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?

  • 1
    \$\begingroup\$ 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. \$\endgroup\$
    – Alaska Man
    Commented May 6, 2019 at 22:21
  • \$\begingroup\$ what is the guide number of your flash? \$\endgroup\$
    – scottbb
    Commented May 6, 2019 at 22:21
  • 3
    \$\begingroup\$ @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. \$\endgroup\$
    – scottbb
    Commented May 6, 2019 at 22:22
  • \$\begingroup\$ 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. \$\endgroup\$
    – doug
    Commented May 6, 2019 at 22:23
  • 2
    \$\begingroup\$ @doug, Please put your answers in the answers section, even if they're short \$\endgroup\$
    – scottbb
    Commented May 6, 2019 at 22:46

3 Answers 3


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.


I am so lazy, so I made these tables.

  1. Normally a table for the sunny 16 rule, marked in beige, 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) and define a base aperture of f11, 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 as an alternative you can use some HSS flash.

  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


It's a simple matter of stop math.

You can use sunny-16 basics of having good exposure with shutter speed and iso at reciprocals (e.g., iso 100 and 1/100s) at the following apertures:

Aperture Lighting conditions Shadow detail
f/22 Snow/sand Dark with sharp edges
f/16 Sunny Distinct
f/11 Slight overcast Soft around edges
f/8 Overcast Barely visible
f/5.6 Heavy overcast No shadows
f/4 Open shade/sunset No shadows

Or, just take a regular shot with automated exposure settings the camera sets, and if it looks good, then do the math to see how many stops it's going to take you to f/1.4 AND however many stops you want to underexpose the ambient.

So, say, you're in sunny-16 conditions. That means, good exposure is at iso 100, f/16, and 1/100s.

If you're stuck at sync speed (1/200s), then you can only whack -1EV off that at base ISO, so the equivalent of bring the aperture to f/11.

Between f/1.4 and f/11 is +6EV.

So you need a -6EV (6-stop) neutral density filter if you're not underexposing the ambient at all. But if you also want to "pop" your subject by underexposing and lighting them more brightly than the background, you'll probably want 7EV or maybe even 8EV.

You can get a set of ND filters of different strengths that you can stack. Or a variable ND filter that uses two linear polarizers so that rotating one over the other adjusts the amount of filtering.

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.

Just me, but this may be wildly optimistic. To darken the background you need to light your subject more brightly than they are by the ambient light. How much darker the background gets depends on how much brighter you can light them.

Speedlights are like the f/3.5-5.6 kit lenses of the lighting world. They're limited in how much light they can pump out. And they're very good at focusing the light, but not so much spreading it out. Using a speedlight in a modifier, to "overpower" the noonday sun in sunny-16 conditions? It's a big ask, and it's typically why folks grab an AD200 Pro (200 Ws) or AD600 Pro (600 Ws) to do this instead of a ~70 Ws speedlight.

You can use a speedlight this way for fill, or as main illumination more in the golden or blue hours, or with your subject in the shade. But playing day for night with the noon day sun is more the province of monolights and pack and head systems, not so much speedlights.

Few last thoughts, while using an ND filter will not put you through the additional -2EV power suck of HSS, it's still going to darken and reduce both the flash and the ambient. And as an additional filter it can still introduce flare, particularly if you're shooting into the sun. And HSS means you don't need the right ND filter combination and that you can actually use a higher shutter speed for things like freezing action in the ambient. You might still want to consider getting a speedlight that can actually do HSS (like, say, a Godox TT600).


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