COMPARATOR 6250

COMPARATOR 6250
The JOBO COMPARATOR is an electronic measuring instrument to establish
exposure times for black-and-white and colour enlarging. Its applications include:
— Determining exposure times for all black-and-white enlargements.
— Determining exposure times for colour enlargements from transparencies
and negatives.
— Determining the correct black-and-white paper grade.
For easy operation the instrument indicates balance by a pair of LEDs.
Contents
1 Operating controls
2 Preparation for use
2.1 Inserting the battery
3 Programming the COMPARATOR
3.1 Entering the exposure value obtained
4 Measuring exposure values
4.1 Constant time readings
4.2 Constant aperture readings
5 Determining the paper grade
5.1 Measuring negative contrast
5.2 Selecting the correct paper grade
6 Care and maintenance
6.1 Battery Test
7 Servicing
8 Guarantee

1 Operating controls
(1) Silicon meter cell
Highly sensitive photocell for spot readings on the enlarger baseboard.
(2) Main switch
Switches on the unit. To switch on briefly (to save the batteries) simply press down the button; to switch on for longer periods
slide the button to lock it down.
(3) LED indicators
These show balance. The instrument is correctly balanced when both LEDs light . up equally brightly.
(4) Seconds scale
Adjustable dial for exposure times from 1.5 to 120 seconds.
(5) Paper speed scale
Calibrated for determining paper grades and to show set paper speeds.
(6) Cursor
Used to adjust the sensitivity of the COMPARATOR and to read off values on the seconds and paper speed scales.
2 Preparation for use
The JOBO COMPARATOR is powered by а 6 volt battery. Once you have fitted a battery and switched on the unit, the
COMPARATOR is ready for use.
2.1 Inserting the battery
The COMPARATOR takes the following battery types: (GP-476) Berex/Daimon/Duracell/ Mallory: Type PX 28; Ucar: Type 544;
Varta: Type 4028; or equivalent batteries of other makes.
To open the unit, push a coin or similar item into the slot at the top of the housing and gently twist it to lever off the base plate.
Place the battery in the battery holder the correct way round (note the ± markings) but do not touch the exposed wiring.
Once the battery is in place, close the unit by pressing on the base plate again.
3 Programming the COMPARATOR
Note: During programming and during all readings keep all darkroom light switched off, as it could affect the reading.
To calibrate the COMPARATOR you must first have correct exposure settings for a given subject.
To establish these settings first make an exposure test without the COMPARATOR
A handy aid for such a test is the JOBO VARIOFORMAT partexpoeure masking frame used together with the JOBO STEP
WEDGE. Then enter in the COMPARATOR the exposure values obtained from the exposure test.
3.1 Entering the exposure value obtained
Keep the en larger magnification and the lens aperture settings the same as for the exposure test. Race the negative or
transparency used for the test in the negative carrier and switch on the enlarger.
Place the COMPARATOR on the masking frame or the baseboard so that the brightest projected image portion with
recognisable detail is located over the meter cell (1).
If you mainly make portrait enlargements, you can select a medium skin tone as a reference value in place of the brightest
image area. Switch on the COMPARATOR with the main switch (2). One of the two LEDs will light up.
Now adjust the sensitivity of the unit by rotating the cursor (6) until the two LEDs (3) show balance, i.e. with both LEDs lit equally
brightly. After this do not move the cursor (6) any more.
You can now switch off the enlarger and the COMPARATOR and switch on normal room lighting or the darkroom lamp.
Next rotate the seconds scale (4) so that the time obtained with the exposure test is exactly centred underneath the cursor (6).
This programs the unit also for the paper type used for the test.
To be able to reset the unit to this program at any subsequent time, mark the following values on the paper packing:
a) The paper speed value below the cursor (6) on the paper speed scale and
b) The seconds value on the seconds scale (4) opposite a given paper speed value (for instance 0 or 10).
4 Measuring exposure values
Race a negative or transparency to be enlarged in the negative carrier of the enlarger, adjust the image size and focus.
Switch on the enlarger and place the COMPARATOR on the baseboard or masking frame so that the brightest area of the
transparency or negative with
recognisable detail is projected on the meter cell (1).
If during programming you balanced the COMPARATOR for a medium skin tone,
again select a corresponding image area for the measurement.
Switch on the COMPARATOR with the main switch (2) and find a new exposure
by either of the two methods below:
4.1 Constant time readings
For this reading method the cursor (6) must be set to the paper speed established during programming.
Adjust the lens aperture of the enlarger to bring the LEDs (3) to balance (both LEDs lit equally brightly).
Now expose the print with this lens aperture setting and with the exposure time used for the exposure test.
4.2 Constant aperture readings
For this metering method the seconds scale (4) must be at the setting obtained during programming.
Adjust the cursor (6) till the LEDs (3) indicate balance.
Now read off the new exposure time on the seconds scale (4) below the cursor (6) and set this value on the exposure timer for
exposing the print.
If you cannot reach balance on the COMPARATOR by rotating the cursor (6) select a different lens aperture.
5 Determining the paper grade
The COMPARATOR can also be used to measure negative contrast (ratio of brightest to darkest negative areas):

The contrast range of the negative determines the correct paper grade needed. With a correct paper grade the lightest and
darkest negative areas with recognisable detail will also reproduce this detail in the print.
5.1 Measuring negative contrast
Place the negative to be measured in the enlarger's negative carrier, adjust the magnification and focus.
Switch on the COMPARATOR and place its meter cell (1) below the darkest area of the projected image with recognisable
detail. Adjust the cursor (6) to balance the LEDs (3).
Note the value on the paper speed scale (5) that is now below the cursor index (6).
Next relocate the COMPARATOR underneath the lightest projected image area
with recognisable detail and once more balance by adjusting the cursor(6).
Again note the paper speed value below the cursor index.
These two values then indicate the ratio of the lightest and darkest negative
areas.
5.2 Selecting the correct paper grade
Read off the required paper grade from the difference between the two paper
speed values, as shown in the tables below.
Example:
Paper speed at darkest point = 14
Paper speed at lightest point = 3
The paper speed difference is in this case 11.
So you would need a »normal« paper grade.
Table for normal black-and-white papers
Paper speed
difference
Over 14
Required
paper grade*
0 Extra soft
12 to 14 1 Special or soft
10 to 12 2 Normal
8 to 10 3 Hard
6 to 8 4 Extra hard
Below 6
5 Ultra hard
Таblе for resin-coated black-and-white papers
Paper speed difference Required paper grade *
12 to14 Soft
9 to 12 Normal
7 to 9 Hard
5 to 7 Extra hard
*Subject to modification
6 Care and maintenance
The COMPARATOR needs no special maintenance. However keep the meter cell window clean as dust or dirt here can cause
inaccurate readings. Preferably clean this cell window with a blower ball. When not in use, store the unit in a cool and dry place.
Protect the COMPARATOR against excessive heat, as well as knocks and im¬pact, as you would any precision measuring
instrument.
6.1 Battery test
To check the battery, briefly switch on the unit with the main switch (6). If at least one of the two LEDs lights up, the battery is still
serviceable. If neither LED lights up, replace the battery (see section 2.1 of this manual). Partially exhausted batteries do not affect the
measuring accuracy of the unit. Check the battery at least once a year for possible leakage. If the COMPARA¬TOR is not in use for
lengthy periods, remove the battery. Battery leakage could ruin the unit.
7 Servicing
If the COMPARATOR should fail to operate or break down even with a service¬able battery, please send it to our servicing
department for checking and repair if necessary.
8 Guarantee
We guarantee the COMPARATOR for 12 months. This guarantee does not cover faults caused by misuse or by leaking batterie