Canon EF 17-40mm f/4.0 L corner distortion

[paulo_cortez]

Hi,

 

Bought recently a 2nd hand Canon EF 17-40mm f/4.0 L and after testing it with a

400D / Rebel XTi, I'm considering taking it to a Canon service. The reason for

that is I'm getting IMHO a lot of corner (barrel?) distortion, even at 40mm and

with small apertures (please check the crop - bottom right, 40mm, 1/400Sec,

F11, ISO 400).

 

My fear is that I bought it for a 5D and as a full frame, I'm expecting a worse

scenario.

 

What do you think? Please tell me something if more crops needed.

 

Thanks for your opinions,

 

Paulo

[shambrick007]

Many quesitons...<p>

What apeture, and what shutter speed? What focal length?<p>

What do you consider the problem to be? The the tilting vertical lines? That's not barrel distortion, that comes from not holding the lens level or perpendicular to the subject? <p> Let's see the whole shot resided for the web.

[steve_dunn2]

<p>You should get significant barrel distortion at the wide end and modest pincushion distortion at the long end; in this regard, my 17-40 functions very much like <a href="http://www.photozone.de/Reviews/Canon%20EOS%20Lens%20Tests/177-canon-ef-17-40mm-f4-usm-l-test-report--review?start=1" target="_blank">the one in this test</a>. As Mr. Hambrick points out, your sample image does not demonstrate either of these; it demonstrates that you weren't shooting straight and level. For some tips about how to shoot valid tests of curvilinear distortion, look at the Target and Bad Target pages within the Calibration section of <a href="http://epaperpress.com/ptlens/calTargets.html" target="_blank">this site</a> (I'd link to them directly but the site is crafted to discourage this).</p>

[g dan mitchell]

<p>Actually, your sample looks pretty good for the corner of an 17-40. No Canon wide zoom that I'm aware of is remotely _free_ of these issues, though some or

better is certain ways (and perhaps weaker in others).

 

<p>Pincushion/barrel distortion (inward or outward "bowing" along the edges) is par for the course on these lenses, even if you carefully level the camera and

shoot a subject in a flat plane. In many cases it is not an issue but if it is you can compensate effectively in post-processing with little or (more likely) no visible

compromise in your prints.

 

<p>You didn't mention <i>perspective distortion</i>, but that what creates the <i>slanting</i> (as differentiated from the <i>curved</i> from

barrel/pincushion) lines in this image. That is an unavoidable effect of shooting a very wide angle lens, and it is especially noticeable when the camera is not

horizontal and/or the subject is not in a plane parallel to the sensor - as is the case in your example. You can reduce this effect by leveling the camera and

centering the subject in the frame, but you cannot entirely eliminate it without special purpose lenses. (The <i>tilt/shift</i> lenses let you compensate for these

effects.)

 

<p>The 17-40 can also exhibit softness in the corners, especially if you shoot it at its larger apertures. Try a shot at f/4 and 17mm to see what I mean. In some

types of photographs this isn't an issue - for example a photo of people with a strong central focus and little of significance in the corners. If you want to diminish

that issue shoot the lens at smaller apertures. In general this might mean trying to shoot at about f/8 on a crop sensor body, though you could go a bit smaller -

say f/11 - and give up only a bit of general sharpness to diffraction while picking up some corner sharpness. On a full-frame body you can stop down more

before diffraction becomes a concern. I don't hesitate to shoot my 5D with this lens at f/16 and I'll shoot at f/22 if the overall effect improves the image.

 

<p>Like virtually all lenses, the 17-40 will vignette in the corners at wide apertures, though I don't think this is your concern.

 

<p>In answer to your concerns about shooting this lens on FF, in my experience it is a "better" lens on FF than on crop. First, it is a true ultrawide in the FF format

while it is merely "wide" on crop. Second, while I could never reduce the softness in the far corners quite enough on a crop body shot at f/8, I can do so on FF shot

at f/16. If you are fine shooting stopped down this is a great FF lens. If you need to shoot wide open a lot you might prefer the 16-35mm f/2.8 L . That lens is

much stronger wide open, though it is a lot more expensive and will not be better than the 17-40 stopped down.

 

<p>Dan

[PuppyDigs]

The crop looks pretty sharp for a pixel-level crop, even in the right corner. I can't see any

barrel distortion. The image is seasick and suffering some perspective distortion. However perspective distortion is not a a lens defect. It's caused by the technique of the photog...

[paulo_cortez]

First, thanks a lot for your ALWAYS very helpful replies, in particular to Dan.

 

As the subject wasn't the building you see in the crop but the roof of an old railway station (in the center), I didn't worry with the building's perspective, therefore the referred "perspective distortion".

 

I chose that picture because of its settings (40mm, 1/400Sec, F11), less prone to distortion, at least theoretically. I would like to have other picture posted but I've already sold my 400D and still waiting for the 5D.