ISO 800 is half as sensitive to light as ISO 1600. A low ISO value (e.g. 100 or 200) means low sensitivity to light. This is exactly what's needed in bright conditions in order to avoid overly-exposed photos. A high ISO value (e.g. 800, 1600 or higher) means a high sensitivity to light.

Can you use 800 ISO in daylight?

Set the Film ISO

If you plan to shoot indoors in low light conditions, film ISOs of 400, 800, or even 1600 are preferred. If you are shooting outside and you have lots of sunlight, try to use ISO 100 film, or even slower (you can find films with ISO 50 or 25).

What is the highest ISO I should use?

While general (professional) candids and documentary photos might be acceptable at ISO 1600-3200, I wouldn't go any higher than ISO 400-1600 for really important portraits.