Optical node of a 17-40 f4.0 at 17mm?

[aaron d]

<p>Anybody know off-hand where the optical node is for the 17-40 when set at 17mm? I have the lens, but the camera is STILL back-ordered, else I'd measure for myself. I will need to be doing panoramas pretty soon and thought I'd try to order the parts from Really Right stuff, but they have two rail lengths. The short would stay out of the photo, but might be too short to position the camera correctly. Other lens I'd use is a 50mm, but I think it would be fine with either...<br>

Thanks in advance!<br>

-A</p>

[evphotography]

<p>It depends on what body you ordered. Is it a FF sensor, then it doesn't perform nearly as well at 17mm, it is pretty soft in the corners. On cropped sensor body, then it performs really well. I found that lens performs best at 20-28mm when stopped down to about f/8-13 on FF, but with cropped sensor body it performs really well at all focal lengths.</p>

[anders_carlsson]

<p>Agree with previous poster. Only to add the obvious: sharpness is probably less of an issue than distortions. The 17-40 barrels quite a lot at the wide end, even on a crop body.</p>

[john_bellenis]

<p>The only way to find the nodal point for sure is to wait for the body to arrive and do a test on the tripod. The nodal point will change with the focal length. If you could borrow a body you can find it with less than one minute of testing (I am assuming you know how to find the nodal point once you have your camera body).<br>

<br /> I would advise against using the 17mm due to the distortion and to use your 50mm vertically - you will need more frames obviously, but that's a good thing. Also, once you set your tripod for the nodal point of the 50mm you can just leave it alone - unlike with a zoom lens. If you are unsure of which rail to buy, I would get both as soon as possible and return one of them the day your body arrives.</p>

[aaron d]

<p>Thanks, guys - this is good advice. The 50 will be the main lens I use (on a FF 5Dii) but I thought the 17 might be good in a pinch for interiors if they come up. I'll do some sperimenting...</p>

<p>-A</p>

[martin-s]

<p>I partly agree with John.<br /> As to the distortion you might want to get a copy of either <a href="http://www.kekus.com/software/plugin.html">LensFix</a> or <a href="http://www.epaperpress.com/">PTLens</a> – depending on your platform – to correct for it.</p>

[joseph_wisniewski]

<blockquote>

<p>Only to add the obvious: sharpness is probably less of an issue than distortions.</p>

</blockquote>

<p>I love it. No less than three people warning about distortion.</p>

<p>Aaron, don't worry about the distortion. Any real panorama software (basically, anything other than PhotoShop's limited built in stitcher) corrects distortion. The "PTLens" program that Martin recommended is based on the "Panorama Tools" engine (that's what PT stands for) using just the lens correction ability of the popular Panorama Tools suite. If you're using a Panorama Tools GUI (Hugin, PTAssembler, PTgui, PTmuc) then just make sure distortion a, b, and c are all "checked" before stitching, and "poof". On wides, I typically select a-e.</p>

[aaron d]

<p>I'm going to try the cheap route first and see where that gets me - Canon's own panostitch. It does OK without node-centered shots but with the foreground/background mismatch. It fixes distortion really well and I imagine sharpness will be plenty enough with a dozen huge files stuck together....</p>

<p>Thanks again all!</p>

<p>-A</p>