Pentax 67: Extension tubes & Exposure compensation

All extension tubes are designed to enable a camera lens to focus closer than a lens minimum object distance (MOD) for more magnification.

Asahi’s “Auto Extension Tubes” are mounted to the inner bayonet of Pentax 6X7 & 67 cameras and permit automatic diaphragm operation with interchangeable lenses of 45mm – 300mm focal length.

Exposure factor

Exposure factor is a value by which shutter speed should be multiplyed to get the film properly exposed when doing a macro or close-up shot with the lens extended by extension tubes.

The further the lens moves forward, the greater the distance between lens and film plane, and the less amount of light reaching film. Therefore exposure must be increased to compensate for the loss of light. This is called “exposure factor”.

To calculate exposure factor properly you need to know three parameters: lens focal length (f), lens helicoid extension (He) and extension tubes length (Te). And there is a formula to calculate it: EF = ( (f + Te + He ) / f )².

We already know the focal length. The Pentax 67 Auto Extension Tubes was released as a set of three lengths: #1 — 14mm, #2 — 28mm and #3 — 56mm (see the top image). Thus, we can calculate the extension tubes length by adding together all the required figures depending on the number of extension tubes we are using. Now all we have to do is to measure the lens helicoid extension.

Lens helicoid extension

How to measure lens helicoid extension with a ruler. Example with Pentax 67 135mm F4 Macro

How to measure lens helicoid extension with a ruler

Most lenses are made in such a way that no exposure compensation is required when you shoot at maximum helicoid extension, because the loss of light in such case amounts not more than 0.3EV, which is easily superseded by the exposure latitude of the film. That is why people typically disregard calculating exposure factor, except for some lenses with the minimum focal distance less than 8× of the lens focal length.

However, it should be figured out in macro photography with the use of extension tubes to accurately determine exposure, as the lens is already extended by tubes and additional helicoid extension leads to a sharp increase in exposure factor.

For instance, the maximum helicoid extension of a 90mm F2.8 lens is 22mm, and in the case of 135mm F4 Macro lens it amounts to 42mm, which is equivalent to tubes #1+2. Thus, exposition should be adjusted in such lenses under maximum extension even when they are used without any extension tubes: +0.5EV and +0.8EV respectively.

Well, it is not difficult to measure helicoid extension with a regular ruler. Please take a look at the image detailing how to do it. I’ve used 135mm F4 Macro for example. In the top image focus is set to the infinity — red mark at 10mm. In the bottom image focus is set to the closest distance — red mark at 52mm. So, helicoid extension is 42mm.


No we can proceed to calculate exposure factor. I’ve made a simple calculator which facilitates this function for Pentax 67 most popular lenses: 90mm F2.8, 105mm F2.4, 135mm F4 Macro, 150mm F2.8 and 165mm F2.8.

Choose lens:
Extension tubes:
Helicoid extension: mm
Measured shutter speed: sec

Exposure factor:
Compensated shutter speed:
Effective lens speed:
Picture area:

To carry out calculations you should first select a lens from the list and a combination of extenstion tubes which you use for your purposes, as well as lens helicoid extension, whose calculation I’ve already described. Exposure factor will be calculated automatically.

You can also specify the shutter speed which you’ve measured with an exposure meter. The appropriate shutter speed will be calculated with regard to an exposure factor.

As a bonus the calculator provides additional useful data for macro work: magnification, an effective lens speed and picture area.

Life size ratio

There is a rule: to get 1:1 magnification with any lens you need to extend it by its focal length. For example, to get 1:1 with 105mm F2.4 you need to use tubes #1+2+3 (98mm) and helicoid extension by 7mm, for 90mm F2.8 you need tubes #2+3 (84mm) and helicoid extension by 6mm, and for 135mm F4 Macro you need to use tubes #1+2+3 (98mm) and helicoid extension by 37mm. In this case the exposure factor will be x4 that gives +2EV of exposure compensation.

Exposure compensation without extension tubes

Yes, you may need it. But it depends on lenses you use and helicoid extension. Better check it with the calculator. In my experience, if the loss of light is more than 0.3EV it is better to adjust exposure.

More lenses

Let me know If you would like me to add other lenses to the calculator. I will do it as soon as I possibly can.

Magnification chart

Lens #1 #2 #2+1 #3 #3+1 #3+2 #3+2+1
45mm F4 0.31 ~ 0.48 0.62 ~ 0.79 0.93 ~ 1.1 1.24 ~ 1.41 1.56 ~ 1.72 1.87 ~ 2.03 2.18 ~ 2.34
55mm F4 (I) 0.25 ~ 0.45 0.51 ~ 0.71 0.76 ~ 0.96 1.02 ~ 1.22 1.27 ~ 1.47 1.53 ~ 1.73 1.78 ~ 1.98
55mm F4 (II) 0.25 ~ 0.49 0.51 ~ 0.75 0.76 ~ 1.0 1.02 ~ 1.25 1.27 ~ 1.51 1.53 ~ 1.76 1.78 ~ 2.02
90mm F2.8 0.16 ~ 0.36 0.31 ~ 0.51 0.47 ~ 0.67 0.62 ~ 0.82 0.78 ~ 0.98 0.93 ~ 1.13 1.09 ~ 1.29
105mm F2.4 0.13 ~ 0.27 0.27 ~ 0.4 0.4 ~ 0.53 0.53 ~ 0.67 0.67 ~ 0.8 0.8 ~ 0.93 0.93 ~ 1.07
135mm F4 Macro 0.1 ~ 0.41 0.21 ~ 0.52 0.31 ~ 0.62 0.41 ~ 0.73 0.52 ~ 0.83 0.62 ~ 0.93 0.73 ~ 1.04
150mm F2.8 0.09 ~ 0.23 0.19 ~ 0.32 0.28 ~ 0.41 0.37 ~ 0.51 0.47 ~ 0.6 0.56 ~ 0.69 0.65 ~ 0.79
165mm F2.8 0.08 ~ 0.21 0.17 ~ 0.3 0.25 ~ 0.38 0.34 ~ 0.47 0.42 ~ 0.55 0.51 ~ 0.64 0.59 ~ 0.72

Offline calculator

Finally, I’ve decided to make a spreadsheet calculator for Microsoft Excel, LibreOffice Calc or similar program. Follow this link to download it. Thanks to the reader who requested this option.


If you are interested in learning more about mathematics of extension tubes, read this article by Douglas A. Kerr “The Mathematics of Extension Tubes in Photography”.

// Sasha Krasnov
Pentax 67 MLU stuck mirror repair

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Pentax 67 lens to Pentax 645 body adapter →

Pentax adapter 645 for 67 lens


  1. Do you have any portrait examples of the 105mm 2.4 while using the helicoid and reverse adapter?

    • Hello Jeff

      I need to browse my archive to find some images. I rarely use #1 for closer focusing with 105mm because I do not need greater scale by some reasons but not because “it’s a bad idea”. So, using #1 with 105mm and ext.tubes #1 & #2 with 165mm is a common practice. All you need uis exposure compensation about 0.5EV or may be 1EV. I personally do not recommend to use helicoid extension tube – you will lose aperture coupling and it is very uncomfortable during the shooting especially wedding sessions.

      And I’ve never used reverse adapter with helicoid extension tube. This combination is intended for scale 1:2 and greater. So, it gives too close focusing.

      P.S. Moreover, for example, Mamiya RB/RZ 67 use bellows focusing which allows for closer focusing than any other Pentax 67 lenses. And it normal for Mamiya and it is normal for Pentax 67 to add #1. I do not see any problem with it.extension.

  2. Sasha, thank you very much for all of the information on the P76 lenses! I’ve found it very helpful when researching which lenses to start with. I’m a portrait photographer getting into medium format now (finally). I already have ordered a 105mm, but I’m trying to figure out which method to use for tighter portraits (face only). Is there an option with extension tube or with reverse adaptor that will allow for a tighter FoV on the 105 lens?

    I’m very familiar with focal lengths and minimum focus distance, but NOT familiar at all with extension tubes and other adapters.

    I thought I could just purchase the 135mm macro, or the 165, etc…. but if there is a solution for the 105mm to get ‘closer’/tighter, and not have to carry another lens — that would be interesting. Although I’m sure I will want to purchase a longer lens for faces, eventually.

    Thanks again! Cheers,

    • Thank you Brian! Considering your question I personally do not recommend to use reverse adapter by some reasons: a) you will need Helicoid Extension Tube at least to make lens focusable; b) also you may need Auto Extension Tube #1 or #2; b) you will loose automatic aperture stop down coupling. Usually Reverse adapter is used for macro work with maginfication ratio 1:1 or greater. If you need to shoot tigther portraits I recommend to use just Auto Extension Tube #1 for 105mm and #2 for 165 lens. It is more convinent and realible way to focus the lenses closer to the subject.

      As for me… I like to use 105mm with Auto Extension Tube #1 in some cases or 135mm Macro. The last one is really underrated lens for portraits but it is very hard to focus it.

  3. I’ve been shooting 4×5 view cameras decided on starting back toward my Old Pentax 6×7 Had these ext. tubes for a long , long time so back to the old basic’s need to be refreshed on so much, From Ditz. to 35mm, now back to the OLD PENTAX 6×7. Thanks for your pages on these Ext. Tubes , never used them from way back then. a new day has arrived , back to new ways.

  4. Need information for lens: 45mm., and 55 mm. using different ext. tube for these two Lens.
    Thank you so much.

  5. Thanks for the technical information. It is rare to find such succinct information that can easily be applied. Shooting 135mm pentax macro with all three rings is tricky- this helps.

  6. Matthew

    Thanks for this! This is very helpful (as was your repair for a sticking mirror/shutter). Do you happen to have an Excel file with the calculator? I’d love to be able to use it without internet access. If not, I’ll just make a rough cheat-sheet for the lenses I have.

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