Back focus
The flange focal length (also known as the flange focal length ) is a term from technical optics . It defines the distance between the image plane , i.e. film or image sensor , and the mounting surface of the lens .
In many optical devices, the back focus is a fixed value. However, this is not always the case.
Photo technology
The back focus refers to the distance between the film plane (or sensor plane) and the contact surface of the lens.
Different camera systems use different support dimensions, which should be adhered to within tolerances of ± 0.01 mm, if possible, so that the focus of the lens corresponds to the distance scale on it. This is important for the use of cameras without direct focus control of the image produced on the surface of the film or image sensor. The function of rangefinder cameras , in particular, depends on the precise adjustment of the flange focal length of the camera and lens. If the back focus of the camera is greater than the back focus for which the lens was designed, the infinity setting of the lens can no longer be achieved, because in infinity setting parts of the lens system reach their shortest distance from the image plane. However, if the flange focal length of the camera is shorter than the flange focal length for which the lens used was designed, the shortest object distance at which the lens can still be focused increases (close focus limit). However, if the difference in the back focus is greater than the distance by which the lens is shifted when focusing, then focusing is no longer possible here either. The difference in flange focal length can be compensated for by using additional intermediate rings.
There are special measuring gauges to adjust the flange focal length, deviations from the camera standard are compensated by the camera service if necessary using thin washers under the bayonet ring.
The position of the film plane is usually marked by a line with a small circle in the middle - usually at the top - on the camera housing. This mark allows a reading accuracy of 0.2 mm at best and is the basis for measuring the distance to the object being photographed equal to the distance to be set on the focusing ring of the lens, particularly relevant for macro shots and other special tasks for calculating the image scale and determining the position of the object plane. With the simplest optics (thin lens or pinhole camera ), this distance corresponds to the sum of image distance + object distance .
In general, it should be noted that single-lens reflex cameras (SLR) have a significantly larger focal length than rangefinder cameras because of the space required for the mirror. This is particularly true for medium format single lens reflex cameras such as the Pentacon Six .
From the point of view of the most diverse possible adaptation options for third-party lenses to a camera system, the back focus of the camera should be as small as possible, which, so that the oscillating mirror does not collide with the lenses, can sometimes only be achieved using a special folding mechanism for the oscillating mirror. Because only if the flange focal length of the camera is smaller than that of the lens, an adapter ring can be built which, placed between the camera and the lens, compensates exactly for the difference in the flange focal length. Otherwise, however, since the image plane of the lens would in this case lie behind the plane of the film or sensor, the possibility of focusing at infinity would be lost. Many medium-format lenses with their large flange focal lengths and image circles can be easily adapted to SLR cameras using special adapters and can even be used as tilt-and-shift lenses , while the reverse is hardly possible.
On the other hand, a larger flange focal length of the camera allows larger swing-back mirrors with otherwise the same structural effort, which is advantageous with regard to the closest possible match between the viewfinder image and the film image. In the case of digital camera systems, it may also be the case that with a shorter focal length the course of some of the light beams that contribute to the imaging is more oblique, which can be problematic for some image sensors, as these usually (unlike classic analog photography on photosensitive film) have angle-dependent sensitivities exhibit.
In practice, however, the advantages and disadvantages of combining different support dimensions are few. At the same time, certain lenses cannot be combined with certain camera systems because their support dimensions are too short or too long. Nikon SLRs , for example, have a rather long back focus, which severely limits the selection of third-party lenses that can be used with these cameras. There are adapter rings with a built-in correction lens for individual cases in which adaptation is desired, although the back focus of the camera is too large for the combination of the selected lens. Without a corresponding additional optical element, its use is limited to focusing at relatively short object distances (close-ups).
Video technology
The back focus is usually fixed on cameras with a permanently installed lens. Interchangeable lenses with a standardized bayonet, for example B3 or B4, can be used on cameras from different manufacturers. With these lenses, the back focus can be adjusted manually, for example, to be able to compensate for manufacturing tolerances and temperature differences.
When adjusting the back focus, the rearmost lens group is shifted in the direction of the lens axis just in front of the image sensor or film. The set focus range is only constant for all focal lengths if the back focus is set correctly.
The flange focal point is at the rear of the lens. Often there is also a snap-in additional ring that is used for macro shots . This ring moves the same lens group as the flange focal distance ring, but, in contrast to this, can be snapped back into the starting position when the macro function is no longer required.
To set the back focus, a picture with strong contrasts is required (usually a so-called Siemens star ). The star is placed 3–5 meters in front of the camera for adjustment. The camera should be at the same height as the Siemens star. The lighting must be such that the image can be clearly seen with the aperture completely open . If it gets too bright, it is advisable to connect an ND filter or adjust the shutter.
Tube monitors that offer the Blue Only function show, due to an actually undesirable side effect, a clear flicker in the Siemens star when the sharpness is right. This property helps when setting the back focus. LC monitors cannot do this, even if they have a "Blue Only" button.
If the back focus is incorrect, once the focus has been set it is not correct for the entire focal length range. This is a problem especially when zooming , ie changing the focal length in "ON", because here you would have to readjust the sharpness manually. You also have to rely on the viewfinder or a monitor when assessing the sharpness of the image , which often does not allow suitable sharpness control.
Overview of common support dimensions
| Mirrorless photo cameras | ||
| connection | Back focus [mm] |
Remarks |
|---|---|---|
| 35mm rangefinder | ||
| Braun Paxette M39 × 1 thread | 44.00 | typical flange focal length for M39 lenses, with the exception of the Leica M39 (also called L39) and the Russian mirror reflex M39 ; a typical M39 lens therefore requires a spacer ring for Leica use |
| Altix | 42.50 | Interchangeable lenses Altix V |
| Contax | 34.85 | Lens bayonet for Carl Zeiss Contax I to III (1932 to 1945), IIa and IIIa (1950 to 1962); compatible with Kiev 1 to 4m (1946 to 1987); conditionally compatible with Nikon-S (flange focal length and lens bayonet are the same, but the distance measurement is only within the tolerance if the lens has a focal length of no more than 35 mm); not to be confused with Contax-G , -N , -645 , -S and -RTS / -MM / -MT / -ST / -S2 / -RX / -AX |
| Nikon S | 34.85 | Conditionally compatible with Contax (flange focal distance and lens bayonet match, but the distance measurement is only within the tolerance if the lens has a focal length of no more than 35 mm) |
| Contax-G system | 29.00 | belongs to a camera system from Kyocera , whose cameras were manufactured in the mid-1990s |
| Leica M39 × 26tpi screw thread | 28.80 | an M39 thread with a 1/26 ″ pitch, also known as an L39 thread for Leica adapters; Lenses with this flange focal length and the very similar metric thread M39 x 1 were made in the USSR and Japan when copying the Leica thread; Experience has shown that both types of lenses are interchangeable, but threads that do not fit exactly on the pitch only grip a small part of the helical line and are therefore less resilient as an odd pair and more prone to wear |
| Nikonos | 28.00 | |
| Konica KM bayonet (Hexar RF) | 28.00 | Compatible with the Leica M bayonet (the difference in the nominal support dimensions results from different measuring methods - Konica measures from the bayonet support surface to the film pressure plate; Leitz / Leica measures from the bayonet support surface to the center between the film guide rail and the film pressure plate - effectively it is the same flange focus) |
| Leica M bayonet | 27.80 | also Voigtländer VM, Epson EM and Zeiss ZM; compatible with the Minolta M bayonet |
| Minolta M bayonet (CL / CLE) | 27.80 | Compatible with the Leica M bayonet |
| Digital mirrorless camera systems | ||
| Fujifilm GFX bayonet | 26.70 | Medium format |
| Samsung NX | 25.50 | Samsung NX series (discontinued in 2016) |
| Canon RF bayonet | 20th | Canon EOS R |
| L-mount | 20th | L-bayonet alliance: APS C: Leica T , CL and TL2; Full format: Leica SL (from 2015); Panasonic (2019) and Sigma (with Foveon - image sensor ) |
| Micro Four Thirds | 19.25 | Panasonic Lumix G , Olympus series EP and OM-D |
| Sony E-bayonet (E-mount) | 18th | also called ILCE and NEX mount; System cameras of the Alpha and Alpha NEX series (APS-C and 35mm format); not to be confused with Olympus E-bayonet and Sony A-bayonet (which was produced for another part of the Alpha series) |
| Canon EF M bayonet | 18th | Canon EOS M and successor (APS-C) |
| Fujifilm X bayonet | 17.70 | Fujifilm X series (APS-C) |
| Nikon CX | 17th | Nikon 1 series (discontinued in 2018) |
| DJI DL | 16.84 | DJI Zenmuse X7 |
| Nikon Z bayonet | 16 | for 35mm format |
| Pentax Q bayonet | 9.20 | Pentax Q |
| Photo cameras with mirror technology | ||
| connection | Back focus [mm] |
Remarks |
|---|---|---|
| 35mm SLR | ||
| Novoflex A bayonet (quick shot) | 63.3 | measured; not to be confused with Sony A-bayonet and Minolta A-bayonet |
| Vivitar T2 screw thread (M42 × 0.75) | 55.00 | not to be confused with M42 × 1 screw thread |
| Praktina clamp bayonet | 50.00 | |
| Contax-N system | 48.00 | belongs to a camera system from Kyocera, the Contax N1 and NX, which were manufactured from 2000 to 2005; Compatible with the Contax N Digital from 2002 |
| Icarex BM | 48.00 | |
| Leica R bayonet | 47.00 | |
| Nikon F bayonet | 46.50 | Ai, Ai-S |
| Olympus OM bayonet | 46.00 | |
| Contarex | 46.00 | |
| Yashica MA bayonet (230AF etc.) | 45.8 | measured indirectly |
| Mamiya ZE bayonet | 45.50 | Mamiya ZE, ZM, NC, Sears CS |
| C / Y bayonet | 45.50 | Yashica and Contax RTS system (a camera system from Kyocera, suitable for Contax RTS I to III and Contax 159MM, 167MT, ST, S2b, RX, AX and RXII from 1974 to 2005); As a special feature of all other 35mm SLR cameras with autofocus, the Contax AX focuses by shifting the film plane instead of adjusting the lens, whereby manual lenses are automatically focused |
| M42 × 1 screw thread | 45.46 | Pentacon (not Pentacon Six ), Pentax, Contax S, Praktica, Zeiss ZS; not to be confused with M42 x 0.75 (T2 screw thread) |
| Pentax K bayonet | 45.46 | also called "PK Mount"; was licensed and installed by numerous manufacturers, u. a. from Ricoh, Chinon, Petri, Samsung |
| Soot. Mirror reflex (Zenith) M39 × 1 thread | 45.20 | can be adapted to M42 with a threaded ring, whereby a slight difference in flange focal distance to the 45.46 mm of the M42 remains (if an adapter is used with a camera with a smaller flange focal length that compensates for the distance to M42, it can be increased by 0 , 26 mm do not use the focus area to infinity - sometimes these adapters are inherently thinner, so that post-processing is not necessary; depending on the lens, the distance setting can alternatively be adjusted accordingly). |
| Exakta bayonet | 44.70 | Exakta-Kine, -RTL and -Varex (VX for short); Most Exa cameras also use the Exakta bayonet (apart from the Exa Ib and 1c models, which are equipped with the M42 thread connection ); not to be confused with the Exakta 66 bayonet |
| Topcon RE bayonet | 44.70 | like Exakta |
|
Rollei 35mm QBM bayonet (e.g. Rollei SL-2000, 3003, SL350, SL35) |
44.50 | of Carl Zeiss AG confirmed value for the QBM bayonet (QBM mount). |
|
Voigtländer QBM bayonet (models: VSL 1 [II. Version] VSL 2 and 3) |
44.50 | identical to the Rollei 35mm QBM bayonet. The wrong value 44.70 was widely distributed by US websites. |
| Minolta A bayonet (AF) | 44.50 | also called A-Mount; AF, AF-i, AF-xi, AF-D, AF-SSM; compatible with A-bayonet from Konica Minolta and Sony ; not to be confused with Novoflex A bayonet |
| Praktica B bayonet | 44.40 | |
| Canon EF bayonet (EOS) | 44.00 | |
| Praktiflex M40 × 1 thread | 44.00 | |
| Sigma SA bayonet | 44.00 | Sigma SA-9, SA-300 |
| Minolta SR bayonet (MF) | 43.50 | Minolta SR, MC, MD |
| Fujica X bayonet | 43.50 | |
| Wrayflex M41.2 thread | 42.05 | |
| Canon FD bayonet (FD / FL) | 42.00 | |
| Miranda bayonet | 41.50 | |
| Konica F bayonet | 40.50 | |
| Konica AR bayonet | 40.50 | |
| Alpa bayonet | 37.80 | |
| Robot | 31.00 | |
| Olympus Pen F bayonet (half format) | 28.95 | |
| Digital mirror reflex (with oscillating mirror or partially transparent mirror) in APS or small image sensor format | ||
| Contax N bayonet | 48.00 | belongs to a camera system from Kyocera, the Contax N Digital from 2002; Compatible with the analog SLR 35mm camera Contax N1 and NX , which were manufactured from 2000 to 2005 |
| Nikon F bayonet | 46.50 | The designation "DX" means that the lens is suitable for an APS-C sensor format like the Nikon Pronea 600i (1996) and Pronea S (1998) / the designation "FX" means that the lens is suitable for sensors in 35mm format |
| Pentax K bayonet | 45.46 | also called "PK Mount" |
| Konica Minolta / Sony Alpha A bayonet | 44.50 | also called A-Mount; the designation "DT" means that the lens is suitable for an APS-C sensor format like the Dynax 5D and 7D , Sony-Alpha SLT-55 , SLT-77 / if the designation is missing on the lens, it is mostly for sensors suitable in 35mm format as with the SLT-99 ; the bayonet is compatible with Minolta A bayonet ; not to be confused with Novoflex A bayonet |
| Canon EF bayonet (EOS) | 44.00 | also EF-S |
| Sigma SA bayonet | 44.00 | Sigma SD9 / SD10 / SD14 |
| Olympus E-Bayonet ( Four Thirds ) | 38.85 | also Panasonic and Leica; not to be confused with Sony E-bayonet |
| Minolta V bayonet | 36.00 | The designation "D" or "DX" means that the lens is not suitable for sensors in 35mm format, but only for an APS-C sensor format like the Minolta Vectis S1 (1996) and Vectis S100 (1997) |
| Medium format single lens reflex | ||
| Mamiya RB67 bayonet | 111.00 | |
| Mamiya RZ67 bayonet | 108.00 | |
| Rollei SL66 | 102.80 | |
| Pentax 6 × 7/67 | 85.00 | |
| Kiev 80/88 thread | 82.10 | Hasselblad 1600F replica |
| Hasselblad thread (1000F / 1600F) |
82.10 | |
| Hasselblad bayonet (200, 500, and 2000 series) |
74.90 | |
| Pentacon Six | 74.10 | also Exakta 66 (not Exakta bayonet ), Kiev medium format; not to be confused with Pentacon M42 |
| Praktisix clamp bayonet | 74.10 | like Pentacon Six |
| Rollei SLX | 74.00 | |
| Pentax 645 bayonet | 70.87 | |
| Contax 645 | 64.00 | belongs to a camera system from Kyocera, whose Contax 645 medium format camera was manufactured from 1998 to 2005 |
| Mamiya 645 bayonet | 63.30 | |
| Leica S | 53 | Digital camera system |
| Video cameras | ||
| connection | Back focus [mm] |
Remarks |
|---|---|---|
| Ikegami | 76.94 | (3.0291 inch) |
| Arri Maxi PL (for 65 mm) | 73.50 | |
| B4 (Sony) via prism | 65.24 | (2.5685 inch) |
| Moviecam BNC | 61.468 | (2.4200 inch) |
| Panavision (PanaMount) | 57.15 | (2.2500 inch) |
| Arri standard | 52.00 | (2.0472 inch) |
| Arri bayonet | 52.00 | |
| Arri PL | 52.00 | |
| BTS | 50.45 | (1.9862 inch) |
| eclair | 48.00 | (1.8898 inch) |
| B4 (Sony) in the air | 48.00 | |
| Aaton | 40.00 | (1.5748 inch) |
| CP | 38.10 | (1.5000 inch) |
| E-mount | 18.00 | (0.708661 inch) also called ILCE and NEX mount; compatible with Sony E-bayonet ; not to be confused with Olympus E-bayonet |
| C-mount | 17,526 | (0.69 inch) |
| CS mount | 12.50 | |
| D-mount | 12.29 | |
Individual evidence
- ↑ http://www.dslr-forum.de/showthread.php?t=782645 DSLR forum, contribution by * Maverick *, December 23, 2008.
- ↑ http://www.natur-makro.de/tipps-nah limit.htm near-setting limit, www.natur-makro.de - Tips for macro photography, Toni Kanitz, 2005.
- ↑ http://www.l-camera-forum.com/leica-wiki.de/index.php/Kategorie:Leica_Schraubgewinde Leica screw thread, Leica Wiki, February 4, 2009, accessed September 1, 2015.
- ↑ Rico Pfirstinger: Inside the Fujifilm GFX 50S. Fuji, September 21, 2016, accessed October 23, 2016 .
- ↑ EOS R System White Adapter. (PDF) In: canonrumors.com. canon USA, September 5, 2018, accessed September 5, 2018 .
- ↑ L-Mount Questions and Answers. Manufacturer website. Retrieved July 12, 2019.
- ↑ a b forum post from October 27, 2006 by Matthias Paul, administrator in the Minolta forum information on the Kyocera Yashica MA bayonet for Yashica AF SLRs
- ↑ Alexander Schulz: Zenit - The history of the Russian single-lens reflex prism viewfinder camera with M 39 lens connection. Lindemanns Verlag, 2003, ISBN 3-89506-242-1 , p. 89.
- ↑ Jupiter 8 (M39 mount) with Sony A7ii Infinity Focus Problem , dpreview.com, accessed July 6, 2018.
- ↑ Jon Fauer: New Lens Mount FFD and ID Chart , fdtimes.com, October 12, 2016, accessed October 23, 2016.
