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 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. 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
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at maximum helicoid extension, because the loss of light in such case amounts not more than 0.3EV, which is easily superseded by the exposure latitude of the film.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 f/2.8 lens is 22mm, and in the case of 135mm f/4 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 f/4 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 f/2.8, 105mm f/2.4, 135mm f/4 Macro and 165mm f/2.8.
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 f/2.4 you need to use tubes #1+2+3 (98mm) and helicoid extension by 7mm, for 90mm f/2.8 you need tubes #2+3 (84mm) and helicoid extension by 6mm, and for 135mm f/4 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.
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.