Role of 6-bit code vs. mount in Vignette correction

[carbon_dragon]

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I'm trying to understand exactly what causes the M8 to attempt to correct vignetting in the M8 for very wide angle lenses. My previous understanding was that the 6-bit code informed the M8 what lens it had, and it then initiated a correction especially for that lens.

 

However, in a conversation I'm having with Popflash, it seems as though Zeiss makes a special version of some lenses, notably the 18mm, which has a variation of the mount (without a 6-bit code) which is supposed to cause the M8 to make a general vignette/cyan drift correction to the picture. Is this possible? If so how is this working? Could someone explain? Thanks.

[ho_co]

The lens flange sets the frame preview lever in one of three different positions, while each lens has its own unique 6-bit code.

 

If I understand correctly, the firmware needs for the framelines and the lens code to agree; if they don't, the camera doesn't recognize the lens.

 

For their wideangles, Zeiss historically didn't necessarily follow Leica's practice. Since you're going to be using an external finder anyway, who cares which finder frame is shown?

 

But now starting with the M8, lens recognition requires a match between the framelines and the lens ID code.

 

In some cases, one can have the standard Zeiss mount coded and manually hold the preview lever in the needed position while shooting. But it's more convenient to mount a coded lens and shoot, so in many cases, Zeiss can offer the alternative mount. For a camera that doesn't pay attention to the lens codes, there's no need to change the mount. For the digital Leica rangefinders, it can be very helpful.

 

Tony at Popflash can probably explain it better than I. Search for coding threads on the forum for more information. See also Carsten Whimster's list at Leica M Lens Codes.

[luigi bertolotti]

Howard explanation is clear, and, I think, correct : the coding is patent-covered, so Zeiss cannot offer it; on the contrary, they can make the mount to activate a certain frame onto M8 (I think the 28), "silently encouraging" customers to have a private-made coding, so having a right correction applied.

Or... they could be even more tricky... ... WATE and Elmarit 21 un-asph have both a "single black bit" code... on the Zeiss 18, make a bayonet apt for the correct frame, and a mount's surface that, "by chance" has "something black" (a tiny screw, a little black logo...) just in the right position... you do not make a six-bit string (patent infringement) but, as happens with some old Leitz lenses, if you set lens recognition ON, the "black thing" is read , and a certain lens is recognized...

[hoppyman]

Howard has provided good information (as usual). For the M8, owing to the agreement requirement, you need to decide which Leica lens that you wish to simulate for correction purposes. I had the same lens from Popflash and their support was superb (everytime I've dealt with them). If you have a mount which brings up the same frame lines as for the Wide Angle Tri Elmar then you can code for that. If you want to code for the newer Super Elmar 18 then you need your lens to bring up the frame-lines for that. Yes in both cases even though you do not get 18mm framelines in the finder you still need agreement. Mine had the mount to allow for coding for the WATE and I found that the correction was excellent. That is what I would recommend. The WATE defaults to 18 anyway in the menu selection.

Forum rules allow for discussion and images from other lenses on Leica cameras so here is an example from that lens (when I had it) on the M8. This should give you an idea that it works well on the M8, it's sharp and contrasty and the correction is fine. Now of course, I'd buy the superb new Super Elmar, but it wasn't available then. Incidentally the M9 does not require the agreement so the framelines make no difference at all and of course you get the same field of view (as this on the M8) with a 24.

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[carbon_dragon]

The lens flange sets the frame preview lever in one of three different positions, while each lens has its own unique 6-bit code.

 

If I understand correctly, the firmware needs for the framelines and the lens code to agree; if they don't, the camera doesn't recognize the lens.

 

For their wideangles, Zeiss historically didn't necessarily follow Leica's practice. Since you're going to be using an external finder anyway, who cares which finder frame is shown?

 

But now starting with the M8, lens recognition requires a match between the framelines and the lens ID code.

 

In some cases, one can have the standard Zeiss mount coded and manually hold the preview lever in the needed position while shooting. But it's more convenient to mount a coded lens and shoot, so in many cases, Zeiss can offer the alternative mount. For a camera that doesn't pay attention to the lens codes, there's no need to change the mount. For the digital Leica rangefinders, it can be very helpful.

 

Tony at Popflash can probably explain it better than I. Search for coding threads on the forum for more information. See also Carsten Whimster's list at Leica M Lens Codes.

 

That was what I thought I understood, but I was somewhat confused by what I got from Popflash. I had asked what "Modified for the M8 rangefinder" might mean for the 18 since I didn't think frame line mattered. He said

 

The lens is not 6-bot coded, as Zeiss does not have the rights to 6-bot code

lenses. The modification is on the flange, and the modified version of the

lens will bring up the wide angle frame lines. With this, the M8/M8.2 will

recognize the lens as a wide angle lens. Whereas a pre-M8 modified Zeiss

lens mounted on the M8/M8.2 would not be recognized as a wide angle and

result in cyan corners on the images. The modification was done by Zeiss at

the time of production.

 

I asked for some elaboration and he told me:

 

The 18mm will bring up the 28/90mm framelines, which is only useful for the

camera recognizing the wide angle lens being mounted. The frameline itsef

is not what makes the M8 change the image, but the notch on the lens that

tells the camera a wide angle lens is mounted happens to also bring up the

framelines. The frameline is more of a way for the user to know that the

lens will work correctly on the camera, and that the camera is recognizing

the lens correctly. The role of the 6-bot coding is to know exactly which

lens is mounted on the camera.

 

That's why I made my post. It sounded like there was something else involved. And I checked the Zeiss site and they DO make alternate variants of the mount for the 18 or so it looks. It made me wonder if there are two indicators on the mount, one the coding and one this "notch". Perhaps the latter gives the firmware the jog to do a general correction and the coding gives it the specific lens to the specific correction? You can see how I'm confused. But I don't want to buy the lens without really understanding the situation. Thanks.

[luigi bertolotti]

Sorry, but the dealer's explanations are not convincing me : AFAIK, to apply the correction for the "cyan corners" (problem which, in turn, arises when you put a UVIR filter on the lens) the M8 must READ a code on the flange AND the VF frame (bayonet mount - activated) must be consistent with the code itself : example :

 

1) Code corrispondent to Elmarit 28 asph - 28mm frame in the VF -----> correction applied

2) Code corrispondent to Elmarit 28 asph - 50mm frame in the VF-------> correction unapplied.

 

Of course with M8 set to "lens recognition ON+UVIR"

 

I have never heard of a "general wideangle correction" : but I admit that never made a specific test... it's easy to verify : I, for instance, can do it with my old Super Angulon 21... it is uncoded : one shot with the 28 frame, another with 50/75 frame hand-activated: with a proper subject is easy to verify if behavior at angles is different.

[earleygallery]

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I think this is wrong - the lens code only is necessary, there is no connection with the viewfinder framelines (if you look at Mark Norton anatomy thread there are switches linked to the lens mount which work for the tri-elmar, as that can only have one code of course).

[luigi bertolotti]

I think this is wrong - the lens code only is necessary, there is no connection with the viewfinder framelines (if you look at Mark Norton anatomy thread there are switches linked to the lens mount which work for the tri-elmar, as that can only have one code of course).

 

James, can be you are right: this question of code AND proper frame hasn't ever covinced me... but I red it more than once in the forum, and never tried to verify it in practice (it is easy) : anyway, the above explanations from the dealer seem to indicate that a correction is applied even without coding.. and this, imho, is uncorrect.... well if tomorrow have time, I'll try to make some tests...

[hoppyman]

James and Luigi, with the M8 you definitely need both the correct frame-lines AND the correct 6 bit code. The firmware requires this logic to work. More precisely any code from a lens that brings up the compatible frame-lines will work. It just isn't logical to mis-code it though. As Howard commented, it is possible to manually move the frame-line selector to get the agreement you need, but it's not as convenient. It is only relevant if you havve the coding as well though.

With the M9 you do not need this agreement, you can fit any M mount coded or non-coded lens and manually select any Leica lens (that was available uncoded, ones that were never sold un-coded cannot be manually selected) and the camera will apply the corrections for that one, whether or not they make sense.

Trust me, I have physically checked this on both my M8 and M9.

In the case of the Zeiss 18 that the OP has mentioned, it normally brings up the 50/75 frame-lines but it can be modified to bring up the 28/90 framelines (The dealer is offering the lens with the 28/90 modification).

If the mount had 6 bit coding added (not by Leica or Zeiss of course) then one mount will work if it is coded as the WATE and the other mount will work if it is coded as the Super Elmar. If you applied the wrong code it just will not work.

It is also possible to purchase an after-market mount which already has the machined pits to allow any coding, of course.

There isn't a 'general'wide angle vignette correction, although for practical purposes it may not differ much from lens to lens (especially if they have similar vignetting characteristics). Having said that the lens DOES work with no coding or correction and the OP may not find the difference significant.

The wide angle Tri-Elmar has a unique relationship in that the variable focal length is not mechanically signalled (unlike with the discontinued Tri-Elmar) and so if the lens gives frame-line agreement AND the WATE 6 bit code then the menu displays a choice to select 16-18-21. Conveniently for people coding that Zeiss 18 as a WATE the default menu choice is 18 so you can ignore the menu option (and get both 18mm in the EXIF and the vignette correction for the 16-18-21 set at 18

[jaapv]

I think this is wrong - the lens code only is necessary, there is no connection with the viewfinder framelines (if you look at Mark Norton anatomy thread there are switches linked to the lens mount which work for the tri-elmar, as that can only have one code of course).

James - sorry - you are wrong. The lens mount must be the corresponding focal length, otherwise the coding will not be read (unless you push the frame selector lever to the right position manually).

[jaapv]

Geoff, I think the actual correction for the different focal lengths of the WATE does not differ. The only rationale for choosing the focal length in the menu is to get it correctly in EXIF.

[carbon_dragon]

So Geoff, if you buy the 18 with the M8 modification (28/90 frameline) then you code it as the WATE. If it is the normal 18 (50/75 frameline) then you code it as the Leica 18. Right? Or you do what everyone else does and get the coding kit and keep trying things till one works.

[Johnny_Johnson]

James - sorry - you are wrong. The lens mount must be the corresponding focal length, otherwise the coding will not be read (unless you push the frame selector lever to the right position manually).

 

If that is the case I would have thought that the converse would be true - if you moved the selector off of its normal position that the body wouldn't read the lens code. I just tried that experiment with three different lenses. With a 50mm Summicron and a 75mm Summarit the body read the code correctly with the switch in both the 24/35 and 50/75 frameline positions. I then tried a 28mm Elmarit and the body correctly identified that lens with the selector in each of the three different positions. I wonder if my M8 body is behaving differently from what would normally be expected or if the relationship between coding and displayed frame lines is a little more complicated than the conventional wisdom would lead one to believe. Or, maybe my original premise is based on faulty logic.

 

Anyone else want to try a similar experiment and report their findings?

 

Later,

Johnny

[sandymc]

The way the M8 (and M9) appear to work internally is as follows: Internally there is an 8-bit code. That 8-bit code is formed from the 6-bit lens id code, and the 2-bit frame selector position code - this is the 8-bit code that's in the MakerNotes. So there are actually four possible look-ups for each 6-bit lens code (of which only three are valid, as there only three frame selector positions). Obviously, the combinations of lens code with "correct frame lines", e.g., a 50mm lens with the frame lines in the 50mm position, bring up the correct lens name and corrections. The "wrong frame lines" may or may not bring up anything at all. E.g., in the case of the original versions of the Zeiss lenses, the combination of the lens code and frame line selector indexes into an empty table.

 

Why do some "wrong" combinations of lens code and frame selector still bring up the correct lens id? I would guess that Leica itself hasn't been 100% consistent about frame selector positions, so they chose to code all of the frame sector positions for some lenses, so the frame selector is effectively ignored.

 

Sandy

[ho_co]

Geoff, I think the actual correction for the different focal lengths of the WATE does not differ. The only rationale for choosing the focal length in the menu is to get it correctly in EXIF.

 

Jaap, I don't think I've disagreed with you before, so I'm probably not understanding fully.

 

You're right that the WATE was designed to have a single correction throughout. But with the M8 and UV/IR-Cut filters, each different focal length gets a different correction. I've messed up often enough to be able to guarantee that. (I can easily enough remember to change two of the three settings--lens, finder, camera--but, man, I mess up one or another of the three every time I use it with the M8.)

 

If, as I assume, the M9 goes with the original design intention, then your statement holds for the M9.

 

But with the M8, there are different corrections. In fact, the "what focal length?" query only arises when one has Lens ID + UV/IR turned on.

[SJP]

I believe the lens recognition function involves two corrections on the M8:

1 - vignetting from the sensor - this is governed by the distance between the rear element and the sensor, which defines the angle of incidence on the microlenses at the edge. AFAIK this is independent of the focal length setting of the WATE as changing that only requires shifing of internal elements. EDIT just examined the WATE datasheet and I am not sure about this, can someone that owns a WATE check?

2 - green/magenta vignetting from the UV/IR filter, this is a function of the focal length. The shorter the focal length the more acute the angle of incidence of the capured light becomes. This correction is the same for all lenses of the same focal length, independent of lens brand etc. I could be different for different brands of UV/IR filters so using Leica filters is the preferred route. When changing the setting of the WATE this correction has to change.

 

Correction 1 could also remove lens vignetting (e.g. on the Noctilux) but I don't think they have included that to keep the "character" of the lens. Note that cornerfix also does these corrections.

[stunsworth]

Correction 1 could also remove lens vignetting (e.g. on the Noctilux) but I don't think they have included that to keep the "character" of the lens.

 

The M8 does remove some of the vignetting from a Noctilux.

[SJP]

The M8 does remove some of the vignetting from a Noctilux.

As it should due to the crop factor, but does it do more than that?

[jaapv]

Jaap, I don't think I've disagreed with you before, so I'm probably not understanding fully.

 

You're right that the WATE was designed to have a single correction throughout. But with the M8 and UV/IR-Cut filters, each different focal length gets a different correction. I've messed up often enough to be able to guarantee that. (I can easily enough remember to change two of the three settings--lens, finder, camera--but, man, I mess up one or another of the three every time I use it with the M8.)

 

If, as I assume, the M9 goes with the original design intention, then your statement holds for the M9.

 

But with the M8, there are different corrections. In fact, the "what focal length?" query only arises when one has Lens ID + UV/IR turned on.

There are three different settings. But users have reported there was no visble difference in correction whether you use one or the other for a different focal length. In other words, you do not need to return to the menu when you switch angles, except if you want to have exif correct.

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Hallo,

Look here: Leica M

[stunsworth]

As it should due to the crop factor, but does it do more than that?

 

Sorry, I meant the firmware removes some of the vignetting.