Hi Everyone, I am working on building an automated 35mm film carrier for DSLR film scanning and could use some help figuring out a method of detecting the edge of a 35mm film frame. I had the original plan of just using a stepper motor to move the film a predetermined amount of space, since there is a standard spacing for film, but with the majority of film cameras being 20+ years old, I am sure there will be some variance in the spacing which will add up over a roll of 36 frames.
So far I have tried using a TCST5250 transmissive optical sensor with a phototransistor output to measure the light area in-between frames, but the light and dark film seem to pass the same amount of IR light. Any and all ideas would be greatly appreciated!
I'm no expert, but presumably, normal photographic film is optimised to have as little sensitivity to IR light as possible, so perhaps it's not a surprise that's still the case after processing.
Perhaps you should try to use optical/visible light to detect the edges of the frames?
It has been several decades since I worked with film. The spacing might be more accurate than you think, if it wasn't there would be a risk of one exposure contaminating another. That being said, it is not impossible though. I would create a bar (24mm) of leds and another bar (24mm) of photo detectors (choose devices that will operate in their normal range as opposed to being overdriven) By knowing (experimentally) the value of clear film you will be able to detect when you are NOT between frames. HOWEVER it is entirely possible that a piece of film could be 100% clear in the 35mm frame. That means you need both a sensor like I mentioned and a counting mechanism using the stepper motors. If the motor says YES and the light sensor says YES then ok, but if the motor says YES and the sensor says NO, then either move film looking for a specific width of clear film OR ???? I hope I have been informative. Of course this is assuming exposed and processed negative film, it will be different for positive like slides.
99.999% of the time they worked very well, but in my many years of film photography I did have the odd slip. The point is you can't rely on the spacing, you need a different system to confirm the stepper position.
There is an industry standard of 4 sprocket holes per frame. The advancing mechanism used that fact to move from frame to frame. Any variance would be from tolerance differences in the mechanics.
Edit: That is for 35mm movie film. I believe it is double that for still cameras.
Edit 2: actually 8 sounds wrong and I can't find any 35mm negs to check. I think it's 4. At any rate it is a standard.
I am not sure you were addressing that to me, but DSLR is NOT a movie camera that runs continuous when the button is down. The OP is concerned with the non-motorized 35mm DSLR where the film was advanced by the thumb of the operator. It was a very well engineered mechanism, but as I said, I have seen it slip VERY rarely and the OP seems to know that as well.
The last years I shot film, I used 35mm hollywood movie film since even under a grain microscope the grain was not visible. I think they started with 70mm and split it into two. I don't know if there were any holes in the movie film, it may have been vacuum advance or they simply punched more holes between the 4. Sorry, it's been about 20 years since I switched from film to digital so the memories are fading and I digitized everything on film I wanted to keep about 2016.
I have some 70mm film backs for my Hasselblad and it is perforated. You can load your own cassettes with movie camera film stock which is desirable since the selection for still cameras is poor these days.
I just measured the sprocket spacing on a Pentax still camera and it does come out to 4 per frame.
I can see several examples on google of 35mm SLR cameras and film cassettes and they have 8 holes. Were some different from others? Hard to see, but clearly more than 4. I count 8
Yes, still cameras are 8. Not sure about movie cameras but the implication seems to be four. When I measured my camera sprocket drive I got the correct number of holes for the 18 x 24 orientation and it works out for the 24 x 36 orientation.
Even though it's a standard, there probably is enough variation in the real world to make orienting by sprocket hole spacing impractical.
They work very well, but you’re dealing with spacing that is ~2mm between frames and these cameras are 20+ years old and have not been serviced. If one camera is advancing 2.1mm, and I am anticipating perfect 2mm spacing, by the time I reach the last frame I’ll be 3.5mm off. This is (probably) an extreme case and I wouldn’t expect each frame to be off.
Can you not eliminate the being off that much by simply advancing the motors the same amount each time that is the same as the camera gears advance the film. I would use at least two cameras, a Nikon, and a Canon as they were the two dominant brands back then.
When ever I visited my local film processing shop, sometimes to to a minor repair, the operator never left the machine while it was printing.
As each frame was presented in the machines "View" screen, the operator made the small alignment corrections to assure a full frame.
That was 20+ years ago, modern times I would assume that image recognition would centre the frame, like the olden days human image recognition.
The film is over scanned, like 38mm and the edges found with software.
Each "automatically centered" image would make alignment easier for the next frame an learning the actual spacing for that roll.
A nighttime image would present no optical sensor detectible edge, but complete image evaluation might work.
