Choose a Lens Aperture (F Stop)
The aperture is a hole which controls the amount of light that passes through to the camera sensor (or film pane for film cameras). It's one of the three key settings of exposure (ISO, shutter speed, aperture).
By adjusting the aperture or f/stop to which it is most often referred, you not only control the amount of light you 'gather' but you also introduce effects on your final image which you will need to understand. Depth of field (DOF, the area of sharpness through the image) is the most important, but there are also optical imperfections or enhancements. Knowing how your camera's lens aperture works will help you make informed choices about what other exposure settings to use and what creative effects or even errors may occur and how these will affect the image.
Steps
- Familiarize yourself with some of the basic concepts and terminology. You'll need to know these in order to make sense of the rest of the article.
- Aperture or stop. This is the adjustable hole through which light passes on its way from the subject, through the lens, to the film (or digital sensor). Like the pinhole in a pinhole camera, it blocks rays of light except those that would, even without a lens, tend to form an inverted image by passing through that central point to a corresponding point in the opposite direction on the film. With a lens, it also blocks rays of light that would pass through far from the center, where the lens glass may less closely approximate (usually with various easy-to-make spherical surfaces) the shapes that would focus it perfectly (usually much more complex aspherical surfaces), causing aberrations.
- Because every camera has an aperture, usually adjustable, and if not, at least has the edges of the lens as an aperture, the aperture size setting is what is normally called the "aperture".
- F-stop or simply aperture. This is the ratio of the focal length of the lens to the size of the aperture. This kind of measurement is used because a given focal ratio produces the same image brightness, requiring the same shutter speed for a given ISO setting (film speed or equivalent sensor light amplification) without regard to focal length.
- Iris diaphragm or simply iris. This is the device most cameras use to form and adjust the aperture. It consists of a series of overlapping thin metal blades that can swing toward the center of a hole in a flat metal ring. It forms a central hole that is perfectly round wide open, when the blades are out of the way, and constricts by pushing the blades toward the center of that hole to form a smaller polygonal hole (which may have curved edges).
- If your camera uses interchangeable lenses, or it is a "bridge" type digital camera, the lens will have an adjustable diaphragm iris. If your camera is a shirt-pocket sized "point-and-shoot" compact model, especially a lower priced model, it may have a "neutral density filter" instead of a diaphragm iris. Also, if the camera's mode dial includes "M", "Tv", and "Av", it almost certainly has an actual diaphragm iris; this applies even on small compact models. If the mode dial doesn't include these three settings, the camera might have a diaphragm, or it might only have an ND filter; the only way to know for sure is to read the specifications in the owner's manual, or read a detailed professional review (Google your camera's model name with the word "reviews", and you will probably find at least two or three reviews on the Internet). If your camera uses an ND filter, your ability to "fine tune" your settings and control depth of field and bokeh effects will be limited to whatever the fixed aperture of the lens provides. NOTE on Mode Dial settings: "M" stands for "Manual" - in this mode you have to set both the shutter speed and aperture. "Tv" is shutter speed priority: you manually set the shutter speed, and the camera's exposure computer sets an appropriate aperture. "Av" is "Aperture Priority" - you manually set the f-stop (aperture) that you want, typically to achieve a specific depth of field, and the camera's exposure computer decides what shutter speed to use.
- Most SLR cameras only close down the iris diaphragm, making it visible from the front of the lens, during an exposure or when the depth-of-field-preview function is activated.
- Stopping down means to use a smaller, or (depending on context) a relatively small aperture (large f/ number).
- Opening up means to use a larger, or (depending on context), a relatively large aperture (small f/ number).
- Wide open means to use the largest aperture (smallest f/number).
- Depth of field is the specific front-to-back area, or (depending on context) the scope of the front-to-back area that appears fairly sharp. A smaller aperture increases depth of field and decreases the extent to which objects outside the depth of field are blurred. The precise extent of depth of field is somewhat subjective because focus drops off gradually from the precise distance of focus, and the noticeability of defocus depends on factors such as subject type, other sources of lack of sharpness, and viewing conditions.
- A relatively large depth-of-field is called deep; a relatively small depth-of-field is called shallow.
- Aberrations are imperfections in a lens's ability to focus light sharply. Generally speaking, less-expensive and more-exotic types of lenses (such as superwides) have more severe aberrations.
- Aperture has no effect on linear distortion (straight lines appearing curved), but it often goes away toward the middle of a zoom lens's focal-length range, and pictures can be composed to avoid drawing attention to it such as by not putting prominent obviously straight lines such as on buildings or horizons close to the frame edges, and it can be corrected in software or by some digital cameras automatically.
- Diffraction is a basic aspect of the behavior of waves passing through small openings which limits the maximum sharpness of all lenses at smaller apertures. It becomes increasingly apparent past f/11 or so, making a great camera and lens no better than a so-so one (albeit sometimes one exactly suited for a specific need such as great depth of field or a long shutter speed where lower sensitivity or a neutral-density filter is not available).
- Aperture or stop. This is the adjustable hole through which light passes on its way from the subject, through the lens, to the film (or digital sensor). Like the pinhole in a pinhole camera, it blocks rays of light except those that would, even without a lens, tend to form an inverted image by passing through that central point to a corresponding point in the opposite direction on the film. With a lens, it also blocks rays of light that would pass through far from the center, where the lens glass may less closely approximate (usually with various easy-to-make spherical surfaces) the shapes that would focus it perfectly (usually much more complex aspherical surfaces), causing aberrations.
- Understand depth of field. Depth of field is, formally, the range of object distances within which objects are imaged with acceptable sharpness. There is only one distance at which objects will be in perfect focus, but sharpness drops off gradually in front of and behind that distance. For a short distance in each direction, objects will be blurred so little that the film or sensor will be too coarse to detect any blurring; for a somewhat greater distance they will still appear "pretty" sharp in the final picture. The pairs of depth-of-field marks for certain apertures next to the focusing scale on a lens are good for estimating this latter measure..
- Roughly one-third of the depth of field is in front of the focus distance, and two-thirds is behind (if not extending to infinity, since it is a phenomenon relating to the amount by which light rays from an object have to be bent to converge at a focal point and rays coming from far distances tend toward parallel.)
- Depth of field drops off gradually. Backgrounds and foregrounds will appear slightly soft, if not in focus, with a small aperture, but very blurred or unrecognizable with a wide aperture. Consider whether they are important and should be in focus, relevant for context and should be a little soft, or distracting and should be blurred.
- If you want great background blur but do not have quite enough depth of field for your subject, focus on the part that will draw the most attention, often the eyes.
- Depth of field generally appears to depend on, in addition to aperture, focal length (longer focal length gives less), format size (smaller film or Make a Sensor Bar size gives more, assuming the same angle of view, i.e., equivalent focal length), and distance (there is much less at close focus distances).
So, if you want shallow depth of field, you can buy a super-fast lens (expensive), or zoom in (free) and set even a cheap smaller-aperture lens wide open. - The artistic purpose of depth of field is to deliberately have the entire picture sharp or to "crop depth" by diffusing distracting foreground and/or background.
- A more practical purpose of depth of field is to set a small aperture and pre-focus the lens to the "hyperfocal distance" (the closest at which the depth of field extends to infinity from a given distance; see a table or the depth of field marks on the lens for the aperture chosen) or to an estimated distance, to be ready to take a picture quickly with a manual-focus camera or a subject moving too fast or unpredictably for autofocus (in which case you'll need a high shutter speed too).
- Remember that you normally won't see any of this through your viewfinder (or on your screen as you're composing. Modern cameras meter with the lens at its widest aperture, and only stop down the lens to its selected aperture at the moment of exposure. The depth-of-field preview function usually allows only a dim and imprecise view. (Disregard any odd patterns in the focusing screen view; they will not appear in the final picture.) What's more, Use a Viewfinder for Artwork on modern digital SLRs and other autofocus cameras don't even show the true wide-open depth of field with a lens faster than f/2.8 or so (it's shallower than it looks; rely on autofocus, not subject to this limitation, when possible). A better option on digital cameras is to simply take the picture, then play it back and zoom in on your LCD to see if the background is adequately sharp (or blurred) enough.
- Understand the interaction of aperture and instantaneous lighting (flash). A flash burst is normally so short that the flash component of an exposure is affected only by aperture. (Most 35mm and digital SLRs have a maximum "flash-sync" flash-compatible shutter speed; above that only a fraction of the frame would be exposed due to the way in which their "focal-plane" shutter works. Special high-speed-sync flash modes use a rapid burst of weak flashes, each exposing a fraction of the frame; they greatly reduce flash range and so are rarely helpful.) A wide aperture increases maximum flash range. It also increases effective fill-flash range by increasing the proportionate exposure from a flash and reducing the time during which ambient light is allowed in. A small aperture may be needed to prevent overexposure in close-ups due to a minimum output below which a flash cannot be reduced (indirect flash, which is inherently less efficient, can help in this situation). Many cameras can adjust the balance of flash and ambient lighting with "flash exposure compensation". A digital camera is best for complex flash setups because the results of instantaneous bursts of light are inherently non-intuitive, even though some studio flashes have "modeling lights" and some fancy portable flashes have modeling-light-like preview modes.
- Test your lenses for optimal sharpness. All lenses are different and are better shot at different apertures for optimal performance. Get out and shoot something with lots of fine texture at different apertures and compare the shots to figure out how your lens behaves at various apertures. The object should be all essentially at "infinity" (30 feet or more with wide-angles to hundreds of feet with tele-lenses; a distant stand of trees is generally good) to avoid confusing defocus with aberrations. Here's some hints as to what to look for:
- Nearly all lenses have lower contrast and are less sharp at their widest aperture, especially towards the corners of your image. This is especially true on point-and-shoot and cheaper lenses. Consequently, if you're going to have detail in the corners of your pictures that you want to keep sharp, then you'll want to use a smaller aperture. For flat subjects, f/8 is typically the sharpest aperture. For objects at varying distances a smaller aperture may be better for more depth of field.
- Most lenses will have some noticeable amount of light fall-off wide open. Light fall-off is where the edges of the picture are slightly darker than the centre of the picture. This can be a good thing for many photographs, especially portraits; it draws attention towards the centre of the photograph, which is why many people add falloff in post. But it's still good to know what you're getting. Falloff is usually invisible after about f/8.
- Zoom lenses can vary depending on how far in or out they are zoomed. Test for the above things at a few different zoom settings.
- Diffraction makes almost every lens's images softer at f/16 and smaller apertures, and conspicuously softer at f/22 and smaller.
- All of this is just something to think about for optimum Compose Photos that already has as good a composition--including depth of field – as possible, and which will not be much more grossly marred by insufficient shutter speed causing camera-shake or subject blur or noise from excessive "sensitivity" (amplification).
- Don't waste film investigating this – check your lenses on a digital camera, check reviews, and in a pinch assume expensive or prime (non-zoom) lenses are best at f/8, cheap simple ones such as kit lenses are best at f/11, and cheap exotic ones such as superwides or lenses with wide or tele adapters are best at f/16. (With an adapter lens on a point and shoot, stop down as much as possible, perhaps by using the camera's aperture-priority mode – look in its menus.)
- Understand aperture-related special effects.
- Bokeh, a Japanese word often used to refer to the appearance of out-of-focus areas, especially highlights because those appear as bright blobs. Much has been written about the details of those out-of-focus blobs, which are sometimes brighter in the middle and sometimes a little brighter at the edges, like donuts, or some combination of the two, but at least one author rarely notices it except in bokeh articles. Most importantly, out-of-focus blurs are:
- Much larger and more diffuse at wider apertures.
- Soft-edged at the widest aperture, due to the perfectly round hole (the edge of a lens, rather than an iris blade).
- The shape of the diaphragm opening, when not at the widest aperture. This is most noticeable at wide apertures because they are large. This might be considered unattractive with a lens whose opening does not closely approximate a circle, such as a cheap lens with a five- or six-bladed diaphragm.
- Sometimes half-moons rather than circular toward the sides of images at very wide apertures, probably due to one of the lens elements not being as huge as it would have to be to fully illuminate all parts of the image at that aperture, or weirdly extended due to "coma" at very wide apertures (which is pretty much only an issue when Take Night Photos With a Point and Shoot Camera).
- Prominently donut-like with mirror-type tele lenses, due to a central obstruction.
- Diffraction spikes forming sunstars. Very bright highlights, such as light bulbs at night or small specular reflections of sunlight, will be surrounded by "diffraction spikes" making "sunstars" at small apertures (they are formed by increased diffraction at the points of the polygonal hole formed by the iris). These will either have the same number of points as your lens has aperture blades (if you have an even number of them), due to overlapping of opposite-sides' spikes, or twice as many (if you have an odd number of aperture blades). They are fainter and less noticeable with lenses with many, many aperture blades (generally odd lenses such as old Leicas).
- Bokeh, a Japanese word often used to refer to the appearance of out-of-focus areas, especially highlights because those appear as bright blobs. Much has been written about the details of those out-of-focus blobs, which are sometimes brighter in the middle and sometimes a little brighter at the edges, like donuts, or some combination of the two, but at least one author rarely notices it except in bokeh articles. Most importantly, out-of-focus blurs are:
- Get out and shoot. Most importantly (in terms of aperture at least), Control your depth of field. It's as simple as this: a smaller aperture means more depth of field, a larger aperture means less. A larger aperture also means more background blur. Here's some examples:
- Use a small aperture to force more depth of field.
- Remember that depth of field becomes shallower the closer you get.If you're doing macro photography, for example, you might want to stop down far more than you would for a landscape. Photograph a Dragonfly often go way down to f/16 or smaller, and have to nuke their subjects with lots of artificial lighting.
- Use a large aperture to force a shallow depth of field. This is great for portraits (much better than the silly automatic portrait scene modes), for example; use the largest aperture you have, lock your focus on the eyes, recompose and you'll find the background is thrown out of focus and is, consequently, made less distracting.
Remember that opening the aperture like this will cause faster shutter speeds to be chosen. In bright daylight, make sure you aren't causing your camera to max out its fastest shutter speed (typically 1/4000 on digital SLRs). Keep your ISO low to avoid this.
- Shoot for special effects. If you're photographing lights at night, have adequate camera support, and want sunstars, use a small aperture. If you want large, perfectly rounded bokeh spots (albeit with some incomplete circles), use a wide-open aperture.
- Shoot for fill-flash. Use a relatively large aperture and fast shutter speed if necessary to mix flash with daylight so the flash isn't overwhelmed.
- Shoot for optimum technical image quality. If depth of field is not of primary importance (which would generally be the case when pretty much everything in the picture is relatively far from the lens and will be in focus anyway), the shutter speed will be high enough to avoid blur from camera shake and the ISO setting will be low enough to avoid severe noise or other quality loss (which would generally be the case in daytime), you don't need any aperture-related gimmicks, and any flash is powerful enough to balance with ambient light adequately, set the aperture that gives the best detail with the particular lens being used.
- Once you've chosen the lens aperture, try making the most of it with aperture-priority mode.
Tips
- Softness from diffraction and, to a lesser extent, defocus (which can create odd patterns rather than softness alone) can sometimes be mitigated by processing such as the "unsharp mask" function in your post-processing software; GIMP and Photoshop being two popular examples. This will strengthen soft edges though it cannot create fine detail that was not captured, and creates harsh erroneous detail if overused.
- There's plenty of wisdom embodied in the old saying: f/8 and don't be late. f/8 typically gives sufficient depth of field for most still subjects, and it's where 35mm and digital SLR lenses are typically at their sharpest (or close to it). Don't be afraid to use it – or program mode (a good mode to leave your camera on for whatever might pop up) – for interesting subjects that won't necessarily stand still for you to adjust your camera.
- Sometimes you have to compromise your choice of aperture to allow an adequate shutter speed or acceptable film speed or "sensitivity" (amplification) setting. You can also just let your camera's auto setting choose something for you to get the shot. Do it.
- If careful aperture selection will be very important to your picture and you have an automated camera, aperture-priority mode or program-shift (scrolling through the combinations of apertures and shutter speeds automatically determined to give proper exposure) are convenient ways to set it.
- All lenses have some distortion in them: there is no such thing as a "perfect" lens, even in Professional models that cost thousands of dollars. The good news is that name-brand lenses, such as those from Nikon, Canon, Pentax, Zeiss, Leica, Sony/Minolta, and Olympus, often have known "distortion correction" profiles that can be downloaded on the Internet and applied in post-processing software (in Adobe Photoshop and Adobe Camera RAW software, for example). Using the capabilities of good post-processing software and camera lens profiles can go a long way toward making photos with a lot of barrel or pincushion distortion look much more natural and pleasing to the eye. In this example of a wide angle panoramic landscape photograph, the problem is that "perspective distortion" and "barrel distortion" is causing the trees toward the outer edges of the image to lean inwards. It's pretty obvious that this is a lens distortion and that it's very unlikely that the trees were actually leaning this way.
- Now, here is the same image after Lens Profile and Vertical Distortion Corrections were applied in Adobe Camera RAW. The trees are now all more or less vertical, both in the center and at the edges of the scene, at the expense of a slight cropping of the image. The photograph looks much more pleasing to the eye, and doesn't have the distraction of the trees leaning inwards
Warnings
- Make "sunstars" with bright points of light, like streetlights, that are not so bright as the sun itself.
- Don't point a tele-lens, especially a very fast or long tele-lens, directly at the sun while attempting to make "sunstars", or for any other reason. You may damage your eye, or the camera's shutter or sensor.
- Don't point a cloth-shutter non-SLR camera, such as a Leica, toward the sun, except perhaps briefly to take a picture handheld, and even then only with a small aperture set. You may burn a hole in the shutter, which would require a somewhat expensive repair.
Related Articles
References
- See Ken Rockwell on diffraction, http://www.kenrockwell.com/tech/diffraction.htm
- http://www.zeiss.com/C12567A8003B58B9/Contents-Frame/696A77A0FB8016CFC125697700547F1F
