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Introduction.
Page 3:
M42 lenses.

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Using M42, "K", "M" and "A" lenses on Pentax
2. "M" lenses.    1. Overview   3. M42 lenses

This is the second of three pages in which I try to provide a more comprehensive look at the ins and outs of using vintage lenses on Pentax DSLR's than is currently (AFAIK) available on the web. This page picks up from page 1, where I described the basic necessities, background and (briefly) technique, using M-mode and the green button to stop-down meter, of using vintage non-AF lenses on Pentax dslr's. See page 1 and the links there for the essential set up.

For "M" lenses read all K-mount manual focus lenses without electrical contacts and 'A' position on the aperture ring.

Pic left is my K-r with my SMC-M 50mm f1.4, a classic and well regarded "M" lens, manual focus of course, no electrical contacts or "A" setting on the aperture ring. Pic right is same lens on my Samsung GX-20 (= rebadged Pentax K20D).

For using this lens both cameras' features and menu are almost the same as that of the K10D described by Robert Donovan. The K-r doesn't have an lcd status screen on top but indications like F-- (= aperture not known) are displayed on the main screen - image right. On the top right of the K-r are the "green button" (G) and the exposure compensation Av [+/-] button. On the K-r it turns out that either Av or the green button can be pressed to achieve metering: in manual mode the Av button defaults to stop-down metering. Also, in practice, I have not found any difference in the operation of the green button according to whether it has been set to Av-shift or Tv-shift in the menu set up. On pressing there is the sound of the aperture being closed down and the shutter speed sets to match the metering.

On the GX20 the green button (G) is smaller and the Av[+/-] button is less handily placed. There are two E-dials (E), one front, one back. Which one is assigned what function varies according to circumstance. Generally the front dial adjusts exposure. Unlike the K-r the Av button does not stop down meter; even in M mode it produces the exposure compensation bar on the status screen and dialing the front E-dial adjusts exposure from the last metered value. Like the K-r I did not notice any difference in the green button function in M mode according to whether it was set to Tv or Av.

Both cameras default to centre-focus: the little red square indicating focus point remains central, and the camera cues (bleep and green sign) are operatonal when the centre is in focus. They also default to centre-weighted metering with "K" and "M" lenses.

Metering in M mode

filmstrip

This "filmstrip" shows test pics with the SMC-M 50mm f1.4, at successive f stops, displayed on the screen. The histogram indicates the accuracy of the exposure. From f1.4 to f2.8 there is a trend to underexposure, which then reverses, at f4 exposure is pretty spot on. From f5.6 to f8 and above tends to overexpose. This pattern, it turns out, is pretty normal. Several photographers have posted about Pentax metering; the graph left of metering with a SMC-A 50mm f1.2 on a K7 is one example. Pentax forums member Lowell Goudge has also explored the metering on the K10D, *istD and K7, using a SMC-M 50mm F1.4, his graph is here. The tests show that:

smc-m 28mm
tamron SP 300mm f5.6

With lens familiarity and a little practice I find I can "green button" and spin the E-dial (E) to change the shutter speed by a requisite number of clicks to get the exposure pretty correct. As mentioned by Donovan with his K10D, there is no indication of exposure accuracy or how far you may have shifted from the metering in the viewfinder or on screen, it's something of a trial and error approach.


samsung GX-20

Using Optical Preview to Meter

There is another approach . For the K-r, first the green button needs to be reassigned from "green button" (= stop-down meter) to "optical preview". This is in the camera-record menu page 4.
Once this is done, pressing and holding the green button will now hold the aperture stopped down to your selected f-stop on the lens aperture ring. And the relationship between the metered exposure (the particular point being that this is a current, and continuously metered, reading) and current shutter speed will be displayed in the viewfinder (but not on screen - screen blanks) as a + or - value. Changing the shutter speed can now be achieved by:

  1. Pressing the Av button. Shutter speed will be set to match metering. Or
  2. Spinning the E-dial. You can judge over/under exposure from the viewfinder display value.
I find that simultaneously pressing and holding the green button and pressing the Av button requires a certain dexterity! It seems to me to make more sense to spin the E-dial to a desired relative exposure value: +0.5, +1.3 etc., as already mentioned a measure of compensation is usually required.

Having only recently figured this out I am still in the process of seeing how well this works and how I like it.

On the GX20, this mode is not as helpful (as mentioned by Donovan with the K10D). The optical preview is available by rotating the on-off switch to the position marked by the 'optical preview' icon. In the viewfinder there is now an exposure bar indicating under/overexposure. But there is no metering. The indication is simply relative to the last time you pressed the green button to obtain a meter reading.

Auto ASA

Differences Between Conducting/Insulating Mount lenses

Some of the earliest K-mount lenses had a black painted mount, like their M42 predecessors (I have noticed this most particularly with third party rather than pentax lenses). Pic shows an example: a macro extension tube. Swappable "T-mount" lens PK bayonets are also invariably painted black. This has consequences: if the mount is non-conducting then the electrical contacts on the camera mount don't detect that a lens is present.

The effect on my K-r in manual M mode is that the green button lapses from optical preview back to stop-down meter. Both the green button and Av will produce a meter value when pressed, but there is no sound: the camera doesn't (try to) stop down. However the camera does stop down when the shutter is pressed (thus leading to an entirely incorrect exposure if the aperture is not set to wide open).

On the GX20, however, the camera doesn't distinguish: the green button operates normally, stopping down in both cases.

There is another issue. These two pics were taken with the SMC-M 50mm 1.4, K-r in A mode ie at f1.4. The one on the right was just with insulating tape on the mount to mimic the effect of paint. As you can see there is a signficant difference in the metering. I shall note here that the official web pages (links below) all say: "If older lenses without an "A" setting on the aperture ring are used in Av (Aperture Priority) Mode, an exposure error may occur." However there was no difference on the GX20.

The most important thing is to be aware that this is an issue, specifically for the K-r and other models that came after the K10D and K20D like the K30, K-x... It pertains most particularly to M42 lenses, of which the majority are painted black, and is discussed in more detail on page 3.

Insulating Mount: "A" lenses

It turns out that "A" mount lenses can also be affected. In my case I noted a problem with an early PKA teleconverter and with a set of extension tubes. Both had a black anodised flange and six PKA contacts (I'm not sure when Pentax put in the 7th data contact: it's there on the 1991 KAF2 mount).

The problem with these was that my GX20 would not operate in Av mode: no aperture was displayed and it was not possible to change the f number on the camera. Conversely, however, there was no problem on my K-r. The solution was to scrape off the anodising opposite the 7th contact to make a connection.

Av (aperture priority) Mode : Why not?

So why is Av mode limited to wide open aperture only with "K" and "M" mount lenses, instead of the range of f-stops?

This pic shows my K-r (KAF2 digital) and ME Super (original K) camera mounts side by side (the evolution of the K mount is described by Djobidar Dimitrov). Both have the open-aperture/stop-down mechanism (highlighted) on the left. The differences are: the electrical contacts on the KAF2 mount; the AF connection lower right on the KAF2; and most significantly for this discussion, a second mechanical aperture connection (arrowed) on the K mount. It is this connection that provides my ME-super with the necessary information to adjust exposure as the aperture ring is moved; the ME-super was designed as an "aperture priority (Av)" camera, and I used it like that for 20 odd years. It is the absence of this connection on the KAF2 mount that has led to it being referred to as the "crippled" K-mount.

This technical barrier to using Av mode with "M" lenses on dslr's is added to by another technical issue. When Pentax moved to the K-A mount in the early eighties, they also made a fundamental mechanical change to the lenses. On "K" and "M" lenses the movement of the aperture lever corresponds to the diameter of the iris. On "A" lenses the movement corresponds to the area of the iris ie directly to f-stop. What this means is that, with a hypothetical "M" lens "modded" to "A" functionality, across the aperture range there would be a varying discrepancy between actual and measured aperture. In other words exposures would be off.

However plenty of modders have tried see here for example (with pics illustrating the exposure at different f stops). And I have to say, since green button metering isn't consistent, the criticism that metering on a modded "M" lens will be subject to an inherent error hasn't much force!

Note too that this issue doesn't affect stop-down metering (as in the "hacks" below).

Av mode: hacks

There are two main work-arounds or hacks that have been tried to get "K" and "M" lenses operating with the aperture range in Av mode: removing the aperture lever so there is no longer a mechanical constraint; and not fully mounting the lens, so the lever simply isn't engaged.
Additionally, early Tamron adaptall lenses were fitted with a M42 style A-M switch, which provides a manual means of stopping down. So with these Tamrons you can work with the aperture range in Av mode (and it provides another point of comparison).

Here is my Ricoh 50mm (Ricoh cameras also used the K-bayonet mount, so Ricoh lenses can be used almost interchangeably with Pentax1). I have removed the aperture lever - easy to do with this lens. Now, as one moves the aperture ring the lens stops down, the viewfinder darkens and the camera metering changes accordingly. These thunbnails show how the metering worked out in Av mode on my K-r for the first four stops.

This is OK - the "stop down" metering is within the range, perhaps half a stop under-exposed, and is pretty consistent. But there is a bug - "mirror flopping". Mirror flopping refers to unusual/unwarranted raising and lowering of the mirror by the camera. In this case, each time I took a pic, a second or two later the mirror would flop. If you google and search the forums there are plenty of people who have experienced flopping in varied circumstances, and I do not know of any completely satisfactory explanation. If you can live with it (and it doesn't seem to harm the camera AFAIK) then this could work.

Experimenting with my SMC-M 50mm not fully mounted (I orientated the lens so the bayonet was just sufficiently engaged for the lens to stay put) produced similar results. I could run through the aperture range and exposures were OK but the mirror flopping was again present. I also tried this with insulating tape isolating the electical contacts and the camera mount except for the 7th "data" contact (see page 3) to see if this would improve matters, but behaviour and exposure was the same. My suspicion is that the K-r firmware is looking for/expecting an indication of aperture lever engagement, and when it doesn't get that it "flops".

With my GX20 the Ricoh 50mm gave these exposures in Av mode. This worked well - the GX20 didn't suffer from any "flopping" issues. My understanding is that the K10D/K20D bodies can be used pretty effectively with these hacks.

Other Models

K10D

Pentax forums member stevebrot tells me:
"On my K10D in M-mode, pushing the green button will always move the stop-down mechanism in the body. This is true even with no lens mounted. Likewise, the optical DOF preview also results in actuation of the stop-down mechanism regardless of mounted lens.
Stop down metering with non-"A" lenses sucks on the K10D and every other Pentax dSLR prior to the K-7. The metering is inaccurate at most apertures and the degree of inaccuracy varies by set aperture and the lens mounted.... The issue has been corrected in the newer bodies, but only in M mode. On those newer cameras, Av with M42 lenses still sucks.
No mirror flop or other mechanical issues, ever."

Other Pages, Threads and Posts on Using M Lenses

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