PetaPixel

6 Things You Don’t Know About Apertures, But Probably Should

aperture1

One of the first things that we learn when we start taking photographs seriously is that ‘aperture’, the size of the hole in the lens through which light passes, controls depth of field.

A large aperture creates shallow depth of field while a narrow one creates wide depth of field. But there’s a little more to aperture than that, let’s take a closer look at this most fundamental photographic control.

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1. Doubling and halving

Apertures are often referred to as stops. Opening up the aperture by one whole stop or 1EV (exposure value) doubles the amount of light passing through the diaphragm, while closing down by one stop halves it.

However, modern cameras are usually set to adjust aperture in one-third stops, something that can confuse novice photographers.

If you wish, it’s usually possible to set a camera to adjust in half or full stops via the custom menu.

The full stop aperture settings that you are most like to encounter are: f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22 and f/32.

Other settings such as f/3.5 and f/6.3 are fractions between these whole stops. F/3.5 could be thought of as f/2.8 and 2/3, for example, and f/6.3 as f/5.6 and 1/3.

Understanding the doubling and halving effect of aperture is helpful when setting exposure and deciding which shutter speed and/or sensitivity setting to use.

If shutter speed is kept the same, the difference in exposure between opening up the aperture from f/8 to f/5.6 is the same as pushing sensitivity up from ISO 100 to 200; the image will be one stop brighter in both cases.

Similarly, if sensitivity is kept the same, the difference in exposure between a shutter speed of 1/125 sec and 1/60 is the same as adjusting from f/8 to f/5.6; again it’s one stop brighter.

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2. F-number

Many novice photographers are confused by the fact that a small aperture has a large f-number or f/number, while a larger aperture has small f-number.

This is because aperture is expressed as a fraction of lens focal length with ‘f’ standing for focal length.

Hence, an aperture of f/11 on a 100mm lens denotes an aperture with a diameter of 100/11, which is 9.09mm. With a 50mm lens the same aperture (f/11) has a diameter of 4.54mm.

Clearly the same amount of light cannot pass through a 4.54mm diameter hole as passes through a 9.09mm, but the exposure at f/11 is the same with both lenses because the light loses more of its intensity as it travels along the 100mm lens.

The loss in intensity follows the inverse square law which says that intensity of the light is inversely proportional to the square of the distance it travels.

It can be shown mathematically as follows:

I=Intensity
D=Distance

I= 1/D2

50mm lens at f/8
Aperture diameter = 4.54
Aperture radius = 2.27
Area of aperture = 2.272xΠ = 16.1883
Intensity = 16.166/502 = 0.006475

100mm lens at f/8
Aperture diameter = 9.09
Aperture radius = 4.54
Area of aperture = 4.542xΠ (22/7)=64.753
Intensity = 64.753/1002 = 0.006475

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3. Diffraction

While closing down the diaphragm to create a smaller aperture increases depth of field it also increases the impact of diffraction which softens the image.

Diffraction is the bending of light rays as they pass over the edge of the aperture blades.

These rays don’t converge on the sensor surface and consequently they give a soft image.

The smaller the aperture, the fewer the number of light rays passing through and the greater the proportion of rays being bent.

As a general rule, it’s advisable to not close down a lens aperture to the smallest value available.

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4. Optimum aperture

Most lenses don’t achieve maximum sharpness when the aperture is wide open, they need to be closed down a little.

Sharpness continues to improve until the impact of diffraction becomes apparent.

Somewhere in between is the lens’s optimum aperture at which it produces the sharpest images.

You can identify the optimum aperture of a lens by shooting a subject with plenty of detail at every available aperture.

It’s essential that the camera is mounted on a solid tripod and that you focus on exactly the same spot.

Then examine the images at 100% on screen to find the image that shows the subject sharpest and check the EXIF data to find which aperture was used.

This is the lens’s optimum aperture.

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5. Blade count and bokeh

The way that out of focus areas, particularly highlights are rendered is referred to as the lens’s bokeh, a Japanese word that is pronounced in a number of ways including boke and bouquet.

Good bokeh is generally considered to have highlights that are rounded rather than having straight sides, for example, forming a hexagon.

The edges of these highlights should also be soft and not haloed or hard-edged. Photographers often describe an out of focus background with good bokeh as being ‘creamy’.

Bokeh is a property of a lens rather than a camera with the optical elements and aperture come into play.

The best results are usually seen when the iris or diaphragm that opens to create the aperture has many blades (9 being typical) that have a rounded edge to create a near spherical opening.

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6. AF and aperture

With the exception of some more advanced models, most modern cameras require an aperture of f/5.6 or larger for the AF system to operate.

However, this doesn’t mean that you can’t shoot at smaller apertures because modern cameras measure exposure and focus the lens when the aperture is wide open, only closing it down to the taking aperture shortly before the image is captured.

Thus if you mount a 70-200mm f/4 lens on a camera and set an aperture of f/11, it will focus the lens quite happily.

However, if you add a teleconverter to double the focal length of the lens, the effective aperture of this optical combination drops to f/8 and many cameras (apart from some more advanced models) will be unable to focus the lens.

Even high-end cameras may only be able to use the central AF point in these circumstances.

Fitting a lens with a wider maximum aperture, such as 70-200mm f/2.8, allows the AF system to receive more light, which can often mean it performs better.

And when combined with a 2x teleconverter the effective aperture is reduced to f/5.6 which means that the AF system is still able to function as normal.


About the author: Jeff Meyer is the editor of PhotoVenture, a photography blog for everything post-capture — improving photos, image management, sharing and more. This article originally appeared here.


Image credits: Aperture by niXerKG.


 
  • DafOwen

    Point 4 : “Most lenses don’t achieve maximum sharpness when the aperture is wide open, they need to be closed down a little.”

    You’ve got that reversed (if I’ve got your meaning correct – maybe not).
    e,g, F22 is closed down but isn’t sharpest, F16 say would be closed down, then opened up a little

  • Ryan.Love095

    Point 2 has a typo in the inverse square law bit:

    I=Intensity
    D=Distance
    I= 1/D2 –> should be I ∝ 1/D^2 or I ∝1/(D*D)

  • sdancer

    Point 5 is IMO stressing the aperture too much. While it’s true that the shape of the aperture determines the shape of the resulting “bokeh blobs”, a lot of the quality of blur is determined by the lense’s handling of spherical aberration (making the balls softer or harder, giving them bright edges, …) and some other aspects like the moulds of aspherical elements producing onion rings.

    I’d have expected to see something about “diffraction stars” in #3 too.

  • http://www.sadiqasyraf.com Sadiq Asyraf

    2. F-number : The formula I = 1/D2 , the 2 supposed to be squared (²) , so could you change it to I = 1/D² or simply put it as I = 1/(D^2) and so on in the calculations so that other people will not be confused.
    BTW thanks, great article.

  • http://www.joshrosscreative.com/ Josh R.

    No. Meaning that if it’s an f/2.8 lens you need stop down to f4 or so for maximum sharpness.

  • http://www.shinyphoto.co.uk/ Tim

    If you graph image-sharpness against aperture, you’ll find it accelerates quickly from poor wide-open through an optimum and then drops-off again as diffraction takes over.
    For an awful lot of APS-C lenses, one can expect peak sharpness to be located a stop either side of f/8.

  • theart

    “the exposure at f/11 is the same with both lenses because the light
    loses more of its intensity as it travels along the 100mm lens.”

    No. The longer lens needs a larger opening to get the same exposure because it has a narrower angle of view. Exposure is dictated by the summation of photons reflecting off of a scene. Less of the scene (smaller angle of view) equals fewer photons.

  • tomdavidsonjr

    Cue the arm-chair optics experts in 3…2…1…

  • http://jtruephotography.com/ Jeremiah True

    I’ve found f8 to be optimal on my 7D.

  • rugfoot

    Is point 6 possibly why it’s so difficult to get good focus with a 50mm at f1.2?

  • josh80

    Not really.. it’s mostly just because the depth of field is so shallow at f1.2 that it’s almost impossible to get what you want in focus, in focus.

  • Zos Xavius

    Because most focusing systems can only see f2.8 at best. Meaning that the image may look in focus to the AF, when in fact it is not. Better light will give the AF more information to work with, but most people use fast lenses for low light, so best of luck. I find it best to MF and chimp with lenses faster than f2.

  • http://www.shinyphoto.co.uk/ Tim

    It’s not so much a function of the camera (except in rare edge-cases) as the lens.

    Just for amusement, one of my favourite lenses peaks at f/5-f/5.6; I used to have one that was sharpest at f/4.

  • http://transienteye.com Mark

    Yes, exactly. If your 100m lens is loosing light because of its length there is something seriously wrong with it.

    The problem with the internet is that anyone can appear authoritative, no matter how wrong they are. It would be helpful if someone would correct the article.

  • http://www.observingtime.com/ agour

    7. Not all apertures are equal. Lenses never move back to the exact same spot, so you can get a tiny amount of exposure variation between pictures. Doesn’t matter for most people, but can ruin timelapse sequences

  • Jason Yuen

    I’d like to think I know a thing or two since I’ve done extensive testing for Aperture Science, Inc. at their Aperture Science Enrichment Center lab in Upper Michigan.

  • http://admiringlight.com/ JordanCS13

    Came in here to say this. Spot on.

  • Mojo

    I didn’t know that. Fascinating.

  • Matt E-D

    One of the benefits of using Nikon AI or AF-D lenses is that you can take advantage of the mechanical aperture ring and it’s “hard stops” to get 100% consistent aperture values from shot to shot.

  • Johan Robertsson

    Also came here to mention this!

  • Johan Robertsson

    that’s why on canon cameras, serious timelapse enthusiasts will press the DoF preview button hold it down and slightly untwist the lens so the electrical contacts are disconnected, this will cause the aperture to stay stuck until you connect the lens properly again.

  • markz

    OK probably pedantically baiting here but that was 6 things, roughly described, I’ve known about aperture since the late 70′s

    as a couple of the more technical points have already been addressed I would add my say that one of the sentences in point 5 should have read:

    “a Japanese word that is mispronounced in a number of ways including boke and bouquet.”

  • markz

    Aperture Science
    We do what we must
    because we can.
    For the good of all of us.
    Except the ones who are dead.
    But there’s no sense crying over every mistake.
    You just keep on trying till you run out of cake.

    ;)

  • Blakael

    I was going to ask about that!! They have 502 and 1002 and was thinking, are they just whacking a 2 at the end…?

    Squared, is what they mean! :)

  • Rick Scheibner

    #6: Huh? Before I purchased my 5DmkII, my Rebel XSi had no problems autofocusing at any aperture setting. I’m not saying it never happens…this is just the first I’d ever heard of it.

  • Jost1

    What to say then? Shrinking down from f2.8?

  • Pete

    I would assume that your camera was auto focusing wide open and then stopping down once you pressed the shutter button… much like the article says.

    Were you using a teleconverter or a lens with a manual aperture?

  • Pete

    I think the article was meant for beginners, so nothing wrong with that. When I was starting out I would’ve loved to hear this info. Also, I don’t think blade apertures have changed that much since the 70s :P

  • Pete

    … but did I tell you about that time I had a run in with “equivalent apertures”! ;)

  • John Tharp

    Understanding equivalency between systems can be important, though! Many people are shooting FF and then picking up a mirrorless APS-C or MFT setup as a light sidekick, and it helps to know just what you should expect so you can make the best of your photographic opportunities :).

  • John Tharp

    There aren’t any AF lenses in the Canon system that have maximum apertures narrower than f/5.6- so this really only matters for users of extenders with already slow lenses, and to them, it matters a whole lot!

  • John Tharp

    Generally speaking, and this is with above average (but not necessarily outstanding lenses) you want to close down at most to f/5.6 on MFT, f/8 on APS-C, and f/11 on 35mm full-frame, in order to avoid the effects of diffraction. You should really only close down further if the added clarity from the additional depth of field is greater than the hit that the whole image takes from diffraction softening, and that’s a case by case basis thing.

  • DafOwen

    My understanding was that they were discussing going from f22 down to f16 – which would be “opening up”.
    But if they are talking about going from f2.8 up to e.g. f4 – then “closing down” was the correct term.

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  • theart

    And everything else in point 2 goes out the window for retrofocus lenses, which is why it fails to explain how a zoom lens can be made with a constant maximum aperture.

  • Bimo Pradana

    Damn, i hate mathemathics..

  • Anonymoused

    Or the alternative title, “6 Things Anybody Who Has Researched Anything About Photography Already Knows.”

  • Arthur Nazarian

    Area of a circle/aperture is pi*r^2, not pi*r, so the given intensities are wrong. The point about the different focal lengths however, still stands.

  • http://www.saleemshaikh.com Saleem Sadique

    I 101% Agree with u @ Matt. i love mechanical aperture Nikon lenses

  • http://www.saleemshaikh.com Saleem Sadique

    Thanks @ Johan, this is wonderful trick Tips.. surely I will try this….

  • christophe

    still not corrected …
    well, writing something unclear, not to say wrong, ok, but one needs to clarify or amends, or else the whole writing quality is subject to doubts …