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Thread: Distance Sensor to lens

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    Distance Sensor to lens

    Hi,
    is there a info at the lens manufactures for the distance between the sensor or film plane and the back of the lens at infinity. With this info i could find out if this lens is possible to use with my camera.
    Especially if i use some short lenses...
    Thank you.
    mueller

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    Re: Distance Sensor to lens

    Some manufacturers do quote a film to flange distance (which is in the case of large format the lensboard), which is probably at infinity.

    A modern lens made for that specific size camera is probably going to fit, except maybe some specialty lenses (maybe macro lenses and some portrait lenses that don't need infinity). When in doubt I would just google if someone has used that specific combination of lens and camera. Usually image circle is a bigger problem than getting the lens on the camera.

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    Re: Distance Sensor to lens

    If you go to Large Format Photography Forum there is an active community of large format photographers who can help you. Someone there is likely to answer questions about a specific lens and camera combination, or point you to resources with information from manufacturers (including historical lens data).

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    Re: Distance Sensor to lens

    The published specification is the "Flange Focal Distance" or "Flange Focal Length".
    This is the distance from the lens mounting surface to the film plane.
    It is not measured from the front or back of the lens.

    In the case of large-format lenses, the distance is measured from the front of the lensboard.

    For smaller cameras like 35mm, the distance is from the mounting surface on the front of the camera body.

    For Nikon F-series lenses, that number is 46.5mm.

    You can find the distance for other small-format lenses here:
    Flange Focal Distance of Photographic Lenses and Cameras

    For large-format lenses, that value is a published spec on the lens data sheet.

    To address your apparent question, you must also consider the image circle diameter of the lens against your format dimensions.

    - Leigh

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    Re: Distance Sensor to lens

    Quote Originally Posted by mueller123 View Post
    Hi,
    is there a info at the lens manufactures for the distance between the sensor or film plane and the back of the lens at infinity. With this info i could find out if this lens is possible to use with my camera.
    Especially if i use some short lenses...
    Thank you.
    mueller
    With digital backs or DSLR cameras on View Cameras with large format lenses, you will need the Flange focal distance less the length of the rear group. This is especially important with wide angle lenses.
    Schneider which uses a traditional design Philosophy, will be closer to the sensor than Rodenstock, which uses a reversed telephoto design in their lenses, and especially wide angles.
    Remember if dealing with a DSLR body that the Canon or Nikon bodies use 44 to 46+ mm deep mirror box and so you need approximately 50mm after subtracting the rear lens group length from the Flange Focal distance.
    The Sony A7 series only requires a clearance of about 21 or 22mm to allow movement, so is better for wide angles.

    You are limited to the 32mm Rodenstock and 35mm Schneider as for the widest focal lengths with the Sony, while the Nikon and Canon, at inifinity, are limited to 90mm and above for movement. The Canon can focus a 72mm or 70mm Rodenstock, but at infinity, little or no movement.

    Hope this helps.
    Rod
    Last edited by RodK; 4th August 2014 at 13:12. Reason: grammar
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    Re: Distance Sensor to lens

    Please shoot me if I'm being a numpty, but I thought the general principle was that infinity was at the focal length of the lens from the focal centre of the lens, and 1:1 was twice the focal length.

    Or is that irrelevant in relation to the lens board, and is only helpful where the lens is symmetrical about the lens board?

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    Re: Distance Sensor to lens

    Quote Originally Posted by shakeshuck View Post
    Please shoot me if I'm being a numpty, but I thought the general principle was that infinity was at the focal length of the lens from the focal centre of the lens, and 1:1 was twice the focal length.
    Or is that irrelevant in relation to the lens board, and is only helpful where the lens is symmetrical about the lens board?
    Sorry, no. (I don't know what a numpty is.)

    Focal length is the axial distance from the rear node of the lens to the focal plane when focused at infinity.
    Every lens has two nodes... front and rear. The rear node is the point from which all exit rays appear to emanate.

    The position of the rear node has no relationship to the physical construction of the lens, although it is at a defined position for any specific lens design.

    An obvious example of this is a 300mm lens on a 35mm camera where the total length of the lens may only be 100mm or so.

    For 1:1 reproduction the rear node is indeed forward of its infinity focus position by one focal length.
    The physical distance moved is always equal to the focal length, but the lens position at infinity focus can be anywhere.

    - Leigh

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    Re: Distance Sensor to lens

    Quote Originally Posted by Leigh View Post
    Sorry, no. (I don't know what a numpty is.)

    Focal length is the axial distance from the rear node of the lens to the focal plane when focused at infinity.
    Every lens has two nodes... front and rear. The rear node is the point from which all exit rays appear to emanate.

    The position of the rear node has no relationship to the physical construction of the lens, although it is at a defined position for any specific lens design.

    An obvious example of this is a 300mm lens on a 35mm camera where the total length of the lens may only be 100mm or so.

    For 1:1 reproduction the rear node is indeed forward of its infinity focus position by one focal length.
    The physical distance moved is always equal to the focal length, but the lens position at infinity focus can be anywhere.

    - Leigh
    Thanks for correcting me, Leigh.

    I assume the rear node moves on a 35mm camera, where there is some focussing mechanism involved - that's 'cheating' in the LF forum.

    Looks like I need to dust off those books on my shelf and reread what I seem to have forgotten!

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    Re: Distance Sensor to lens

    Quote Originally Posted by shakeshuck View Post
    I assume the rear node moves on a 35mm camera, where there is some focussing mechanism involved - that's 'cheating' in the LF forum.
    All lenses work the same way.

    To focus closer than infinity you move the rear node away from the film, which means moving the mechanical lens assembly by some means.
    That's true on all cameras, all lenses, and all formats. Zoom lenses work by playing tricks with internal magnification ratios.

    The mechanism is different, using a helicoid on smaller formats and bellows on LF.

    All my 26 lenses work that way on my 4x5, 5x7, and 8x10 LF cameras.

    - Leigh

    4x5: Sinar F2, Zone VI, Tachihara, Plaubel Peco Provia, Speed Graphic;
    5x7: Tachihara;
    8x10: Sinar F2, Tachihara.

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    Re: Distance Sensor to lens

    Quote Originally Posted by mueller123 View Post
    With this info i could find out if this lens is possible to use with my camera.
    Especially if i use some short lenses...
    Thank you.
    mueller
    As post #2 says, it is the image circle that is usually more of a concern especially if you use camera movements, and even more especially if you are using wide angle lenses. On the other hand you can get most types of LF lenses simply to focus using varieties of lens boards.

    Steve

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    Re: Distance Sensor to lens

    Quote Originally Posted by Leigh View Post
    All lenses work the same way.

    To focus closer than infinity you move the rear node away from the film, which means moving the mechanical lens assembly by some means.
    That's true on all cameras, all lenses, and all formats.

    The mechanism is different, using a helicoid on smaller formats and bellows on LF.

    All my 26 lenses work that way on my 4x5, 5x7, and 8x10 LF cameras.

    - Leigh
    You're suggesting all lenses - including 35mm and MF - focus essentially by using a variable-extension-tube mechanism. If that were true, all lenses would change in length when focussed. And they don't.
    I assume the rear node changes on 35mm/MF through the use of IF (in some cases), moving just some elements/groups and not all?

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    Re: Distance Sensor to lens

    Hmm.

    What I've seen mentioned in this thread as the "Flange Focal Distance" is more commonly known as the "mount register" ... the distance from the lens mounting flange outer surface to the image plane. This term is commonly used in helical mount lens systems on smaller format cameras, perhaps flange focal distance is what the LF manufacturers use. There are various different aggregations of mount register available on the web, mostly for small and medium format camera systems with helicoid lens mounts. One such is Camera Mounts Sorted by Register : http://www.graphics.cornell.edu/~westin/misc/mounts-by-register.html

    "Rear Node" ... It's the lens primary nodal point that matters, not the "rear node." The primary nodal point is the point in the light path through a lens to the image plane where all the ray trace paths cross—for example, what enters the lens at upper left is imaged at the lower right, and what enters the lens at the lower right is imaged at the upper left. The point where these two ray-traces cross is the primary nodal point.

    The focal length of a lens is defined as the distance between the primary nodal point and the image plane when the lens is focused at infinity. This is true whether the lens is a prime or a zoom. In a zoom lens, reorganization of the element groups' spatial relationships in the lens change the primary nodal point.

    There are three kinds of focusing mechanisms in common use: unit shift, front element, and internal.

    - Unit shift is used universally with LF cameras and works with all lenses. Basically, if you shift the lens unit away from the focus plane such that the distance between the primary nodal point and the image plane is greater than the focal length of the lens, the focus distance is decreased. In unit focusing lenses with helicoid lens mounts, the overall lens length grows as you focus closer. The vast majority of quality lenses in the world are unit focusing lenses. There are variations on the design of unit shift—incorporation of movements of rear- or middle-element groups relative to the infinity position ("floating element" designs) in helicoid mount lenses allow optical corrections and optimization for different focus distances on some high end lenses. The Nikkor 24mm f/2.8 of 1966 or so was one of the very first unit focusing lenses that included this kind of corrective re-adjustment of lens geometry to improve optical correction at close focus distances.

    - Front element focusing is used in many many lenses, usually on fixed lens cameras but often on less expensive interchangeable lenses too. A simple example are the three element Tessar and Triotar type lenses used on the Rollei 35 cameras. In this focusing system, as the front element is shifted away from the rest of the lens elements, the focal length of the lens is decreased, which means the distance between primary nodal point and the image plane is increasing, which in turn pulls in the focus distance. Lenses with front element focusing also grow in length as you focus closer, but there are more constrained limits to how close a lens can focus with this design before image quality across the image plane is seriously degraded. However, it is a popular design due to its low cost.

    - Internal focusing mechanisms are used exclusively on fixed and high-end interchangeable lenses as the mechanism involved is complex. With internal focusing, the same sorts of mechanisms used in unit shift with corrective movements of the lens groups (floating element designs) are used along with optical adjustments similar to front element focusing. Such lenses are necessarily helicoid mount constructions, with complex helicoids that move the internal elements at differing rates to provide high quality optical performance across the focus distance range. Some lens designs have combined internal focusing along with zoom operation as well. They're usually fairly expensive lenses. Some examples are the Micro-Nikkor 200mm f/4 ED-IF AI-S, the Zeiss Vario-Sonnar 9.7-48.5mm f/2-2.4 used in the Sony DSC-F707, and the new Summilux 23mm f/1.7 ASPH used in the Leica X typ 113.

    A good source of information on lens designs can be found in Lens Design Fundamentals by Rudolf Kingslake. It covers these sorts of design fundamentals.

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    Re: Distance Sensor to lens

    Quote Originally Posted by Godfrey View Post
    What I've seen mentioned in this thread as the "Flange Focal Distance" is more commonly known as the "mount register"
    I suppose that depends on one's definition of "common".

    You'll find Flange Focal Distance on every lens datasheet, as a defined parameter.

    I've never seen "mount register" mentioned anywhere.

    Quote Originally Posted by Godfrey View Post
    "Rear Node" ... It's the lens primary nodal point that matters, not the "rear node."
    Again, a non sequitur.
    Every lens, even a single element, has two nodes, designated "front" and "rear".
    Lens datasheets give their precise position, shown as H and H'.

    The front node is the point into which all incoming rays converge.
    The rear node is the point from which all outbound rays emanate.

    These two nodes could be coincident in a theoretical "thin lens", but not in a real lens.

    Quote Originally Posted by Godfrey View Post
    The focal length of a lens is defined as the distance between the primary nodal point and the image plane when the lens is focused at infinity.
    Short answer... wrong.

    Long answer:
    The focal length is defined as the distance from the rear node to the image plane along the optical axis at infinity focus.

    Are you some kind of academic dude?
    It doesn't appear that you've ever read a lens datasheet.

    And yes, I have Kingslake and several other lens design texts.

    - Leigh

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    Re: Distance Sensor to lens

    Quote Originally Posted by shakeshuck View Post
    You're suggesting all lenses - including 35mm and MF - focus essentially by using a variable-extension-tube mechanism.
    If that were true, all lenses would change in length when focussed. And they don't.
    You're making a common error in confusing the external package with the optical device.

    The package is just a tube to keep the parts from falling on the floor.

    How it interfaces with the actual lens mechanism depends on the design.

    - Leigh

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    Re: Distance Sensor to lens

    I hope I haven't started a flame war here, Leigh - your response to Godfrey seemed a bit harsh.

    Quote Originally Posted by Leigh View Post
    Every lens, even a single element, has two nodes, designated "front" and "rear".
    Lens datasheets give their precise position, shown as H and H'.

    The front node is the point into which all incoming rays converge.
    The rear node is the point from which all outbound rays emanate.

    These two nodes could be coincident in a theoretical "thin lens", but not in a real lens.

    - Leigh
    I appreciate that I'm trying to have a discussion on a topic I know little about, but this doesn't make sense mathematically; if you're crossing ray paths twice (or any even number of times), your final image wouldn't be inverted...
    ...or am I missing something again?

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    Re: Distance Sensor to lens

    Thank you for going to the trouble of explaining the different focussing types, Godfrey.

    I will indeed find a reference and get myself better versed.

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    Re: Distance Sensor to lens

    Permit me to expand a bit on the subject of "Principal Planes" and their relevance.

    This is a Nikon large-format lens of "standard" design, a Nikkor-M 450mm f/9 (see note*):

    The Principal Planes are identified on a lens diagram by dots on the optical axis, labeled respectively H and H' (H Prime).
    The dots identify the front (H) and rear (H') nodes, at the intersections of the optical axis and the two planes.
    As mentioned earlier, the lens focal length is from the rear node (H') to the image plane, measured along the optical axis.

    Here's a Nikkor-T 500mm f/11 telephoto design for comparison:

    The blatantly obvious difference is that the two Principal Planes are waaay forward (to the left) of the front element, not within the lens body as for the Nikkor-M design. This is typical of telephoto lenses.

    This lens has a focal length of 500mm, but a physical length of only 129.5mm.

    Note also the Flange Focal Distance, called out at lower right. That is the shutter mounting plane.
    On the standard lens, the FFL is almost equal to the optical focal length (428.4mm v. 450mm), while on the telephoto they are very different.

    I chose to illustrate this post with large-format lens diagrams because they have far more detail than those for fixed-format lenses.
    The optical principles involved are the same regardless of the intended format.
    Different applications may dictate different optical designs.

    - Leigh

    *Note:
    The diagram is a cut-away view of the lens mounted in a shutter.
    The upper half shows the lens elements themselves, while the lower half shows the outer view.

    NB: Nowhere in these or any other lens data sheets in my library is there any mention of "mount register".
    Last edited by Leigh; 30th November 2014 at 16:30. Reason: corrected spelling of "principal"

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    Re: Distance Sensor to lens

    Kingslake:

    Section 3.3 The Gaussian Optics of a Lens System

    Quoting the third paragraph* (from my hard copy):
    "... The paraxial portion of this locus is a plane perpendicular to the axis and known as the principal plane, and the axial point itself is called the principal point, P2. The paraxial image point F2, which is conjugate to an axial object point located at infinity, is called the focal point, and the longitudinal distance from P2 to F2 is the posterior focal length of the lens, abbreviated f'."

    {Italics original, red highlight mine}

    From Kingslake, Rudoph and Johnson, R. Barry, "Lens Design Fundamentals, Second Edition", SPIE, Bellingham, Washington, 2010, page 67.
    ISBN 978-0-12-374301-5

    - Leigh

    *Note: The quote is abbreviated because it starts with a discussion of the object side of the lens, which is not relevant to this discussion.

    FWIW... The term "mount register" does not appear in the index of that book.

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    Re: Distance Sensor to lens

    Quote Originally Posted by shakeshuck View Post
    I hope I haven't started a flame war here, Leigh - your response to Godfrey seemed a bit harsh.
    Leigh always responds to my posts like a jerk. I just ignore him. His obstinate churlishness don't bother me one wit. Sometimes he might have something useful to say, which would be interesting to discuss if he didn't comport himself as such a little snot nose.

    G

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    Re: Distance Sensor to lens

    Quote Originally Posted by Godfrey View Post
    Leigh always responds to my posts like a jerk. I just ignore him. His obstinate churlishness don't bother me one wit. Sometimes he might have something useful to say, which would be interesting to discuss if he didn't comport himself as such a little snot nose.
    G,

    You obviously did not read my two posts immediately preceding yours.

    Running your mouth before educating your brain is really foolish.

    It makes you look like a dumb-mudgeon.

    - Leigh

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    Re: Distance Sensor to lens

    I found this page (unfortunately not complete) that illustrates the node points:

    johnlind.tripod.com/science/sciencelens.html

    This shows how the two node points essentially act as one light path, which explains the still-inverted final image I was expecting.

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    Re: Distance Sensor to lens

    Quote Originally Posted by shakeshuck View Post
    I found this page (unfortunately not complete) that illustrates the node points:
    Photographic Science: Body
    That's a good basic treatise on the subject.

    In particular, it points out that the coincidence of the two nodes is based on the theoretical "thin lens" abstraction.
    That concept is convenient for analysis of optical systems and ray tracing.

    Quoting:
    "Further, it is assumed to be a theoretical "thin" lens; one without any appreciable thickness.
    "In this case the "front lens node" and "rear lens node" are located inside the single lens element at the same point."

    The "thin lens" convention does not contradict in any way with the "real lens" analysis that I mentioned earlier.
    In fact the latter is derived from the former.

    - Leigh

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    Re: Distance Sensor to lens

    Quote Originally Posted by Leigh View Post
    I suppose that depends on one's definition of "common".

    You'll find Flange Focal Distance on every lens datasheet, as a defined parameter.

    I've never seen "mount register" mentioned anywhere.
    Like this?

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    Re: Distance Sensor to lens

    Quote Originally Posted by Oren Grad View Post
    Like this?
    OK. That's just the Flange Focal Distance, as on this Rodenstock datasheet:

    Same parameter, same values, in the same position on the datasheet. Samo samo.

    - Leigh

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    Re: Distance Sensor to lens

    Quote Originally Posted by Leigh View Post
    OK. That's just the Flange Focal Distance, as on this Rodenstock datasheet:
    Yup - the terms are interchangeable, which was the point.

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    Re: Distance Sensor to lens

    Quote Originally Posted by Oren Grad View Post
    Yup - the terms are interchangeable, which was the point.
    Nope. Mount Register and Optical Register have one word in common, the other different.

    There's no reason to assume that the two phrases are synonymous.

    - Leigh

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