Clarification - grad filters with slide film

Slide film may record let's say 5 stops. But ambient light is actually a lot more than that in stops. The grad reduces by two stops the brighter areas in the sky. (or one or three stops depending on the grad). That way, the film won;t blow out the highlights when you open up the aperture to catch more of the shadow areas.

Alan, let's say I yesterday summer here southern hemisphere. I metered the water with my Sekonic meter and I metered the sky it was 4 stop difference. Slide film can capture that. Grad filter still required?

Would the sky still have detail in it but it would look brighter? So if I used a grad it would make the colors more punchy?

The water reflection is giving so much light that it appears you're OK without a grad. I'd bracket in any case. I shoot with 120 roll film.

I always shoot to get the exposure correct never to punch up the colors. Of course, I shoot Velvia 50 which is pretty punchy to begin with.

Velvia has about 5 stops maximum of usable range. Provia 100F has a bit more in my experience.

Anyway, the answer is it all depends on the conditions you find yourself in, and the effect you want in the end photo. my technique is to spot meter a tone I want to be middle grey (a patch of grass or green foliage in the light you are exposing for is pretty good for this in landscapes). Remember that value or log it into your meter's memory if it can do that, then spot meter all around the sky to determine the maximum value above middle grey you previously set. Then use an appropriate strength or grad to bring the sky values down into the usable range of the film; this could be anything from 1-4 stops (or more) depending on conditions and how dark/saturated you want the sky to be.