[Rob Barron]

I have tried both the Sigma 105mm and the Canon 100mm, both fairly extensively as I was leant them by a friendly retailer to decide upon which one to buy.

I don't think there is a lens of similar price that can touch the Canon for sharpness, it is stunning. The bokeh of this lens is excellent as well and the contrast between the soft backgrounds and the sharp main subject is enough to make your eyes bleed!

The Sigma is sharp too, no problems on that front. It is very contrasty but no problems with that. My main problem with the Sigma was the autofocus on it. Now, for people who want to use the lens ONLY for macro, they won't be bothered as the vast majority of macro fans tend to use manual focus anyway. But if you want to use this very capable lens as a short telephoto as well, and why not given that on a DSLR it is effectively a 160mm f2.8 lens, the hunting will get annoying. It drove me nuts in just a few days.

The Canon has a street price about £80 above the Sigma but I would still go for it if your budget allows. The USM is near-silent and well worth having. The Sigma does not have HSM and overall this shows in the AF being less than snappy. The clarity of the Canon lens is second to none and I use the Canon MR-14EX ringflash which attaches quickly and easily. I cannot say whether this ringflash attaches to the Sigma, never tried that.

Horses for courses really. Is the Canon worth its money? Without a shadow of a doubt. Is the Sigma a good lens? Certainly. These are both lenses that will produce sharp macro shots at true 1:1. They both have f2.8 though I must say I felt the bokeh was smoother on the Canon. I tried this by taking shots with both lenses of identical subjects and printing them out. There is a difference, not just with a microscope!

I would definitely advise against the 60mm macro lens for reasons already stated: you can't get close enough to insects and other small wildlife without them fleeing as soon as you get up close. The 100mm lets you stay back and get the shot you want.

Would I recommend a zoom with macro facility? Not if you can afford to avoid them. Why? Well, a few reasons really:

~ They are not prime lenses and so are never going to be quite as sharp even if it's a top quality lens.
~ The aperture will invariably be smaller than you can get on a prime lens. Macro shots work so much better when you can through the BG right out of focus and keep all the ettention on the sharp subject . This is harder when the maximum aperture is often f5.6 or smaller.
~ Zooms with macro are rarely true macro. Macro is generally accepted to be 1:1 life-size reproduction of the subject on the sensor. As an example, the Sigma 75-300 APO Macro Super, a very good lens for its money, calls itself a macro lens but only does 1:2, half life-size. I have even seen some macros that only do 1:4, that is really just a close-up lens.
~ Zooms with macros have the macro at their telephoto end. The lens mentioned above can only switch to macro between 200 and 300mm. Whilst having a bit of distance is excellent, that is a bit too far, especially when combined with a DSLR and multiplied by 1.5/1.6

Having said all that, if your budget is restricted to buying one lens at the moment and you need to cover a good focal range as well as have macro, definitely get something like the Sigma and if you want to get in closer, stick a +4 dioptre close-up filter on the end!

There is quite a range of choice out there but if you aim to specialise in macro, I think it is fair to say you need a macro lens. If macro is something you are enjoying as one genre among many, perhaps one of the compromise options is what you need.

Cheers,
Rob