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Naming of neutral density filters for DSLR photography

Planning to invest in a decent filter system to include in your photography gear? I am sure the nomenclature might have confused you a bit. In addition to my own ND filters set, I got lucky to be able to borrow a few grad filters from my friend who is a professional wedding photographer This allowed me to fiddle around a bit with different level of optical density and thus served as the photography inspiration behind this post. 
                               
naming neutral density dslr filters
Long exposure shot using a neutral density filter
The naming of the neutral density filters (either gradual or non-gradual) is a kind of myth, but this short and sweet post will make it easy for you. Some filter manufacturers name them as per optical density of the filter while others focus on f-stop number. The table below relates optical density, filter factor and f-stop number for neutral density filters.
Optical density of filter:

0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3.0 

Filter factor:

2 4 8 16 32 64 128 256 512 1024

F stop:

1 2 3 4 5 6 7 8 9 10

Example: 0.9 ND filter. This means it is a 3 f-stop ND filter. Similarly, a 10 f-stop ND filter would have an optical density of 3.0. So we have got the basics of neutral density filters, let's now talk a bit about the main motive behind spending money on density filters. There are certain aspects of photography which require a use of ND filter. For example: 
1) Outdoor portrait photography: Neutral density filters help for outdoor portraits in broad daylight. Example: Sun is behind your model or subject. In this case, a balance between ambient light and subject exposure can be achieved by using a fill flash and ND filter. Fill flash will prevent shadows on the subject face and pop out the subject more. Another way can be setting exposure to the background which would require a very high shutter speed and great flash power. 
2) Landscape photography. Shooting in harsh daylight sun can be made possible by using ND filters of higher optical density. Capturing sunset over the beach using ND filter gives a nice and silky effect to waves and leads to properly exposed sky. ND filter actually allows a longer exposure as it minimizes the amount of incoming light. 
Pro Tip: If you find it hard to focus post attaching a higher value ND filter, focus before on a point (either manually or auto) and then switch your lens to manual mode. Slide the filter in front of your lens and boom. Another way can be bumping up the ISO all the way to allow your camera focus through the filter and then lowering it back. There is a huge debate about using screw on density filters vs square shaped filters which require a holder system such as Lee SW150. 


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