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Friday, 22 November 2024

Sigma 18-50mm f2.8 DC DN/C lens for Canon RF mount. User review 22 November 2024

 



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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