Back focus

from Wikipedia, the free encyclopedia

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
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
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)
Hasselblad bayonet
(200, 500, and 2000 series)
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
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

  1. DSLR forum, contribution by * Maverick *, December 23, 2008.
  2. limit.htm near-setting limit, - Tips for macro photography, Toni Kanitz, 2005.
  3. Leica screw thread, Leica Wiki, February 4, 2009, accessed September 1, 2015.
  4. Rico Pfirstinger: Inside the Fujifilm GFX 50S. Fuji, September 21, 2016, accessed October 23, 2016 .
  5. EOS R System White Adapter. (PDF) In: canon USA, September 5, 2018, accessed September 5, 2018 .
  6. L-Mount Questions and Answers. Manufacturer website. Retrieved July 12, 2019.
  7. 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
  8. 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.
  9. Jupiter 8 (M39 mount) with Sony A7ii Infinity Focus Problem ,, accessed July 6, 2018.
  10. Jon Fauer: New Lens Mount FFD and ID Chart ,, October 12, 2016, accessed October 23, 2016.

Web links

Wiktionary: Back focal length  - explanations of meanings, word origins, synonyms, translations