Just thought I'd share the results of a little experiment, as Internet advice regarding film heat tolerance seems to include everything from 'always keep film cool in an ice chest and develop immediately' to 'let it bake away and don't worry about it.' This past summer I took a family trip to the southwest USA, camping much of the time and keeping my extra 35mm Velvia 50 (all fresh, about a year before expiration) bundled in extra clothes in my duffel in the car trunk. Daytime temperatures were over 100 degrees F on several occasions but a thermometer stored with the film never made it out of the 90s. Obviously whatever rolls I was using each day were exposed to the ambient 100F+ for hours. I didn't expect any problems and didn't have any, so I decided to keep one of the extra trip rolls in the glove box of my car for a few weeks after we got home.

Unfortunately for the test, summer temps never made it out of the 80s here in the southern Appalachians, but a temperature logger I kept with the film still showed highs in the glove box of over 120F, and at least 100+ total hours of temperatures over 100 degrees F. The remainder of the time temperatures fluctuated between the 60s and the 90s every day and night. More subjectively, getting into the hot car and seeing the film baking away in the glove box day after day definitely would have caused me to avoid that roll for any important work. I finally took the film out, shot the roll in various lighting situations with variously colored subjects, and took some comparison shots at the same time on a 'normal' roll that had been treated with respect in the refrigerator and air conditioning.

I got both rolls back yesterday and I honestly could not detect any issues with the heat-exposed roll when I looked at it alone. Colors, contrast, Dmax, etc were all what I would expect from Velvia 50. When I mixed the 'heat' roll slides up with the 'normal' roll slides and compared the two without looking at my notes, I did notice that the 'heat' roll consistently had an extremely slight difference in contrast (ever so slightly less than the normal) and maybe an extremely slight shift toward increased saturation of warm colors in some areas (though images shot in neutral-toned fog looked exactly the same on both rolls). I really can't emphasize the slightness of the difference enough - I've seen similar differences between rolls just from processing variations, which may have actually been the case here.

In the past I have been pretty casual about leaving a camera with Velvia in my car in the summer when the roll is not particularly important, always without issues, but I never actually measured temperatures and compared with film that had been kept cool. Now I'm pretty convinced that I don't need to take any special precautions with fresh film that may be exposed to 100F+ temperatures for days at a time, other than keeping it out of direct sun and insulating it as well as possible, and the results will be practically indistinguishable from film that's stored 'by the book.'