My special thanks to David Kirby B.Sc. for help with the English version.

 

 

Canon EOS 5D (original EF mount)

 

 

Canon EOS 5D (Minolta MC/MD mount)

In this article I will review the possibility of mounting all 35mm manual focus Minolta lenses (Auto Rokkor, MC/MD Rokkor, MD) on a modern Canon Full Frame (24x36mm) DSLR, in this case the well known Canon EOS 5D.

Some technical details will be included here to help explain how the adopted solution was arrived at and also to demonstrate how we can transform the Canon 5D into a "customised" DSLR that can accept any Minolta manual focus lens. Two main constraints were accepted in implementing this solution. Firstly, it should only use a mechanical adapter, with the aim of maintaining all the original optical features of a Minolta lens, in particular the infinity focus and the angle of field of the lens. Secondly, it must not involve any changes to the MC/MD Rokkor lens mounting bayonet.

This solution was based on introducing minor changes to the second version of the EOS-MD flange [2] designed for the APS-C DSLR. Accordingly, the new flange was called "EOS-MD type 3". It was designed therefore, with an optical register of 43.5mm [2] using a mechanical adapter with no optical elements. Focus to infinity is possible even with minimum f-stop values (i.e. f1.4 or f1.7 for example) because the lens works to a "real infinity" distance. Also, the distance between the focal plane (the 24x36mm CMOS sensor) and the subject in focus corresponds with the figure we see on the distance scale on the MC/MD lens barrel (this is useful, for example, when using hyperfocal distance with a wide angle lens). In this way we can preserve all the original optical features of the MC/MD Rokkor lens on a Full Frame DSLR and in fact the angle of view, the depth of field, the focus to infinity, the out-of-focus (bokeh) and other optical features are "all" fully maintained by the new flange. Further, it does not introduce any loss of light or crop factor to the final digital image.

As mentioned before, from a design point of view, the EOS-MD type 3 flange is basically a type 2 [2] with some changes. What are these changes ? Firstly, the CANON 5D's original EF flange is 0.02mm shorter than the one for the APS-C DSLR, so the thickness of the type 3 flange was changed accordingly. Secondly, the point of stop of the MC/MD lens on the type 3 flange was changed and now the lens is aligned with the vertical axis just as it would be on an original Minolta SLR film camera. This is necessary in order to avoid "vignetting" when using extremely wide angle lenses like, for example, the MC Rokkor 16/2.8 fisheye lens with "fixed" lens hood.

Now, let us briefly review the basic details about the optical register [2] of a lens.

 

fig. A - the optical register

 

fig. B

 

 

The optical register: what does it means ?

The possibility of using a lens (with say, register X) on a DSLR (with register Y) is essentially linked to the concept of optical register.

The optical register is the distance (in millimetres) between the focal plane (CMOS sensor plane) and the flange's top plane (see the figure A). If the lens register is greater than the DSLR register (i.e. X>Y), the adaptation is easy to make and we can simply use an adapter ring on the DSLR's original flange [2]. If the lens register is shorter than the DSLR register, which is the case with Minolta MC/MD lens and the Canon 5D, it is necessary to modify the flange [2] (see the figure B).

The effect of reducing the register from 44mm to 43.5mm makes it possible to achieve a "real infinity" focus with a Minolta MC/MD lens at a fully open aperture, e.g. f1.4, without the need for any internal lens. Therefore the Minolta MC/MD Rokkor can be used without compromising either the optical design or the original optical quality in any way. Note that "real infinity" is quite different from the "virtual infinity" obtained, for example, by stopping down a 24mm lens at f16 and focussing on a subject 10 metres away[4]. Remember, a subject is only in focus on one plane, i.e. the main plane of focus, at a fixed distance from the object focal plane (the film or sensor) in accordance with the distance setting on the focus ring. The depth of field (and the circle of confusion) gives us the impression that the image is in focus a bit less than and a bit further than the distance of the main plane of focus from the object focal plane (the sensor). So, "real infinity" means that the infinity focus is not only achieved by stopping down the lens but is also possible with the aperture fully open when lens is set at infinity or just before infinity distances.

Therefore, from the point of view of optical performance, we can say that when mounted on the EOS-MD flange type 3, a Minolta MC/MD lens's behaviour is the same as it would be on a Minolta SLR film body such as the X-700.

What are the main aspect about

the type 3 flange ?

During the design phase of the EOS-MD type 3 flange some significant points were considered:

1) All The MC/MD Rokkor lenses must not suffer any alteration to the mounting bayonet. This choice was made because Rokkor lenses lose their monetary value when changes are made to the bayonet and at the same time they can increase their value over time, especially for rare and higher quality versions. By contrast DSLRs, both APS-C and Full Frame, lose their value over time so ultimately it is better to make changes to the camera whilst preserving the MC/MD lenses in their original state.

2) The internal geometry of a Canon DSLR [3] is the only one compatible with the shape of the Minolta MC/MD mount.

3) It must not behave like conventional adapter rings which, when mounted on the EOS body, restrict use to only short distances (macrophotography or just a few metres) or indeed the type of adapter ring with internal optical elements that allow you to reach "real" infinity focus but at the cost of poor optical quality of the final digital images. Also, they must avoid causing any loss of light due to the thickness of the adapter ring or add any multiplier factor to the focal length. On the contrary, performance must be the same as when using the MC/MD Rokkor lens on a Minolta SLR body in terms of optical behavior and infinity focussing.

4) It must not involve any modification to the electronic circuits inside the CANON DSLR and the installation steps must be fully or partially reversible (see below for details).

5) It permits the following lenses only:

  • All Minolta Manual Focus lenses (SR mount): Auto Rokkor, MC/MD Rokkor lenses including the MD type.
  • All universal lenses with Minolta MD mount (Vivitar, Tokina, Tamron Adaptall 2, ect)
  • All M42 (42x1) lenses (Zeiss Jena, Pentax Takumar, Russian lenses, Pentacon, etc.) using the M42-Minolta MD adapter "on" the EOS-MD flange

6)The Canon 5D (MK I and MK II) uses a larger mirror than the APS-C DSLR. It comprises two different parts: a plastic support which holds the mirror, (in dark gray in fig. C) and a thin glass "true" mirror, (in light gray in fig. C). In order to use Minolta MC/MD lenses on the flange type 3, both parts need to be trimmed by about 2.5mm (see fig. D for details) otherwise the mirror would collide with the base of the Minolta MC/MD lens as it flips up during exposure (see fig. D).

Now we should consider: is this mirror trimming a reversible process? It is possible to give three different answers to this question.

6.1) It is totally reversible if we replace both parts: the plastic support and the thin glass. Obviously this is the most expensive choice. The mirror will be as good as before the change.

6.2) It is partially reversible, by replacing only the thin glass and retaining the trimmed plastic holder. This option is less expensive and again, the mirror would still be as good as before even if the underlying plastic support remains trimmed.

6.3) It is not reversible if we leave both parts altered. This option has no further costs but may be noticeable when we replace the original Canon flange and lens (see fig. F).

Note that the trimming process does not affect the final image or the image that we would see through the viewfinder. In fact, the missing part of the mirror (about 2.5mm along the front side, see fig. E, F) would still be masked by the thin metal frame of the focus screen.

7) When using the EOS-MD type 3 flange, focussing the Minolta MC/MD lens is MANUAL ONLY and aperture values can only be set by stopping down manually.

8) The EOS-MD type 3 flange has a mechanical lock that sets the stopping point for the Minolta MC/MD lens but it does not have the traditional "lock pin". The Minolta MC/MD lens is held firmly (to prevent movement during focussing and stopping down) to the flange by a combination of friction and pressure effected by a sturdy steel ring. Is there a risk of the lens dropping out of the camera during focussing operations? No, because the lens is very tight on the flange when it is already near the point of stop and the pressure is at maximum when the lens reaches the point of stop.

9) The Canon 5D operates correctly only in the Manual mode (M) or Aperture Priority (Av) and light metering can be set to different modes (matrix, center, semi-spot, spot). For example: setting the 5D to Av mode and using a Rokkor wide-angle lens at hyperfocal distance can be useful for a dynamic subject that moves within a certain distance range. This is an example of true automatic mode operation using a manual focus lens !

And what about focus operations ?

Is it possible to achieve precise focussing using this flange with a manual focus lens like the MC/MD Rokkor? The answer is yes.

Because the original 5D focus screens are not accurate enough to use with a manual focus lens it is advisable to replace the original focus screen (i.e. the Ee-A) with a split screen (easily available new on www.ebay.com by searching with "eos split screen") or with the original Canon Ee-s. The split screens are, in general, accurate and precise but sometimes I have found that they can introduce a slight "backfocus". Obviously this backfocus (and also frontfocus) is an issue not related to the optical register modification introduced by the EOS-MD flange but to the thickness of the split screen which may be different from the original one. Further, the split screen on the 5D (and also on other DSLRs) introduces some exposure error in low light of about 2 f-stops, especially using the "spot" mode, because the spot area used by the DSLR's lightmeter and the split/microprism area can overlap and give a false reading. So, what is the best lightmeter mode? I have found that the "matrix" setting on the 5D (and also on other DSLRs) lightmeter gives the best results overall.

We can understand from the fig. G what happens inside a DSLR.

 

fig. G - inside a DSLR

In order to get perfect focus the path P1 must be equal to path P2, so P1 = P2.

If P1 > P2 then we get backfocus. This is a situation that can occur using a non-original focus screen (i.e. a split screen).

If P1 < P2 we have frontfocus.

Obviously it is possible to correct the backfocus by making an appropriate adjustment to P1 by adding a shim (see fig. H) between the split screen and the pentaprism (in order to obtain P1 = P2). Did you know that this is the procedure used by Canon service themselves by using a proprietary shim (see the Canon 5D service manual on page 11, CB3-2850-000)? So, we can make our own shim by taping the outer perimeter (only !) of the split screen, on the side facing the pentaprism itself, with a thin strip 1 mm (max 1.5 mm) wide of adhesive tape (my advice is to use the Scotch Magic 3 with thickness = 0.05mm). Calibration consists of finding the correct thickness (usually just 1 or 2 layers of tape would be necessary, with thickness equal to about 0.1mm) to get the focus correct by using the scale at 45 in the following article (see page 18). Note that this thin strip around the split screen will not be visible in the viewfinder because it is covered by the thin metal frame that holds the split screen below the pentaprism.

Conclusion

Perhaps you are thinking: can I do all the above changes to my Canon 5D by myself? How difficult are they to do? Again, there are two responses.

YES, they are quite easy to perform if you have good skills in dismounting cameras and have good tools for trimming the mirror.

NO, there is some risk of damaging the camera (the mirror in particular) if you do not have those skills and tools. So what do you do? It would be much better to take your own (expensive!) Canon 5D to a camera repair laboratory.

 

Finally, we have shown that a CANON 5D camera can be customized in order to mount all MC/MD Rokkor lenses by using a new EOS-MD flange, the type 3, allowing us to enjoy "all" the optical features of an original Rokkor lens on a digital sensor with 24x36mm format.

Do you think it is a dream? Are there still doubts about that ?

Again ... I hope not !

fig. C- The shape of Canon 5D's original mirror

fig. D - The 5D mirror lines of cut (in red bold)

fig. E - The shape of mirror once shaved.

fig. F - The Canon 5D's mirror once shaved

fig. H - Two split screens: without thickness (left),

with small thickness on the outer perimeter (right)

fig. L- Original shims for the Canon EOS 5D split screen adjustment (0.03mm e 0.05mm)

 

 

 

The first pictures of Rokkor lens on Canon 5D

Minolta MC W. Rokkor 17mm f4 @ f8 (Canon EOS 5D, 1/640s, 100iso)

 

Minolta MC Rokkor PF 58mm f1.4 @ f1.4 (Canon EOS 5D, 1/8000s, 100iso)

 

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