How a film camera superimposes the date onto photos

today on Applied Science we're going to take a look at the hardware that superimposes the date on to film photographs this is something I always wondered about as a kid because the print is actually on the negative and so the height of these characters is super tiny and I've never seen an LED display or anything that's this small and as it turns out the thing that does this is a super miniature very simple LCD projector that's with a folded optical path that's powered by an incandescent light bulb so this turned out to be a pretty interesting teardown and I thought you'd find it interesting too I bought this camera myself sometime around 1990 and as it happens 2019 is actually the highest year that this thing can register so the way this works is we set the date on the back here and we've gotten up to 2019 and then it cycles around back to 1990 again so I've outlived to the camera and it's now's a good time to do a teardown at the time this was a big purchase for me I think I seem to remember this being about twenty or thirty dollars but the point is that the twenty or thirty dollars is not a lot for a camera even in 1990 and this feature of having the date super imprinted was not restricted to just high-end models so the technology that goes into here has to be cheap or else it wouldn't work it's been a long time since I've seen a roll of film but I actually happen to have one still kicking around in the closet which is great because we can see how this is going to work so we load the film in like this and it's pulled across by these cogged wheels here and the way that you do this is just sort of leave a little tail of film in there and it will wind it up I was actually very proud that this camera was Auto loading and auto advancing fully motorized which was nice because my previous camera was manual so I thought this is a huge upgrade but anyway we have these electrical contacts here and spring-loaded pogo pins here which will probe in a minute and then importantly there's this spring-loaded pressure plate on the back and the point of this is to keep the film very flat if this is out of plane at all the picture won't be in focus so there has to be this spring-loaded plate here it's critical that the film is really flat and just look at this it's not immediately obvious where that date super super imposing comes from right there's a little window here just as we can see the what kind of film you have loaded there's a little light seal around there so that when you look in the light doesn't go in and spoil the pictures but other than that it's it's a very thin sort of back here and it's not obvious where the date comes from so we'll pull off this pressure plate and keep digging into it to see what's going on this plate is spring-loaded and I've already kind of pulled it apart a little bit so I'm just gonna get the rest of it off here but this this was actually mounted quite a bit more securely than that makes it look and now it's starting to become a little bit more obvious there is a window here that looks suspicious and that window lines up with this window in the pressure plate and so this is starting to seem like this is where it is and remember the pictures will actually be upside down relative to the scene that you're taking because the lens right so it is upside down and the date is in the lower right corner of the picture when it's printed so it's in the upper right corner here because it's mirrored I am the film and then there is a suspicious looking door over here so let's open this up and that does in fact turn out to be a battery chamber so there's a tiny CR 1025 battery in the back and clearly that powers the date because I pulled the battery out and now this is off but there's also batteries in the camera there's two doublea's that drive the motor and and the shutter in here and so we'll probe this next and figure out between the battery in these contacts what the electrical input is to this date back system okay I've got the scope hooked up to these spring-loaded pogo pins here with the ground clip on the rightmost pin and there's four here but there's only two terminals in the camera so I'll just kind of align it so we get a signal there and then when I hold the shutter button down there's about 2.25 almost two and a half volts applied to these terminals and it's always there as long as the shutter button is held down it's not like a controlled timed pulse or anything so originally I thought that the camera might be controlling this the exposure produced by this date superimpose hardware but it looks like that's not the case there's no timing being done in the camera body so as long as you're holding the button down it's applying voltage to the back and then I originally thought that maybe the power for this date superimposing hardware is coming from the camera since there's two large double-a batteries in the camera body but there's only that tiny coin cell in the back I was thinking maybe it's doing something clever by sending power to the back but as we'll see that it's actually not quite the case it's just a signal and we'll tear this down and see what's next once removing the pressure plate from the back of the film it's pretty obvious where the screws are so we'll just go ahead and take those out and then this whole thing comes out and we can see that there's some typical conductive rubber puck buttons and on the backside of this we're finally getting somewhere oh what's interesting is that the module itself that creates this this date thing is actually kind of separate from the camera and it's not surprising because you'd imagine that the manufacturer that makes these date modules wants to put these in all kinds of different cameras and so it makes sense that there's kind of one module and then they just put this in all kinds of different cameras prototype models I really love how tiny the display is there's just something about having this like micro seven segment display that's really cool and it's completely unpowered now and we can see all the digits which indicates that this is probably some kind of LCD tech it shows up even better on the camera maybe due to something with the polarization or something like that but let's connect some power to it and see if it'll work outside the camera okay we've got this thing hooked up one set of leads goes to a DC power supply just to simulate the coin cell and the other set of leads goes to a pulse generator simulating pressing the shutter button once a second just to get this thing to trigger often and at first it doesn't really seem like anything's happening but if we look close there is something coming out the window but it may not be what you'd expect like it doesn't look like a seven segment display and there's a good reason for that the fact that this is off the film plane adds an additional optical constraint here right so let me get a seven segment display out so we can see what's going on here imagine that this projection screen is the film plane and that this little seven segment display is what we're using to impress the date on to it you can see that if we're held back a little ways the image is not very good right because the light is spraying out from this display in all directions it has to be in contact with the film plane to get a good transfer but that's physically not possible with our setup because in the camera the pressure plate is pushing the film down and then the electronics have to be offset behind that pressure plate by a couple millimeters maybe even three or four millimeters at least and the only alternative was speed to have the electronics floating on the spring itself which is one idea but that means floating the battery in the circuit board and everything else and that does sound kind of difficult so actually this date superimposition is done with a projector the optics are not like a 7 segment display purposefully it doesn't produce a nice easy-to-read image like this it's actually shooting the light out like a projector so that it forms an image on a plane that's a good you know three or four millimeters away a tiny little micro projector so to see this projector in action what I'm going to do is add our simulated film plane here this projection screen it's basically tracing paper and I'm going to turn the room lights out and turn that camera exposure up to see if we can see this thing in action so sure enough when we add a projection screen suddenly this makes sense where it's actually producing the image a few millimeters away from the screen so it's not really like a seven segment display that's meant to be looked at it really is much more like a projector so let's open this thing up to see how they built a projector in such an incredibly thin and incredibly low cost device okay so we'll open this up and we can see that this side still has the LCD in there and it's kind of an unusual LCD it's quite dark it reminds me of like a welding helmet like an automatic welding helmet you can hold this up to the light and almost nothing comes through so it's it's a little bit of an unusual LCD and then on this side the circuit board there's a light source behind here and the light comes up through this window shines through the LCD and that's what makes this projector so if we carefully lift the circuit board off without disturbing the other stuff behind it we can see that there is a little mirror in there it's actually a little tiny glass mirror and another LCD with another set of zebra stripe rubber conductive connectors and on the other side of this is the LCD that the user sees to set the date on the back of the camera so where is the light source well first of all there's no optics the only thing that's in here is a mirror and then the LCD we've seen there's no lenses or focusing or anything like that and the light source is here it's actually this cheap incandescent grain of wheat bulb and remember this is 1990 so LEDs were around but not nearly as common as today but I think there's another reason that they wanted to use an incandescent bulb like this and that is that it's a point light source in those days pretty much all the LEDs were very soft diffused LEDs and we had a very generally a large light source but for this projector basically what this is is a shadow puppet projection system so the way this works is if you have a very distant light source and you put something in the path you can project the image you know a little ways away but if your light source is close or if the image that you if the thing that you're projecting is close then the image is not so good and eventually it goes away entirely so the idea is that without any optics at all or with a point light source that's ideally pretty far away you can make this projector system pretty clever actually I think it's a very good use of low cost tech to make this very effective system so since this turned out to be just a shadow puppet projector you might be thinking that we could do something similar with an all mechanical device instead of an LCD and you'd be right I did some research and as near as I can tell the oldest or the first date the acker data back for a camera was the Nikon M f10 and it appears to be entirely mechanical you actually have these cute little plug-in modules where you can set the date or use an analog wristwatch sort of module and it shines light through the hands of the watch or through these little shutters mechanical shutters to imprint the date or the time onto your photos unfortunately this whole thing sells for about a thousand dollars on eBay so it'd be kind of inappropriate to buy one just for a teardown but maybe someday or maybe if people are super interested how we can make that happen I thought we'd have some fun with this thing and so I had dremeled a slot in the back case here so instead of the internal light source with the mirror I've cut a hole where there may lurk mirror used to be and then set up this diffused light source behind it so that the light shines through the LCD and actually discovered something in the process you can see that the update rate is much slower for the tiny LCD than it is for the big LCD I have the button wired up to the signal generator so it just cycles through because it looks cool and then I you know after doing that I noticed that the update rate was slow on this tiny LCD so there is something weird going on it's not a totally normal LCD it's probably optimized for really good light blocking and and slow update because what ends up happening is the thing changes the state of the LCD and then just flashes the light to expose the film but it is cool because I can change the color of the LEDs behind there of course with the modern LEDs you get all these colors but after doing this I was thinking yeah wait a minute how come how come every superimposed date on a piece of film I've ever seen is always that red color originally I was thinking maybe it was because they used red LEDs but that's not the case they're using white light so why is it not white check this out I think I actually figured it out why remember that how this works is the film is loaded in the camera like this this is the photosensitive side and this is the back of the film and the date is projected through the film to the photosensitive layer and look what happens if we project light through there it's that orange color again so if we're on this is a very cool cool white source so it really is orange if we're on this side of the film shooting light through then the light that manages to make it through to the photosensitive layer is in fact pretty orange and you know this makes sense you've ever seen a negative for color film it always has this orange color everywhere so I think at least in part it's actually the color of the base of the film itself that's imparting this orange glow and that's part of the reason why the date is always orange on these films so anyway it was something I've been wondering about for a long time hope you found that interesting and I will see you next time bye

Channel: Applied Science

Views: 370,027

Keywords: camera, date back, data back, tiny, LCD, LED, display, teardown, applied science, ben krasnow, electronics, film, film camera, 35mm camera

Id: ezME4_xMMnk

Length: 14min 11sec (851 seconds)

Published: Sat Oct 12 2019