jbl Posted January 24, 2013 Share #1 Â Posted January 24, 2013 Advertisement (gone after registration) I have a couple of lenses 50mm or longer that don't have a magic number on them. Does this mean they're exactly on the focal length? Â One of them is a Noctilux, so I just figured they had zero tolerance on that lens, but I wasn't ever sure. Â -jbl Link to post Share on other sites More sharing options...
Advertisement Posted January 24, 2013 Posted January 24, 2013 Hi jbl, Take a look here magic number question. I'm sure you'll find what you were looking for!
jaapv Posted January 24, 2013 Share #2 Â Posted January 24, 2013 You have discovered Russian fakes! Link to post Share on other sites More sharing options...
01af Posted January 24, 2013 Share #3 Â Posted January 24, 2013 I have a couple of lenses 50 mm or longer that don't have a magic number on them. Does this mean they're exactly on the focal length? No. Â Â One of them is a Noctilux, so I just figured they had zero tolerance on that lens ... It's not 'tolerance' ... as in, some arbitrary, random deviation from the target value. Instead it's a design decision, and all lenses from one production batch will stick to the chosen value very strictly. The chosen value may vary by one or two tenths of a millimeter between production batches, but not as a 'tolerance' but to deliberately adapt to variations in production conditions, such as availability of materials or machinery. Â According to Leica's data sheets, the Noctilux-M 50 mm 1:1 is 52.4 mm and the Noctilux-M 50 mm 1:0.95 Asph is 52.3 mm. Link to post Share on other sites More sharing options...
Bill W Posted January 25, 2013 Share #4 Â Posted January 25, 2013 Speaking of design adn the way Leica does things. Years ago I had an R lens which I believe was a 250 (might be wrong) but I bought it used. I noticed that it would focus beyond infiniti. I had used Nikon equipment beofre this and they stopped exactly at the infinit mark. I inquired with a dealer and Leica and they said it was normal and just the way Leica designed the lens. I believer a lot of their R telephoto lens designs were this way. Good thing was that they took fantastic photos. Link to post Share on other sites More sharing options...
jbl Posted January 25, 2013 Author Share #5 Â Posted January 25, 2013 Canon lenses go to infinity and beyond. I've never really understood why but I feel like I heard it had to so with humidity. Thinking about it now, I have no idea. Â -jbl Link to post Share on other sites More sharing options...
jaapv Posted January 25, 2013 Share #6 Â Posted January 25, 2013 Metal expands when it gets hotter, so at high temperature the lengthening of the barrel will shift the focus, most noticeable on long lenses. Thus many long lenses of many brands can be focussed past infinity. It is even the rationale behind the white colour of Canon tele lenses. Link to post Share on other sites More sharing options...
Michael Geschlecht Posted January 26, 2013 Share #7 Â Posted January 26, 2013 Advertisement (gone after registration) Hello Everybody, Â Another reason for having the focus throw go beyond Infinity with longer lenses is: Â A person using focussing aids then has a longer focus throw in both directions when rocking back & forth just short of infinity. This helps them to more precisely determine the point of best focus at this distance. Â Best Regards, Â Michael Link to post Share on other sites More sharing options...
Bill W Posted January 26, 2013 Share #8 Â Posted January 26, 2013 All of the reasons make sense now. I guess I did not consider expansion affecting focus. Link to post Share on other sites More sharing options...
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