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01-31-2009, 12:10 PM   #1
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Removing paint from M42 lenses: resale value?

All,

In an extension of the tin-foil hack to enable Av mode on screw-mount lenses, one can also physically remove the paint from a bit of the lens itself. It works; I've tried it on a cheap Vivitar.

(If it's unclear what I mean, I can take and post a photo.)

I'd also like to hack my SMC Takumar 50mm, but as that's a much nicer lens, I worry about the resale value. Anyone have thoughts on how that will be affected by this hack?

Thanks,

Reid

01-31-2009, 12:45 PM   #2
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re-stain should be easy...

QuoteOriginally posted by rpriedhorsky Quote
All,

In an extension of the tin-foil hack to enable Av mode on screw-mount lenses, one can also physically remove the paint from a bit of the lens itself. It works; I've tried it on a cheap Vivitar.

(If it's unclear what I mean, I can take and post a photo.)

I'd also like to hack my SMC Takumar 50mm, but as that's a much nicer lens, I worry about the resale value. Anyone have thoughts on how that will be affected by this hack?

Thanks,

Reid
I think it is reasonable to recolor any area from which the finish has been removed with a black magic marker & mentioning to the purchaser it has been touched up. First do an experiment to determine which pins must be shorted so there's only a small area where metal must be exposed.

Also, it isn't clear to me on which bodies any shorting need be done (My K100D needs some shorting for sure...but I hear conflicting info on others.)
01-31-2009, 12:45 PM   #3
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Any sanding, grinding, cutting or stripping done to a lens will impact its resale value. This is particularly true if the lens is in excellent cosmetic shape and/or is potentially collectible. Unless you are planning on keeping a lens and using it forever, you had best leave it "stock" and forget doing a "custom" job.

Steve

(I like my used lens purchases to be as close to new condition as possible...unless I get them for free or nearly so...)
01-31-2009, 12:57 PM   #4
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Out of curiosity, why are you doing this mod to M-42 lenses? AV mode already works using stop-down metering with no modifications required. I can see how shorting the contacts might be useful in M mode, but not in AV.

Are you doing something special?

Steve

01-31-2009, 01:33 PM   #5
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QuoteOriginally posted by stevebrot Quote
Out of curiosity, why are you doing this mod to M-42 lenses? AV mode already works using stop-down metering with no modifications required. I can see how shorting the contacts might be useful in M mode, but not in AV.
On my K200d, the metering is accurate in M mode but not in Av mode. Adding a tinfoil shim makes it accurate in Av too. (More in my previous thread.)
01-31-2009, 01:54 PM   #6
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QuoteOriginally posted by stevebrot Quote
Out of curiosity, why are you doing this mod to M-42 lenses? AV mode already works using stop-down metering with no modifications required. I can see how shorting the contacts might be useful in M mode, but not in AV.
Steve, as far as I can tell (as a result of experiment as well as reading), my K100D at least requires shorting contacts to meter correctly & continuously in Av mode.

This may or may not be true of other Pentax models, but I am not lying. My K100D requires electrical contact with ANY lens to meter properly in Av mode. K100D is referred to hereafter.

In Manual mode it meters incorrectly until the AE_L button is pressed, contact or no contact.

If a piece of paper is inserted preventing electrical contact with ANY LENS, metering is incorrect on my camera in Av mode- pressing the AE-L or optical preview button has no effect until the camera is switched to manual mode. It'd be helpful to know if your camera behaves the same.

I just tried the paper trick with an SMC-F 50 1.7 in Av mode; with paper interfering with electrical contact the camera metered my computer screen at 1/500 sec. without paper it metered at 1/180sec. This is pretty definitive I'd say; I promise, I did not make this data up.

With my camera it is not necessary to use manual mode or the AE-L button if there is electrical contact, in which case Av mode meters properly.

We need to get this cleared up; until then let's qualify our statements using camera models.
01-31-2009, 02:53 PM   #7
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QuoteOriginally posted by newarts Quote
Steve, as far as I can tell (as a result of experiment as well as reading), my K100D at least requires shorting contacts to meter correctly & continuously in Av mode.

This may or may not be true of other Pentax models, but I am not lying. My K100D requires electrical contact with ANY lens to meter properly in Av mode. K100D is referred to hereafter.

In Manual mode it meters incorrectly until the AE_L button is pressed, contact or no contact.

If a piece of paper is inserted preventing electrical contact with ANY LENS, metering is incorrect on my camera in Av mode- pressing the AE-L or optical preview button has no effect until the camera is switched to manual mode. It'd be helpful to know if your camera behaves the same.

I just tried the paper trick with an SMC-F 50 1.7 in Av mode; with paper interfering with electrical contact the camera metered my computer screen at 1/500 sec. without paper it metered at 1/180sec. This is pretty definitive I'd say; I promise, I did not make this data up.

With my camera it is not necessary to use manual mode or the AE-L button if there is electrical contact, in which case Av mode meters properly.

We need to get this cleared up; until then let's qualify our statements using camera models.
The original post concerned M-42 lenses in AV mode. Those operate with a fully manual aperture when using the K-mount adapter. Unless you are always shooting full open, shorting the contacts will buy you nothing. Shorting the contact is only really useful with Pentax-M and Pentax-K series lenses.

Of course, what can I know, I don't have a K100D?

Steve

(Does shoot extensively with M-42 and non-A contact K-mount glass...have also done a bit of research regarding the metering issues with these vintage lenses...is also familiar with shorting the contacts...)
01-31-2009, 03:30 PM   #8
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QuoteOriginally posted by stevebrot Quote
Unless you are always shooting full open, shorting the contacts will buy you nothing. Shorting the contact is only really useful with Pentax-M and Pentax-K series lenses.
Hi Steve,

While this may be true on your K10d, on my K200d that's simply false. I double-checked today and my statements above and in the other thread remain true.

I'm not expecting automatic stop-down. What I'm after is correct metering for whatever aperture I've manually selected on the lens.

Reid

01-31-2009, 03:41 PM   #9
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QuoteOriginally posted by stevebrot Quote
The original post concerned M-42 lenses in AV mode. Those operate with a fully manual aperture when using the K-mount adapter. Unless you are always shooting full open, shorting the contacts will buy you nothing. Shorting the contact is only really useful with Pentax-M and Pentax-K series lenses.
Steve, I'm sorry to tell you that what you say is simply not true in the case of the K100D (and K200D according to rpriedhorsky - I don't think s/he's making it up either.) when using an m42 lens with a flangeless adapter.

In Av mode, with electrical contact, an m42 lens will meter correctly open or closed down; without electrical contact it will not meter properly in Av mode.

Pentax K and M mount lens automatically short the contacts when mounted because of the chrome plating on their base. If you cover the chrome plating on a K or M lens with a non-conductor they'll work exactly the same as a m42 with no contact (they won't meter correctly in Av mode.)

I don't think I can say it more clearly.

Dave
01-31-2009, 04:23 PM   #10
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I would cut out a ring of tin foil about the size of the camera's mount, spray it with 3M77 spray adhesive and glue it onto the lens. The adhesive comes off with mineral spirits so it's totally reversible. The tinfoil is probably not thick enough to alter infinity focus.
01-31-2009, 04:52 PM   #11
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QuoteOriginally posted by Just1MoreDave Quote
I would cut out a ring of tin foil about the size of the camera's mount, spray it with 3M77 spray adhesive and glue it onto the lens. The adhesive comes off with mineral spirits so it's totally reversible. The tinfoil is probably not thick enough to alter infinity focus.
That's a good idea & certainly worth a try.

If the aluminum foil tears & wrinkles too easily, one might use some thin steel or bronze shim stock. It looks like a ring with an ID of 41.5mm and an OD of 53-54mm would be about right.

Yes, those dimensions are ok; I cut such a ring from .001" steel shim stock & it works well on a super-tak 50/2, which, as Mike Cash says, is too small to cover the contacts unaided.

Dave

Last edited by newarts; 01-31-2009 at 08:52 PM. Reason: correction & confirmation of measurements.
01-31-2009, 06:56 PM   #12
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QuoteOriginally posted by rpriedhorsky Quote

I'd also like to hack my SMC Takumar 50mm, but as that's a much nicer lens, I worry about the resale value. Anyone have thoughts on how that will be affected by this hack?
That particular lens isn't wide enough to cover the contacts anyway, is it? So would there be any point to sanding it?

I'm really against doing this sort of thing, but as a firm believer in the sanctity of private property rights I believe you can do whatever you want to do with it. But first at least make sure there would be some point to it.
02-01-2009, 12:42 AM   #13
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QuoteOriginally posted by newarts Quote
Steve, I'm sorry to tell you that what you say is simply not true in the case of the K100D (and K200D according to rpriedhorsky - I don't think s/he's making it up either.) when using an m42 lens with a flangeless adapter.

In Av mode, with electrical contact, an m42 lens will meter correctly open or closed down; without electrical contact it will not meter properly in Av mode.

Pentax K and M mount lens automatically short the contacts when mounted because of the chrome plating on their base. If you cover the chrome plating on a K or M lens with a non-conductor they'll work exactly the same as a m42 with no contact (they won't meter correctly in Av mode.)

I don't think I can say it more clearly.

Dave
What do you mean, by "not meter correctly"? I am not trying to be demeaning, sarcastic, or pedantic. There are issues with stop-down metering with many Pentax dSLRs, but those issues exist for all Pentax lenses capable of stop-down metering (M-42 and all K-mount lenses with an aperture ring) regardless of whether the base is conductive.

Are you saying that shorting the "A" contact will correct the meter inconsistencies present when using non-A lenses? If so, ignore the rest of this message and PM me to tell me more. If not, read on...

*****************************************************************

In regards to my personal experience...

I don't have a K100D or K200D. All I have is a K10D and a couple of M-42 lenses and several non-A K-mount lenses that I use for the majority of my serious shooting. Neither of my M-42 lenses have a conductive base and both operate just fine in AV mode. (At least as fine as my non-A K-mount lenses, but that is another story...I will be posting some exposure bias plots sometime in the near future for all of my manual lenses.)

In regards to the Pentax-A mount, a little historic reference might be helpful here. Please don't be insulted if you already know this stuff. The important thing to remember is that the Pentax-K and Pentax-M lenses were designed before the "A" mount was developed and that both, along with the M-42 lenses supported AV exposure automation by design. Subsequent variations of the K-mount, with the exception of the "crippled" versions allowed for full forward and backward compatibility between cameras and lenses for all variants.

The "A" variant of the K-mount was designed originally to allow for full program exposure automation. This required that the mount signal both minimum and maximum aperture to the camera body. The body also had to know whether the mounted lens supported aperture control from the body (not true for Pentax-K or Pentax-M). That last piece was done via the "A" contact.

The "A" contact is recessed into the mount on the body. The original Pentax-A lenses had a short pin that protruded to make contact ONLY when the aperture ring is at the "A" setting. A circuit through the "A" contact signaled the camera to control the aperture from the camera body. No contact meant that body control and thus program automation was not possible. This allowed full backward compatibility to M-42 and non-A, K-mount lenses while enabling program exposure for the newer lenses. Pentax-K and Pentax-M DON'T make contact even though many (not all) have conductive bases. That is the whole purpose of the design. Conductive bases ARE required for the "A" setting to work, however.

Information regarding maximum/minimum aperture was communicated by a pattern of insulated points on the lens that corresponded to electrical contacts on the camera. The base would conduct at all points except for where it was insulated. Unless contact is made through the "A" contact, all the others would be ignored. This scheme has been modified somewhat on the FA and DA lenses, but is still pertinent for this discussion.

On the original A and FA(non-crippled) bodies, K-mount lenses with aperture set through the lens ring communicated aperture information (not the actual f/stop, but the offset from maximum aperture) via a mechanical linkage that worked a variable resister in the camera body to bias the meter. This mechanical linkage is missing on the FA(crippled) mounts found on current Pentax dSLRs. As a result, all non-A lenses must meter in stop-down mode when used with the crippled mount on these cameras.

There is one other piece to the puzzle that is important. FA and DA lenses always short the "A" contact. Information regarding the aperture ring position, max/min aperture, and other data is transmitted through a dedicated data pin.

So what does this all mean?
  • When you insulate your FA lens from the body with a sheet of paper, the camera thinks it has a non-A lens attached. Usually this means a significant underexposure at large apertures with fast lenses and other metering issues.
  • When you mount a non-A lens with a conductive base, the camera thinks that it has a non-A lens attached (no "A" contact means that the body cannot control the aperture). Metering may be flaky...
  • Ditto for non-A lenses with non-conductive bases
  • Ditto for M-42 lenses with non-conductive bases
  • Ditto for M-42 lenses with conductive bases
When you manage to "short" the "A" contact with a lump of foil or some-such when using a non-A lens, the body behaves (near as I can tell from my experimentation) as follows:
  • All exposure modes are enabled as if an A-mount lens is mounted
  • Since no data pin or insulating points are present, the body assumes the min/max aperture hard-coded into its firmware. The maximum is f/1.2 and this is the value that is displayed on the top LCD on my K10D. I don't know what the minimum is.
  • The body assumes an auto-aperture lens with the aperture full-open and meters accordingly. If in AV mode, changing the aperture with the thumb wheel will result in a corresponding change in the shutter speed (duh...).
  • Actual taking aperture at exposure time with K-mount lenses may be somewhat variable since the actuation lever movement is calibrated to the A-mount standard. X amount of movement of the actuation lever translates to Y aperture on the lens. The Pentax-K and Pentax-M do not comply with that standard and may be all-on or all-off or somewhere in between depending on the aperture ring position and the aperture chosen on the body.
  • M-42 lenses may meter accurately (I have not tested this) in AV mode if the body thumb wheel is set to the lens maximum aperture and the M-42 lens is set for manual aperture operation.
The last point relates to my original comment. This is essentially the same as operating the same M-42 lens in AV mode without shorting the "A" contact. As such, I can't see why the bother. Unless, of course, it might magically correct the lack of meter linearity that we all see with stop-down metering. In which case, you have my attention.

Steve

(Studied the original concept of the "A" mount with considerable interest back in the day when it was first introduced...)

Last edited by stevebrot; 02-01-2009 at 01:03 AM.
02-01-2009, 03:02 AM   #14
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I did not see anyone mentioning, that shorting the contacts also makes Trap Focus work.

Otherwise camera fires always, when you press the shutter in both, AF and MF mode. Even if the subject is OOF (AF confirm not lit up) in AF mode.

As for metering. On my k100ds tried shorting the A pin with screwdriver. When doing so exposure is more or less correct and consistent (as much as it can be with such lenses..), when the pin is not shorted with some lenses i get up to 2..3 stops underexposure in Av and a little less in M.
Also, most of the time it is very unpredictable and you cant just always add 2 stops to get correct exposure. In Av even if you could, there is no headroom left for ordinary EV comp.

As for the price of M42 lenses. I think it matters for some more expensive takumars, zeiss and similar lenses, wich are real quality lenses. On the other hand you can see even such beauties as 31/1.8 ltd "raped" to get working on other mounts. Most of my M42 lenses for example, is just a cheap alternative to modern lenses in those focal lengths i use rarely. How much cheaper a 20$ smc takumar 55/1.8, or some 20$ 135/2.8 revuenon can get? It isn't anything that rare also, unless it is some new old stock, with box, caps and all the papers. Wheter you keep/sell the lens as a collectable or actually want to use it makes the difference.

Last edited by ytterbium; 02-01-2009 at 03:10 AM.
02-01-2009, 07:47 AM   #15
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QuoteOriginally posted by Just1MoreDave Quote
I would cut out a ring of tin foil about the size of the camera's mount, spray it with 3M77 spray adhesive and glue it onto the lens. The adhesive comes off with mineral spirits so it's totally reversible. The tinfoil is probably not thick enough to alter infinity focus.
I have always wondered if anyone has tried the aluminum tape used in duct work (NOT duct tape). It is aluminum backed with adhesive, but is still fairly thin. The adhesive is fairly strong, I'm just not sure how easy it would be to remove the glue after a while, since it, like the nastier duct tape is meant to create a stronger seal with heat fluctuations. Maybe it is worth a shot with my vivitar 28/2.

-southy
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