Long Shutter Speeds

[End Game]

Also just try a lenshood, might do a lot, might do a little but always worth having

 

[EOS_JD]

I am considering buying a Hoya ND filter but was wondering which one to get, the ND4 or ND8.

Now if I am understanding this correctly, the ND4 will reduce my ISO by 1/4 so if shooting at ISO 100 it would reduce it 75. Correct?

Would the ND8 be enough or should I go with the ND4?

I want one because I recently was on vacation and was trying to get some shots on the beach of the water and could not get the smooth look of the water I was looking for without over exposing. I don't expect to be using it much in bright light. Mostly early morning and evening.

Your ISO is not affected in any way by the use of a filter although you can amend the ISO to get a correct exposure. It's the overall exposure that is affected and if you reduce the light then you need to compensate by amending one (or more) of the three variables (ISO, shutter speed, aperture).

In the above case the OP wants long shuter speeds so using the filter he's set a low ISO (say ISO100), a small aperture (like f16 or smaller) and I'd probably let the camera do the work to evaluate the appropriate shutter speed. If you cannot slow the shutter speed enough, you may need another ND filter.

Keep your eye oin the exposure bar in the viewfinder of your camera as in most situations it will advise where a correct exposure is. I'm guessing your bar (when shooting the water) will be way off to the right indicating over exposure.

Exposure compensation is used to make the tweaks if needed. Learn how to read exposures. Bryan Petersen's book "Understanding Exposure" is likely to be of great benefit to you.