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Converters often get bad press. Perhaps they deserved it about 20 years ago but modern ones are really much better than those early models.
The Sigma EX 1.4x APO Converter is an excellent and extremely versatile little accessory capable of extending the range and usefulness of many lenses and in my opinion, a valuable and essential addition to the kit bag.
It is of EX designation which in Sigma terminology means 'Excellence' and indicates that it is considered to be within the top quality professional range of Sigma lens products. It has the familiar smooth black 'crinkle' finished associated with the EX range and both looks and feels very good quality. It is extremely compact and will take up very little room in the outfit bag. It would be churlish to leave it at home! I purchased my example over a year ago and it is now indispensable, always with me whatever kit I am using on any given day, full-frame or aps-c dslr.
It is not suitable for use with lenses of shorter focal length than 50mm and in any case, this would be pointless. It is also not suitable for use with all lenses being incompatible with some. You need to check Sigma's website for compatibility and also, your own particular camera manufacturer's website to ascertain just which lenses are suitable for use with converters/extenders.
Whilst this converter can be useful for lenses of between 50mm to 100mm, unless these are fast lenses (large maximum aperture), some of the benefit will be lost. 50mm lenses should have a maximum aperture of f/1.4 preferably, so that when the converter is attached, the 1 f/stop of light loss provides a still useful effective maximum aperture of f/2, the 50mm f/1.4 lens effectively becoming a very useful 70mm f/2 portrait lens. A 100mm lens needs to have a maximum aperture of f/2, so that it converts to an effective 140mm f/2.8, a great 'head shot' portrait lens with very shallow DOF (Depth-of-Field).
Things become even more exciting when using this converter on APS-C sensor DSLR's when that 50mm f/1.4 'standard' lens becomes a far more exciting 105mm f/2 lens (Nikon APS-C), 112mm f/2 (Canon APS-C) or 119mm f/2 on Sigma DSLR's. That 100mm f/2 lens I mentioned earlier will become an effective 210mm f/2.8 lens (Nikon APS-C), 224mm f/2.8 lens (Canon APS-C) and a 238mm f/2.8 on Sigma DSLR's. Quite useful.
The converter is also very useful when paired with a good macro lens, such as Sigma's 70mm f/2.8, 105mm f/2.8 or 150mm f/2.8 Macro lenses, where it not only provides additional working space (through longer effective focal length) but also, larger than 1 to 1 'life-size' reproduction. Great for insects and the like!
Most people however, I would think, will use this converter with either a longer telephoto prime lens or, one of the few compatible telephoto zoom lenses, when it can very usefully be employed to extend the range, particularly at the longer end of the focal length range.
I use my Sigma EX 1.4x APO Converter mostly with my Canon EF 135mm f/2 L USM lens, where it effectively becomes a 190mm f/2.8 lens on my full-frame EOS 5D and a very useful, 300mm f/2.8 with very close focusing. Of course, it has the additional benefit that it is far more compact than any normal 300/f2.8 and still performs superbly.
On my Canon EOS 400D (APS-C), I often use the converter paired with a superb Sigma EX DG 100-300mm f/4 zoom, producing the equivalent of 224-672mm f/5.6 zoom lens which still retains the full AF facilities. Now, try to imagine just how much such a beast (if it existed!) might cost? You get my point? I have used this combination on my full-frame Canon 5D as well, although this is no longer necessary as I have recently purchased a Canon EF 100-400mm f/4.5-f/5.6 L USM zoom which effectively, duplicates the previous Sigma 100-300/1.4x pairing. However, I have just carried out exhaustive testing of my new Canon 100-400 L lens with the converter and I am extremely impressed with the results. This combination provides a lens with an effective zoom range of 140-560mm, albeit with reduced aperture values of f/6.7 to f/8 which prohibits auto focusing. Happily, it's no problem using manual focus with this combination, The extremely limited depth-of-field making the image appear to literally 'snap' into sharp focus visually.
I have also experimented using the same combination on my APS-C 400D, when it effectively provides a zoom range of 224-896mm, again at reduced aperture values of f/6.7 to f/8. This provides a huge range and still produces very high quality indeed, thanks to the cropped effect of the APS-C sensor utilising only the central 'sweet spot' of the image circle.
Performance, with all the variations discussed above is really extremely good. Several of the lenses mentioned do not appear to lose any quality whatsoever when used with this converter. Others only very slightly. Using a converter magnifies any lens flaws or faults and providing you match the converter with good lenses, you will be guaranteed good results. The only thing I have found with ALL these combinations is that diffraction, which is evident at the smallest apertures of all lenses, is considerably more noticeable when the converter is attached. This is however, completely normal and only to be expected.
If you have lenses in your kit which are compatible with this converter, then get one. Very highly recommended.
I am so impressed, I am now considering purchasing the Sigma EX 2x APO Converter. I know you lose two effective f/stops but hell, it's got to be fun hasn't it?