Arca Brainbox utility?

steve_singleton2 · April 22, 2000

Before buying an Arca 6x9/4x5 FC, I was intrigued by the Brainbox accessory. This mechanical aid offered to help measure and calculate "depth of field, extension and swing angle." This may represent Dumbo's Magic Feather for a LF beginner like me, but I'm curious. One dealer told me Arca has discontinued the Brainbox; but B & H still lists it as a special order item for $235. My question is whether anyone has used the Brainbox, knows how it compares in convenience and accuracy to Sinar's near-far focusing system, and whether it is or is not available.

ellis_vener_photography · April 22, 2000

I am told by the factory that i bought the last "Brainbox" in the US. This was a couple of years ago. Supposedly the design is being revamped and updated. If

you can find one then yes I think it is worth it. It is very convient: it slides onto the rail behind the rear standard/ function carrier. The way it works is like so:

you focus on the nearest point you want in focus and slide the brainbox up to the back of the rear function carrier. You then focus to your far point by driving

the rear function carrier forward and push a button on the side of the brainbox. A geared arm shoots forward and makes contact with the back of the function

carrier. you then look at the scale for the format you are using and decide what size circle of confusion you want to work with (there are two scales for each

format 1:4 (or close-up) or infinity (general work) you read the f-stop against the marker line and that is the f-stop you'll need for the depth of field you want.

You then divide the distance between the two points in half (there is also a scale in cm) and drive the function carrier back to that halfway point (let's say the

distance between the two points is 2.6cm, you then focus back till the distance is 1.3cm) (all of this is much easier done than described). it is as accurate as

the Dof scale on my Sinar C (which was a hybrid of a Sinar F front standard and a P rear standard.)

I've never used it to determine tilt or swing angles.

jeff_taugner2 · April 22, 2000

The Brainbox has not been available for at least 4 or 5 years.

Arca-Swiss also has no plans of making a new version. Sorry. Jeff

masayoshi_hayashi · April 24, 2000

Steve, I'm one of those people who need brain.But here is worth a

try.

You could try to make a Sinar equivalent DOF scale on your rear

function carrier focusing knob and tension control knob. Figure out

the distance on the rail per turn of the focusing knob, or

distance/turn where delta is depth of focus. Then use the equation

delta = 2*c*N to get the minimum f-number N to cover the depth of

field required, where c is circle of confusion you want depending on

the choice of your format and how much you want to enlarge.

<ol>

<li>Tape on both the rear function carrier tension control knob for

focusing and focusing knob. So you can mark on the tapes. Make sure

the tension control knob right next to the focusing knob is relatively

tight enough to minimize back lash (maybe not that much even when

tension is loose).

<li>Convert the distance on the rail to turns on the knob by simply

turning the knob and looking at the calibrated scale on the rail. Mark

every 1 mm delta for example on the left side of the focusing tape.

<li>After you transfer the delta to turn on the focusing knob,

carefully transfer the 1 mm marks on the focusing knob (angles) to the

tension knob. When you do this angle transfer, make sure you look

directly down to marks. You now transfered 1 mm marks on the tension

knob.

<li>Remove the tape on the focusing knob.

<li>Mark f-numbers N's on the left side of the tape, based on N =

delta/2/c for a given delta.

<li>Mark a line somewhere by a marker on the left side of the focusing

knob to indicate where you start focusing and to align the f-number N

= 0 on the tension knob to the line on the focusing knob.

<li>You now have a Sinar equivalent DOF knob.

</ol>

Caution: Because the size of the two knobs are different (tension

control knob being bigger than focusing knob), carefully transfer the

mark on the focusing knob to the tension knob at the same angle.

Note: the above calculation does not take magnification m into account

as Sinar doesn't (but supplemented by another knob). You need to use a

more accurate formula: N = delta/2/c/(1+m) but notice the minimum

f-stop N for the required depth of field decreases as magnification m

increases. So you're guaranteed to have the required depth of field at

any magnification for a given delta, using the earlier formula: N =

delta/2/c. Notice this equation is the same as N = delta/2/c/(1+m)

when m = 0. But of course you need to compensate exposure time for the

bellows extension so use the delta/2 position of the rear frame to

figure out the extra stops to be added to time. To convert the bellows

extension to stops, use one of the methods described in the <a

href="http://www.cs.berkeley.edu/~qtluong/photography/lf/bellows-factor.html">Bellows

extension exposure compensation</a>.

The DOF knob is still good in approximation even when standards are

tilted/swung so you can still use the DOF knob. When you use

tilt/swing, the way you determine the DOF using the knob is different

from the way you'd do with no tilt/swing.

<ul>

<li>When no tilt/swing then focus on farthest and nearest you want in

focus, using rear focusing knob. (the plane of focus is vertical to

optical axis)

<li>When tilt/swing, then focus on the two farthest angle planes from

the optimum plane of focus or two objects on the corresponding to

which intersect the corresponding planes of focus. The "Farthest"

anlge plane is far in angle sense.

Let me know anyone if anything is wrong. I think actually try making

one for myself this afternoon....

masayoshi_hayashi · April 24, 2000

</ul> Did it fix the indentation? Anyway don't make your decision

solely based on the lack of DOF calculator on the camera. Arca F-line

is something more than that.

masayoshi_hayashi · April 24, 2000

The procedure 5 should read

Mark f-numbers N's on the left side on the [tension] tape, based on

N = delta/2/c for a given delta. Sorry if there's any confusion

arised

bill_glickman · April 25, 2000

Steve.. there is many ways to skin a cat. For DOF and lens tilt,

I do the following which is much faster. Buy a programable

calculator and program in the formula for DOF... then use a range

finder to determine distances in your scene, or if you are shooting

in a studio use a tape measure... enter the near pt. and the far

point in the equation and press compute and the calc. will provide

you the answer at what distance to focus at. This works 98% of the

time for me... unless you are shooting the Grand Canyon and the

formula asks you to focus at dead air. Who cares about extension, if

you are focussed at the right distance, you will automatically have

the proper extension. As for tilt, same thing, enter the formula, or

do the simple math in your head... fl/(jx5) where J equals the

distance in feet under the lens in which the plane of sharp foucs

intersects, so a 150mm lens at 10 ft below the lens requires a 3 Deg.

tilt... from their you need to examine the gg to get it just

right.... The beauty of a programable calc. is you can program

anything in it, and also if you get a snazzy one that has a numeric

solver like some of the TI's, you enter a formula and enter in all

the variables except one, then solve for that one... without re

writing the equation to solve for each variable individuly.. better

than a spread sheet! Hope this helps...

masayoshi_hayashi · April 25, 2000

Ok, I tried the procedure I described above yesterday. There are still

some typos and minor improvements in the above procedure. I figured

out the delta/turn to be about 4.75mm ±.25mm experimentally

when a strip of standard laser paper is placed on the focusing knob

and glued the ends together. One whole revolution of the knob gives

about 20mm (2cm) delta on the rail.

Method 1

<ol>

<li><li>Calculate delta = 2*c*10^(1/2*log(2)*Av) where Av = {5, 5_1/3,

5_2/3, 6, 6_1/3, 6_2/3, 7, 7_1/3, 7_2/3, 8, 8_1/3, 8_2/3, 9, 10_1/3,

.....}. Av = Aperture value in APEX notation. Av = {5, 6, 7, 8, 9,

...} are equivalent to N = {5.6, 8, 11, 16, 22, ...}.

So Av = {5, 5_1/3, 5_2/3, 6, 6_1/3, 6_2/3, 7, 7_1/3, 7_2/3, 8, 8_1/3,

8_2/3, 9, 10_1/3, .....} is equivalent to N = {5.6, 5_1/3, 5_2/3, 8,

8_1/3, 8_2/3, 11, 11_1/3, 11_2/3, 16, 16_1/3, 16_2/3, 22, 22_1/3,

22_2/3, .....}. Check by Av = log(N^2)/log(2). You now have delta-Av-N

conversion chart.

<li>Tape on both the rear function carrier tension control knob for

focusing and focusing knob. So you can mark on the tapes. Make sure

the tension control knob right next to the focusing knob is relatively

tight enough to minimize back lash (maybe not that much even when

tension is loose).

<li>Convert the distance delta on the rail to turns on the knob by

simply turning the knob and looking at the calibrated scale on the

rail. Make sure you always look at the tick you're referring in a

consistent manner. For example, close one eye and look directly

straight down into the tick while consistently positioning your head

at the same point relative to the top of the frame. Mark the starting

point, then mark deltas on the left side of the focusing tape. You

don't need to label deltas by f-numbers at this point.

<li>After you transfer the deltas to the focusing knob, carefully

transfer the marks on the focusing knob (angles) to the tension knob.

When you do this angle transfer, make sure you look

directly straight down to marks towards the center (axis) of the

knobs. You now transfered the marks to the tension knob.

<li>Remove the tape on the focusing knob.

<li>Mark corresponding f-numbers N's, using the chart you made in

procedure 1 for a given delta, starting from N = 0 (equivalent to

delta = 0).

<li>Mark a line somewhere by a marker on the left side of the focusing

knob to indicate where you start focusing. Then starting from that

point, mark a line for every 90 degrees using different colors for

each angle. For example, 0 degree = Red, 90 degrees = Green, 180

degrees = Blue, 270 degrees = Yellow. So one of the lines can be

easily seen from you when you're taking pictures. You use different

colors to remind yourself which line you focused first. Had they been

all the same color, you would not remember which line on focus knob

you started with when focused a far [near] object and then focused a

near [far] object.

<li>You now have a Sinar equivalent DOF knob.

<li>You might want to protect marks from use of tension knobs by clear

tape.

</ol>

Method 2

A better DOF knob without a use of tension knob but a bit more

elaborate. Method 2 can be applied to any LF/MF cameras with geared

focusing knobs:

<ol>

<li>Do procedure 1 in Method 1

<li>Cut out the 1/4 width of the rubber piece on the left side of

focusing knob. Leave 3/4 on the knob as it is.*

<li>Make or find a right size of plastic C-shaped ring (open ended

ring) piece. The right dimensioned ring should rotate on the knob by

hand but stays on the focusing knob so that the ring rotates together

with the focusing knob otherwise.

<li>Mark the starting point starting from any tick on the calibrated

scale on the rail (defining where 0 is), then mark deltas on the left

side of the focusing tape and corresponding f-numbers using the

conversion chart you made just before.

<li>Mark a line somewhere on the right corner close to you of the main

body of the rear function carrier. Basically the line should be

somewhere you can comfortably see at your working angle when you're

actually taking a picture. This line is your reference point whenever

you set the f-number N = 0 on the ring. Equivalent to every 90 degrees

on focusing knob in Method 1. Think you have 4 moving zeros in Method

1 and 1 fixed zero in Method 2.

<li>Cover the ring by clear tape to avoid marks and f-numbers washed

away by use.

<li>You now have a Sinar equivalent DOF knob.

</ol>

* Not recommended to cut more than 1/4 width of the rubber piece. The

fraction 1/4 is based on experimentally how much fraction of the

focusing knob I use to focus. When focusing, you don't want to touch

on the ring so it does not rotate accidentally and screw up DOF

calculation. So the further the ring away from your finger focusing,

the better. I'll work out procedure 1 and post it later.

masayoshi_hayashi · April 25, 2000

Ok, I tried the procedure I described above yesterday. There are still

some typos and minor improvements in the above procedure. I figured

out the delta/turn to be about 4.75mm ±.25mm experimentally

when a strip of standard laser paper is placed on the focusing knob

and glued the ends together. One whole revolution of the knob gives

about 20mm (2cm) delta on the rail.

Method 1

Specifically for Arca

<ol>

<li>Calculate delta = 2*c*10^(1/2*log(2)*Av) where Av = {5, 5_1/3,