3-stop ND filter question

[batmura]

I have been using a 10-stop B+W ND filter for a while and I'm very pleased with it. The only thing I don't like is having to focus before mounting the filter on the camera at times, so I was wondering if a 3-stop filter would still get the same results when used with longer exposures. Would I this way be able to have my camera auto-focus with the filter on it or would manual focusing still be a must? Thanks.

 

[Robin_Usagani]

No, you can still focus no problem with only 3 stop.

 

[batmura]

No, you can still focus no problem with only 3 stop.

Thanks! It's good to hear the camera will auto-focus and I'll be able to see through the viewfinder. Can I also get the same results with longer exposure times, i.e. 60 seconds on a 3-stop instead of 20 seconds on a 10-stop?

 

[Robin_Usagani]

I think you got the basic wrong. If it takes 20 sec with 10 stop, you need a lot quicker shutter for 3 stops (too lazy to do the math but it will only be a split second).