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How to use/meter Manual & M42 Lenses on all Pentax DSLRs (K-1, K-3, K-5, K-30, etc)
Posted By: Adam, 08-12-2010, 04:24 AM

Many Pentax DSLR owners want to use M42 screwmount (Takumar) lenses, or M or K manual lenses, on their cameras because of the low cost and relatively high image quality of these lenses.

If you're wondering whether or not these lenses can be used with Pentax DSLRs (or the K-01), then the answer is yes! Pentax as well as third-party manual and screwmount lenses can easily be mounted on any Pentax DSLR (such as the K-50, K-500, K-30, K-5 series, K-r, K-x, K-7, K10D, K100D, K200D, *ist D, etc.) Just follow this guide!

Modern Pentax DSLRs use the Pentax "K-mount", which employs a bayonet and therefore differs significantly from the M42 screw mount. The older manual M and K (SMC Pentax-M, SMC Pentax) lenses actually use the bayonet, so they will not need an adapter - you can skip straight to the lower portion of this article (starting at "Important!") for information on how to meter with those lenses. Screwmount lenses usually have "Takumar" in their names, and in order to mount screwmount lenses on your k-mount body, you'll need a Pentax k to m42 adapter. Pictured above is the genuine Pentax adapter, which is ideally the one you want to get. Unfortunately, it has been discontinued and can only be purchased second-hand. However, similar third-party adapters are also available. Caution: Many third-party adapters, such as this one, have a protruding flange which will prevent you from focusing all the way to infinity. If you want to buy a third-party adapter (they're generally cheaper), make sure that they don't have this flange. Here's an example of a good third-party adapter.

Once you have your adapter, the next step is to install it on your camera (it can easily be put on and removed on-the-fly). Check out the m42 to k adapter manual. After you've installed the adapter, you'll want to mount the lens. This is done by screwing it into the camera until the lens feels firmly attached. The focusing window and lens ring should line up with the camera just like any other lens. Now that your lens is mounted, let's talk about how to take photos with it.

Important! The hard part is to get the camera to actually fire when a manual lens is mounted. In order to accomplish this, enter your camera's custom function menu, select the "Using Aperture Ring" setting (usually at the end of the menu, #21 on the K-7, #27 on the K-5, #27 on the K-3, #26 on the K-1), and set it to 2 (allowed). Once you do this, the shutter will at least fire, as it wouldn't have with this setting disabled (you would simply have seen an F-- indication on the top LCD/info screen). The setting description should read: 'Shutter will release when aperture ring is not set to the "A" position' when "allowed" is selected. Also note that the mount on the lens must be conductive for electrical current so that it shorts the electrical contacts on the camera body. All Pentax manufactured lenses have a conductive mount, but some third party lenses do not in which case the area of the mount touching the contacts must be sanded down.

K-30, K-50, K-500, K-70, K-S1, K-S2 and K-01 users: make sure you also set your green button "action in M/TAv Mode" to Tv SHIFT. This is found under the button customization menu (page 3 of the main menu) on the K-01 or as a custom function on the K-30, K-50 and K-500. On the K-S2 and K-70, look under the e-dial programming sub-menu under button customization in the record menu.

Finally, ensure that auto ISO is disabled.

Now, let's discuss metering. Since manual lenses don't feed aperture data to the camera, the only way for the camera to check how much light is being passed through the lens is to measure the light while the lens is stopped down. Follow this procedure to properly meter with a screwmount, M, or K lens:

___0. Ensure that the "Using Aperture Ring" custom function is set to "2 (allowed)" (K-30/50/500/01 users must also ensure that the green button is configured to Tv Shift in M/TAv Mode) as described above
  1. Set your camera to M mode using the mode dial (your camera won't fire in other modes*)
  2. Compose and focus your image.
  3. Using the aperture ring (the ring at the very back of your lens; it will have numbers such as 2.8, 4, 5.6, 8 written on it), select your desired aperture setting. Note that the smaller the aperture number is, the more light passes through the lens, and the blurrier the out of focus areas of your photograph will be (and vice-versa).
  4. [Screwmount lenses only] Switch the diaphragm clutch on your lens to "Manual" (you can leave it on Auto when composing and focusing if you don't want a dark viewfinder).
  5. Measure the light by either pressing the "Green Button", or pushing your power button to DOF preview mode (only available on high-end bodies). Your camera will automatically set the shutter speed for you.
All that's left now is for you to press the shutter release button to take your photo. Congratulations- you've now learned how to use M42 and M & K manual lenses with Pentax DSLRs!

*Screwmount lenses may also be used in Av mode since they are always stopped down to the aperture you will be shooting at (unlike M&K lenses, which are stopped down only when the shutter is released or when you meter as described above).

Note: if your aperture ring has an "A" on it, instead of doing stop-down metering as per this guide, you'll want to set the ring to "A" and use the camera's scrollweel to adjust the aperture via Av mode.

Click here if you found this article helpful!

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Last edited by Ole; 02-12-2011 at 11:46 AM.
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06-26-2013, 04:38 AM   #166
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Thanks bluenose! I did wonder if I should just go sod all and set it at 60... I'll give it a go and see what happens.

06-26-2013, 07:20 AM   #167
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QuoteOriginally posted by a96agli Quote
Thanks bluenose! I did wonder if I should just go sod all and set it at 60... I'll give it a go and see what happens.
Glad it was of help. I have read/heard that when using a zoom you should set it to the actual focal length or the longest length the zoom will extend but I like the Pentax approach - pick the middle focal length and just shoot.

07-22-2013, 02:54 PM   #168
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Great simnple t the point Bob
08-07-2013, 10:12 AM   #169
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I have a question about this. If I buy a 3rd party adapter (the OEM version is not realistic, pricewise), is there anything I shoud look for and/or avoid, other than making sure I get one that can focus to infinity? Thanks!

08-07-2013, 10:32 AM   #170
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It appears that most of the 3rd party adapters (focus to infinity) have a tendency to get stuck making it difficult to remove the lens. The one that SRB in the UK sells seems to be the exception. Otherwise it is advised to remove the spring clip on the adapter - it's just held on with a screw. Use the search function on "M42 stuck" and you should find a number of threads on this problem.

The OEM adapter does show up in new stock once in awhile at a decent price at online camera stores.

Pentax M42 adaptor|M42 lens| Pentax K
08-19-2013, 05:41 AM   #171
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Just bought a K-30 and updated it to the latest firmware (1.04). I've been testing it with a K-mount Pentax-M 28/2.8.

EDIT: spoke too soon - turns out it only meters correctly in M mode - in Av mode it is, in fact, metering with the lens wide open, which would make sense.

After setting the aperture setting on the camera, I did not have to change anything with the green button - it worked as intended out of the box.

Last edited by mabman; 08-19-2013 at 06:09 AM. Reason: Further testing contradicted myself :(
08-29-2013, 07:47 AM   #172
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I found this article very helpful. I was able to use a prime M lens on my Kx camera after reading this. My question is how do you use the M lens with the built in flash?

Last edited by Tom S.; 08-29-2013 at 07:53 AM.
08-29-2013, 10:12 AM   #173
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The built-in flash should always fire at full power. You'll have to divide the Guide Number by the distance to subject to get the f-stop.

08-29-2013, 07:26 PM   #174
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QuoteOriginally posted by Not a Number Quote
The built-in flash should always fire at full power. You'll have to divide the Guide Number by the distance to subject to get the f-stop.
Thank you.
08-30-2013, 05:18 PM   #175
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Thanks Adam!
this forum is so friendly!
09-02-2013, 09:37 AM   #176
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What Green Button???

QuoteOriginally posted by Not a Number Quote
From button customization in the Rec Mode 3 menu select green button
I'm reading a lot about this "Green Button" and how to set up it's functions... alas; my K-100D has no such GB, and seems to have assigned it's functions somehow to either the "pre view" stop on the power switch that goes around the shutter button or the little square "Av" [+/-] button located just SouthWest of it.

Have experimented with these but can't really tell what effect they have on exposures (if any) and they don't seem to help matters any.

About the only luck I've had thus far is using a screw mount 35mm WA pre-set lens with M42>Kmount adapter and setting "M", setting aperture the old fashioned way.

This set up has been the only way I've been able to use the flash to get a decent picture.

I've noticed that using "Heritage" lenses on this DSLR (don't know if it universally applies) cuts field of view / picture size down considerably, making them to some extent "Narrow Angle" lenses. This WA somewhat compensates for that phenomenon but not quite as much as I'd like it to.

I still have to guess at the exposure settings though; if there is a TTL light meter in this camera it would be helpful, but I have yet to find it. Any suggestions?
09-02-2013, 09:47 AM   #177
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The K100d doesn't have a green button, but apparently you kan use the AE-L button for stop down metering in M-mode. For an aps-c dslr you have to multiply the focal length by 1.5 to get the equivalent field of view of a film slr or a full frame dslr (which Pentax doesn't have (yet). 35mm on dslr is equivalent to 52.5mm on slr.
09-02-2013, 10:11 AM   #178
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Yes, the AE-L button serves this function in M (manual) mode. See page 146 in the English edition of the user manual. See page 145 for manual metering. The K100D does not have meter bars but shows in digits the EV difference. For non-A type lenses use stop-down meter in manual mode. Set the DOF preview to optical and activate the DOF (on main switch of the K100D). This should stop down a k-mount lens and activate the meter. Screwmount lenses have to be stopped down manually. Adjust the EV by changing the shutter speed from the eDial or the aperture using the ring on the lens.
09-07-2013, 12:00 AM   #179
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What about for Pentax istDS?
12-01-2013, 06:22 PM   #180
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Green Button - M42 lens

K-01 - I set the Green button to TV Shift. But seems to do nothing. Metering is whatever I have the F-stop set to. Regardless.

So I reset Green Button to allow toggle on and off Focus peaking. Which it does fine. Everything else seems to be as before.

Why do I need to set TVshift for M42? What am I missing?

Last edited by cahudson42; 12-01-2013 at 06:23 PM. Reason: add K-01

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