What ND filter strength do I need to shoot at f/1.4 in daylight with flash at 1/200s?
Asked 5/6/2019
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I want to use a Sigma 30mm f/1.4 for outdoor portraits in bright daylight, but I’m limited to about 1/200s because of normal flash sync speed with a speedlight and softbox. My goal is to shoot wide open for shallow depth of field, keep the ambient background somewhat darker, and use flash to light the subject. How do I calculate the ND filter strength I need, and roughly how many stops of ND are typically required in sunny conditions?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
7y ago
2 Answers
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You've discovered the wonderful, yet complex, world of mixed lighting - congrats!
Quick example using Sunny 16 rule...
- Starting Parameters (proper ambient exposure): ISO100, f/16, 1/125
- Adjusting to open up for bokeh: ISO100, f/1.4, 1/16000
Now, obviously those new settings have gone into theoretical territory with that shutter speed value. So, let's add in some ND and bring it back down. If you are wanting to have a darker ambient / more flash style of photo, then you're going to have to underexpose by a stop or two (or three). Let's assume two and redo our starting calculation...
- Starting Parameters (underexposed ambient): ISO100, f/16, 1/500
- Adjusting to open up for bokeh: ISO100, f/1.4, 1/64000
Still in theoretical territory. But, let's add some ND to get you back down under your maximum flash sync speed:
- Adjusting with ND: ISO100, f/1.4, 1/125, 9stops ND
So, there you have it. If proper exposure were a Sunny 16 kind of day, and if you wanted ambient to be two stops underexposed, then you'd need 9 stops of ND and one very powerful flash. In fact, I'd actually bet that a speed light won't have the juice to overcome 9 stops of ND - but you can ask about that calculation in another question, if you are so inclined.
If your starting exposure changes, however, then so too does your ND need. For this reason, trying to actually calculate the 1 ND filter to rule them all is an educational exercise at best. In reality, you should either carry multiple filters (3 stop, 6 stop, 9 stop [thus allowing for combos ranging from 3 to 18 stops]) and/or a variable neutral density filter.
Originally by user67377. Source · Licensed CC BY-SA 4.0
user67377
7y ago
0
Generated from our catalog & community — verify before relying on it.
Use stop math from a normal ambient exposure, then count how many stops you need to reach f/1.4 at 1/200s.
In bright sun, Sunny 16 says a normal exposure is about ISO 100, f/16, 1/100s (or close). Opening from f/16 to f/1.4 is about 7 stops more light. Going from 1/100s to 1/200s removes 1 stop, so you still need about 6 stops of ND just to hold normal ambient exposure.
If you want the background darker by 1–2 stops, add that too:
- normal ambient at f/1.4, 1/200s: about 6 stops ND
- ambient 1 stop darker: about 7 stops ND
- ambient 2 stops darker: about 8 stops ND
So in full sun, expect roughly 6–8 stops of ND depending on how much you want to underexpose ambient. In less bright conditions, you’ll need less.
A practical approach: meter the scene normally, then count the stops from that exposure to your desired aperture/shutter/ISO. Also note that high-speed sync is an alternative, though it reduces flash power.
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