What is this tool I used to have access to called?

[williaty]

In art school, we had this magic box in one of the darkrooms. You'd open a little door, insert a piece of film, tell the box what (nominal) speed of film you had, push a button, and it would create something like 12-14 little rectangles of gradually increasing exposure on the film. Then you'd process the film and read the values on the densitometer to evaluate both the EI of the film and whether your developing was creating a reasonable CI.


1) What the heck is the little magic box called?

2) Does anyone still sell little magic boxes today?

3) Is there a way to fake the little magic box if it turns out that they no longer are available for purchase?

 

[removed account4]

do you put your film in after it has been developed into negatives ?
maybe it is a densitometer ?

i worked with a guy and we went to a rental darkroom one night and he gave the guy
his negative and it spun around this thing with a probe that gave him a starting point for exposure.
years later i was told it was some sort of densitometer/
==
whatever it was, it sounds like something very useful !
why don't you contact your instructor
and ask what that device is called
i haven't a clue.

 

[Vaughn]

Photograph an evenly lit wall (you can throw camera out of focus for this, too) and with each frame change the exposure one stop. Develop. Then read it on a densitometer.

I would use only two or perhaps three different shutter speeds that are relatively accurate to each other (not at either end of its range) and use the f/stops to do most of the heavy lifting.

 

[williaty]

do you put your film in after it has been developed into negatives ?

You put the film in the magic box to expose it, therefore it comes before developing.

maybe it is a densitometer ?

Nope, I remember which toy that one is. That's for reading density off developed film.

why don't you contact your instructor
and ask what that device is called

The intervening years make that a little hard these days :lol:

 

[calebarchie]

Sensitometer

 

[eddie]

I'm not familiar with your machine, but it sounds like using a step wedge in a light box will do the same thing.

 

[Mr Bill]

1) What the heck is the little magic box called?

Sensitometer (like calebarchie said). The ones I've been familiar with have not been made for a long time.

You can do the second part of the test by obtaining what they call a "step wedge," essentially a strip of film with varying density steps on it, then "contact print" this onto your test film. The variation in exposure steps is locked in to the step wedge, so that part is known - the contrast can therefore be measured, and this is the important thing in determining the development time.

 

[calebarchie]

You may still be able to find relatively new ones that were used in dentistry and radiology industries.

 

[Bill Burk]

What you describe is called a sensitometer as has been pointed out. There are several threads regarding them.

A couple I enjoyed...

Enlarger ----> Sensitometer

(there was a url link here which no longer exists)

The Great Sensitometer Shootout

(there was a url link here which no longer exists)

Flash as Sensitometer

(there was a url link here which no longer exists)

So you could make one yourself.

Keep in mind, as a hobbyist or even a professional photographer you may not require a precise, calibrated light source. My EG&G sensitometer can not be considered calibrated, but I find in practice that it gives me very consistent results. I have tried to estimate of the amount of light it produces at its 1/100 second setting (in meter candle seconds). But have concluded that without actual calibration, I have to be satisfied knowing only within plus or minus 1/6 stop how much light I am getting. That is close enough for me.

As you develop occasional test strips and measure the contrast, one of the strips might be special because it might happen to have density measurements that meet ASA/ISO requirements. ASA requirements are very specific but just to give you an idea, such a negative will have a contrast of approximately 0.62 gamma or contrast index (I call it "hitting the ASA triangle).

My best estimates for myself (of the amount of light put out by the sensitometer) come from that special negative, where its curve crosses 0.10 over base plus fog. That point relates by virtue of ASA/ISO film speed, to the meter candle seconds that hit the film.

Film manufacturers have to know exactly how much light is hitting the film because they might have to prove the film meets the specifications or Quality Assurance may send it back for being out of spec. But if you got film the usual way for an advanced amateur or professional... since the manufacturer sent it out... you can trust that an ISO 400 film will be 400. The manufacturer can't work backwards this way to figure out the light. They have to calibrate the sensitometer machine. As a consumer, you can work backwards... And as I said, it may give you an estimate that is within plus or minus 1/6 stop, but that is pretty good for an estimate.

 

[Bill Burk]

... the contrast can therefore be measured, and this is the important thing in determining the development time.

This is the most important thing.