Utilizing an ND Filter for Long Exposure Photography - MIOPS
ProMaster 52mm Variable ND #9538 - The Film Center
What are CENTER neutral density (CND) filters?
Firecrest Neutral Density 39mm to 77mm Filter (Open Box)
Amazon.com : H&Y 100x150mm HD MRC Center Graduated Neutral Density 1.2 Square Filter (4-Stops) with Magnetic Filter Frame for 100mm K-Series Filter Holder : Electronics
Zeiss 72mm Center Filter for 15mm f/2.8 ZM Lens 1364316 - Adorama
Heliopan 67mm ND 0.45 SH-PMC Center Filter 706751 V B&H Photo