/ and distortion seems to pop up often.

Thanks heaps Doc, that's cleared it up quite a bit

Welcome to the wonderful world of photo - it is somewhat like boats - there is always a bigger, faster, etc. Like boats some of my best boating memories are in a $200 aluminum canoe gliding gracefully along a river with a cooler full of my favoriate drink. 2nd was the "ride along" an a multi zillion dollar America's Cup Racing yacht - $$ had nothing to do with my enjoyment.

Each lens and boat has a purpose - I suppose I could take my fishing gear out on the America's Cup yacht but somehow I don't think it would be as effective as in the canoe. Likewise, I could get the $12,000 telephoto (or the multi-million Leica tele that is mounted on a Mercedes 4 wheel drive) and take a portrait with it, but likely would need a cell phone to talk with the subject.

Here is the primary test - look at the picture. Do like it? Y/N.

Composition is going to be the 1st thing you notice and that is completely independent of equipment. Effectively tied with this is exposure, you have alot of flexibility in exposure compared to just a few years ago.

2nd is going to be subject matter it self - a cute baby pic vs. a grand vista of Mt. Somethingorother.

3rd is focus (this order might be debatable) and how crisp/clear the subject is. Again, at size of enlargement bill board, computer monitor, or snap shot you intend for it to be viewed by others.

At this point 99.9999% of the people of passed the snap shot on or have clicked "next" in the slide show.

For the .0001% starts the critical evaluation.

Through the composition, exposure, focus elements the gear you have will take you a long way.

Experience and practice will improve these elements as quickly as anything else - for inspiration cruise through some older national geographic magazines - they shot film, no auto exposure, no auto focus. And just about all the lens today are better than the ones used then. You will find some pretty amazing pics.

Have fun,

Yeah that's what I was thinking. I've noticed the lines thing on some of my friends shots, it looks like when you don't have your AA/AF turned up high enough on a game etc.

I know some people don't care about it etc but I guess I just wanted to know how to spot it soi can become better etc. I haven't noticed it on my lenses so far, but the only time I use full zoom is for portrait shots etc