One of the main
complaints by users and reviewers about the Canon RF mount crop sensor
system has been the paucity of lens offerings.
When the EOS R7 and R10 were announced in 2022 only two
zooms were made available. Since then Canon has added two more zooms. But each
of the Canon offerings to date is a utility style model with compact dimensions
and a small variable aperture diaphragm.
Eventually Canon decided to license its proprietary lens
mount and autofocus technology to selected specialist lens makers with Sigma
being first to offer attractive products.
Sigma has announced a roadmap of autofocus crop sensor
(RF-S) lenses for Canon starting with the subject of this review,
* 18-50 [FF equiv 29-80mm] f2.8 DC DN/C followed by a
* 10-18mm [FF equiv 16-29mm] f2.8 DC DN/C and in due course four primes,
* 16mm [FF equiv 25.6mm],
* 23mm [FF equiv 37mm],
* 30mm [FF equiv 48mm]
* 56mm [FF equiv 80mm].
The primes are f1.4 DC DN/C models.
These lenses have a rubber O ring around the edge of the
metal mount and are stated by Sigma to be dust and splash resistant.
They do not have an optical image stabiliser. They can be
used on any Canon RF mount camera but are best mounted on the EOS R7 which has
IBIS.
In one move Sigma has provided most of the previously
missing lenses needed to make the Canon
RF-S system attractive to enthusiast photographers.
This move is a win-win for all concerned.
Canon’s crop sensor bodies, especially the R7, become more attractive to buyers, Sigma gets to sell
more product and us consumers now have access to a larger and for some, more
appealing range of lenses than had previously been available.
Like the other lenses in this list the 18-50mm f2.8 has
already been available for Sony E, L Mount and Fujifilm X mount for several
years, so its optical characteristics are well known and favourably reviewed.
Of greater concern is the autofocus performance. The
background to this is that many consumers have reported that third party lenses
often fail to focus correctly on modern mirrorless cameras due, presumably to
the complexity and proprietary nature of the many high speed calculations
involved.
I am pleased to report that autofocus performance of the Sigma 18-50mm on
my Canon EOS R7 is absolutely fine with no problems appearing after several
hundred test frames in bright light, near darkness, low and high contrast, on
people, places and things.
Single AF is snappy fast and reliably accurate. Human and
animal face/head/eye detect works just fine, as it does with Canon branded
lenses.
Servo AF also works very well with face/eye detect and
accurate focus being achieved even in low light and when the subject is
backlit.
On the R7 in dark mode |
Description
The RF mount version of this lens is styled like Canon’s own
RF-S models, with an angled step up from the 61.5mm outside diameter of the
lens barrel to the 69mm diameter of the outside of the mount. This allows us to
distinguish on sight between the RF mount version of the lens and versions for
other mounts which is handy as there is no label on the lens telling us which
mount it fits.
Sigma lenses are made in Japan to a high standard. Even
those in the “Contemporary” class which is Sigma’s consumer level designation
are well made and finished. The barrel
is polycarbonate and the mount metal with a thin rubber O ring around the edge.
As with Canon’s RF-S models, there are no switches on the barrel. AF/MF is
selected in camera.
There is no optical stabiliser in the lens.
The zoom action is very smooth and works “backwards” as do
all Canon zooms. The focus/control ring turns smoothly. The supplied lens hood
( Canon, are you paying attention ??) fits with just the right amount of
resistance.
The lens cap is nicely designed with grippy edges on the
center pinch tabs. This is welcome as
removing the lens cap is an action we must perform thousands of times.
The filter is 55mm. All my tests were done with a Hoya HD2
UV filter in place.
Mounting onto the R7 was quite stiff initially but freed up
a bit after being removed and re-mounted a few times.
The overall size of the lens is very compact for a constant
f2.8 standard zoom. At 74mm in length it is the same length as the Pana/Leica
12-35mm f2.8 for Micro Four Thirds and
half the price. It is dramatically smaller than the Canon EF-S 17-55m f2.8
which was my standard lens on the EOS 20D and 40D for several years.
Just for reference the R7 + Sigma 18-50 f2.8 is also very
much lighter and more compact than an approximately equivalent full frame kit
such as a Canon EOS R5.2 with the RF 24-70 f2.8 and is a quarter of the price.
Optics
There are 13 elements in 10 groups with 3 aspherics and one
SLD element.
Sigma has posted a schematic of the lens construction and
MFT charts.
Like almost all current model zooms the Sigma 18-50mm f2.8 relies on post capture processing either in camera for JPGs or in post processing for Raws, to rectify distortion, aberrations and corner shading.
In Adobe camera Raw the results are pleasing with almost
complete elimination of distortion and partial correction of corner shading and
aberrations. Full correction is usually not too difficult in Camera Raw.
On the test chart the lens is very sharp in a large central
area of the frame at all focal lengths right from f2.8. Corners and edges on
the long side are a bit soft at 18mm and f2.8, becoming sharp by f5.6. This
softness is much less evident when photographing normal subjects than it
appears on the finely detailed test chart.
The edges and corners are sharp at 24, 35 and 50mm right
from f2.8.
Diffraction starts to affect rendition of fine details from
around f10.
The bokeh looks nice to me with no obvious ni-sen or
unpleasantness at any focal length or aperture.
Various types of flare can be induced when the lens is
pointed towards bright lights. Against the sun this is mostly veiling flare
although shaped and colored flares can be induced if we deliberately try to do
so.
Some reviewers have reported a bit of longitudinal chromatic
aberration at wide apertures although I have not found this to be a problem.
Lateral chromatic aberration in the form of mild purple
fringing can be seen on foliage against the light at 18mm. This is easily
corrected in post processing.
Focus breathing is sufficiently well controlled that it is
rarely an issue in practice.
I tested the effectiveness of the R7 IBIS with my usual
protocol. I found that with the IBIS ON I am able to get sharp pictures hand
held at a shutter speed about three EV steps slower than is possible with the
IBIS switched OFF. This is quite
significant and makes the R7 the preferred camera body for this lens as the
R10, R50 and R100 do not have IBIS.
18mm f2.8 |
28mm f2.8 |
50mm f2.8 |
I like to explore lens characteristics further by photographing cut grass to reveal the distribution of sharpness towards and away from the focus point at each focal length and aperture.
An ideal lens is equally sharp from one side of the frame to the other in an even relationship to the plane of focus at each aperture. In the real world some lenses depart markedly from the ideal, exhibiting variable complex distortions of the zone of sharpness as the focal length and aperture are altered.
In Canon RF world I have found the RF 35mm f1.8 and the RF 24-105mm f4-7.1 STM are lenses which do have troublesome shifts in the distribution of sharpness as the focal length and/or zoom alters.
Fortunately the Sigma 18-50mm f2.8 is one of the better
zooms which I have tested with no extreme distortions or shifts in the
field of sharpness.
Comparison with Canon RF-S 18-150mm f3.5-6.3
The comparison between these two lenses is an instructive
example of lens design in practice. The two lenses are about the same size,
mass and price and each takes a 55m filter.
The Canon prioritises focal length range over wide aperture
and adds an image stabiliser.
The Sigma prioritises wide, constant aperture but must
accept a much smaller zoom range in order to achieve this.
On the test chart the Sigma is slightly sharper at all
comparable focal lengths and apertures except 18mm at f3.5 where the Canon is a
touch sharper at the edges.
The Canon is better for close-up and semi-macro work, with
the ability to render small subjects about twice as large on the sensor as the
Sigma at a workable distance from the subject.
Both lenses are routinely capable of producing excellent
results on the R7 in just about any photographic situation. We cannot really say one is better than the
other. They are both very good within their envelope of capabilities.
I recommend the Canon lens for outdoors, close-ups, travel
with a one body/one lens kit, amateur sports and action, and on the R10 or R50.
I like the Sigma for indoors, low light, softer backgrounds
and best possible image quality on the R7.
Conclusion
The Sigma 18-50mm f2.8 is a welcome addition to the range of
lenses suitable for Canon RF mount crop sensor cameras (RF-S). It is
particularly suitable for the R7 which has IBIS and the lens has enough
resolution to make good use of the high pixel count sensor on the R7.
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