Film's properties are known and understood and there aren't any "super films" that are better or more capable than another country's. Lenses are also understood.
With electro-optical (EO) sensors, great care must be taken to reduce the quality of the final product when it is publicly released so that adversaries do not gain a complete understanding of the what the sensors are capable of.
Film creates "better" images, but modern EO sensors are more capable in certain circumstances.
There is all kinds of computational and electronic trickery one can do to obtain images that may be impossible to capture on film that you want to keep secret, like fusing short wavelength IR with visible light or using it to discipline visible light to correct or reduce atmospheric distortion. Other EO technologies can determine what an object is made of from great distances.
Technologies like that you want to keep secret.
In a hypothetical Cuban Missile Crisis set in 2019, US analysts would have visible, near- and short wavelength-infrared, thermal, and pan-chromatic imagery to look at, but the 2019 version of Adlai Stevenson would still only show the visible images at the UN.
There were superfilms, film stock sensitive in the infrared/UV spectrum and filters to optimize such film. There was also film sensitive only to very specific colors. Releasing images from these, at any resolution, would indeed give away much of the program's abilities/goals.
The ability to more easily declassify film stock is due less to the technology and more to the bureaucracy within intel communities. The film stock is owned by a single agency and so the declassification authority is relatively straightforwards. Digital imagery is shared instantly with a host of different agencies, many of whom still do not talk to each other regularly, and is stored in countless archives. Declassifying a digital file is therefore an administrative burden in comparison to a roll of film kept by a specific agency.
Yes but the use of a particular technique in a particular location/time would divulge the specific collection goals of an operation, something that often remains classified long after the operation itself has been acknowledged. So while the existence of UV film is no secret, knowledge that it was being employed over a specific site at a specific time can be.
Less opportunity for sigint types to deduce information about the image source from the images and less opportunity for reversing whatever algorithm/processes were used to reduce quality (in order to make a "releasable" copy) in order to re-create that information.
I probably shouldn't have said it's "easier" to declassify. It's more of simply being a matter of deciding that the NIIRS (basically resolution) rating of the imagery from the OBC is low enough relative to other sensors and collection platforms that it won't provide much of an advantage to an adversary.
The embedded Linux system that was added to the OBC also provided a continuously variable Velocity over Height (V/H) control that allowed the OBC to collect imagery at lower altitudes than what it was originally designed for. This improves the quality of the imagery but at a lower area coverage rate as a tradeoff.
Film from the OBC is digitzed, ortho-rectified, and exploited as soft-copy. Back in the day it was done on mechanical light tables.
