Pentax 6x7 extension tubes and exposure increase factor

I recently acquired a set of Pentax extension tubes for my 6x7. I haven't had time to expose any film. I did mount tube #2 behind my 150mm/2.8 lens. I liked what I saw at minimum distance.
Now to my question: What is the exposure factor for each tube? No, I don't have a metering prism. Just my trusty Weston & Gossen meters.
Thanks for your help.

Exposure factor...

Extension length / FL = EV increase in exposure

1/10-1/5 = 0.5EV increase in exposure
1/4-1/3 = 1EV
1/2 = 1.5EV
1 = 2EV
1.5 = 2/5EV
2 = 3EV

Let b = the length of the tube and measure E = extension of the front of the lens forward if you use any of the lens’s helicoid extension.

The number of additional stops needed is

Δf = 2*ln[(b + E + f)/f]/ln2 Note: ln = natural logarithm (key on caculator)

The time factor = [(b + E + f)/f]^2


Example 1: b = 45mm, E = 0 (lens set to infinity), f = 100mm.

Δf = 2*ln[(45 + 0 + 100)/100]/ln2 = 2*ln[145/100]/ln2 = 1.07 stops.

Time factor = [(45 + 0 + 100)/100]^2 = [145/100]^2 = 2.1


Example 2: b = 82mm, E = 38mm, f = 100mm.

Δf = 2*ln[(82 + 38 + 100)/100]/ln2 = 2*ln[220/100]/ln2 = 2.3 stops.

Time factor = [(82 +38 + 100)/100]^2 = [220/100]^2 = 4.84

If you use more than one tube, then b = sum of the lengths.

For example, if your tubes were 45mm and 82mm, then b = 45mm + 82mm = 127mm

Thanks gentleman. Math is always a fall back poisition for me. I was hoping that someone actually uses the tubes and had measured/calculated the increase for each tube. I'll get out my mm scale rule tonight and muddle though the arithmatic. I'll probably be able to guesstimate within + or - a 1/2 stop.
That said, do the retrofocus/telephoto formula lenses as found on SLR cameras behave the same as a normal formula lens like those found on view cameras? You know, like a 150mm view camera lens that actually focuses 150mm from the film plane at infinity?
Thanks.

There is no fixed factor for a given length tube. Note that it’s necessary to know

1. The tube length (or sum of tube lengths if more than one tube is used).

2. The lens extension if any is used.

3. The focal length of the lens.

That’s because the SUM of all three of these distances determine the forward displacement of the lens from its infinity position.

Only by knowing the forward displacement of the lens from its infinity position can we accurately determine the light falloff as it travels the extra distance from the lens center to the film. That’s what determines correct exposure.

The light doesn’t fall off quite as fast as the formula suggests for retrofocus lenses. In this case the formula recommends somewhat more exposure than necessary. But that’s usually a modest overexposure and isn’t a problem on negative film.

For use with transparency film that requires tightly-controlled exposure, the use of exposure tests on the same film or referring to the data sheets that were originally supplied with the extension tubes will give the most accurate exposure information. That information must be measured for each lens. There is no single factor that works the same for all lenses.

If you’re using negative film, then the calculated values are useful and fairly accurate for most lenses of normal and longer focal length. The only time they will recommend moderate overexposure is when using relatively short (wide angle) retrofocus lenses.

The retrofocus design made necessary by the clearance of the swinging reflex mirror is a significant factor in determining the exposure for wide angle lenses, but doesn’t make much difference for longer lenses that have little or no mirror clearance problems to begin with.

Thanks Ian. I'll get out my ruler and calculator tonight.

Note to self, don't buy extension tube without TTL metering..

No need for the calculator. The Pentax 6x7 extension tube manual lists exposure correction factors for most of the lenses in the series (including the 150/2.8), when using the tubes individually or stacked in any combination. Thanks to M. Butkus, you can find a copy of that manual online in pdf form, here: http://www.butkus.org/chinon/pentax/pentax_6x7_extension_tubes/pentax_6x7_extension_tubes.htm

Happy shooting!


Jeff

SMBooth said:

Note to self, don't buy extension tube without TTL metering..

Click to expand...

Now you tell me.
Doing that would be almost like Digital, hey?
The Zone VI, Technika and Speed Graphic don't have TTL metering.
The M5 do!

Jeff,
Thanks! I can't believe I didn't search for the instructions? This is my second set set of tubes. Neither came with the instructions.
It's all clear as mud now!

I will be very interested to hear how you like the results! The extension tube set is something I have been eyeing for a while myself. For hiking, it would see that going with a tube or two would make for a lighter, more compact, and perhaps more convenient solution than adding a macro lens to my already-too-weighty pack of gear. My only qualm with the tubes has been that I am not sure how the image quality compares to the dedicated macro lens.

So... you have two sets you say? Hmmm...

Venchka said:

Jeff,
Thanks! I can't believe I didn't search for the instructions? This is my second set set of tubes. Neither came with the instructions.
It's all clear as mud now!

Click to expand...

Second set seperated by decades. First set long since gone along with the camera and lens. Dumb! When I could, I went back to the beginning. The P 6x7 was good in the early 70s. Still good today.