Hand rolled Vision3 500T film, but I would like to know what is the best ISO to use, 400 or 800, and why?
There are a couple of considerations here...
When Kodak Vision3 500T is used for stills photography, very often it has been pre-treated to remove the RemJet anti-halation layer, to make it compatible with the standard C-41 process. This process effectively increases the sensitivity of the film to ISO 800. CineStill 800T is Vision3 500T with the RemJet layer removed. (If the RemJet layer hasn't been removed, the film shouldn't go through a regular C-41 process.)
Since this is a tungsten-balanced film, it's possible you will use a 85 filter with it. If that's the case, obviously then the filter factor comes into consideration too.
If "none of the above", why not just shoot it at ISO 500? If your camera can't set ISO 500, just set the camera to ISO 400 and "underexpose" by 1/3 of a stop. You can use an exposure compensation setting if available, or else just manually set the aperture narrower or shutter speed faster than recommended – the film will get the right exposure.
The Rem-Jet (removable jet black backing) coat is a dispersion of lamp black (soot) in a binder of cellulous acetate phthalate. This binder is an “acid plastic”. It can be softened and washed away using an alkaline solution. Machine processing uses a pre-bath to temporarilly harden the film so it better withstands transport in a fast moving film processor. The pre-bath softens the Rem-Jet and spinning rollers, much like paint rollers scrub off the Rem-Jet.
You can make an alkaline solution and hand buff with a well washed “T” shirt. This can be performed the film has been processed but before drying. Water 27 to 38°C (80 to 100°F) 800 mL 800 mL Borax solution 15 g/L use 20g Sodium Sulfate (Anhydrous) 100 g Sodium Hydroxide 1.0 g Water to make 1 L
The Rem-Jet serves to protect the film from exposure from the rear. Many motion picture cameras with thru-the-lens viewfinders leaked light if the photographer looked away. It serves as an anti-halation backing. It protects the film when large rolls are loaded and unloaded in subdued light.
Does anyone know if removal prayer to exposure has an effect on film speed? If so, why is this?
The answer by osullic touches on some excellent points. This depends on whether the anti-halation layer is still present at the time of shooting or not and the chemistry the film will be developed in. I'll add some thoughts under the assumption that there is no other choice than to shoot at ISO 400 or 800 and DX hacking, manual ISO setting and exposure compensation are out of the question.
If the anti-halation layer is removed prior to using the film, which I consider somewhat doubtful if this is a DIY project where you handroll 500T from a bulk roll, you are probably better off shooting at ISO 800 if you intend to develop in C-41. The lack of anti-halation layer could result in halos around strong highlights. This is a look that can be appealing in its own right but might become too pronounced when overexposing at ISO 400. A reason to believe this is that CineStill rates their 800T film at ISO 800, while it's simply Kodak Vision3 500T with the remjet removed. To be honest I'd be surprised if they did much more than removing the remjet to make the film suitable for C-41 processing as you can't simply change a film's chemistry, but I could be sorely mistaken. CineStill probably did a good amount of experimenting at various ISO ratings and with different development times to find what seems to work best, so it's a nice starting point. But Vision3 500T was never intended to be used without remjet backing or developed in C-41 so it remains a matter of compromise. You'd develop the film as if it were ISO 800, so no push/pull development relative to how you shot it.
If the remjet is still on and you must choose between ISO 400 or 800, then shooting at 400 would be an overexposure of 1/3rd stop while 800 would be an underexposure of 2/3rd stops. If you want to know the math to get at this I'll be glad to explain further on request. Film tends to do better with some overexposure rather than underexposure. Unlike digital sensors where highlights can clip into pure white, blown highlights on film tend to roll off more gracefully. Film usually has more latitude on the overexposure side than on the underexposure side relative to the rated box speed. This coupled with the fact that an overexposure of 1/3rd stop is a less dramatic departure from the "ideal" exposure than underexposing 2/3rd stops makes it the more sensible option. From the Kodak brochure of Vision3 500T:
A white card is 2 1⁄3 stops higher than normal exposure, and there are at least 3 1⁄2 stops above that for capturing specular highlight detail. A 3-percent black card is 2 2⁄3 stops below normal exposure. There are at least 2 1⁄2 stops of latitude below that for capturing shadow detail.
So about 5.83 stops latitude overexposure and 4.83 stops latitude underexposure. You're better off erring on the side of overexposing. Given how forgiving the latitude is the 1/3 stop from ISO 500 to 400 isn't a huge deal.
Development time and/or temperature could be altered to compensate for this if necessary. For example, the CineStill color simplified two-bath kit prescribes a development time of 3.5 minutes at 39°C/102°F and a development time of 2.75 minutes for pulling one full stop at the same temperature. So a good guess for pulling by 1/3 stop to compensate for the slight overexposure would be to take the difference between 3.5 and 2.75 minutes (= 0.75 minutes or 45 seconds), divide by 3 (= 0.25 minutes or 15 seconds) and subtract that from the standard development time. This gives you 3.25 minutes.
If you're developing using RA-4 paper chemicals which some people claim provide better results than C-41 then you're already on experimental ground. In that case go by testimonies online (Flickr can be a good resource for such discussion) to get some idea of how long and at which temperatures to develop when shooting at various ISO ratings.
In case you can develop using ECN-2 chemistry for which Vision3 film is intended your best bet is to consult the official Kodak resources. Currently you can find info at this link: https://www.kodak.com/uploadedfiles/motion/h2407.pdf. It doesn't mention anything about compensating for under- or overexposure. Since ECN-2 chemistry in quantities suitable for hobbyist use is a rarity (but available through some channels at the time of this writing) once again you'd be at the mercy of the experience of other photographers for getting reference data and would have to experiment yourself. At that point it is much easier to just find cartridges with DX coding for ISO 500 or hack it together yourself than trying to figure out how to make it work at different exposures.
Other than the exposure and development the final result will also depend on whether you make prints from the negatives or use digital scanning. One method might be able to get better results than the other for either overexposing or underexposing.
Finally remember that if you don't remove the remjet you shouldn't have the roll developed by any lab that isn't specialized in handling motion picture film or doesn't have provisions for removing the remjet. Doing so would at best ruin their C-41 chemistry and at worst damage their development machinery, which could be a costly mistake. Either find a service that offers ECN-2 processing at film quantities for a photographer or do the development yourself.
The bottom lines are:
- Film usually deals better with overexposure than underexposure.
- ISO 400 is only 1/3 stop overexposed from ISO 500 so a better choice than 800.
- Since Vision3 film was made for machine processing in specialized ECN-2 chemistry in large quantities at tight tolerances, any deviation from this is an experiment. As it goes with experiments you're best off trying things and seeing what works until you're satisfied with the results, or find something unique along the way that you love.