How much more light? How much less?

At this point, we know what’s bigger and smaller, faster and slower. We know that f/4 is a bigger opening than f/8, and therefore lets more light in. We know that a shutter speed of 1/200 is faster than 1/100, and therefore lets less light in. We know that ISO 400 is faster than ISO 200, and therefore captures more light.

You’re with me, right? Good.

I’m about to throw a little bit of math at you, but I promise it’s only a tiny amount. No, really. All we really have to do is multiply by 2 or divide by 2. That’s easy, right? Take a deep breath. Ready? Let’s go.

How much more light does a shutter speed of 1/100 let in compared to a shutter speed of 1/200? The obvious guess is twice as much, and it’s precisely right. Any shutter speed that’s half as fast will let in twice as much light. Any shutter speed that’s twice as fast will let in half as much light. So 1/200 is half as much light as 1/100. 1/400 is half as much light as 1/200, and is therefore 1/4 as much light as 1/100. Double the bottom number means half as much light. Easy, huh?

Let’s look at ISO next. Do you think that ISO 200 is twice as much light as ISO 100? Bingo! Just like shutter speed, changing the setting by 2 means twice as much light, or half as much light. ISO 200 is half as fast as ISO 400. ISO 100 is half as fast as ISO 200, so it’s 1/4 as fast as ISO 400. Again, all you have to do is multiply or divide by 2, and you’re good.

Now aperture. Do you think f/4 is twice as fast as f/8?

Sorry, but this time it’s a tiny bit more complicated. When it comes to aperture, making the opening half as big means you’re only going to get 1/4 as much light. Making it twice as big means four times as much light. If you don’t want to know why, skip the rest of this paragraph. Our aperture is basically a circular opening. Remember back in high school, the area of a circle is pi-r-squared? Changing from f/4 to f/8 means that the radius of the opening is divided by 2. Because the area is changed by the square of the radius, the area of the opening is divided by 4. The amount of light coming in is directly related to the area of the opening, so the amount of light coming in is also divided by 4.

As it turns out, the way to cut the light in half is to change the aperture by a factor of about 1.4. In this case, f/5.6 is half as much light as f/4, and f/8 is half as much light as f/5.6.

Let me put it all into a nice tidy table, in case that’s easier to follow:

setting Starting setting 1/2 as much light 1/4 as much light
ISO 400 200 100
Shutter Speed 1/100 1/200 1/400
Aperture f/4 f/5.6 f/8

Now, a tiny bit of jargon. Whenever you increase or decrease the amount of light by a factor of 2, that’s called a stop. A shutter speed of 1/400 is one stop faster than 1/200. An aperture of f/4 is one stop wider than f/5.6. ISO 100 is one stop slower than ISO 200.

In summary: for shutter speed and ISO, changing the number by a factor of 2 means twice as fast or twice as slow. For aperture, changing the number by a factor of 2 means four times as fast or four times as slow. Half as much/twice as much is really all of the math you need to know in order to be a competent photographer.

A bonus note for those of you who are vaguely mathematically inclined. Remember that I said that changing the aperture setting by a factor of 1.4 meant twice as much or half as much light? The astute reader will note that 1.4 is (approximately) the square root of 2. That’s not a coincidence.

Next lesson:  How the settings play together


Filed under Aperture, ISO, Lesson, shutter speed

13 responses to “How much more light? How much less?

  1. Steve Green

    Hi Patti:

    Great job–I’m sending all my photographically-challenged friends to your page. Slight error: you said:
    “As it turns out, the way to cut the light in half is to change the shutter speed by a factor of about 1.4. In this case, f/5.6 is half as much light as f/4, and f/8 is half as much light as f/5.6.”

    I’m sure you meant “….is to change the aperture….). I knew what you meant, but you probably want to avoid confusion, right?


  2. stopshootingauto

    Thanks Steve, you’re right! I’ve corrected the entry. That’s what I get for typing quickly and not proofreading carefully enough.

  3. Jeff

    You said:
    “For shutter speed, changing the number by a factor of 2 means four times as fast or four times as slow.”

    Should that be for Aperture?

    By the way, until I read your articles I was completely confused about how exposure worked….thanks so much for the simple explanations.

    Keep it up.

    Thanks again,

  4. stopshootingauto

    Thanks Jeff! You’re right, that was a brainfart. I’ve updated the entry, and will go write “I will proofread my entries” on the blackboard 100 times.

  5. Tim

    Hi Patti,

    Does using the flash ever factor into choosing shutter or aperture? Does the exposure meter compensate for the flash?

    Thanks for this site. I have learned a pile and am starting to enjoy photography.


  6. stopshootingauto

    Tim, that’s a great question. The short answer is “it’s complicated.”

    I’m not an expert on flash photography, but it absolutely does matter. One thing to be concerned with is the fact that your shutter speed needs to be slower than the sync speed required for your flash. If you can’t sync the flash faster than 1/250 sec, then trying to take a picture at 1/1000 just isn’t going to work well.

    I believe that if you have a system that does TTL metering, then the camera will take the flash into account when it does its calculations, but I’m pretty sure that it’s way more complicated than I could describe right now. Here’s an article I found that should give you better information:

    Honestly, I’m kind of a hack when it comes to flash. Mostly I use my flashes in manual mode off-camera, and just screw with them until I get the right setting. I do this for hardcore macro work, and I mostly do OK with it. The other thing I use flash for is snapshot-level work, and as much as I hate to admit it I just stick the flash in auto mode and let it do its thing. I haven’t yet invested the time to really understand flash photography yet, mostly because I do very little of it. If I ever do, I’ll digest it and toss it on the blog.

  7. Erica


    I was out in the sunshine today, and wanted to try to take a shot of a creek (with slow shutter speed). I put the camera on shutter speed priority, and took a few shots at different speeds.

    I think because of the sun, my photos were WAY over exposed.

    this is my unedited:

    and this is my photoshop fix: (which I’m still not thrilled with)

    How should I meter and shoot this situation in the future? (too much light, slow shutter speed)

  8. stopshootingauto

    Erica, too much light is rarely a problem for photographers, but occasionally it is.

    You did everything right. You set the camera to ISO 100, which is usually (but not always) the slowest ISO available. You put the camera in shutter priority mode and picked a good shutter speed to get the water motion, and the camera responded by picking f/32, which is no doubt the smallest aperture available on that lens. It wasn’t small enough. I bet that the camera’s viewfinder was complaining to you that the shot was going to be overexposed.

    Basically, you’ve run out of room for your camera to reduce the light. You can try a bit faster shutter speed, but you probably won’t be able to capture the water’s motion.

    What now? As it turns out, there’s a product made exactly for this purpose– it’s called a neutral density filter. It’s basically a solid gray filter that goes on the front of the lens to darken an image. The most common use is for exactly what you want, but they occasionally come in handy for other things.

    I recently bought a set on Ebay from seller fotodiox_gear. Here’s an example:

    They aren’t optically the best quality filters in the world, but since I only expect to use them occasionally I think they’ll be good enough. If you get square-style filters, you’ll also need a filter holder and an adapter ring that screws into the front of your lens. Here’s an example:

    but of course you need to be sure to get the right size ring for your lens.

    If you just want one ND filter, it’s less expensive to just get a single filter that screws onto your lens. Here’s one:

    There’s also something called a graduated neutral density filter that’s similar, but it’s half gray and half clear. You might use this for a sunset or something where you only want to darken half the image.

  9. Erica

    thanks!! that makes perfect sense, I was guessing it was something like that. I will have to get some filters… lol, they are on the ever-expanding wish list :)

  10. Paula

    I LOOOVVEEE your site and super helpful information!

    In the 5th paragraph you state “ISO 100 is half as fast as ISO 100, so it’s 1/4 as fast as ISO 400.” Is this a typo? Shouldn’t one of those ISO 100’s be ISO 200? Maybe the 2nd one? Or am I getting confused?

    Thank you for your blog! I refer people to it often.

  11. Yeah yeah yeah.. its me again…(what can i do, you’re the best :) )
    Just wondering, do you have a suggestion box here or some request area? If ever, have you done flash photography?
    Low light and night photography on the run is killing me. Either the people I photography go blind with the flash, or they will be blurred due to long exposures.
    Do you know how to setup a flash that is far away from your camera so that the light does not go “In Your Face-FLAT”? ..perhaps setting it up like 9 o’clock of your subject?

    Anyway, if you don’t cover such, its ok..(just wanted to try to ask)
    By the way, I have captured the moon’s image ok, thanks to you! :)

    • stopshootingauto

      Flash photography is not my strong suit. I’ve done some, but I’m far from an expert.

      However, the place you want is a blog called Strobist– They’re all about shooting with off-camera flash and doing it well.

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