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I'm currently shooting with a Sigma 30mm f/1.4 lens for Canon. I love its wide open aperture, especially in low light. But I realized that using this open aperture is next to impossible in brighter daylight.

Even cranking the shutter speed up to 1/4000th results in blown out pictures. I’d like to be able to use this lens in the daytime along with a speed light with a softbox so that I can create portraits where the background/ambient exposure is darker, and then fill in my subject with my speed light.

But my speed light can only handle shutter speed of 1/200th of a second. My question is, if I get an ND filter to darken things up so as to allow for underexposed ambient mixed with properly exposed flash...How many stops of ND should I be going for?

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    You do not have to use it wide open at 1.4 in the daylight. How about f8 or f16 ? if you still want to use 1.4 then stop the aperture down until it is the correct exposure and the count the stops. that is the ND strength you need. Go back to 1.4 and put the ND filter on and meter. – Alaska Man May 6 at 22:21
  • what is the guide number of your flash? – scottbb May 6 at 22:21
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    @Alaskaman If I'm not mistaken, I think Mike was specifically trying to go for shallow depth of field at ƒ/1.4, not to use it as an exposure control. – scottbb May 6 at 22:22
  • You're going to have to go to 5 to 7 stops of ND but you may have a problem with the speed light / softbox intensity. Especially if you want the background significantly darker than the subject. – doug May 6 at 22:23
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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.

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I am so lazy, so I made these tables.

  1. Normally a table for the sunny 16 rule, where you have your Iso fixed at 100 is:

Sunny 16 table

Each column is a stop, and I marked in green the 200 which is a common sync speed these days.

  1. So, if I now fix my shutter speed at 1/200 (besides the Iso at 100) I can now have a table for ND filters, marked in x (times) values and rND.

Table of ND filters

You might need to adjust depending on "how darker" you want the ambient light.

  1. Remember that you can use at some extent an HSS flash with an HSS remote trigger.

  2. Take a look at these questions regarding stacking ND filters: Will stacked ND filters perform as well as a single 10 stop ND filter? and polarizers: Can I stack ND Filter and Polarizer together? , and search the tags https://photo.stackexchange.com/questions/tagged/neutral-density https://photo.stackexchange.com/questions/tagged/nd-filter

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