cirke Posted November 25, 2015 Author Share #21 Posted November 25, 2015 Advertisement (gone after registration) there is nothing written on the new 50 apo summicron Quote Link to post Share on other sites More sharing options...
Advertisement Posted November 25, 2015 Posted November 25, 2015 Hi cirke, Take a look here written on the side. I'm sure you'll find what you were looking for!
gyoung Posted November 26, 2015 Share #22 Posted November 26, 2015 Is that when focused at infinity? If I remember my optics from the course donkey's years ago correctly, its part of the definition of focal length that it is measured at infinity. So it cant be any different simply because its not focused at infinity. Gerry 1 Quote Link to post Share on other sites More sharing options...
Peter Branch Posted November 26, 2015 Share #23 Posted November 26, 2015 (edited) Hello Exodies, Engraved focal lengths are for the lens focused at Infinity. Regardless of Brand/Format. Best Regards, Michael It does not matter where this lens is focused - its focal length remains constant. It is not a floating elements design which may vary its focal length, slightly, depending on the focus setting. The figures, such as 57 which in this instance implies a focal length of 135.7mm to the nearest 0.1mm, are reportedly used to adjust the profile of the cam which connects to the rangefinder. Edited November 26, 2015 by Peter Branch Quote Link to post Share on other sites More sharing options...
lct Posted November 27, 2015 Share #24 Posted November 27, 2015 Ever heard of lens breathing?http://bobatkins.com/photography/technical/focus_breathing_focal_length_changes.html 1 Quote Link to post Share on other sites More sharing options...
pico Posted November 27, 2015 Share #25 Posted November 27, 2015 (edited) Thanks, lct, and others for the needed corrections. This issue has gone around in other threads for years. Focal length changes as one racks out the lens to closer distances, the f-stop diminishes as well, although for most of our purposes neither really matter. One thing never settled was brought up when I posted a picture of the scale on my 75mm Summilux. (The 00 under the t in feet.) Does that suggest the 75mm is perfectly 75mm? Someone suggested that the engraving standard was not followed in some Canadian made lenses. I still do not know. Edited November 27, 2015 by pico Quote Link to post Share on other sites More sharing options...
lct Posted November 27, 2015 Share #26 Posted November 27, 2015 (edited) Yes i must have a couple of lenses like that. A true 75mm in your case. Now many lenses have not those engravings: 21/3.4 asph, 24/3.8 asph, 35/1.4 FLE, 35/2 asph, 35/2.5, 50/2 apo, 50/2.5 and probably others. Edited November 27, 2015 by lct Quote Link to post Share on other sites More sharing options...
pop Posted November 27, 2015 Share #27 Posted November 27, 2015 Advertisement (gone after registration) Focal length changes as one racks out the lens to closer distances, the f-stop diminishes as well, although for most of our purposes neither really matter. Focal length does not change by moving the lens as a whole forward and backward. Focal length is the distance from the lens to where all the light rays meet that have entered the front of the lens in parallel to the optical axis. This property is independent of the position of the lens and does not directly refer to the distance from the lens to the image of an object in front of the lens. The aperture, OTOH, is expresssed as the ratio between the diameter of the diaphragm and the image distance. It willl, of course, grow less with increasing image distances. Taking a photograph at the scale of 1:1 the image distance is twice the focal length, hence the aperture is only half the value it has at infinity. It lets pass a fourth of the light. That's quite noticeable, onless you measure the exposure through the lens when many photographers tend to ignore it because it's all accounted for. Quote Link to post Share on other sites More sharing options...
Exodies Posted November 27, 2015 Share #28 Posted November 27, 2015 Does anyone publish charts of distance and focal length for lenses? Perhaps not very important wrt the image, but fascinating non the less. How much does it change for a 50mm lens? Quote Link to post Share on other sites More sharing options...
pop Posted November 27, 2015 Share #29 Posted November 27, 2015 Does anyone publish charts of distance and focal length for lenses? Perhaps not very important wrt the image, but fascinating non the less. How much does it change for a 50mm lens? Sorry, I don't understand: how much does what change for a 50mm lens? Quote Link to post Share on other sites More sharing options...
lct Posted November 27, 2015 Share #30 Posted November 27, 2015 Field of view depends on focus for most if not all lenses i guess. Wider at infinity in my experience. Easy to check with any TTL camera. Quote Link to post Share on other sites More sharing options...
pop Posted November 27, 2015 Share #31 Posted November 27, 2015 Field of view depends on focus for most if not all lenses i guess. Wider at infinity in my experience. Easy to check with any TTL camera. Easy to check with any camera, but cheaper with a digital one and more interactive with a TTL one. As a matter of fact, the effect is quite easy to understand. Stand in a room at some distance from the window, preferably a narrow one. Take a step towards the window and you see more of the landscape outside. Take a step backwards, away from the window, and you see less of the landscape. As the lens travels closer to the sensor or film when set to infinity, the FOV will be wider. Always. This applies to lenses without any floating elements, of course. Quote Link to post Share on other sites More sharing options...
gyoung Posted November 27, 2015 Share #32 Posted November 27, 2015 For once Wikipedia has it clear:- "When a photographic lens is set to "infinity", its rear nodal point is separated from the sensor or film, at the focal plane, by the lens's focal length. Objects far away from the camera then produce sharp images on the sensor or film, which is also at the image plane." Field of view will change when focussing closer than infinity, but focal length does not, it is by definition measured at infinity. I doubt focus breathing is in Cox's optics, a modern term derived from a poor understanding of the subject IMHO Gerry Quote Link to post Share on other sites More sharing options...
Exodies Posted November 27, 2015 Share #33 Posted November 27, 2015 What is the focal length of a 50mm lens at its closest focus distance Quote Link to post Share on other sites More sharing options...
gyoung Posted November 27, 2015 Share #34 Posted November 27, 2015 What is the focal length of a 50mm lens at its closest focus distance 50mm, see above, focal length does not change with focus distance. Gerry Quote Link to post Share on other sites More sharing options...
lct Posted November 27, 2015 Share #35 Posted November 27, 2015 (edited) Because focal length is measured at infinity. At 0.7 metre the FoV of a 50mm lens must be around 60 or 75mm depending on lenses i guess. Here Elmar 50/2.8 on Fuji X-E2 (0.7m & infinity). Welcome, dear visitor! As registered member you'd see an image here… Simply register for free here – We are always happy to welcome new members! Edited November 27, 2015 by lct 1 Quote Link to post Share on other sites Simply register for free here – We are always happy to welcome new members! ' data-webShareUrl='https://www.l-camera-forum.com/topic/253086-written-on-the-side/?do=findComment&comment=2938137'>More sharing options...
jdlaing Posted November 27, 2015 Share #36 Posted November 27, 2015 Because focal length is measured at infinity. At 0.7 metre the FoV of a 50mm lens must be around 60 or 75mm depending on lenses i guess. Here Elmar 50/2.8 on Fuji X-E2 (0.7m & infinity). FoV_xe2_5028_1m_web.jpg FoV_xe2_5028_inf_web.jpg Focal length is measured from the point of convergence inside the lens to the film/sensor plane. Quote Link to post Share on other sites More sharing options...
pico Posted November 27, 2015 Share #37 Posted November 27, 2015 50mm, see above, focal length does not change with focus distance. Gerryter The definition of focal length is certain, however I think what matters to us is how a lens behaves when focusing on distances less than infinity. As near focused, angle of acceptance diminishes as if it were a longer lens, and that's what matters. What does not matter is that the book definition remains unchanged. Quote Link to post Share on other sites More sharing options...
Peter Branch Posted November 27, 2015 Share #38 Posted November 27, 2015 (edited) This thread has two distinct subjects; the meaning of the numbers on the lens and what does the result actually mean? Regarding the engraved numbers this has been the subject of numerous threads and is extensively covered by E. Puts in his books. There are two things to note firstly that the conventions are, in some instances, different for different focal lengths . Secondly that Leica Germany used a different convention from Leica Canada. Setting aside the complication that lenses with moving elements may slightly change their focal length as the focusing mount is moved a given lens has a fixed focal length. Focal length is defined in terms of image magnification and follows the rule that (1/u) + (1/v) = (1/f). Where "u" is the Object distance, "v" is the Image distance and "f" is the focal length. All measured in the same units. If "u" is Infinity then (1/u) is zero, thus (1/v) = (1/f) so "v" = "f". Where people seem to be getting confused is to equate "v" with "f". This is only true when the object in focus is at Infinity. In practice since computers don't like working with Zero or Infinity, "u" at Infinity is often taken to be (100 X f). With Leica one has to be careful about the distance markings on the lens. Leica measure the distance from the Object plane to the Image plane, which is "u" + "v" + "δ". Where "δ" is the distance between the Entry and Exit Nodes of the lens, which can be negative when measured in the direction from Object to Image! It is this distance which is engraved on the Distance Scale. Edited November 27, 2015 by Peter Branch 2 Quote Link to post Share on other sites More sharing options...
pico Posted November 27, 2015 Share #39 Posted November 27, 2015 There are two things to note firstly that the conventions are, in some instances, different for different focal lengths . Secondly that Leica Germany used a different convention from Leica Canada. Different? Really? In what way? Quote Link to post Share on other sites More sharing options...
gyoung Posted November 27, 2015 Share #40 Posted November 27, 2015 Because focal length is measured at infinity. At 0.7 metre the FoV of a 50mm lens must be around 60 or 75mm depending on lenses i guess. Here Elmar 50/2.8 on Fuji X-E2 (0.7m & infinity). FoV_xe2_5028_1m_web.jpg FoV_xe2_5028_inf_web.jpg You should be measuring field of view in degrees as an angle, not confusing it with the effect of a longer focal length on the same format. Field of view for a lens of the same focal length will depend on format as well as focus distance, but it does not affect its measured focal length, which is an absolute. Gerry Quote Link to post Share on other sites More sharing options...
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