Be more confident: How to Choose a Lens Aperture (F Stop)

How to Choose a Lens Aperture (F Stop)

One of the most important settings on any
adjustable camera is the size of the hole (known
as the "aperture") through which light passes on
its way from the subject, through the lens, to
the film. The size setting of this hole,
customarily referred to as the "f/stop" in
reference to a standard measurement or simply the
"aperture", determines depth of field, manages
the effect of certain lens imperfections, and can
add certain special effects such as spiked
"sunstars" around bright highlights. Here's how
to choose the best one.

!! 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 [1]* 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 [2],
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/ numbers are a ratio of the diameter of your aperture
compared to your lens. Hence, a larger number is a smaller_
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 [3] 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).

* Most SLR cameras [4] 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.[1] [5] 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).

Understand depth of field. Depth of field is, formally, _the range
of object distances within which objects are imaged with acceptable
sharpness [6]_. 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 [7] 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.[2] [8] 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.[3] [9].

* Roughly one-third of the depth of field is behind 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 [10] 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.

Using a longer focal length and a smaller aperture defocuses
backgrounds just as well as a longer focal length; compare shots
taken at 50mm at f/1.8 (top), and 200mm at f/5.6 (bottom) Depth
of field generally appears to depend on, in addition to aperture,
focal length (longer focal length gives less), format size
(smaller film or sensor [11] 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
[12] (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 [13] 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,
viewfinders [14] on modern digital SLRs [15] 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 [16] 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 [17] 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 [18] 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.

* _ Falloff, which is darkening towards the edges of a picture,
is common when shooting lenses wide-open.*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 [19]; it draws attention towards
the centre of the photograph, which is why many people add falloff
in post [20]. 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 clarity
of a picture [21] 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 [22], 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 [23] – 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 taking pictures of lights at night [24]).

* Catadioptric (mirror) lenses give donut-shaped
bokeh.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).

Get out and shoot [25]. Most importantly (in terms of aperture at
least), Control your depth of field. [26] 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:

* The left shot was done at f/2, the right shot was made at
f/16. Note how the letters furthest from the camera are brought
into focus by stopping down.*Use a small aperture to force more
depth of field.*

* Depth of field is shallower the closer you are to your
subject. This shot was made at f/6.3, and depth of field is
perhaps a millimeter or two.*Remember that depth of field becomes
shallower the closer you get.*If you\'re doing macro photography
[27], for example, you might want to stop down far more than you
would for a landscape. Insect photographers [28] often go way down
to f/16 or smaller, and have to nuke their subjects with lots of
artificial lighting.
Large apertures force backgrounds to be thrown out of focus; this
is great for portraits, as in this shot made at f/2.*Use a large
aperture to force a shallow depth of field.* This is great for
portraits [29] (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 [30].

!! Video !!

!! Tips !!

* _ The real world is not a boring test subject. Use any of your
apertures if that\'s what you need.If careful aperture selection
will be very important to your picture and you have an automated
camera, aperture-priority mode [31] or program-shift (scrolling
through the combinations of apertures and shutter speeds
automatically determined to give proper exposure) are convenient
ways to set it.

* Sometimes you have to compromise your choice of aperture to allow
an adequate shutter speed or acceptable film speed or
"sensitivity" (amplification) setting, or just let your camera
choose something for you to get the shot. Do it.

* 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.

* 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
the GIMP and Photoshop. This will strengthen soft edges though it
cannot create fine detail that was not captured, and creates harsh
erroneous detail if overused.

!! Warnings !!

Make "sunstars" with bright points of light not so bright as the sun.

* Don't point a tele-lens, especially a very fast or long tele-lens,
at the sun attempting to make "sunstars" or for any other reason.
You may damage your eye, or the camera.

* 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 WikiHows !!

* How to Clean a 35mm Film Camera and Lens [32]

* How to Buy Lenses for Your Digital SLR [33]

* How to Adapt if Your Lens Is Too Sharp [34]

* How to Buy a Good Used Camera Lens [35]

* How to Attach a Lens to the Nikon DSLR [36]

!! Sources And Citations !!

!! Article Tools !!

* Read on wikiHow

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Friday 28 January 2011

How to Choose a Lens Aperture (F Stop)

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