Floating element: optical effect (FL,f)?

<p>Pure curiosity here: previous posts have explained that a normally focusing lens increases effective focal length slightly when focused close (while an internally focusing lens changes its effective f/stop, I forget which way). So what then is the effect when a floating element is added for close focus? Would such a lens (like the 75/2 M) experience a greater or a lesser change, compared to one (like the 75/1.4) that had none? (The possible relevance here being accuracy of framelines.)</p>

 

<p>Eric I don't know if this is true from a working stand point but it may very well be optically. Ive used both lenses on a tripod and never saw it on the neg. but with all the other factors with a rangefinder cameras mask system, I don't even know where to start. Damn fine lenses for a great camera system so I live with its limitations, hell the m system deals with my limitations so we'll call it even. :>)</p>

<p>I think that a normally focusing lens changes both it's effective focal length and effective f-stop while internally focusing lenses may be more like front-cell focusing lenses on old folder cameras - these change <em>nominal </em>focal length and aperture, but retain their effective value. Then, lenses with floating elements still move their whole lens block, so I assume their effective focal length changes too? I don't know if there is a sharp definition between "internal focus" and "floating element" but these phrases probably indicate different design goals - one goes mostly for ease of autofocus and weatherproofness, the other mostly for optimum sharpness at all distances.</p>

<p>If I understand correctly, lens designers use floating elements, usually movable elements at the rear of the lens, to improve the close-range performance of lenses that would otherwise suffer a bit from limitations of the lenses' basic design. In other words, a lens with floating elements should offer somewhat better optical quality at minimum focusing range than an otherwise comparable lens without floating elements. At longer focal ranges, there would be little if any difference in performance. I doubt that the use of floating elements has much if any effect on the accuracy of viewfinder framelines, though. </p>

<p>Internal focus is different. That is when designers use the internal movement of movable elements, usually in the middle of the lens, to focus the lens at all focal ranges, rather than moving all lens elements or the front lens elements. In addition to helping keep lenses weatherproof, it may help to maintain the balance and handling qualities of large, heavy telephotos.</p>