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05-24-2018, 12:11 PM   #1
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Pentax “red dot” lenses

Hi all. My first post with my “new” Pentax MX. The manual says to align the red dot on the body to the red dot on the lens and twist. I currently have no lens. How do I identify a SMC K- mount lens with the red alignment mark. I would like to get the 50mm f/1.2 but want to stay true to the original “red dot” lenses. Many thanks!

Last edited by BigMackCam; 05-24-2018 at 12:53 PM. Reason: Corrected spelling in title
05-24-2018, 12:15 PM   #2
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Virtually all Pentax lenses have the red dot, old and new.

Here is the original K series if you want to get those:

Pentax Lens Search |

You may also be interested in the M series:

Pentax Lens Search |

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05-24-2018, 12:37 PM - 2 Likes   #3
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QuoteOriginally posted by khking Quote
original “red dot” lenses
"red dot" is not a type of lens. It is just the mark on camera and all K-mount lenses that help with alignment.

When buying your lens use caution as some sellers do not understand the distinction between F/1.2 and f/2. You will often find f/2 lenses listed as f/1.2 either in error or by deliberate deception. The 50mm f/1.2 runs around $500 the f/2 usually less than $50 in excellent condition.
05-24-2018, 12:47 PM   #4
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Your best bet would be to find a lens here in the for sale portion of the forum. I have gotten most of my lenses that way. No worries buying here if you follow the rules on sending money.

05-24-2018, 01:18 PM - 1 Like   #5

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Even 3rd party brands, like Sigma, Tamron, Tokina, etc. use the red dot indication for their Pentax-mount versions of their lens designs. Usually, the red dot is located on the lens's mounting flange facing into the camera. Some have the red dot on top of the lens body near the edge. I recently purchased a Sigma 17-50mm f/2.8 pro-style zoom lens, which one reviewer complained had no red dot. But I found it instead has a white dot on top at the base of the lens body- just as good or maybe better for visibility. More recent Pentax lenses also have a corresponding white line there as well as the usual red dot. In all cases, the idea is for a convenient visual reference in lining up the parts to snap the lens onto the camera.
05-24-2018, 01:50 PM   #6
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The K and M series lenses also have a raised white plastic dot on the side of the lenses in addition to the red dot on the mount. The A series used a dirty yellow colour instead of white. The old Pentax manuals inform that this was to help align the lens with the camera when it was too dark to see the red dot on the mount itself. You are supposed to align the white or yellow dot with the lens release lever on the older cameras.

The 31mm Ltd and 77mm Ltd sport a green plastic dot instead of white/yellow.
05-24-2018, 02:00 PM   #7
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Sometimes the red dot is white but no worries. All K mount lenses fit fine.
05-24-2018, 03:16 PM   #8
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For a MX, this is the one you'll be looking for most likely:

SMC Pentax 50mm F1.2 Reviews - K Prime Lenses - Pentax Lens Reviews & Lens Database

If you plan on moving to a later body (or Pentax DSLR), there is also a Pentax-A version of the lens:
SMC Pentax-A 50mm F1.2 Reviews - A Prime Lenses - Pentax Lens Reviews & Lens Database

This is much more usable on modern Pentax equipment, but the price tag tends to be higher as a result. If you are adapting to other systems or using on the MX, it is immaterial; the lenses will function the same. I haven't used either, but the optical design is the same and they should be very, very similar in use.

05-24-2018, 05:49 PM   #9
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The raised plastic dot on the side lining up with the release button was copied from the Leicaflex mount, introduced in 1964. Leica used a red plastic dot, which became their trademark.
I’d heard it mentioned that Pentax based the K mount design on the Leicaflex, and they do look very similar, except for the aperture sensing cams.
05-24-2018, 05:55 PM   #10
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these charts might be of interest and help

" Pentax Lens Compatibility Chart
Copyright 2006-2016 All rights reserved.

Pentax bayonet lenses labeled "SMC Pentax" are referred to as "K" lenses in the table below. Other Pentax bayonet lenses have the lens designation as a part of their name as in "SMC Pentax-FA." Note that "star" lenses work like their "non-star" counterparts: DA* lenses work like DA lenses, FA* like FA, F* like F, A* like A, and M* like M.
Exceptions: FA and F "soft" lenses behave like M lenses in terms of metering. "

[ look at the bottom line of the chart for the MX film camera ]

The Pentax Camera Lens Compatibility Chart

" Pentax K-Mount Lens Series Explained: The differences between various Pentax lens series

In a nutshell

There are several series of Pentax K-mount lenses; the ones currently being produced include the DA (digital), DA L (cheap digital kit), D FA (full-frame, digital optimized), and FA (full-frame) series. While all Pentax K-mount lenses ever produced are compatible with all current Pentax DSLRs (in some cases with restrictions in that not all exposure modes of the camera are supported by old lenses), there are significant differences between each lens series and each has different features. Furthermore some of the newer lenses do not work with older cameras. We'll be exploring the differences between the various K-mount series lenses in this article "

Pentax K-Mount Lenses Explained: The differences between various Pentax lens series

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