180mm? Which one really?

[SVP]

Angenieux P21 180f/4.5 (wide-open):


Voigtländer Apo-Lanthar 180f/4 SL (wide-open):


Both pics shot from the very same viewpoint / same camera-to-subject distance / with the same camera (Nikon D2H).
Ok, due to the crop factor, the f.l. of both effectively is 270mm instead of the nominal (FF) 180mm, but my point is: why the P21 appears to be of longer f.l. than the Apo-Lanthar? Which one really is 180mm? Any relevant data available?

[iangreenhalgh1]

They look very close in length to me. It would be unusual for a 180mm lens to be exactly 180mm. They all differ, even in the same batch. Often makers used to write the precise f.l. on the optical block, for instance, I have a Wollensak 540mm lens that had 544.21 written on the block, so it's a bit longer than the specification but still within tolerance. So if one of your lenses is, let's say, 184mm and the other 178mm, on your 2x crop camera, there is a 12mm difference.

[sichko]

Another possibility is so called "focal breathing" - variation in 'focal length' with subject distance - which might be different for the two lenses. See, for example : http://www.bobatkins.com/photography/technical/focus_breathing_focal_length_changes.html

[ForenSeil]

+1 for both Ian and sichko
You can be sure that both are not exactly 180mm

Most makers have wide tolerance when engraving focal length on the lens; Saying "200mm" simply sounds better than "197,5mm" etc.
Same with 50mm and other lenses, many are actually 52mm or even 48mm and so on
It's an unwritten rule that you can round up/down the focal length in 5mm steps above 35mm focal length if you want, that's why 42mm, 46mm, 51mm, 56mm, 83mm and so on lenses don't exist, they are sold as 40mm, 45mm, 50mm, 55mm and 85mm.

Same (but less excessive) is often made with F-Stop numbers, for example Minolta MD 50mm F1.4 is actually F1.45 and 51.2mm if I remember correctly.

Difference between internal focusing- and external focusing lenses can be very huge aswell; is one of them focusing without extension? Focal lengths engraved are usually measured at infinity and they can change while focusing.

PS: Also with ultra wide angle lenses lenses can have the same focal length but very different effective visual angle due different types of projection (gnomic vs fisheye projection etc.); it's not always easy to measure the exact focal length here.

PPS: Especially with ultra large aperture lenses exactly the same model of a lens can have slightly different focal lengths due slightly different temperatures while grinding the lenses and mirrors etc., that's normal also for first party makers. I heard of cheap chinese mirror lenses which can have more than 10% tollerance in focal length within the same lens model.

PPPS and slightly offtopic: Also with some ultra large aperture lenses (like my Celstron C8 ) you can see the focal length and the focus very slowly changing while cooling or warming the lens when taking it outdoors, as the sides of the mirror are shrinking more than the center, making a slightly shorter focal length.

[cyrano]

What one do you like?

[hifisapi]

Nothing can touch my Tamron SP LD IF 180mm F2.5 limited edition lens. its simply awesome.

[SVP]

Many thanks for the replies.
I'm aware of the deviations between real (precise) f.l. and the nominal (declared) one, hence my request for the lens data if available; I simply didn't expect the difference to be that obvious (ok, I admit the 1.5x crop factor helps boosting the effect to some extent). I still wonder which one of the two is closer to the nominal 180mm ...

[hifisapi]

some lenses, especially IF lenses, vary in focal length depending on focus distance.