Understanding Light Stops

One thing that seems to baffle a lot of new photographers is understanding the term “stop” as it pertains to a measurement of light. We see this word used over and over with regards to shutter speed, aperture, film speed, filters, lighting, and other ways that light is used. You may hear a phrase like “you should expose one side of face 1-2 stops under the lit side of the face and expose the background 1 stop under the main subject”. For many people that’s about like asking them to solve a complex calculus problem. So what does it all mean and how do we use this information? That’s exactly what we are going to look at in this article

Stop the Madness
So, what is a stop exactly? Simply put, if you take a given amount of light and double it, you are moving up one stop, if you cut the amount of light in half then you have gone down a stop. So a stop isn’t a specific amount of light, instead it is a measure of difference between the amount of light. To be completely technical about it, the term “one f-stop” refers to a factor of 2 (approx. 1.41) change in f-number, which in turn corresponds to a factor of 2 change in light intensity. So how can we visualize this easily? If we have a single light bulb as a base light source, then two light bulbs would increase the light by 1 stop. Going to 4 light bulbs doubles the output again giving us 2 stops, to get to 3 stops we then need 8 light bulbs, etc.

So let’s take the sentence in the introduction, if we wanted to expose one side of the face 1 stop under the other side of the face, what we are saying is we want half the amount of light on the shadow side that we have on the lit side. Pretty simple concept huh?

For the most part, we are actually going to be doing a lot of guesswork unless we have good light meters and can measure each piece of the scene. Using our DSLRs we can zoom into each area of the scene and look at the exposure meter in the camera for a pretty good meter reading as well.

How aperture and shutter affect light
Ok, so now we have a basic understand of what a stop is, now we need to see how to control this using the two basic controls, shutter speed and aperture. To illustrate the effect of aperture, we have created this chart that shows the amount of light that is gathered at typical aperture settings. While each camera/lens may have slightly different available aperture numbers, they are typically in multiples of 1.4 (as described in the previous section). As you increase the f-stop number to the next 1.4 increment, you halve the amount of light.

The effect of shutter speed is the same concept. Every time you double the shutter speed, you decrease the amount of light in half. If you cut the shutter speed in half, you double the amount of light.

Again, you can see this represented by a similar chart.

Where this gets interesting in combining aperture and shutter speed, you can get the same exposure with different combinations of aperture and shutter speed. For example, a shot taken at 1/500 at f/2.8 and a shot taken at 1/60 at f/8 will have the same equivalent amount of brightness, what will change is the depth of field.

Let’s take a look at one of our favorite studio lights, the Westscott TD5 Spidelite. The light head has 5 bulds in it, you can control the center bulb, and two sets of two bulbs. To show how this affects stops we can use the different combinations to control how many stops of light we have:

If we only turn on the center bulb and use that as our base reference point, we can then use the different variations to come up with the following amounts of light changes:

1 Bulb

2 Bulbs

3 Bulbs

4 Bulbs

5 Bulbs


1 Stop

1.5 Stops

2 Stops

2.25 Stops

How can we use this information?

If you have a light meter than putting this information to use is just a matter of measure-adjust-repeat until you get the desired light readings you want. With a modern DSLR, we can do this with the camera’s light meter with fairly good results.

Let’s say we want to use a two-light setup to light a subject’s face and we want one side 1 stop darker than the lit side. To begin with we zoom into the lit side of the face (or use the Photovision Digital Calibration target aimed at the light source , the small size works best for this) and adjust the lights until you get a good exposure reading. This is the time to make the camera adjustments to center the exposure in your camera’s meter.

Good Exposure Reading

The next step is to meter the shadow side of the face, using the same technique of zooming into the subject or a target to get a meter reading. This time however we want to underexpose by one stop.

-1 Stop Under Exposed

At this point we now have the camera setup for the right exposure on the lit side of the face and we checked to make sure the shadow side was one stop underexposed, we are now good to take our shots and be confident that the lighting will turn out good.

Hopefully this has helped to explain the concept of stops as it pertains to the relative amount of light and how we can use the tools we have available to make sure we have good lighting and exposures.


Kerry Garrison lives in Castle Rock, Colorado with his wife and two dogs. With 10 years of experience shooting products and 5 years of experience in the wedding industry, Kerry brings a good deal of technical know-how and can explain topics in easy-to-understand terms.

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20 Responses

  1. Anonymous says:

    oh btw my strobe light is Ba Wang SL-150 Studio Flash

  2. kgarrison says:

    Make sure your shutter speed is no faster then 1/200th. Any faster and the camera wont sync with that flash. If you are in manual, aperture priority, or shutter priority modes then the built-in flash wont pop up.

  3. Anonymous says:

    Make sure your shutter speed is no faster then 1/200th. Any faster and the camera wont sync with that flash. If you are in manual, aperture priority, or shutter priority modes then the built-in flash wont pop up.

  4. darren says:

    hi there i have a sony a200 slr and a strobe in a softbox running in wireles mode, i wanted to how to turn the onboard light off and have the strobe only doin the flash work

    and if i go close to the person its takes alovely shot, but only coz the built in flash is closer.
    With me further away and the strobe doin the light work the photo again is very poor,
    i cnt work out how to create shadows u have expland up there

  5. Anonymous says:

    hi there erm ive tried wat u siad but the Ba Wang SL-150 Studio Flash strobe wont flash unless the built in slr flash is on, and in order to have it on the flash need to be up.
    im sooo lost lol

  6. Anonymous says:

    oh rite yeh thank you sooo much, i thought that myself and have already ordered the shoe adapter 2days ago so i cnt find out if it works with the cord till it comes, but now uve told me, im over the moon that its gonna work and ive was doin the rite things, again thank you, ill let u no how it goes.

    ps ill let ppl know about this site and put u on my myspace if u want.
    ppl help me then i like to help ppl.

  7. Anonymous says:

    oh another question, if i just use the lamp on the Ba Wang on one side of a person. can i still get good quality shadow shots. if so wat aperture and shutter speed would u recommend. or should i just play around with the settings. soz im all new to this, lol

  8. kgarrison says:

    You must be using the optical slave on the Ba Wang, in order to only have the Ba Wang flash go off you need to use the Sync cord that came with it. Do use the PC Sync on a Sony A200 you will need to buy a PC Sync Adapter that will plug into your camera's hot shoe. Your local computer store should be able to hook you up with the right part.

  9. darren says:

    and does my slr and the Ba Wang have its own trigger in them coz the Ba Wang strobe works without any other sync cords or triggers when i press the shoot botton on the slr

  10. DAVE says:

    you started great..then you went down hill….

  11. Shari says:

    Very nice tutorial! Thank you for making lighting so much clearer!

  12. Great tutorial, I'm sending the link to my wife because if I try and expain it she won't understand. All I get from her when we go out on shoots is "My images are too dark!". Hopefully, once she reads this all will become more clear.

  13. Roger says:

    This is a great article. Thanks for the post.

  14. Roy Filters says:

    Good info on the f-stops. Getting the technical right is usually the easy part but judging the amount of light needed to produce a good effect is the problem for most people learning photography.

  15. tom says:

    Thank you for the explanation on 'stops' But if you are using a 2 strobe setup and your main light reads f-8, then why should the fill light read f 5.6 if opening up the lens 1 stop doubles your light. It would seem that it should go in the other direction..or read f-11.

    • kgarrison says:

      If you have both lights on then the combination of both lights would go to F11. If you are trying to get a 1 stop ratio then you only meter one light at a time and if the key is f/8 than the fill should be f/5.6.

  16. Tim says:

    Hi KerryG,
    While your written description ("…they are typically in multiples of 1.4…") seems correct — though I've never seen a lens that WASN'T graded in standard f-stops — the visual illustration used is incorrect. F-stops are f/1.0, f/1.4, f/2.0, f/2.8, f/4.0, f/5.6, f/8.0, f/11, f/16, etc., which represent the multiples of 1.4 (1.4 x 1, 1.4 x 2, 1.4 x 3, etc.). The illustration instead uses adding of 1.4 to each f-stop.

    Or put another way:
    f-stops are calculated by powers of √2 (which roughly equals 1.4). For example, √2⁰ = f/1.0, √2¹ = f/1.4, √2² = f/2.0, √2³ = f/2.8, etc.

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