Nikon D800E manual lens focus problem - flange distance of 46.67mm

Discussion in 'Nikon' started by james_symington|1, Jul 16, 2012.

  1. As it turns out the problem - reported in an earlier thread - I am having with my manual focus lenses on the D800E appears to be that the Nikon standard flange distance is 46.67 (as against the 46.50mm you read about everywhere). Zeiss ZF lenses are apparently set to 46.50mm (which I thought was the standard) and hence this annoying problem of all my lenses focusing beyond infinity. The Nikon tolerance is +/- 0.015 apparently.
    I guess I just have to eat humble pie with Nikon.
     
  2. Are you saying that the flange on the D800E is not 46.50mm anymore?
     
  3. You or someone has their math backwards, at a flange distance of 46.67mm, a lens set for infinity at 46.50 will never reach infinity focus.
     
  4. Hi Jose and Bob,
    Nikon just wrote to me and said that the flange distance is 46.67mm on the D800/E (and presumably everything else?) and Zeiss wrote to me a few days ago to say they make their lenses based on 46.50mm.
    I am merely repeating what I have been told and - on this occasion - it is not me that has this backwards. :)
     
  5. It is pretty normal that lenses focus beyond infinity. This is designed-in to cope with tolerances and temperature effects. If your lenses do so, don't worry. If the flange distance really were more than 46.5, the lenses wouldn't reach infinity anymore. Therefore I highly doubt that the information you got from that Nikon guy is correct.
     
  6. Well after perusing my Nikon F service manual, it specifies measuring the film-flange distance to the outer film rails, not the inner ones. 35mm film is nominally 0.14mm thick, add that to the film-image plane distance of 46.50mm and you get 46.64mm pretty close to what Nikon told you, but that is the wrong number in this situation.
    The outer film rails make it much easier to measure the distance with a depth gauge micrometer and having the camera body sitting face down on a surface plate.
     
  7. Whilst I agree that many lenses (AF, ED, APO etc) are supposed to go beyond the ZFs are supposed to hit infinity exactly on the hard stop - whereas they have been going quite a way over on the D800E (probably 2-3mm of focus ring rotation).
    I never noticed such a problem on my D700 and D3 whereas I noticed it within a minute of using Live View for the first time on the D800E. This has precipitated these discussions with Zeiss and NPS and I don't know who to believe any more.
    Apparently the source of this info is one of the senior technicians in Nikon UK. The only reference to this distance I have found is this:
    http://www.fredmiranda.com/forum/topic/665776/0
    What, if anything, can be made of this I don't know.
     
  8. The 46.67mm were already mentioned here: http://www.photo.net/nikon-camera-forum/00On4l and also here: http://www.astropix.com/HTML/I_ASTROP/FILM/FOCUS1.HTM
    @James: since the lenses worked with correct hard stop on your D700(?), this appears to be a tolerance issue and could potentially be solved by shimming the mount on the D800E - maybe this is worth pursuing?
     
  9. So, how does the Hard Stop (system) on Zeiss lenses cope with thermal expansion?
    So -15C to +40C has no thermal effect of these lenses? Sorry, they're v.good, but I don't believe it.....!
    .....or all that hokum about lenses going 'beyond infinity' to allow for it, is just a poor excuse for very poor tolerances!
    I don't think Zeiss use expansion-free materials.....do they??
     
  10. With the only exception on the ED lenses, all my MF Nikkors have hard infinity stops.
     
  11. Mike: I assume that any lens with a hard stop has to be designed such that thermal expansion is negligible. It's not unusual for lenses to have a hard stop at infinity, so this can't be new. Sure the D800 is going to be better at showing up focus errors than most, but James's lenses appear to be much farther off than I'd expect.

    However, it's also true - as Bob says - that the flange distance of the camera must be shorter than that for which the lenses is designed, not (as suggested by the reported numbers) longer - infinity is coming up with the lens racked farther than minimum distance from the sensor than it would be for infinity.

    Nikon seems to have some really weird QC issues with the D800, but a mount this far off seems very strange.
     
  12. @James: since the lenses worked with correct hard stop on your D700(?), this appears to be a tolerance issue and could potentially be solved by shimming the mount on the D800E - maybe this is worth pursuing?​
    Hi Dieter,
    I have taken my D800E three times into Nikon UK to get this sorted but they run their tests - whatever they are - and come back to say that my camera passes them all with flying colours. This is why I have been in touch with Zeiss as well to make sure I am not missing something. This latest email from Nikon about the 46.67mm has now got me thoroughly confused though. I'll have to see what they come back with tomorrow.
     
  13. So, how does the Hard Stop (system) on Zeiss lenses cope with thermal expansion?​
    The same way that it does/did on all my Bronica ETR & SQ , Zeiss Hasselblad V, Zeiss Contax, Leica R, Leica M, Mamiya 645, Pentax 67 & some 645 etc lenses that I have had over the last 15 years that all had infinity hard stops. What they do to combat it I have no idea but I have never had this problem before - presumably use material that expands only negligibly as Andrew suggests.
    The same ZFs that give me grief on my D800E worked fine on my D700 which alas I sold as part of this upgrade. I can easily accept that the D800E is going to be much more demanding to focus relative to anything else I have had before but the extent of this issue seems to indicate there is something out on the camera.
     
  14. I never bought the "explanation" that AF lenses don't have "hard infinity stops" because of thermal expansion - manual focus lenses didn't seem to have the problem. I am not saying - since different materials are involved - this isn't a valid explanation. But my guess was always that AF lenses focus "beyond" infinity to allow the AF system to go past the "sharpest" focus from either direction to determine the correct focus - even when the subject is at infinity
    @James: I am just guessing but since tolerances are involved (and "unfortunately" third party lenses) it seems possible that though the camera is "within specs" the lens/camera combo is not. Suppose the camera is at one end of the tolerance range and the lens at the opposite - either is fine, but together, they are out. Did you send your lenses with the D800E to Nikon when they did the three attempts to fix the problem?
    The 46.67mm have me baffled as well - but it seems from the few references linked to in this thread, that it isn't the flange-to-sensor/film distance but the one to the outer guide rails in a film camera (and whatever that distance is in a digital camera (back of sensor?)). In a film camera, that would equate to the maximum distance the focused image would ever have to be at - and likely the outer rails were chosen because the distance can easily be measured. No idea what the equivalent reference point in a digital SLR is - but it seems possible that the same reference distance was used as before. In any case, I don't think that Nikon made such a significant change - this has got to be an issue of the lens/camera combo being out of their combined tolerance range.
    .
     
  15. Hi Dieter,
    Yes they have had my 50mm f1.4 ZF and 25mm f2 ZF2 - but I get this same issue on the 35mm f1.4 and 100mm f2 as well to the same degree. To have one or two lenses out a bit I can believe but to have all four out sounds like there is something up with the camera particularly as I have used the 50mm and 100mm for years on the D700 without noticing this whereas I spotted a problem immediately on the D800E.
    Also when I used my AF Nikkor 50mm f1.4 G it achieved infinity focus just shy of the infinity mark too. But Nikon maintain that the camera passes all their tests.
    I would be very curious to hear from other ZF users if they are having this same problem with the D800E. If it is a problem with the way ZF lenses are made then I would imagine I would not be the lone voice on this forum raising this problem. I can only conclude that my camera sits precariously at the edge of (or beyond) its allowable tolerance and hence the "it passes all its tests" spiel I get from Nikon.
    As for the 46.67mm business heaven knows where that comes from. If it is rubbish then unfortunately it is Nikon UK's senior technical guy who is coming out with it - which may explain lack of progress in getting this fixed.
     
  16. I have just tested two of my MF Nikons, and the flange up to the pressure plate is definitely longer than 46.50mm... 46.67mm makes more sense.
    But this plate lean on that outer guide rails, leaving a channel for the film to pass; there are another couple of rails, where the film emulsion is in contact. This rails are in another different (shorter) level. So I still think that the real flange (mount to emulsion film side) is "still" 46.50mm (at least on film cameras).
    BTW, a 50mm lens @ f2.8 reach infinity at an hyperfocal of 30meters (for a "standard" CoC of 0.03mm). My D700 confirm focus at such distance, and the infinity mark fall in the "middle dot/line" in the barrel.
    If you set the CoC on 0.015mm (for e.g., a 16x20" print), the hyperfocal is at almost 60meters... so I think it`s better not to have a hard stop to reach that point (and to have LiveView or a better system to focus at exactly that distance). The AF system doesn`t distinguish between 30 and 60 meters (at least on my D700 with the 50/1.4AFS), the infinity mark stays in the middle. If the camera can take a pic with such accuracy level, is another topic.
    I know this is not an issue with James, because "his infinity" doesn`t even reach the infinity mark on the lens.
    --
    I would be very curious to hear from other ZF users if they are having this same problem with the D800E.
    I`m now curious, too. Also waiting to hear from other users.
     
  17. Expansion co-efficient of Duralumin is 0.00225%/degree K. So a two inch long lens will expand by just over 1 micron per degree C. I don't think that's a very big deal, and certainly not enough to account for a noticeable shift in the end stop. Plus that expansion would be spread from back of lens to front and would alter the separation of the glass elements as well as just extending the lens. It's possible the element shift would partly counteract the physical expansion?
    I don't suppose James fancies ramming the end of a depth gauge against the sensor face to test the actual lens-flange to sensor distance once and for all? But then there's the IR and AA filters in front of the sensor to take into account as well.
    BTW, my own experience with Nikon UK is that they'll say anyththing to avoid addressing a real problem. Standard procedure appears to be:
    1) Blame the customer or throw some doubt on the fact that it's anything to do with their camera/lens.
    2) Keep the equipment for "testing" or "awaiting parts from Japan" for such a long time that the customer is just glad to have their gear back again.
     
  18. P.S. James, UK consumer law states that for the first 3 months after purchase it's up to the seller to prove to the buyer that the goods are fit for purpose. Your contract is also with the retailer and NOT with Nikon UK. Therefore it's up to the vendor to prove to your mutual satisfaction that the camera performs as it should. Since no proof appears to have been forthcoming, the vendor is in default. If you let this ride for more than 3 months, then the burden of proof of being unfit for purpose shifts to the purchaser (you!). DO NOT, let those shifty sods at Nikon UK keep your camera until the 3 months is up. Get your local trading standards officer involved if necessary.
     
  19. Rodeo Joe - thanks for the tip. I have kept Wex Photographic in the loop from the start so will be onto them unless Nikon suddenly have a change of heart. I am off on a month long trip to Hawaii at the end of next week and need to get this sorted quickly now. As I spend an awfully large amount of money with them I'm pretty sure they will be obliging.
    More exciting updates on this story tomorrow I suspect. Along with the arrival of a Sony RX100 which I hope will be problem free...
     
  20. I just tested three of my Zeiss ZF lenses; 21/2.8, 35/2.0 and 100/2.0 on D800E and all focused on their hard stop infinity. The focus point was set on a horizon (high rise buildings and trees) at least 2-4 km away. Tests were done through OVF (difficult to notice any small focus changes when reaching or 1mm near the hard stop mark at that distance) and using LV + maximum magnification and 10x magnifying glass.
    I also used two Leitax-ed Leica Summicron-R lenses; 35 and 50mm. Both reached infinity at their hard stop infinity marks as well.
     
  21. Wow - thank you very much Jack.
    Were they shot wide open?
    I am going to give Nikon UK a great big b@ll@cking today I feel...
    James
     
  22. James, I shot at f/8.
    Do you experience focussing beyond infinity issue using both Live View and OVF? Just a thought, but there may be a chance that mirror alignment in your camera may be off a bit. When I received my D800E I noticed a slight front focussing with most of my lenses (focussing through OVF didn’t match magnified focussing via LV) A simple adjustment of mirror angle corrected this issue allowing accurate focusing via OVF using MF lenses.
     
  23. I`m now at work, where I can measure with way more accuracy the flange on my F3.
    Mount to "pressure plate" distance: 46.67mm ("outer rails", if you like).
    Mount to inner rails distance: 46.50mm, (the inner rails are the rails where the film stands, on the emulsion side).
    As far as I know, "flange focal lenght" means lens mount to film distance (or sensor equivalent).
    If they use the same name for whatever other parameter, it`s beyond me.
     
  24. OK Nikon came back and corrected their statement of yesterday - it is 46.50mm:
    "Hi James,
    You are correct in that the distance from the lens mount to the focal plane is 46.5mm however our technicians need to ensure the rear of the sensor is actually 46.67mm. Sorry for the misleading information. We can confirm however that the depth on your camera is 100% accurate."
    Still doesn't explain why my lenses all remain so far out.
     
  25. Hi Jack,
    Thank you again for looking at this. There was a mirror alignment issue as well but they sorted it.
    Could you possibly have another go when you have a minute and just test where infinity falls when you shoot wide open as depth of field at f8 will mask problems to a certain extent? If you get sharp focus at the infinity stop then I still have a problem in spite of their assurances that my camera is bang on.
    Many thanks,
    James
     
  26. I'm wondering whether the reason Hard Stops normally seem to work, sorry James, on all the lenses you mentioned, inc Medium Format ones, is the much longer helix 'throw' of MF lenses negating the thermal expansion and so not pushing/pulling the elements very far and also allowing for more flexible (sloppy!) tolerances in AF lenses.
    I suppose as AF lenses focus by electro-optical feedback, they need to know when it's in focus and who cares where on an arbitary rotational scale!
    I guess as these MF lenses are designed for film and DSLRs, that if the pressure-plate is 46.67mm and the rails that the emulsion would rest on are 46.50mm, I'd think as long as the flange>focal plan is between 46.50 and 46.55, (leaving the rest to the film backing and any anti-halation layers) it would be just fine.
    The distance in film cameras is flange to emulsion face, that's easy. But how do you account for all the filter stuff, esp IR Blockers (my converted D50's was about 3mm thick!) when you measure a distance to a sensor surface you can't actually reach?
    Maybe your E version with it's extra Anti-Anti-Aliasing filter is a distance 'problem'? No-one's taken account of the difference between a D800 and a D800E. Stranger things have happened....especially at Nikon UK!
    It would explain why they work OK on your D700!!
    EDIT. The 'rear' of the sensor at pressure plate distance.. err... OK??? The sensor is 0.17mm thick, just like film! Wow, who'd have thought that.
     
  27. You may well be right Mike - I simply don't know. I also haven't understood exactly what tests Nikon have performed on my camera. Still curious to see if Jack can come back with some findings when shooting at infinity wide open.
     
  28. So Nikon UK have backed down on the register distance? I should think so too! I just rammed a digital caliper down the throat of my old FM, and got register readings of 46.52mm with film in, and 46.68mm without. So it looks like the 46.67 mm figure is actually the old film camera lensmount-to-pressure plate nominal distance. This leaves Nikon to explain how come the back of the digital sensor is only 0.17mm away from the true image plane. If that were possible then we could just slip one of Nikon's super slimline sensors into our old film cameras and be shooting digital with them!
    Much more likely is that the guy at Nikon UK is simply lying through his teeth and they've neither checked your camera nor have the proper equipment to do so.
    As I said before, the burden of proof of the camera being "fit for purpose" is with the vendor in the period immediately after purchase. You need to ask polite but searching questions of Nikon UK. Such as: Can they provide a readout of the measurements they've taken from your camera? If not, why not? Can they provide any hard evidence at all for the camera being within spec? Can they give you details of how the measurements were taken? How can the back of the sensor be only 0.17mm away from the focal plane? In law, all that is WEX's job, so don't let them fob you off to Nikon UK either. Even a local garage will show you a printout of your car's engine management system if asked. Does Nikon UK not have far more sophisiticated equipment to diagnose their cameras?
    Time Nikon UK was made more accountable to its customers, especially since they don't seem to be fighting our corner on pricing and allocations either.
     
  29. James,
    Wide open only Zeiss 35/2 required 0.5 mm of focus barrel counter-clockwise turn to perfectly focus on infinity target. Other lenses were right on the hard stop.
    Did Nikon service confirm their testing method? I believe service technicians hookup camera's output to an external display to run calibration tests so it may explain different results than via viewfinder.
    Let us know how it turns out.
     
  30. Fascinating thread, everyone.

    Firstly, good luck with Nikon UK. I've generally got on okay with them, but they're the only people ever to have cleaned my D700's sensor, and the fact that they're reporting it as (slightly) scratched makes me a little worried. I've had good experiences with Wex, so I'm sure they'll back you up (although they may not currently be in a position to give you a replacement).

    Mike: There's no thickness difference between the D800 and D800E filters - that's the point of the weird AA filter on the D800E.

    I could certainly believe that the difference between the 46.67 and 46.50 figures is one of film thickness (Wikipedia actually says 0.14mm). I'm also intrigued by the assertion that the sensor is exactly this thick, which sounds like a remarkable coincidence but more like gibberish.

    I'm still of the opinion that the camera is wrong; I've no idea whether there's a shim behind the sensor that can be made thinner to fix it (if "infinity" is short of infinity, we know the sensor isn't far enough from the lens). I'm now concerned that you've given Nikon UK a hard job to do, because as of yesterday they're also supposed to be checking out my 135 DC to tell me whether its LoCA is "normal". I'm slightly less confident of the answer than I might have been before this thread...
     
  31. About the sensor thickness, my guess is that they are playing "Chinese Whispers" between the technician and the receptionist, or whatever the interlocutor is. I bet he/she is someone with a quite limited photographic experience, and probably based (if so) in the use of a mobile phone camera.
     
  32. Thanks very much Jack - a bit of sample variation at play there on the 35mm f2 I suspect but the adjustment to correct is still tiny compared to what I have to do. Good to know that the other two ZFs hit infinity perfectly on the hard stop where none of my four ZFs (not to mention a fifth Voigtlander 40mm) do that.
    Lloyd Chambers at diglloyd.com also has confirmed to me that my camera is obviously out given my description of the circumstances. This makes me all the more angry with Nikon UK for fobbing me off with nonsense and just trying to make me go away with a camera they must know to be wrong.
    Good luck Andrew!
     
  33. James, Could you bring few lenses to your local Nikon dealer and try them on their D800/D800E demo just to confirm? If your lenses work fine on the demo you could request a replacement of your camera.
     
  34. Pardon the question after all of you have weighed in with very detailed data. Will a new Zeiss 28 f2 function according to spec on a D800E? Is it worth it to use, for 20x24 fine art prints, in place of a 20-35 f2.8 Af-S or a 28-70 f2.8 AFS. Thanks.
     
  35. For the record my CV 58/1.4 does focus beyond infinity with my D200 (infinity is roughly towards the middle of the infinity symbol). Rather than a perpetual back and forth with Nikon, if your other gear will focus at infinity without hassle, I'd just adjust the infinity stop on the ZF lenses and be done with it.
     
  36. Hi Steve,
    Please understand that if you focus carefully in Live View your results from high end lenses will sparkle and the 28mm f2 is very very good. I had one before I rationalised my too large collection.
    Hi Alex,
    All my lenses focus beyond infinity on the camera so the question is which bit of kit is out? If I adjust my lenses they might not get to infinity on other bodies...
     
  37. Steve`s question is related to the tripod and focus accuracy "initial topics" with the D800/D800E.
    The larger you print, the smaller CoC you have to set. It also implies reduced "depth of field" at the film plane; a focusing error will rapidly send the focus plane out of this margin.
    Under certain situations, the only way to get focus accuracy is to use Live View. The AF could turn simply useless. So, if you have to scrap every line pair out of a lens, you need first to assure a perfect focus accuracy (and obviously, to avoid camera shake).
    Mike commented above about a "reasonable" tolerance of 0.05mm on the film plane. I don`t have a calculator here, but if the film plane is at 46.55mm, (instead of five mm shorter), it could mean that a bonus of luck is needed even for something near a 16x20" print, to have it "correctly" sharp under "normal" conditions. (If the CoC for such size is 0.015 and I recall it right, at e.g. f2.8, we will have a "film depth of field" of 0.08mm, that is 0.04 (=d/2), in front and behind the film plane).
     
  38. Steve,
    I don't know if you are subscriber to diglloyd.com but Lloyd has just done a piece on the 28mm ZF and he loves it. http://diglloyd.com/
     
  39. I have taken back my camera from Nikon this morning as I have a month long trip to Hawaii next week which I need the camera for and there don't seem to be any replacements around. I guess I'll just have to live with it for another bit and hopefully borrow somebode else's D800E to check behaviour on that too.
     
  40. Wex Photographic have come through with another D800E that is quite a bit better and actually hits infinity spot on with the 50mm f1.4, 35mm f1.4 and is only fractionally out on the 25mm f2 and the 100mm f2.
    Thank heavens....
     
  41. James, I suppose this shows how broad 'within calibration' means!
    Glad it seems to have been sorted. Take loads of pics in Hawaii and tell us how it behaved.:)
     
  42. Will do - just have to get through another bl@@dy week at work though!
     
  43. Glad it all worked out for you James, but unfortunately this sorry saga just confirms my low opinion of Nikon UK's "service". Respect to WEX though. They may well get my business soon.
     
  44. Wouldn't surprise me if they didn't have the parts kit and service manuals yet. :(
     
  45. And at what apertures? Wide angle lenses at f11-16 will achieve focus on everything from 1m to infinity anyway.
    Is it me or do newcomers to photography insist on trying to shoot landscapes wide open or what?
     
  46. Depth of Field might extend from 1m to infinity @f16 on a wide angle, but the plane of focus is exactly that, a plane....... ie a single distance away from the lens depending where the focusing helicoid is set. Apparent focus either side of that plane is just an illusion defined by the CofC, the smallest point deemed to be a point not a blob.
    To determine focusing problems, it is normal to test the lens wide open so as to minimise the DoF and emphasize where the plane is. In James's case the Helicoid Infinty Stop mismatched the actual Plane at Infinity due to the flange>sensor distance being at the extreme end of tolerance...or beyond :)
    And I think newcomers and veterans alike can shoot at any aperture they choose. There are no rules, just methods of achieving what you want. If you want to highlight a single grave stone in a large cemetery, you shoot wide open. If you want to capture the full scale of the cemetery sharply, you'd probably stop down to f16. Both are correct.
     
  47. Francisco: I've seen a lot of landscapes shot with 35mm DSLRs at f/16 and f/22. Guess what: they're soft, because they're diffraction limited. Ansel got away with f/64 only because the film area was so huge. The D800E is apparently indistinguishable from a D800 by f/8 due to diffraction. When I shoot landscapes with my D800E, will I be shooting wide open? Probably not, although it's possible if I get a Zeiss 21mm. Will I be shooting at f/11-16? Only if depth of field trumped aperture. And then, I might consider focus stacking.

    It does appear that James was seeing behaviour that was a long way out of spec. I don't think I own any lenses that have hard infinity stops, but if I did, one primary purpose I'd be putting them to is astrophotography (because it's hard to see to focus properly, especially without ruining your night vision). And I'd certainly not be shooting them at f/16 for that.
     
  48. I believe lenses that focus beyond infinity has more to do with infrared focusing than thermal expansion.
    Some lens designs are better and do not exhibit shift in focus with infrared.
    One possible guess for your D800E focusing problem is that the replacement glass for the removed filter is missing resulting in shorter mount to sensor distance.
     
  49. Richard: There is no "removed filter" on the D800E - it has two filters, just like the D800, but they cancel rather than complement each other. The lenses I've seen with infrared marks generally require that you focus short of infinity when in infrared, although I'm prepared to believe that this could be different with different (especially superachromatic) designs.

    I like the argument that the AF system likes to be able to over-shoot, but it may also be simply easier (and more accurate) to let the autofocus system handle infinity focus and leave the markings as approximate (I have Canon lenses without any markings at all!) than to calibrate the lens accurately enough to make the markings work.

    As an aside: Is "infinity focus" through a few miles of sea-level(ish) atmosphere - for example, if I'm standing here and taking a photo of Snowdon (which is probably about twenty miles away) - the same as "infinity focus" pointing upwards at the sky? I'm assuming the difference is negligible (and much smaller than the difference between pointing straight up at the sky and looking at stars on the horizon); just curious.
     
  50. Infinity and Beyond! Thanks to Buzz Lightyear for that......


    If, as Rodeo so nicely set out before, the idea of thermal expansion being insignificant, the main reason/excuse for helicoid 'slack' at the far end has to be for correction of flange>sensor distance 'errors'.
    I know the basis for focusing a modern multi-group-multi-element optic isn't quite the same as moving a single lens element to and fro, but for non-zoom lenses, it's close. As long as the lens node can be set the 'correct' distance from the sensor, focus can be achieved. It doesn't particularly matter where the flange is in that set-up.
    Afterall, shimming the flange to the MAX, is an extension tube........:)... No Infinity though..:-(
    The other way round, is much, much more of a problem, flange-skimming is not really an option!
    _____
    Andrew, I guess that would mean, astrophotographically speaking, you could focus your 400mm 2.8, via live view, on some handy bit of the moon and apply a blob of super-glue...in focus for stars forever!!
    ...... however, that lens might be (will be?) out of focus on any other camera with a minutely different flange distance, particularly James's 'old' D800E!
    Someone with a mathematical mind could work out the rotational movement required to move the plane of focus from you>your shed>snowdon>stars. The 'negligible' bit might start getting as 'negligible' as the thermal expansion bit before mentioned. I fear 'negligible' x 'negligible' might be significant! DofF is not going to help with out-of-focus stars!
     
  51. Mike: I promise, when I eventually get the 400 f/2.8 that I've been wanting ever since I started photography, that I won't superglue any of it.

    Incidentally, Thom Hogan notes that the current 400 f/2.8 isn't quite as sharp "at infinity" as at moderate distance. I'm choosing to believe that's a deliberate sop to astrophotography, because it strikes me that, if I had absolutely perfect lens resolution, my D800E would have been quite a bad choice for stars (point light sources, a bayer sensor and no low pass filter are a bad mix). But I could always adjust the focus slightly to solve that. Should I risk my D800 on my Dobsonian (aperture, yes; equatorial mount, not so much) I somehow doubt the optics are going to manage to produce this problem.
     

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