Which lens is
common question asked by both inexperienced and experienced shooters is
'which lens is best', usually with a specific focal length in mind? Understanding the factors or qualities that makes one lens different to another allows you to
better define the question in the first place. There
generally isn't such a thing as the 'best' lens in any particular focal
length simply because it's very uncommon, and even impossible, for any single lens
to have a superior performance in every possible respect.
So the question should really be altered to reflect ones particular
needs or desires. A better question might be 'which lens has the
smoothest Bokeh', or 'which lens is sharpest wide open', or similarly
specific questions depending on the intended application.
Fixed focal length (prime)
or variable focal length (zoom)
focal length lenses are often called primes. Primes lenses are
generally easier to design and manufacture than zoom lenses which
need more elements in more complex optical and mechanical designs.
This allows primes to be made into much faster lenses such as F1.4
or faster, or into special purpose lenses such as tilt or shift
lenses. Zoom lenses might only have a maximum aperture of F2.8, at
best, so they are not the best choice for low light photography
where maximum lens speed is required.
Zoom lenses excel where maximum flexibility and speed is required as
their focal length can quickly be changed to frame or compose a shot
appropriately. Composing a shot with a prime lens would potentially
require you to move, to get closer or further from the subject or to
actually change the lens, by which time the opportunity to shoot may
have passed. Most modern zooms are virtually as good as their
equivalent prime lens.
Speed, largest aperture
||One of the
main characteristics of any lens is it's speed or maximum aperture.
A lens might be available in many versions where one version might
have a maximum aperture of F1.4 whilst another might be F2.0, 2.8 or
slower. The lower the aperture number, such as F1.4, the larger the
physical aperture or opening which therefore allows a greater amount
of light to pass through the lens. This has several effects. Firstly
it allows the lens to be used in darker situations than a slower
lens, such as F2-F2.8, but it also allows the lens to have a
shallower depth of field (less in focus) which allows the subject to
better stand out from it's background. This later quality is often
the main reason a fast lens is chosen over of a slower lens. Slower
lenses are also simpler and cheaper to make so they are often also
an economical option offered by lens makers however they are not
necessarily poorly performing second rate lenses as some might see
them. Their simpler design often allows them to perform better than
their faster and more expensive relatives.
Manual or Auto Focus
lenses have Auto Focus (AF) nor is AF always needed or even
desirable. Manual focus (only) lenses are actually quite common,
especially older lenses which are often adapted to modern digital
bodies. Manual focus allows greater focusing precision than AF
however manual focus is more difficult and slower so it is not well
suited to sport, photojournalism or many fast paced subjects such as
children or pets.
Stabilization (IS or VR) is used to eliminate camera shake at slow
shutter speeds so it gives a sharper image in situations that might
ordinarily have resulted in blurry images due to camera shake. IS
does not work well with fast moving subjects, it only reduces the
effects of camera shake. For example, if you shot 2 different
subjects but in the same light and with the same shutter speed and
aperture then you would potentially get 2 different results. If the
first subject was stationary, such as people sitting at a table at a
wedding reception, then they would potentially be rendered quite
sharply but if shot people dancing on the dance floor, in the same
light, then the dancers would probably be blurry due to their
motion. So IS does have it's uses but it also has limitations.
is one of the most noticeable and important lens qualities but it is
not always the simplest to judge. A lens might be very sharp in the
central pert of the image but much less sharp near the corners and
this may or may not matter depending on the way the lens is to be
used. Some lenses also have field curvature where the sharpness is
not in a flat plane but in a curve. A lens with pronounced field
curvature would be unsuitable for photographing flat subjects, such
as art work, where the field curvature would cause the edges/corners
to be out of focus compared to the centre of the image. Some lenses,
such as enlarger, projection and reproduction lenses are designed
specifically with field flatness in mind.
Does the lens perform best
at close range or at infinity
lenses are designed to perform optimally when focused at infinity so
their performance can diminish noticeably when the lens is focused
at medium distances or closer. One of the reasons that specific
'Macro' lenses exist is because these lenses are designed to perform
optimally at very close focus distances, although most macro lenses
work quite well for medium-infinity focus applications too. Some
macro lenses even have adjustable or floating elements that move,
either automatically or are adjusted manually, to compensate for
lens aberrations at the particular distance being used. The distance
at which a lens is typically used should be considered when choosing
a lens. Normal use, which may constitute subjects from several
meters away all the way to infinity, would suit normal, that is non
macro, lenses. Macro lenses may have an edge in terms of sharpness
when shooting portraits at close distances however many portrait
photographers don't like biting sharpness in their portraits as this
can highlight blemishes way too much.
Minimum focusing distance
||This is an
important consideration if you intend shooting subjects at close
range, such as portraits, product shots or macro/close up images.
Specialized Macro lenses not only have a very close minimum focus
distance (often abbreviated to MFD) but often have a focusing
helicoid (the thread inside the focus ring) which is much finer, ie
with a longer throw, in the close focus range and often not as fine
at the infinity side of the helicoid. This does make macro lenses
harder to focus accurately at longer distances but then they are
not really designed for that application.
vary in cost from a few dollars to a few thousand dollars. At times,
ie under specific conditions, the performance of the cheapest lens
may be almost indistinguishable from that of the most expensive
lens, however, you generally do get what you pay for so there is
almost always some kind of justification for the price paid.
Price is not always a reflection of the mechanical or optical
quality of the lens however an expensive lens will often, but not
always, be made from better quality materials and to a higher
mechanical/optical standard. The price of lenses on the second hand
market is a reflection of their desirability, not necessarily
quality, so any lens which is in great demand will command a high
price whilst a lens which has lost favor with the market will
potentially be very cheap.
The cost of new lenses can vary significantly but again, you do
generally get what you pay for. Some major manufacturers make very
similar lenses but in different price ranges. The cheaper lenses,
some times termed kit lenses, are often made from plastic instead of
metal and are often slower (smaller) maximum aperture compared to
the more expensive offerings which may have image stabilization,
better/stronger construction, better optical performance and faster
(larger) maximum apertures.
be a very important consideration and can be overlooked by many.
It's importance will be a very personal decision and may be
altogether irrelevant to some people whilst a primary consideration
essentially the out of focus area of an image which can be rendered
in different ways by seemingly similar lenses. Any lenses Bokeh may
be determined by factors such as lens design (lens aberrations), the
shape and/or number of aperture blades, the aperture used, the
distance to the subject and the position of the out of focus areas
relative to the focus plane ie in front of or behind the focus
plane. All of these are factors in the resulting Bokehs quality so a
lens might have 'good' Bokeh in certain circumstances but relatively
'bad' Bokeh in others. There isn't really any such thing as 'good'
or 'bad' Bokeh however most people would consider 'smooth' or
'creamy' Bokeh to be pleasant as it tends to be least distracting so
helps a subject to stand out. Conversely 'bad', 'poor' or 'harsh'
Bokeh is generally quite noticeable in an image so is often
distracting and undesirable, although it can also add to the
interest in an image if used appropriately.
Bokeh can be a primary consideration in lens choice, over sharpness
and other qualities, as a lenses Bokeh will potentially determine
the overall 'look' of an image shot with that lens. A lens with
'poor' Bokeh might be described as harsh, jagged, double edge or
doughnut shaped. In all these cases the Bokeh would potentially be
distracting from the subject being photographed.
A common misconception is that 'good' Bokeh is simply the same as
having very shallow
depth of field, and that the more blur the better, but in fact you might
have a shallow depth of field but a very harsh Bokeh which many
would consider undesirable.
There are many forms of distortion but in all cases distortion
changes the shape of the subject being photographed. Any distortion
is undesirable if you are photographing geometric subjects such as
architecture, art work, products etc because the distortion can
change their shape to a degree where it is noticeable and
undesirable. On the other hand, many subjects are not adversely or
noticeably affected by distortion unless the distortion is quite
extreme so the subject matter should dictate the acceptable amount
and type of distortion from any lens.
Chromatic aberration (CA): Visible light consists of many colours
and lenses do not behave equally to all colours or Wavelengths of
light, such as Ultraviolet or Infra Red.
Chromatic aberration is where light is focused in slightly different
places depending on it's colour. Apochromatic (APO) lenses are those
which focus all (visible) colours in the same place, resulting in
reduced or non existent CA.
Spherical aberration is where the shape of the lens itself cause the
light passing near the edge of the lens to focus in a different
place to light passing closer to the centre of the lens.
Spherical aberration can be corrected to some degree by using
Aspherical lens elements.
Vignetting: Vignetting is the effect where the corners of the image
are some times darker than the centre of the image. It is usually
most noticeable with fast lenses or at large apertures and is
reduced or eliminated when the lens is stopped down. Vignetting in
lenses is unavoidable
however some lenses do have less vignetting than others so it is
still an important consideration for some types of photography.
Flare, Contrast and lens
Flare is caused by bright light sources reflecting
off lens surfaces and bouncing around inside the lens, usually
resulting in a reduction in contrast. Sometimes reflections of the
light sources themselves can appear in the image and this is the
more common and noticeable form of flare. Lens flare is a much more
common problem than most people appreciate as it's reduction in
contrast can often be subtle compared to the common idea of flare
such as when the sun is in the shot and it's reflections are very
obvious. If you shoot at night or in difficult or contrasty lighting
conditions then it is important to use a lens which has very good
Modern lens coatings and designs have resulted in lenses with
excellent flare control compared to similar lenses from previous
generations but with simpler and often ineffective coatings.
||Focus shift is an effect of Spherical
Aberration. It typically affects very fast lenses and results in the
focus plane changing or shifting when a lens is focused at it's
maximum aperture and then stopped down. The effect of stopping down
moves the focus plane slightly towards infinity and can result in an
out of focus image. Focus shift is quite common but it is not well
understood and certainly not always recognized as even existing.
This some times leads to confusion and misunderstandings about a
lenses performance which may not be as sharp as expected whilst in
fact the plane of sharp focus is simply not where one thought it
should be. It's quite common to see the focus plane in a portrait
slightly behind the eyes, where presumably the photographer intended
the eyes to be in focus, and this is often how focus shift manifests
itself in the real world.
Focus shift is not the same as 'front/back focusing' which is a
common problem with AF lenses which may not be properly calibrated
to a camera so the AF always focuses slightly in front or behind the
subject. A lens can conceivably have Focus Shift and front/back
focusing issues as they are different and unrelated problems.
Movements such Tilt and/or Shift
Some lenses are designed specifically to allow the
lens to tilt and/or shift. Tilt and shift are different in their
effects and application so it is not necessary to have both hence
some lenses can only tilt and some can only shift and some lenses
can do both. These movements (tilt or shift) allow the image to be
controlled in specific ways which would otherwise only be possible
with a View or Technical camera. Tilt/Shift (T/S) lenses are usually
very high quality lenses because the act of tilting or shifting a
lens can result in reduced image quality unless the lens is of very
hi performance to begin with.
Tilt is most commonly used to control the focal plane such as to
have the foreground and background in focus at the same time without
stopping down the lens. Shift is usually used to reduce or eliminate
converging verticals such as when looking up at a building.