I know this may sound like a broken record, but my search on here and at WallyWorld turned up nothing! Where do you find the 35-40mm filters again? I'd like to try one on the next camera to see if it makes a difference.
There are several different filters to choose from. A skylight 1-A filter really makes the blue in the sky pop out. UV filters remove the haze you see in some pics. Try one of each to see which one works the best for your film and setup.
Hoya makes decent filters for a good price. Tiffen is another lower priced filter. Heliopan is top of the line glass. As yotee pointed out earlier, regular window glass can be very harsh in how it sees light. All window glass is not the same. I'm surprised the factory cam makers haven't added this feature to their units.