Using Hassy extension tube with PME 45 prism

[nelson_tan]

Hi guys,

 

Read the manual of the meter-prism PME 45 for Hasselblad, and it seems

that the user must key in the maximum aperture of the lens before

using the meter. This is done to "calibrate" the meter to the

brightness of the lens, crudely speaking.

 

Does anyone know if I should add in the compensation of a extension

tube when using one with the lens? I'll be using a extension tube 32E

with a 120mm f/4 Makro Planar. My guess is that I need to add in the

compensation, meaning that I will have to key in a "working aperture"

smaller than f/4, to recognize the light loss due to the extension

tube.

 

If I'm right, then how many stops do I have to factor in, when using a

32mm extension tube with the 120mm f/4 makro planar? Thanks in advance

for any help rendered!

[michael_heal]

<p>I don't think you need to enter the correction factors when metering through the lens, but if you do (or if you want to use an external meter), here they are. I've given them relative to the distance reading on the lens's distance scale. Obviously, the actual distances will be much shorter than those shown on the scale.</p>

<ul>

<li> If you can make corrections in 1/2 stop intervals, open 1/2 stop whenever the focusing ring reads more than 4 meters. Open 1 stop when it reads less than 4 meters.</li>

<li> If you can make corrections in 1/3 stop intervals, open 2/3 stop when the focussing ring reads more than 2 meters. Open 1 stop when it reads less than 2 meters.</li>

<li> If you want to use only one correction, opening 1 stop provides the best overall compromise. The maximum exposure error is 1/3 stop over, 1/6 stop under.</li>

</ul>

<p>If you want to calculate these numbers yourself, try the following formula:</p>

<blockquote>

<p>correction = 2 ( log( <em>f</em> + <em>x</em>) - log( <em>f</em> ) ) / log(2)</p>

</blockquote>

<p>where <em>f </em> is the focal length in millimeters and <em>x</em> is the total extension of the lens from the body in millimeters, including the contributions of both extension tube and helix. The helix of this lens can alone provide a maximum extension of approximately 28 mm, meaning that <em>x</em> can vary between 32 and 60 mm with the 32 mm extension tube in place.</p>

<p>To calculate the distance settings from the extensions, you need to know that</p>

<blockquote>

<p>distance = ( <em>f</em> + <em>y</em>)² / <em>y</em></p>

</blockquote>

<p>where <em>y</em> is the amount of extension in millimeters provided only by the helix. In this case <em>y</em> = <em>x</em> - 32.</p>

[squareframe]

Nelson,

 

you do not have to do a thing. the F-max setting merely sets the open-aperture boundary as reference for setting your lens. set it at f4 and leave it. enjoy the PME-45 .. it is a wonderful addition to a beautiful camera.

[graham_middleton]

To clarify - hasselblad cameras with CF lens and a Meter such as the PME, measure the light on the viewing screen. In order to do this the lens must be wide open (also to allow for easiest viewing) and the F stop of the lens (which does vary) must be notified to the meter. So now we can get our basic exposure. However the extension tube will not have an effect because the meter will just see the image as darker and suggest a longer exposure.

Now if you are using a 120 lens with a 32 tube I guess you are shooting close-up? In which case I would suggest using an ambient meter and bracketing as the built-in meter would be reading larger areas of the subject which may not be typical (and interpret the ideal grey point as something lighter or darker). Hope this takes some of the mystery out of the process - and makes sense!

[squareframe]

in *all* situations, the meter is a suggested value only, and must always be assessed by the photographer and compensated against, according to the overall dynamics across the metering frame. the F-max parameter is only to reference against, and assumed to be the metered aperture. unfortunately, it is something that you will forget to set when changing lenses, as you will forget to change ISO settings when you switch backs. luckily, there is a mode that can be set that will always display these two parameters for 1.5 seconds on activation. even so, the mind will easily ignore the prompt.

 

large format experience really helps attention to detail and avoidance of error.

 

well, most of the time.