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Controlling the faucet – Learning ISO | Day 4

water flows from a faucetYesterday, we imagined a faucet when we learned about shutter speed. We learned to fill our cup by controlling our shutter speed. Leave the faucet running too long, the cup overflows – your image is overexposed. Not long enough, the cup doesn’t fill and your image is underexposed.

If shutter speed refers to how long you leave the water on to fill the cup, ISO refers to the flow of water.

ISO is a measurement of how sensitive your camera is to light.

The lower the number, the lower your camera’s response to light. The higher number, the more sensitive it is to light. And the pros say “I. S. O.” not /eye-so/. ISO stands for International Organization for Standardization – and for the purposes of languages around the world has adopted the acronym ISO. Think of ISO like receptors of light – you can have 100 receptors on your sensor or 400 or more…

If ISO 100 is a slow flow of water, each increment doubly increases the flow: 200, 400, 800, 1600, 3200, 6400 +

The higher the ISO, the faster the water pours out of your faucet. And in direct relation – the higher the ISO, the faster you can shoot.

If the water comes out very fast, it isn’t necessary to leave the faucet on very long to fill your cup.
If the water flows more slowly, you must leave the faucet running for a longer period to fill the same cup.

Together, ISO and shutter speed control how much light gets to your sensor.

But there is a trade off. With increased ISO comes quality deterioration of what you capture. The higher the ISO, the more noise (sometimes still called grain from the good ol’ film days) you will see in your photos. So while ISO of 100 or 200 may look like glass, an ISO of 1600 may look like sand.

Most photographers try to shoot with as low ISO as possible, but seek machines that will give them flexibility to use higher ISOs without visible noisy, grainy photos. This sensor quality makes big price point difference when shopping for a camera! It is a big reason why a camera costs $1000 or $5000. It’s also why wedding photographers can get beautiful clear photos in a darker church while most consumer cameras get noisy images – pro photographers invest in machines with sensors that handle ISO better. The lower the quality of a camera’s sensor, the sooner destructive grain becomes an issue at lower ISOs.

“So, why are my photos better outdoors than indoors?”

Because generally, outdoor means better light. Even inexpensive cameras can handle well-lit situations easily. As the light decreases, so does the performance of most cameras. Outside, you can shoot fast at a low ISO. Indoors, you (or your camera) often must compensate by increasing ISO and slowing shutter speed to allow in more light.

camera dialHigh ISO = noise. Slow shutter = blur. It’s the perfect recipe for disaster. If you have an inexpensive camera that doesn’t handle noise well, try to keep your photo opps outdoors or during well-lit daylight hours. You’ll be much happier with your results. Don’t be afraid to direct your subjects to a place you know your camera can perform better.

Time to play!

Grab that trusty camera, and put it on program mode (not Auto, which will choose the ISO for you).
Set your ISO to the lowest number you can see. On Nikon that will be 200, on Canon it will be 100. Your camera may vary.
Take a picture, and note the shutter speed your camera chooses.
Stay in the same place, shoot the same subject again.
Move the dial one notch – doubling your ISO to 400 or 200. (Some cameras may have middle ISO numbers, but for this exercise, double the ISO.)
Take a photo, and note the new shutter speed.

You will notice – each increase is twice as sensitive and each decrease is half as sensitive. Without getting too technical at this point, it means you can get the same mathematical exposure by adjusting the ISO and shutter speed in doubles and halves.

Imagine the correct exposure of an image is ISO 400 at 1/100 sec.
You can halve the ISO and double the shutter and get the same exposure – ISO 200 at 1/50 sec. Using our analogy, you reduce the flow of water in half, you have to leave the faucet on twice as long to get the same results.
Likewise you could double the ISO and halve the shutter speed – ISO 800 at 1/200 sec. Meaning, you double the flow of water out of the faucet, and you only have to leave the faucet on half the amount of time.

In the above example, what’s the correct shutter speed if we move the ISO to 1600?
Which ISO would we choose if our shutter speed is 1/800 second?

So far, so good? Tomorrow begins the last piece of the technical puzzle!

Please visit my girls who are sharing this 31 Days journey with us:

31 Days of Grace :: Chatting at the Sky
31 Days to an Inspired Table :: My First Kitchen
31 Days to a Less Messy Nest :: Nesting Place
31 Days of Living Simply :: Remodeling This Life
31 Days of Autumn Bliss :: The Inspired Room
31 Days to More. . .With Less :: Beauty and Bedlam
31 Days to a Better Photo :: My 3 Boybarians
31 Days to Stress Free Entertaining :: Reluctant Entertainer


  1. My husband’s always trying to explain ISO to me, but this is brilliant. Thank you, Darcy!

  2. This is brilliant Darcy! I can’t wait to go play with my camera! 🙂

  3. Once again, perfect explanantion!

  4. I’ve done a lot of reading about the technical trio for photographs. This is, by far, the best explanation of ISO I’ve ever read. Thanks.

  5. Yeah ! it’s perfect ! Thank you Darcy ! ♥

  6. This series is gold. Gold I tell you.

  7. I don’t get ISO at all. As soon as I start messing with it, all my pictures turn up all black, no matter what shutter speed I use. I’m going to play around today and hopefully I’ll figure out what I’m doing wrong.

    • Black photos could be a result of shutter speed being too fast for the ISO. The solution would be to slow down the shutter, or if necessary, increase the ISO.

      Darkness tells me light isn’t reaching your sensor. So we need to use one of our tricks to allow more light in.

      What are you shooting with?

  8. I just discovered your blog, and BOY am I glad I did 🙂 I foresee you being a daily stop for me now! Outstanding blog!

  9. Darcy – this is SO helpful. I can envision that faucet and actually remember what is what. 🙂

  10. same here, i’ve tried to read up on this to make since but the water flow is really a great analogy. It just makes since. I will have to make up a song about halving the ISO (eye so) and doubling the speed. I’ll have to change my terminology if I want to sound professional 😉

    Thanks loving this challenge!

    • Love that Christy – I make up songs for stuff I have to remember, too.

      And you know the annoying thing Dora and the map do to remember steps? I do that too.

      Whatever helps, right? 😉

  11. i agree, love the way you explained this, and it makes it so much easier to understand.
    and math? seriously? i need math to take pictures? i hate math. ;D

  12. Darcy, I cant thank you enough for this explanation. I dont have it down yet but I will as I go over and over it. I do have a question-probably past my understanding- would there be any need to change settings if shooting the same photo more than once if light source remained the same? I have seen photographers standing in place then snap, adjust, snap, adust, snap etc. Again thank you so much for being willing to share your knowledge in an understandable way.

    • They may be adjusting ocal points not necessarily exposure – and that means their hands would constantly be in action.

  13. Thank you for your tutorial… I’m really enjoying it. I’ve displayed more exciting pictures of my stapler on my blog. I must say, it’s the most pictures of desk accessories I’ve ever taken in my life, LOL! http://playdoughinmycarpet.wordpress.com/31-days-photo/

  14. I took a photo class a few years ago and the teacher said that there isn’t a big difference between 200 and 400 and that it is better to use the lowest possible. I typically always try to shoot at ISO 200 (I have a Nikon) unless the lighting is really poor. If shooting in decent light, is typically a rule of thumb to try using the lowest possible ISO like that teacher said, or not necessarily? I guess what I am saying is that I understand the concept of ISO…and that a higher ISO is necessary in lower light, but they are just numbers to me – 200, 400, 800, etc. So I have trouble knowing which to use in which amount of light…how do you know what the correct exposure is? Your example is ISO 400 at 1/100 sec. or ISO 200 at 1/50 sec. But if they produce the same “correct” exposure…why is one better than the other? Sorry, not trying to be difficult!!! Just want to FULLY understand 🙂

    • ISO 400 is double ISO 200, so there’s a noticeable difference. There is no such thing as the correct exposure – although admittedly, some are more right than others.

      I explain more later but reasons why you would choose one of the other might include:
      children or pets in the photo – you cannot shoot kids at 1/50s unless you are trying to show motion blur.
      if your camera is noisy at ISO at 400, then shoot ISO 200 for the same exposure.

      Generally the conditions of the subject will tell you which formula is better.

      I hope over the next 6 days especially, it will help fill in the gaps for you.

  15. I shot photos this afternoon in manual for the first time ever. THANK YOU for this series. What a gift you are sharing. Can’t wait to get them on the computer and keep learning more and more all month long!

  16. Love this! Thank you so much- it’s just what I need!

  17. Darcy, I love you. That is all. You are amazing and I can’t wait to give every single person who ever asks me how to use their camera your blog address with a link to this series. You rock.


    • Love you too, Em. You put a huge smile on my face, because you know – you pretty much rock my world.

  18. stepping n late, but this is a great series. thanks!! i have a much better understanding of ISO now!

  19. and finally the light has switched on… thank you Darcy, this is an invaluable series!

  20. Great take away from this one – use the PDF version on the camera manual. When I used the search function I found what I thought was some lost ISO ease-of-adjustment from the D50 to the D5000 that I didn’t pick up on when reading the hardcopy of the manual at first.

    Glad to have my convenient ISO adjustment back thanks for the exercise.

  21. i’m *thisclose* to crying. lol i used MANUAL on my camera and i THINK i know what i was doing! :o) thank you, thank you, a thousand thank you’s!

  22. hm! i always shoot in aperature mode (love short DoF), and always w an ISO around 650 or 800 (nikon d200)(my subjects are usually moving, and i’m usually outdoors), never controlling shutter speed… i think this just might help me up the game! :))
    especially w stills. ok.. i’m getting excited. been reading this stuff for years and just getting a grasp of it through your blog now! thank-you! 🙂

  23. Jane Taylor says:

    Greetings from South Africa!!
    THANK YOU so much for & sharing this absolutely invaluable information. I am a novice (just upgraded from a point & shoot to a Nikon D5000) and just love that you’re writing in “plain English!!!” Thank you, thank you, thank you!!!

  24. Thank you Darcy for this website. Its so nice of you to post these things. I have a Nikon D80 and have been trying to learn it from this 50 year old brain for about 3 years. I’ve worked on Manual mode but so often my pictures are less than desired. They just don’t seem to “pop”. Someone sent me this link and so yesterday I started studying.

    Thank you

  25. I saw your series back this fall during the 31 Days of _______, but I just had a point and shoot camera then. I recently got my first dSLR, so I am now going through your lessons, one each day and learning so much! Thank you!

  26. *DING* There’s my lightbulb moment. The faucet analogy makes so much sense to me! I have read so much stuff on the Internet about taking better photos, and this is the clearest explanation I’ve read yet. Thanks so much!

  27. Rachael says:

    This is a really great analogy! Thank you so much!!

    So are the answers ISO 1600 = shutter speed of”1/400 sec” and shutter speed of 1/800 sec = “3200 ISO”? Wanting to make sure I really get it. heh!

  28. Thank you thank you for this wonderful guide! I’ve read explanations of ISO in multiple books, and never grasped it until I read this. Your best advice so far has been to get out there and try it! Thanks!

  29. I love how you’ve set up the 31 days of learning. I’m just getting to know my camera and this makes it easy, understandable and not as overwhelming when you can learn a little bit every day. Only on day 4, but loving it so far. Thanks!!

  30. This was the best explanation I have ever read about ISO!! I have read a lot of them and understood enough to know how to set my ISO with noise and light in consideration but this was by far the best explanation. I “get” it now! =)

  31. The analogy is great – It makes sense now.

  32. I found your site about a week ago. I think I asked the googleverse ‘how to take better pictures indoors’ and your blog was one of the first sites that popped. You speak of this magic light bulb. Well, when I got to the ISO day, my wittle bwain turned on and my light bulb nearly exploded! I’m like the rest of the masses, been taking pictures for years, they look good, some really good and I have no idea how I made it happen. But, like most everyone else, I wanted those real neato ones, with the fuzzy background and crystal clear subject up front. So, a couple years ago I got a little kid dslr, one of the Canon T series. I was taking better pictures, but the indoor scenes were just killing me! I bought this slr camera- it was supposed to automatically make all my shots great! What was the friggin’ deal??!??!! I’m a good girl, I’ve always been a manual reader. I’d read that thing making dinner, getting ready to nod off, watching tv. Sure, I learn a ton by punching buttons, but I always head back to the manual. So, I knew my buttons and what did what, for the most part. I still carry around my cheat sheet for aperture and shutter speed. I’d tweak and move and turn lights off and turn lights on and use the flash and start over from scratch and I’d still get too dark or yellow or just crap. They’d be good enough for the memories, like everyone else’s indoor shots. But, I wanted something better than that. Well, your ISO day helped me out 180 degrees. I knew about it, what is was, just never messed with it. Right after I read your post, I whooped out my camera and started mashing buttons. Whadaya know!! The test shots looked pretty much like real life colors! No blue and yellow hues overtaking the picture. I just sat there thinking ‘You doofus. This was here, it’s own little button, even has a label that says ISO, and you never touched it. Doofus.’
    Well, needless to say, I’m SOOOOOO very thankful you did this blog. Even if it took me a few years to find it. Any other time I asked a question about photography, the answers were ALWAYS so very technical, I just quit reading and went back to punching buttons and taking more pictures outside than in. You talk like a normal person and put in terms that non-photags can understand. Thank you again!!!


    • Hi Erin!

      So glad it’s helpful! Love to hear stories like this.

      Congrats on the lightbulb moment!

  33. So, in one sentence you say that the pros say I.S.O. and not EYE-SO. And then in the next sentence you call it an ACRONYM. Don’t mean to talk semantics here, but an acronym is a word that is formed from the first letters of a groups of words. The word is MEANT to be pronounced.

    Other acronyms are SONAR, LASER, and NATO. None of which are referred to as S.O.N.A.R or L.A.S.E.R.

    Also, many pros I know pronounce it ISO, and a few others I know say I.S.O.

    To each his own.

    • Please examine the following list of acronyms.

      ETA. No one says etta. Estimated Time of Arrival.
      FDA. No one says the fe-dah. Food and Drug Administration.

      There may be geographical predominance in other areas for languages nuances, like ISO. So be it. In the US, at the time this was written (Sept 2010), the predominate use of the term was/is still ISO. Regardless, it is irrelevant to the content of the article.

      Lastly, I offer this series freely, without compensation as a resource for new learners. It took an enormous amount of time to create. I hoped it would be source of education for those just taking their first steps into better understanding their cameras. I, quite frankly, don’t care how you choose to say it. It simply doesn’t matter.

      FWP. Clearly. 😉