One of the problems many
Digital SLR owners face, unless they have
one of the few models using a full frame sensor the same size as a 35mm film
frame, 36mm x 24mm, is that many of the
lenses currently available are designed to be used with a sensor area larger
than that fitted to their camera. So when they fit a lens on the
camera, the field of view is not the same as that usually obtained
with the focal length. Although there are some benefits to be gained
from the use of current 35mm lenses, especially for
owners with existing lens systems, one of the big drawbacks is the
difficulty in obtaining good quality wide-angle views. The answer
from lens makers at first was to make 35mm 'ultra-wide' lenses
that when used on DSLR's with smaller sensors gives a smaller but
still useful wide-angle
capability. But there are problems with this approach
One is that digital sensors need the
light paths to hit them at right angles to get the best response and in
general most 35mm lenses are not designed to do this because film
doesn't have this requirement. It's not so much of a problem with
the longer focal lengths, but as the focal lengths shorten, the angle
at which the light paths strike the sensor become increasingly oblique, resulting
in reduced image quality. This generally shows as lower contrast and
less sharpness.
This is why almost all digital cameras
produce better images at longer focal lengths than they do at the
shorter ones. It's not the only reason, pixel frequency/pattern
frequency is another, and that is also tied to the image view, with wide
angle lenses producing the wrong mixture of low pixel and high
pattern frequency. Please refer to Sensors
. Another problem is the very large front elements that
are involved with wide and ultra-wide angle lenses and the
difficulty that can result in using them in anything other than
'flat' light conditions. Avoiding flare is a constant problem, and
while its a pain using a prime wide-angle, with a zoom wide-angle
its worse because the element can still collect and bounce the flare
around inside the lens, even if your using another focal length on
the lens.
A further difficulty is that digital
camera sensors are more highly reflective than film and this can
have an adverse effect on an image if a poor quality lens is
used. If you look inside a camera with the lens removed you'll see
all component surfaces inside are dark matt coloured to prevent as
much stray light bouncing around as possible when the shutter is open. Reflective
light is very degrading to image quality.
So a welcome development has been the
arrival of an increasing number of lenses made specifically to suit
digital cameras. A distinction
needs to be drawn between those lenses made specifically for a
certain sensor size, and those which have been modified 'to suit
digital cameras'. Lenses which fall into the latter category
are ones which are still designed for use with a 35mm sized sensor but where the light
paths have been corrected to fall at a better angle on the digital
sensor and use extra coatings to improve image quality. Lenses
built to suit the correct sensor size also of course have these
qualities, but the advantage is, or should be, that smaller lighter
lenses result. The focal length quoted will still be the same, but
as the lens only has provide an image which covers a smaller area -
the size of the smaller sensor - the optics used, and especially the
front element, can be made smaller.
However, even if you own
a camera that uses dedicated lenses, those matched to the size of
the sensor, you still feel at odds as the lenses are marked with the correct focal length for the
size of the sensor - as do all compact cameras - which has no
meaning for the average user used to the 35mm designations and the
corresponding angle of view. So the
convention has arisen whereby the lens focal lengths are also given
as the equivalent to 35mm sizes. Digicams usually have this equivalent
marked on them, DSLR lenses don't.
Most
DSLR's are given as
having a focal length multiplier of a certain size in order that it
can be worked out roughly what the focal length/field of view are,
in 35mm terms, and this is relative to the size of sensor used. Thus
Canon's 1D has a 1.3x, all Nikon, Pentax and Fuji's a 1.5x, Canon's
APS-C SLR's a 1.6, whilst Sigma's have a 1.7. The 4/3rds sensor is
half the area of a 35mm frame so a 2x factor gives an equivalent
focal length. The smaller the sensor, the larger the multiplier
factor will be.
Whilst the use of lenses
made to suit the sensor size has its advantages, so does the method
of using 35mm lenses with smaller sensors. Apart from the obvious
one of increasing apparent focal lengths by in-camera 'cropping' of
the image produced, it has the added advantage that as the image comes
from the best portion of the lens it is less likely to exhibit the
common lens faults of barrel and pincushion distortion, and corner
shading. Although corrective lens elements are used to reduce these
faults as much as is possible, as lenses produce their best image
quality at the centre which progressively reduces towards the outer edges, it follows
that any image cropped from a smaller central portion will have
higher image levels and lower distortion. It also means that
smaller, lighter lenses can be used. For example, a 400mm
f5.6 lens becomes the equivalent of a 600mm f5.6 lens when used on a
D-SLR with a APS 'C' size sensor, yet is lighter, smaller, and
considerably cheaper.
Lens
focal length comparisons
To help you work out the
comparable focal lengths for the different DSLR sensor formats, below are two
tables of lens focal lengths and their equivalents at the
different multipliers. One for prime lenses, one for zooms. While not every single length is
covered we hope there is enough of a spread to cover the vast
majority of sizes.
*In all cases the
figures are rounded to the nearest whole number.
NB. A very important
factor to bear in mind is that depth of field, as we explained on
page 1, is related to focal length. So if you have a 28mm focal
length lens, and on a APS-C sensor camera it thus becomes the equivalent of a
42mm lens in respect of field of view, remember, the depth of field
is still that for a 28mm lens over the aperture range and focusing
distance. In other words it's greater than a 42mm focal length lens
would produce on a 35mm full frame DSLR. This is
an especially important point with
regards to telephoto lenses. A 400mm focal length might be the equivalent of a
600mm in field of view terms on a 1.5x multiplier APS-C DSLR, but it's still a 400mm for depth of
field. This is why APS-C sensor DSLR's have such an advantage with
DOF over 35mm DSLR's. And 4/3rds DSLR's have an even greater one.
This is all explained on page 1, Digital
Camera Lens basics
Prime Lenses.
Prime
Lens
1.3x
1.5x
1.6x
1.7x
12mm
15mm
18mm
19mm
20mm
15mm
19mm
22mm
24mm
25mm
18mm
23mm
27mm
29mm
30mm
21mm
27mm
31mm
33mm
36mm
24mm
31mm
36mm
38mm
41mm
28mm
36mm
42mm
45mm
48mm
35mm
45mm
52mm
56mm
59mm
43mm
56mm
64mm
69mm
73mm
50mm
65mm
75mm
80mm
85mm
70mm
91mm
105mm
112mm
119mm
85mm
110mm
127mm
136mm
144mm
100mm
130mm
150mm
160mm
170mm
135mm
175mm
202mm
216mm
229mm
180mm
234mm
270mm
288mm
306mm
200mm
260mm
300mm
320mm
340mm
300mm
390mm
450mm
480mm
510mm
400mm
520mm
600mm
640mm
680mm
500mm
650mm
750mm
800mm
850mm
600mm
780mm
900mm
960mm
1020mm
Zoom Lenses.
Zoom
1.3x
1.5x
1.6x
1.7x
12-24mm
15-31mm
18-36mm
19-38mm
20-41mm
15-30mm
19-39mm
22-45mm
24-48mm
25-51mm
18-35mm
23-45mm
27-52mm
29-56mm
30-59mm
24-70mm
31-91mm
36-105mm
38-112mm
41-119mm
28-90mm
36-117mm
42-135mm
45-144mm
48-153mm
35-105mm
45-136mm
52-157mm
56-168mm
59-178mm
80-200mm
104-260mm
120-300mm
128-320mm
136-340mm
80-320mm
104-416mm
120-480mm
128-512mm
136-544mm
100-300mm
130-390mm
150-450mm
160-480mm
170-510mm
100-400mm
130-520mm
150-600mm
160-640mm
170-680mm
135-400mm
175-520mm
202-600mm
216-640mm
229-680mm
170-500mm
221-650mm
255-750mm
272-800mm
289-850mm
Camera Lens
Multiplication Factors - LMF.
1.3x
:- All Canon 1D series
1.5x :- All
Pentax, all Fuji, all Sony, all Nikon DX series
Although it has always
been the fact that some lenses are better than others, that the
best lenses use the best quality optical glass and design and cost
more, nowhere has this been more apparent than when a digital SLR
is used. Digital sensors are much more demanding in terms of the
quality of light that they are supplied with, much more sensitive to
small changes in light, and there is a marked
difference in the quality of images using an 'ordinary' lens, and
one made to the highest standards.
At one time prime lenses
were regarded as providers of the best quality images. More recently
zooms have become almost universal in use, their image quality and convenience
being seen as more than acceptable. Currently however, thanks to the
arrival of DSLR's, prime lenses are staging something of a comeback
as users re-discover the superior image quality they deliver with
digital sensors because of their higher resolving powers.
It
is also becoming clear that a high resolution lens is more important
than the number of pixels on a sensor, and that as pixel counts
rise, so must the resolution of the lenses used in order to make the
most of the increase in pixel count. Replacing the lens on your DSLR
with one of higher resolution will be more beneficial in terms of
the increase in image quality, than changing the DSLR for one with a
higher pixel count, and then using the same lens on it. However
as DSLR sensor counts rise above the 10mp mark these remarks need
some further qualification.
The
first is that increasing the resolving power of a lens can be
achieved two ways. One is by using better quality optical design,
more expensive materials, better quality elements. Another is by
increasing the contrast the lens provides rather than increasing the
glass quality. The second method is used with cheaper lenses since
it costs less. Whilst either method was acceptable to a degree with
past film camera use, using high contrast lenses with digital
sensors leads to reduced image quality. Sensors need high
resolution, but not high contrast, which can materially reduce the
tonal range produced.
The
second point is that as sensor counts have risen past that 10mp
figure, so it has become apparent that even the best quality lenses
made to date struggle to produce the level of resolving power
needed. This is not a factor that only affects the smaller sensor
DSLR's, for it is the relationship between sensor count and the
resulting pixel size in percentage terms that matters here. Not the
actual pixel size, but the percentage of space each pixel occupies
on the sensor in relation to the image circle the lens produces.
Anyway,
the bottom line is that lenses with ever higher resolving powers are
required if the increase in sensor resolution is to be of benefit by
leading to finer image detail capture. But the conundrum is that
using these new very high resolving lenses which the camera makers
are now introducing with older DSLR's with lower resolution sensors
of the past, 6/8mp/10mp, tends to lead to reduced image
quality.
It's
nothing that is cut and dried, with a distinct separation, but
instead a blurred dividing line, and that's part of the problem. You
can't say for example that past film design made lenses just don't
resolve enough, but that all later digital design lenses do. A
recent lens review of a camera makers entry level lenses found that
an early digital type just wasn't good enough for the latest 14mp
sensor camera to arrive, and if used would result in an image no
better than that from a 10mp sensor. We
ourselves found back in 2003/2004 with the purchase of our first 6mp
DSLR, that a digital designed lens bought specifically to be used
with it performed far worse than the best quality film lens designs.
Users of Canon's full frame cameras, the 5D and 1Ds series, have
discovered that each and every lens type tends to perform
differently on the cameras and the only way to find which one's are
the best to use is simply one of trial and error.
What
the upshot of all this will be is unclear at present, but we would
suggest the following. That when you are buying a lens
to use with a DSLR you consider very carefully all the points we
have raised. If it is an older camera with a relatively low pixel
count, be cautious with regard to the latest revised lens designs
with high resolution. And if you
are intending to buy a new high resolution DSLR, whatever the sensor
format, APS-C or Full Frame, you discount the idea of using any
current lenses you have with it, older film types or newer digital
versions, and make your buying
decisions around the premise that you will be purchasing the latest
new design lenses to use with it. You may find that your current
lenses are good enough, which will then be a bonus, but not the huge
disappointment it would be if they weren't, but that you had assumed
that they would be.