Re: [Rollei] RE: Do Rolleinars require exposure compensation?

[Richard Knoppow <dickburk >]

At 10:23 AM 03/26/2002 -0500, you wrote:
>I don't understand the difference between the use of bellows and the
>rolleinars. I also use a Mamiya TLR, and you have to admit the inclusion of
>bellows into the design was great. The bellows do require exposure
>compensation as stated previously, and the compensation factors are
>indicated on the side of the camera as you extend the bellows.
>
>I'll try another test roll of Provia tonight, and bracket the exposures.
>For an indoor still life lit from above (no flash), using Rolleiflex with
>rolleinar #2 about 12-18 inches from subject, would you do an incident
>reading or reflected light reading and why?
>
>Thanks again,
>R.J.
>
  Exposure compensation using bellows is due to the inverse sqare law
applied as the lens is moved further away from the film.
  A supplementary lens changes the focal length of the lens. If the lens is
used at infinity focus its speed also changes along with the focal length,
as one would expect, close-up attacments making the lens FL shorter and the
lens faster. However, when its used at a _fixed distance_ the increase in
speed of the lens with a close up attachment exactly compensates the
bellows factor, so no exposure adjustment is necessary. 
  I hadn't thought of it before but the same effect should take place for
front-element-focusing lenses, since they focus by changing focal length
rather than lens position.
 In fact, a bellows factor exists for any distance less than infinty,
however its negligible until you get close enough for about 1:5
magnification ratio. 
  
- ----
Richard Knoppow
Los Angeles, CA, USA
dickburk  

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