77mm ND8+ND64+ND1000 Lens Filter Kit with 3 Vacuum Cleaning Cloths and Filter Pouch, 24 Layer Multi-coated HD Optical Glass, Nano-Dazzle Series

SKU: SKU.2048V1

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The most popular ND filter for waterfalls is a 3-stop (0.9) filter. However, you can use higher versions, up to 10-stop (3.0) filters, to shoot exposures of over 30 seconds in the midday sun. The darker the filter, the slower the shutter speed you can use. 
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The 77mm filter size will fit your camera. 
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Yes, 82mm filter size is the one for your Sony. 
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These are constant at 3 stop, 6 stop and 10 stop. 
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It's recommended to buy the largest filter size and use it with the stop down filter ring on your other lens. Learn more about the step down ring here https://www.kentfaith.com/SKU0801_11-in-1-step-down-ring-set.