Simple guide to aperture, f stop numbers and depth of field

An easy guide to understanding depth of field and f stops

I was asked by a friend this morning about aperture/f stop and depth of field, as he was confused by some of the more generic articles on the Internet.. I thought it would be a great idea to post it here, as it may help someone else!

If you are struggling with shutter speed click here for more info..

Ok F stop, such as f1.8 of f22 is the measurement of the size of the little ‘eye’ of your camera, this is the aperture, the hole in which light is let through the lens and into the camera. The numbers, such as f1.8 or f22 etc corresponds to the size of the hole, the SMALLER the f stop number (IE f1.8) the BIGGER the ‘eye’/aperture…and vice versa a LARGER f stop value such as f22 is a physically SMALLER size..

So when you have your aperture set at it’s ‘widest’ setting (for instance f1.8) you are trying to let in as much light as possible.

So how does f stop relate to depth of field (DOF)?
Ok so when you are reading small print, or trying to focus on something small you find yourself squinting, yeah? The reason you are squinting is because you are making your ‘eye’ / aperture as SMALL as possible..because when the aperture is small you can focus much further.
Shallow depth of field
On the contrary, if you have a LARGE aperture such as f1.8 your depth of field becomes very limited, you’ve seen photo’s where the item is in focus and the background is blurred? This is exactly what is happening, because the aperture is LARGE, the depth of field is SMALL, or limited.

In a model photoshoot example, if you had your aperture set to f1.8, you may find that if you focus on the end of her nose, that even just a small way back such as in line with her ears, will be slightly out of focus.. this is becuase the aperture is too BIG (the f stop number is SMALL remember!).. so to get more of her in focus you would increase the aperture…. the smaller you make the aperture (the bigger the fstop number) the MORE will be in focus.

If you had 7 people standing one behind the other and you wanted to get all the faces in focus, you would have to have a SMALL aperture (High f stop number such as f22) to get it all in focus… whereas if you wanted and arty picture you might choose a big aperture such as f1.8 to focus on one face at the front and the the others would blur.

If you set a lens to an f stop of f1.4 this will allow twice as much light through (to hit the sensor) than f/2. Each stop from that is a square root of 2 (Well, actually 1.4).. so the next number in the list allows twice as much light through:

(Wide ‘eye’ – ‘fast’ – shallow DOF) f/1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, etc. (Small ‘eye’ – ‘slow’ – large DOP)

So I hope that has helped someone who didn’t quite understand before!

Just remember the SMALLER the f stop number IE f1.8, the BIGGER the size of the ‘eye’…which results in a SMALLER depth of field…
and vice versa.
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25 thoughts on “Simple guide to aperture, f stop numbers and depth of field

  1. Thanks for this explanation – I’ve recently been bitten by the photography bug, and was struggling with this concept…the squinting analogy nailed it for me.

    Some lovely shots on here by the way…!

    Dan

  2. This is the 10th time I’ve came only to try to understand this concept and the first time I’ve found an article that’s made sense. Thank you!

  3. If it is not too much to ask… with your articles could you give side by side comparisons of different aperture’s or Shutter Speeds or ISO… for a better understanding.

    GREAT ARTICLE THOUGH… I THINK I GOT SOME IDEA OF WHATS GOING ON!

  4. The explanation is FAB. You are truly knowledgeable otherwise you couldn’t have given such a classy explanation.

  5. I have been taking a photography class and your post summed up everything I’ve been learning. You made it very easy to understand.
    Thank you.

  6. Chris

    What a brilliant website, I have just started my first city and guilds course and i love all photography and eventually would love to work as a wedding photographer. I can see your website is going to help me very much.

    Gaynor

  7. hey thanks so much…

    i’ve been confused on the camera iso, shutter, f value, etc for quite sometime..

    today bought a super budget camera and decided to find some info on the specs of compact camera that i bought…. have been looking around just couldn’t spot the difference on all compact camera that i’ve viewed…

    i believe my next aim will be slr camera…

  8. wow, thanks for this information…now, I know how to captured flower…and those small things…

  9. Great tutorial as usual. What if you have 7 people standing in a line one behind the other and you just want to focus on the face of the fifth person and render the 4 people in front and the 2 people behind out of focus/blurred. How is this achieved? Thanks.

  10. Thank you! In answer to your question, you want to adjust your aperture for the DOF to be shallow enough, (So that’s closer to the f/2.8 rather than f/22 end). Depending on lens length and distance, I would suggest starting your aperture at around f/2.8 and seeing what the results are. Not enough blur each side then make it 1.8, if there is too much blur then perhaps try f/4 of f/5.6… hope that helps

  11. So glad I happened upon your website. Your explanations and analogies have clarified and solidified concepts I have been struggling with. Thank you!
    Gianna

  12. This is a great and simple explanation spelled out in simple terms, but I have a question… I have found that when I go down to a F1.8 ( the more light comes into the sensor) see I have learnt something!! to get the acquired DOF, I notice that it often just whites out…. how do I get the colour balance back? Is it normal to take the shutter speed right up to 1/2500 more or less? I have changed the Fstop, but then I loose the true effect that I set out to achieve?

    I look forward to your response

    Cheers Maree

  13. I just thought of a rhyme to help me remember f stops, “Small to focus all” meaning, small f-stop to have more in focus in picture. Would that make sense? I was reading the example of 7 people in a group you would want to use a small fstop to get them all in focus.

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