The osullic answer touches on some excellent points. It depends on the presence or absence of the anti-halation layer at the time of shooting and the chemistry in which the film will be developed. I will add some thoughts on the assumption that there is no choice but to run at ISO 400. or hacking 800 and DX, manual ISO setting and exposure compensation are out of the question.
If you remove the anti-halation layer before using the film, which seems to me somewhat doubtful if it is a DIY project in which you maneuver 500T on the fly, it is probably better to turn 800 ISO if you intend to develop in C -41. The absence of anti-halation layer could lead to halos around strengths. It is a look that may be attractive in itself but may become too pronounced when overexposed to ISO 400. We can think that CineStill evaluates its 800T film at 800 ISO, while it is simply Kodak Vision3 500T with the removed remover. To be honest, I'd be surprised that they're doing a lot more than removing the remission to make the film suitable for C-41 processing, because you can not just change the chemistry of a movie, but I could to cheat on me. CineStill has probably had a lot of experience with different ISO ratings and different development times to find what seems to work best. So it's a good starting point. But Vision3 500T has never been designed to be used without jet support or developed in C-41, so it's a compromise. You would develop the movie as if it were the ISO 800 standard, so no push / pull development compared to the way you filmed it.
If the rematch is still on and you have to choose between 400 or 800 ISO, shooting at 400 equals overexposure by one-third, while 800 is under-exposure by two-thirds. If you want to know the calculation to get there, I will be happy to explain more in detail on request. The film tends to do better with overexposure rather than underexposure. Unlike digital sensors in which highlights can be cut in pure white, highlights blown on a film tend to unfold more easily. The film generally has more flexibility on the overexposure side than on the underexposure side compared to the rated speed of the box. This, coupled with the fact that an overexposure of 1/3 stop is a less dramatic departure from the "ideal" exposure than under-exposing 2/3 of the shutdown actually makes it "worse". most sensible option. From the Vision3 500T Kodak brochure:
A white card has an amplitude greater than 2 to 13 normal exposure and at least 3 stops above that required for capturing details of specular highlights. A black card at 3% corresponds to 2 2/3 stops below a normal exposure. There are at least 2 1/2 stops of latitude below those used to capture shadow detail.
So, about 5.83 stops the overexposure to the latitude and 4.83 stops the underexposure to the latitude. You'd better be wrong about overexposure. Since the leeway of a third of 500 to 400 ISO is not so complicated, it does not matter.
The time and / or the development temperature can be modified to compensate for this if necessary. For example, the CineStill Simplified Two-Color Color Kit prescribes a development time of 3.5 minutes at 39 ° C and a development time of 2.75 minutes for a complete shutdown at the same temperature. So, a good idea to shoot by 1/3 stop to compensate for slight overexposure would be to take the difference between 3.5 and 2.75 minutes (= 0.75 minute or 45 seconds), divide by 3 ( = 0.25 minutes or 15 seconds) and subtract only standard development time. That gives you 3.25 minutes.
If you develop RA-4-based chemicals that some people think are better than C-41, you're already on the experimental ground. In this case, check out the online testimonials (Flickr can be a good resource for this type of discussion) to get an idea of the duration and temperature of temperature creation when shooting at different ISO ratings.
If you can use the ECN-2 chemistry for which the Vision3 film is intended, it's best to consult Kodak's official resources. Currently, you can find information on this link: https://www.kodak.com/uploadedfiles/motion/h2407.pdf. He does not mention anything about the compensation of under or overexposure. Since ECN-2 chemistry in quantities appropriate for amateur use is a rarity (but available through some channels at the time of writing this article), again, you would be at the mercy of the experience other photographers to obtain reference data. experience yourself. At this point, it's much easier to simply find DX coded cartridges for ISO 500 or assemble them yourself rather than trying to figure out how to make them work at different exposures.
In addition to exposure and development, the end result will also depend on printing from negatives or digital scanning. One method may provide better results than the other in case of overexposure or under exposure.
Finally, remember that if you do not remove the jet of ink, the developed roller should not be developed by a non-specialized film processing laboratory or do not provide for the removal of the jet of paint. 39, ink. At best, this would destroy their C-41 chemistry at best and at worst their development machines, which could be a costly mistake. Either you are looking for a service offering ECN-2 processing for a large number of films for a photographer, or you are in charge of development.
The bottom lines are:
- The film generally treats overexposure better than underexposure.
- ISO 400 is only 1/3 overexposed stop of the ISO 500, so a better choice than 800.
- Since the Vision3 film was manufactured for machine processing of ECN-2 specialty chemicals in large quantities with tight tolerances, any deviation from it is an experiment. As for the experiments, it is better to try and try to see what works until you are satisfied with the results or find something unique that you like on the way.