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I have owned a lot of macro lenses, the canon 100mm, the 100mm L, the sigma 150mm, and the tamron 90mm
all have their foibles, the canon L's foible is only it's price.
When the only bad thing you can say is that it's expensive, you know you're reviewing a quality product. The IQ from all the macro lenses is exceptional, all are equal. The AF is a little better on this L version, very quick, the canon non L and sigma 150mm are about equal, and the tamron is noisy and slow.
The 4 stop IS is mainly useful shooting the lens as a portrait lens, allowing you to drag the shutter and let in more ambient light- if you shoot with strobes then both canon lenses make good portrait lenses. Don't bother shooting sports with IS on, 1/30 is too slow for most moving people.
The best thing about the 100l is the improved bokeh- the non L has a octagonal apperture at small appertures, this can make your bokeh unattractive and it actually ruined a lot of my images, the 100mm L is CLEAR WINNER on this factor alone.
If you're on a strict budget, and you're only shooting bugs with strobes at f22 then the non L is a good option, but if you want a more versatile lens the L version is the way to go
This is a 100mm Canon prime lens intended for macro photography (generally photographing still objects with a very short depth of field).
These large Canon prime lenses have a habit of being very heavy, however the first thing I noticed when picking this up, has how light it was, compared with many of the L Series range. It's a big lens & is probably not ideal for carrying around casually snapping some pictures, but it definitely has it's uses. The plastic build quality is the reason for this, however it isn't a major fault & doesn't really affect your use in any way.
The zoom ring is very smooth which is essential for this kind of lens where the depth of field is very shallow. The margin for error is very small so you need to know you can rely on the lens to work for you in critical moments, in my experience this lens will do that. I don't typically use macro lenses regularly, I only hire them when needed for a job, however I have worked on a number of product photography shoots in my time, where this kind of lens was used extensively.
The image quality is as high as you would expect, although it is obviously related to the camera body you're using the lens with (a better body will likely render a better result to a cheaper model). Images are sharp with wonderful rendering of colour, and live up to the high standards set by all Canon lenses.
The autofocus is fact & accurate as usual with Canon lenses, although as it's not the top of the shelf lens in this category, there is probably room for improvement, but I have not experienced any issues. Although, my use has mainly been in a very controlled environment, with a slow method of working. In a more fast paced shooting environment, the focusing speed would be put to the test in a more intense way.
The lens comes with IS (Image Stabilisation), which improves the sharpness of the camera when handheld, meaning camera shake & blurring is minimised. I haven't used a similar lens without IS so I can't really comment on the difference, but I can say I didn't encounter any issues with image quality in this area.
While this is a very good lens, it has quite a narrow field of use & I would think carefully before investing in it. I prefer a more multi purpose lens personally, but it depends on the kind of photographs you want to take.
Lens construction: This is an L lens and it's very well built, as you would expect from Canon's premium lens range. It's surprisingly light, however, which is handy for steady shooting for long periods without tiring, but the plastic construction (apart from the metal lens mount, where you NEED metal to protect against wear) makes the lens feel a little cheaper than its L-series brothers. The focus ring is smooth and is capable of tiny adjustments - vital for macro work where the depth of field is so low.
Features: Focus limiter which works very well for 0.3 - 0.5m, as this lens is also a great performer as a short telephoto so it would be annoying to have to wait for it to cycle through the whole range. I have experienced this on macro lenses without a limiter (Pentax DFA 100mm f2.8 Macro) so its inclusion here is welcome. Autofocus is accurate and fast, though not significantly better than other macro lenses which would be a real trump card over the cheaper non-L, non-IS alternative. It's also got instant override, so you can make tweaks yourself - vital for macro work where the depth of field is so low.
Image stabilisation: this lens uses a unique IS system designed specifically for macro, with correction for the frame pitching movements that are magnified by taking close-up pictures. It works excellently when I've had occasion to use it, though if you're using flash for macro it's not necessary. Also it doesn't counter movement within the frame, so a light breeze amongst the flowers you are shooting will still render your shots blurry despite the IS. See below for comparison to the Canon alternative lens, the 100mm f2.8 Macro.
Image quality: World class. Colours and saturation are superb, but the big hitter is the sharpness - perfect, so-sharp-you'll-cut-yourself edges with no noticeable aberrations either. Truly a great lens at a focal length and aperture that lend themselves to high quality (aperture doesn't go too wide, focal length is not loo long or too short)
Comparison: The main comparison is the non-L, non-IS, Canon 100mm f2.8 macro - which also has brilliant image quality. Those using APS-C sensor cameras could also consider Canon's 60mm macro. Neither have IS, and I would say that's not a huge issue, so unless you really feel you'll need it they both seriously undermine the 100mm macro L IS.