In this tutorial on creating christmas themed images the author describes how she adjusted her exposure settings to add more weight to the christmas lights

She says she went from f/2.8 and 1/3 of a second to f/3.2 and three seconds exposures. It seems the difference in aperture (not even a doubling in light intensity) would not compensate for the increased light intensity of the ten times decrease in shutter speed. Is this correct?

source https://expertphotography.com/how-to-photograph-christmas-lights/

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    myself, i'm wondering what 1/3 c is supposed to mean. you translate it as 1/3 of a second, but is that a given? – ths Mar 1 '19 at 17:01
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    If you search this site for 'Mixed Lighting' you'll get a ton of info as well. Also, this blog is almost legendary when it comes to learning: strobist.blogspot.com – OnBreak. Mar 1 '19 at 18:51
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    @ths if not a typo...obscure reference to c as the constant speed of light maybe? – OnBreak. Mar 1 '19 at 19:31
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    @Hueco: I don't think you want a camera with a shutter moving at relativistic speeds... – Vikki Mar 2 '19 at 4:38

So, what you have here is a mixed lighting situation. The background and star notes are being lit via flash (check out their shadows. Nice and soft and from top to bottom. The main bulb cluster is on the left and yet it has no impact on those shadows) and the bulbs themselves are being lit...by themselves :-).

(There's probably a speedlight with a soft box on it or some other softening light modifier and it's placed north of the shooter, aiming down. When looking at lighting, always look to the shadows to get an idea of where it came from and how soft it was)

In these types of images, the flash is controlled by:

  • its power level
  • aperture
  • ISO

The amount of light captured by the bulbs is controlled by:

  • ISO
  • aperture
  • shutter speed

As you can see, there's overlap there but also two unique controls: the bulbs are the only thing affected by shutter speed while the flash is the only thing affected by changes to its power level.

So, in changing the exposure from f/2.8 to f/3.2 — the shooter brought down the amount of light processed from the flash (you can see that the highlights on the notes are less hot in the second example). They brought it down by 2/3 of a stop.

Now, that would also bring down the light captured from the bulbs by 2/3 of a stop. BUT, they also brought the shutter speed from 1/3 to 3s (~just over 3 stops), increasing the the total amount of light captured from the bulbs by ~2.5-3 stops.

So, in the end, the amount of flash captured was brought down while the amount of ambient (the bulbs) was brought up.

See technique 2 in my answer here. It's a similar mixed lighting shot and another idea for you to test mixing flash and bulbs. Mixed lighting is a whole subset of lighting technique, there is a lot to it, but damn if it isn't a lot of fun.

  • An excellent method, and an example of why not being scared to learn to use a flash is such a useful tool in photography. I would also find it highly ironic if the article writers took this root, given that they opened the piece with basically "don't bother with flash..." - Feels like a good reminder to take photo articles with a grain of salt I guess? – TheLuckless Mar 1 '19 at 18:41
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    @TheLuckless Definite yes on the NaCl. Though, I did check out the article and it does mention using a speedlight as a main. That being said, it does far too little to actually describe the method, especially to a technical enough degree for a beginner to replicate. So, yea, this is a bit too photo-click-baity for my tastes. Good thing SE exists :-) – OnBreak. Mar 1 '19 at 18:50

We don’t know exactly what 1/3 C means but we can surmise it a typo and should read 1/3 of a second.

Assuming this is true, how come the second shot, using a shutter speed of 3 seconds, is not overexposed? This might be a mistake, however, the text of the example you cited reveals the author used an electronic flash as the main light source.

In your mind, the difference between 1/3 of a second and 3 seconds is 9X. True, but this is photography- think of it as 3 stops --- additionally, an electronic flash outputs a flash of light that averages about 1/1000 of a second in duration. The key to understanding the difference in these two shots ---- the flash contributed equally to both images. It flashed and then extinguished. The difference in exposure is simply the accumulation of the ambient light, not the flash. We don’t know much about the ambient light except to say, it was likely feeble. Stopping down the lens reduced the exposure.


Edit: reading the complete article in detail over lunch rather than the section in question and skimming the rest, the main source is a speedlight, which I missed while searching for more uses of 'flash' from the start of the article. - See Hueco's for the more detailed answer of what's going on with the dual exposure.

Possibly a typo in the article (Photography websites aren't exactly renowned for rigorous peer review or anything.) or they've glossed over some additional post work.

  • Second photo does kind of look like it may have a gradient filter applied, and/or its levels played with. The main point of that section seems to be referring more to the concept of using a longer shutter time to capture more of that glow/scattering effect from the lights, moreso than a focus on nailing 'correct' exposure (article section TL:DR boils down to "Seeing weak effects from the light? Change to a longer exposure time and adjust as needed.)
  • Do you capture more glow because it is assumed you lower the aperture size? – Levon Mar 1 '19 at 17:08
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    Partly. The 'glow' that surrounds a light source is from different forms of light scattering - Increasing capture time grabs more of it from more places. Similar thing with light painting in a way - You're not recording 'a' glow, you're recording the glow over time as light gets scattered to your lens from different points. (You could also add fog to give more 'stuff' for the light to scatter off of, but that gets tricky to control.) Think of the difference in photographing a swarm of fireflies at 1/1000th of a second vs 10 seconds. Odds are more will be 'on' during some part of the 10s. – TheLuckless Mar 1 '19 at 17:21

The 1st image seems to have exposure compensation set to 1/3 to brighten it. (1/3c) Turning the compensation off makes a darker image. Now going to a smaller aperture means a longer exposure which means the Xmas lights have more time to contribute to the ambient portion of the light. The brighter xmas lights makes the background even darker for the same full image meter exposure. The longer exposure also means less flash is used and thus even bigger differences in bright ambient Xmas light and flash lit blue background.

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