Date: Sat, 14 Mar 98
From: Stephen O Gombosi sog@rmi.net
To: hasselblad@kelvin.net
Subject: Re: Re: 120 CF Makro
Pat wrote:
>you wrote:
>
>>The 120 CF Makro does require
>>some exposure compensation over the course of its focal distance. I don't
>>remember if the Wildi manual mentions this but I think the recent "Eye on
>>Closeup" from VC does state this in a footnote. The adjustment is ~1/2 stop
>>between infinity and 2.6 feet.
>
>Between infinity and 2.6 feet? Wouldn't that be virtually the whole range of
>the lens? I think someone said the lens had a minimum focus distance of 2 feet
>7 inches (or thereabouts)... so if it needs a 1/2 stop for the whole range,
>why not just print the scale a 1/2 stop over and be done with it? (Kinda like
>the view camera lenses that have the f-stop scales put at differing areas
>depending on which lens the shutter is in...)
Okay, folks...in *theory* you need some exposure compensation when focused
at *any* distance other than infinity, with *any* lens, but the amount of
compensation needed is so slight at "normal" distances that it can be
effectively ignored. How much compensation do you need? Well, that depends
strictly on the ratio of the iris->film plane distance to the focal length:
effective aperture = (f-stop set on lens)*(F+L)/F,
where F is the focal length of the lens, and L is the additional extension
required to focus at the "non-infinite" distance you're working at. How does
this relate to magnification? Well, magnification is given by the following
formula:
M = L/F
Since (F+L)/F = F/F + L/F = 1+M, the effective aperture can also be given
by:
effective aperture = (f-stop set on lens) * (1+M)
What does this mean in real life? Well, if you're shooting at a 1:1
reproduction ratio, then:
M = 1 = L/F, so L=F.
Your effective aperture (lets call this "A") is related to your indicated
aperture (let's call this "a") in this way:
A = a * (1+M) = a*2
So, if your indicated aperture is f/8, your *effective* aperture is f/16
and you need to apply 2 stops of compensation.
At half life-size, M = 1/2, so
A = a * 1.5,
So your f/8 indicated aperture is effectively f/12 and your compensation
is slightly more than a stop.
At M = 1/4.5 = 0.222, which is the closest focus for the 120,
A = a*1.222,
so your indicated f/8 is an effective f/9.778, and your exposure compensation
is a hair less than a half-stop (.4938, if you're terminally picky).
At infinity, your required compensation is always 0, since the effective
and indicated apertures are identical.
That's how all these compensation charts are calculated.
What about Proxars? Well, the Hassleblad nomograms are a bit confusing.
Proxars
don't alter exposure because they don't affect the iris->film plane distance
(actually, it's a bit more complicated than that since diopters work by
decreasing the effective focal length of the lens, but when you run through
the algebra it's a wash). As the *text* in Wildi (3rd edition, page 258)
says:
Close-up lenses do not alter the exposure, you can use the lens
settings obtained from your normal exposure meter.
Why do the nomograms in Wildi's book and the Hassy closeup brochure
seem to indicate otherwise? Because they're only intended to indicate
magnification range for the Proxars, not exposure compensation. They
need to add a footnote or something to clarify the situation.
Of course, the easiest thing to do is just meter through the lens. If you
do this, you can just use the indicated settings regardless of the
magnification or the means used to attain it.
Steve
From: "P.H.Groepper" pgroeppe2@estec.esa.nl
[1] Re: Exposure compensation chart for bellows extension
Date: Mon May 11 1998
Can anyone tell me is there such a thing as a simple exposure
compensation chart for bellows extension on a 4X5 camera?
here it is:
Lens-to-film dist. exposure value (EV) compensation
------------------- ----------------------------------
Focal length x 1 -> 0 (i.e. don't change at infinity)
" x 1.41 -> 1 (i.e. open 1 f-stop)
" x 1.73 -> 2 (i.e. open 2 f-stops)
" x 2 -> 3
Focal length x Factor -> Factor x Factor - 1
Note, that exposure compensation is never negative, minimum compensation
is zero at infinity setting. Also, it's not specific to any camera or
film size.
You can refine the above table yourself with the formula. It's practical
to make the table directly fit for a given focal length. You then need
one table for each focal length.
cheers, Peter
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