Real focal length of 17-40/4L lens

<p>Do you know what is real focal length of 17-40 lens? Is it really 40 on the long end or maybe more 38 or 42?</p>

<p>The FL stamped on the lens is usually a nominal measure, not an exact measure, so you're probably right, but not necessarily so. So of the test labs actually measure the "true" FL. You might dig around to see if you can find a review where they did that. </p>

<p>Is it important for some reason?</p>

<p>http://www.photozone.de/canon_eos_ff/427-canon_1740_4_5d<br /> will give you necessary information. Somewhere the mostly meaningsless (Swedish:meningslös, as Jons puts it in the <em>Seventh Seal</em>) actual focal length will be found somewhere on the "internets".</p>

<p>I would only guess too that atmospheric and other external conditions make make the true FL of any lens ever so slightly variable.</p>

According to this Lens Rentals article, the 17-40 F4L has an actual focal length of 17.5mm to 39mm. This is

described about 1/4 down the page at the beginning of the section named "MTF Charts". Hope this is helpful.

 

https://www.lensrentals.com/blog/2014/07/canon-wide-angle-zoom-comparison/

<p>I may be wrong here, but I believe I've read that the industry standard is that the actual focal length of a lens should be within ±5% of the advertised focal length.</p>

 

<p>Also, keep in mind that focal length is measured with the lens focused at infinity. The effective focal length can change (significantly in some cases) as you focus on subjects at closer distances.</p>

<p>Thanks guys for your responses. Thanks Kenneth for your link. I just need to shoot with a lens around "perfect standard" and this one is going to work.</p>