Keys to Properly Exposing a Photograph 2: f/stop

20 02 2012

Welcome to Part 2 of “Keys to Properly Exposing a Photograph.” To start at the beginning, CLICK HERE

Understanding exactly how f/stop work behind the scenes is VERY complicated. I’ve spent countless hours throughout my career studying it. If you’re looking for the most comprehensive, in-depth, intense explanation of f/stop. It lives here. Really, follow that link. It will tell you everything you could possibly want to know.

Freddy Knew You'd be Back

Now, if you’re reading this, you either have returned with an amazing understanding of the complex inner-workings of your SLR camera, or your back here waiting for me to give you the simplified version. Well, here it is.

The f/stop, as you will use it, is a range of seemingly random numbers from 1 to 22. (1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22.) (Some lenses will go higher than f/22. Thanks for the correction Rob) They represent how open your shutter (eyelid) will be when your sensor (eye) captures the picture. So why the weird increments? The answer is complicated. The simplest way I can say it is the f/stop is a ratio which describes the relationship between the diameter of the aperture (how wide your eyelid is open) and the focal length of the lens (how long your lens is.) The longer your focal length, the bigger your aperture needs to be to have a low f/stop. This is why a 70-200mm f/2.8 is so much bigger around than a 50mm f/1.4.

So again, why the weird increments. Now we’re getting to the bottom of this. We know an f/stop is the relationship between the diameter of the aperture and the focal length. Let’s say a 200mm lens, is set at f/4. The diameter of the aperture is going to be 200 divided by 4 which equals 50. (200/4=50) So, the diameter (distance across) of our aperture will be 50mm. Remember A = Pi times r squared? Long story short, the area of the hole letting the light in for our 200mm lens at f/4 is about 2000mm. Solve that for one stop down, f/2.8, you get about 4000mm, f/2 and you get about 8000mm. Summary, every stop down doubles the area of the hole letting the light in. This effectively doubles the light on your sensor, and this is what makes your picture brighter!

  • Remember when I told you your shutter goes in increments of doubles and halves too? Theoretically, if your image is properly exposed at (1/125 and f/2.8)… It will also be properly exposed if you double your f/stop (open the aperture twice as wide) and double shutter speed (leave the shutter open half as long) i.e. (1/250 and f/2.0) or if you halve your f/stop (open the aperture half as wide) and halve your shutter speed (leave the shutter open twice as long) i.e.  (1/60 and f/4)
So what’s the catch you say? The price of this extra amount of coveted sunlight? This one is a bit different. The way it effects your image is it creates a shallower depth of field. This means when you may have been able to focus your camera on something 10 feet away before and still see something 40 feet away clearly, now you may blur out nearly everything that isn’t exactly 10 feet away. The explanation for this is more complicated so we’ll put it off for another day. Keep your eyes open for more on that in a “Bokeh” post sometime in the near future.

A Short Depth of Field Makes The Background Unfocused

Bokeh is The Quality of the out of focus part of the image

The other catch is cost and weight. To get a low f/1.4 on a 50mm you need an aperture diameter of about 35mm. If you wanted a 200mm f/1.4 your diameter jumps up to 142mm. That aperture hole would need to be nearly 6in across. Such a lens does not exist that I’m aware of but it would cost a pretty penny. Just for a 70-200mm f/2.8 you’re getting into a $1000-2000 lens that weighs in excess of 3lbs. If you’ve got the money, (and the upper body strength) go for it. Here my equipment review post where I talk about mine along with the rest of my gear.

I know what you’re thinking. “Arley, what if I don’t have a lens with a low enough f/stop to put my shutter speed fast enough to freeze action in moderate light?” That’s where ISO comes into play. In the next installment, we’ll dig into the catch all that can add the most light, though at a steep price.

I would love to hear your feedback in the comments or on Twitter! If you enjoyed this post, consider clicking a share button below to let your friends know. I’m a big fan of being “Pressed” so if you think you’re readers would enjoy this and you could use some fresh content from a friend, you’re more than welcome to it. I don’t mind you using excerpts or images from anywhere on the blog either as long as you link back to me in some way.

I’m always working toward new features and better tools on the blog but these things cost money so if you would like to help, consider a donation via the PayPal button in the right sidebar. (Really want to launch a video series, got my fingers crossed 🙂 )


The next part in the series is ISO




13 responses

20 02 2012

Here might be a challenge for you. I have dyslexia. Which is made worse with numbers, fractions, decimals, etc. I have troubles trying to grasp the concept of the f.stop and trying to achieve what I want to do/envision because my brain just fumbles everything up. So I’m basically reading “hmmm…yeah…ok…wait….what…..hmm…..ok… get a shallow depth of field or to get a large depth of field you need to change your f.stop to number….what….huh….hmm….”
Can you explain this to me basically how you might explain it to a nine year old? So that I might be able to finally wrap my mind around it? Nobody has really been able to do it… which is understandable seeing as how it’s all about numbers. And trying to explain it without numbers is almost oxymoron.

20 02 2012

Hi Rebekah, your f/stop is a circle that let’s light in. When your circle get’s bigger, you get more light and a shallower depth of field. When your circle gets smaller you get a deeper depth of field but less light.The bigger the number after the slash gets f/2, f/2.8, f/4 the smaller your circle gets.

To get a really big circle, f/1.4, you need a really wide lens that costs a lot of money 🙂

Depth of field is how far in front and behind what ever your focusing on is still in focus.

If you’re using a longer lens, it takes a bigger circle to get the same amount of light as a shorter lens. It still gets the same f/# though, like f/2 or f/2.8

I hope that helped to clear things up, let me know if you have any other questions and I’d be happy to help you 🙂

20 02 2012

I’m enjoying these articles. Maybe I should point my wife at them when her eyes gloss over whenever I talk photography (although I do just think she’s being stubborn, it’s not that difficult to understand some of this stuff).

Just a slight correction on your post above, however – aperture isn’t limited to f/22, one of my lenses stops all the way down to f/32 when it’s bright enough!

20 02 2012

Thanks for the catch Rob, you’re absolutely right. I’m gonna tweak that to correct the mistake.

20 02 2012
Rebecca Booth

Very insightful and informational. Thanks for the great explanation, i’ve always had an extremely basic knowledge of the f/stop even though i use it regularly 🙂

21 02 2012

I was the same way for a long time. It’s one of the photography tools that you can use without understanding but you can use it so much more effectively and better informed when you really know what it does 🙂

21 02 2012

Thank you very much for this tutorial. It helps me to read things several times…it’s all making very good sense now! Thanks!

21 02 2012

Hi Judy, I’m glad it was a help for you, thanks for stopping in

27 02 2012
Keys to Properly Exposing a Photograph Intro: The 3 Kings « ShootAbout

[…] F/stop is a source of confusion and distress for many learning photographers. There are complicated formulas, charts, and diagrams. But for now, here’s the simplest explanation I can offer: A lower f/stop means your aperture (the hole your light comes through) stays larger, and a higher f/stop means it closes more. So, the advantage of a low f/stop is a bigger hole which, you guessed it, lets more light in. The “price” here is a narrow depth of field, I’ll explain more about this and a few of the limitations of f/stop in part 2. […]

27 02 2012
Keys to Properly Exposing a Photograph 1: Shutter Speed « ShootAbout

[…] Keys to Properly Exposing a Photograph Intro: The 3 Kings Keys to Properly Exposing a Photograph 2: f/stop […]

4 03 2012
Romancing Venice « Art Gluttony

[…] (f stops) which determines the depth of field of a photo i.e. how much is in focus.  (Visit Shootabout if you’re interested in learning more about this, he explains it in easy to understand […]

31 05 2012

Mastery of the “three kings” means mastery of the art of photography. Your explanations are basic and right on point. Keep up the good work.

1 06 2012

Thanks, I agree with you technically speaking. When you can control the “Three Kings” you open the door to creating something closer to your vision, you still need your vision though 🙂

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