Bellows Factor

[Dave]

Lindy,

The first step is to focus your camera at infinity (about 100 feet is good
enough for optical infinity). Then take a measurement from some reference point
on the lens to some reference point near the ground glass back. This should be
close to 127mm for your lens. Doesn't really matter though just remember where
you measured from and to and what the number was.

Now, when you extend your bellows for close up work take another measurement.
Here is the mathmatical relationship:

(bellows extended^2) / (bellows distance at infinity^2) = exposure factor
in other words the extended bellows measurement squared over the normal bellows
measurement squared equals the exposure factor.

Say your measurement at infinity is 127mm and when you are doing a close up shot
the extended bellows measurement was 180mm, then the formula would look like
this:

(180^2)/(127^2)= 2.01

or.....

32,400/16,129=2.01

1 stop more exposure = 2x the light value

this means (in this example) you need to expose the film for 2 times longer than
your meter reading OR open the lens one stop. Rather than do an awkward
calculation every time, you can make a little ruler out of cardboard or mat
board and do the math some rainy evening and mark of the ruler in various
exposure factors. Mark it off for say 1.5X, 2.0X, 2.5X, 3.0X etc. and throw it
in your camera case.
------------------------------------------

If you doubled the bellows extension (254mm) from it's normal value (127mm) you
would need 4X the light or 2 stops:

254^2/127^2=4.00

Also, there is a comercial exposure ruler that is available, but I forget who
makes it. I think it is kind of a slide rule type gadget, but you can easily
make a ruler yourself.

Regards,

Dave

jli...@my-dejanews.com wrote:

> I'm just getting into large format. I've acquired a Crown Graphic in super
> condition with a 127mm Ektar lens. My question is at what point does bellows
> factor become a factor,and is there any quick and dirty rule of thumb for
> compensating for bellows extension.
>
> Lindy
>
> -----------== Posted via Deja News, The Discussion Network ==----------
> http://www.dejanews.com/ Search, Read, Discuss, or Start Your Own

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[Frank]

I have the ruler... an Imagequest bellows compensation and hyperfocal
focusing aide

They are great, but hard to find.

--
Frank Filippone

[jli...@my-dejanews.com]

[Normand Gobeil]

im kind of biginner too but i will tell you an easy rule i learned from
an "old pro"
first,extention shorter than 137 mm with your lens does not make
difference.
1.take the mesurement of bellow in inches
2.lets say that the number you obtain is an f stop(!!!)ex:6.5
3.137mm(your lens)gives with a non scientific calcul 5.5 inches
lets say that it is also an f stop
the difference between is the exposure compensation
in the example i gave you it gives around 1/2 stop

warning:this is totally unscientific and it works most of the times
with popular focal.
for better and more precise work dont use this method.

[Michael S. Briggs]

jli...@my-dejanews.com wrote in message <706dtd$r44$1...@nnrp1.dejanews.com>...

>I'm just getting into large format. I've acquired a Crown Graphic in super
>condition with a 127mm Ektar lens. My question is at what point does
bellows
>factor become a factor,and is there any quick and dirty rule of thumb for
>compensating for bellows extension.

There is a simple rule for deciding whether a correction for bellows
extension is needed. If the object focused on is more distant than 10
times the focal length of the lens, the required bellows correction will be
1/3 stop or less. So for a 127 mm lens, if the object is more than about
4 feet away, you don't need an exposure correction (unless you think you
need to determine the exposure to better than 1/3 stop).

--Michael

[Richard Eskite]

After a while, you get to the point where you can determine bellows factor for the
lenses you use most of the time by just looking at the camera. My assistant has
developed an uncanny ability in this area. Extreme camera adjustments and wide
angle lenses occasionally throw him off, so that's when we use the Calumet Bellows
Factor Finder Kit. This little gem consists of a 2"x2" plastic square you place at
the focal distance and a ruler you use to measure the size of the square on the
ground glass. The ruler is calibrated to give you the exposure correction needed
and it is 100% accurate. Another scale yields reproduction ratio (on which exposure
compensation for bellows draw is based). I've used the other methods mentioned,
especially converting the lens focal length/bellows extension measurement to
f-stops, and have found the bellows factor finder kit to be much more convenient.
The BFFK works with lenses of any focal length as it gives you a direct meaurement
of repro ratio. It also works for any camera that allows access to the ground
glass. Just my $.02.

[HRfoto]

>Re: Bellows Factor

>(bellows extended^2) / (bellows distance at infinity^2) = exposure factor
>in other words the extended bellows measurement squared over the normal
>bellows
>measurement squared equals the exposure factor.

I have used the same formula for years. It works great and is a lot easier to
use than the formulas requiring reproduction ratios.

>Also, there is a comercial exposure ruler that is available, but I forget who
>makes it.

This is available from Calumet for only a few Dollars. Call them at
1-800-CALUMET

Heinz Richter
HRphotography

[Brian Ellis]

If you don't mind spending about $150, buy the Casio pocket computer loaded with Phil
Davis' exposure program from Darkroom Innovations. It includes any necessary bellows
compensation in the exposure readout so you don't have to do any computations at all.
The computer does loads of other nice things too and for me has been well worth the
money. In addition to calculating the "correct" exposure, it also gives you development
times (after you've done the necessary testing) and much other useful stuff.

[Rich Satterlee]

HRfoto (hrf...@aol.com) wrote:
: >Re: Bellows Factor

: Heinz Richter
: HRphotography

There is a little ruler in the Kodak field guide (circa, 1957 or thereabouts).
Like the calumet ruler, it uses a known reference size target of 2 inches. You
place the target in the field of interest, lay your ruler over the ground
glass and read off the bellows factor (in stops). A couple of notes.

1. Works for any focal length lens. It measures the magnification factor with
the known target size.

2. I made my own rulers with a couple of pieces from the tubes that integrated
circuits come in and marked the scales with a "Sharpie" indelible pen. I
threw one in each of my LF camera bags/boxes.

3. The target size is two inches. Which is the same as a 35 mm slide mount.
Throw a couple of the slides (duds) in your LF camera bag and you are set.

Hope this helps,

Cheers,

Rich S.

[Tim Atherton]

Try the Quick Disk homepage. I have found this (free) little gizmo
invaluable, as well as idiot proof (important in my case!).

http://members.vienna.at/salzgeber/disc/

Tim A

jli...@my-dejanews.com wrote in message <706dtd$r44$1...@nnrp1.dejanews.com>...
>I'm just getting into large format. I've acquired a Crown Graphic in super
>condition with a 127mm Ektar lens. My question is at what point does
bellows
>factor become a factor,and is there any quick and dirty rule of thumb for
>compensating for bellows extension.
>

[DEBEANDRAY]

On page 38 of the kodak Pocket photo guide has your answer. Publication AR-21
. If you have a 210 m lens that is an 8 inch lens. Average distance to film
plane is 8 inches for a normal exposure. A greater or less would require
compensation in that exposure you will need to determain.