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With all of the talk of the Sigma 120-300 of late on the forum I borrowed one and ran it through a number of tests compared to the Canon 300 f/2.8L IS.

Tests were done using an EOS 1Ds Mark II at f/2.8 and f/8. The Sigma was zoomed to its longest focal length (marked as 300mm) for all tests:

Focal Length:
Either the Canon is much longer than 300mm or the Sigma is nowhere near it. It has often been documented that 300mm is optimistic for the Sigma on the long end but I was shocked by how much. If we assume the Canon Prime lens's focal length to be correct at 300mm than the Sigma's focal length at the longest 300mm setting is only 259mm. I checked the zoom ring and optics for binding and the lens was operating correctly. My conclusion is that the Sigma is nowhere near 300mm and should be considered a 120-260mm f/2.8 zoom lens. Just to double check this, I mounted a 1.4x on my 70-200 f/2.8L lens and as expected, the reproduction size was actually larger on the 70-200 with 1.4x (280mm) than the Sigma at 300mm. Somebody in marketing got way optimistic on that one.

Resolution:
Using the standard resolution chart, the Canon and Sigma's are equivalent in the center with the Canon having a very slight but visible edge in resolution and sharpness at the edges and in the corners. While under most circumstances this would not even be noticeable, there is a difference. The difference is measurable at both f/2.8 and at f/8 although at f/8 it is nearly negligible.

Contrast:
Contrast is marginally better on the Canon but not enough to make a real world difference

Distortion:
The Sigma exhibited ever so slight Barrel distortion at 300mm although less than the 70-200 with 1.4x exhibited. The Canon was nearly perfect.

Chromatic Aberration:
Negligible on both lenses

Flare:
A subjective test gives the nod solidly to the Canon lens with less and better controlled flare.

Performance with Canon 1.4x II:
Both lenses performed well with significantly higher magnification on the Canon due to Sigma's mis rating of the focal length.

Overall, the Canon 300 f/2.8L is far superior to the Sigma primarily due to the much longer reach and secondarily due to slightly better optical performance in every test. Overall the Sigma is a very sharp and versatile lens with some flaws such as not having a focus limit switch but it is not what it is marketed as - a 120 to 300mm lens, unless the Canon is actually a 340mm lens. However tests against the 70-200 w/ 1.4x and 300f/4 shows the Sigma to be the one that is inconsistently rated for focal length.

That is astounding, E.J. I have heard some of the 70-300 lenses as being closer to 70-285 in terms of field of view, but 260 ...

Eric

Hey E.J., Do you think that this might be or could be limited to the lens you borrowed or do you think this is going to be the same with all the Sigma 120 - 300 lenses of same model?

I have read many places where people say the lens is really a 270 or 280 but this one measures out at 260. I don't think its unique to this sample. This is assuming the Canon 300 ni actually 300. If its 310 then 270 is reasonable.

Thats disappointing to see. You'd expect it to at least be a 285 or 290..not 260...

Thanks much for running that test, E.J.

260mm down from 300mm is a pretty significant amount for marketing slippage. That's a major downer. I wonder if the Sigma is really 120mm on the short end, or is that optimistic as well?

The optical performance doesn't sound too bad considering expensive Canon prime vs. cheaper Sigma zoom. And I suppose 520mm @ f/5.6 (with a 2X TC) with a short minimum focus distance beats my current hand-held limit of 400mm @ f/5.6, if the resolution & contrast are at all comparable. Hmm...

A little more insight from measuring the front elements of these lenses. the sigma is only 96mm across so in order to be a true f/2.8 lens, the focal length can not be more than 268mm. If the lens were any longer, it would no longer be a f/2.8 lens. Perhaps this is why sigma chose to not make it a true 300mm lens. Interestingly enough, the Canon front element at about 102mm would restrict that lens to 286mm at f/2.8 so I'm guessing the lens is actually more like f/2.94.

Is it legal for them to advertise this leans as a 300mm on the long side of the zoom? Doesn't seem right to me.

NOTE: by no way am I countering what E.J. is saying, just thought I would share an observation.

The focal length shortfall of the Sigma is amazing! I am always dubious of Popular Photography's lens tests (since they get a lot of advertising money from many manufacturers - including Sigma), but thought I would check what they say. For the "Sigma 120-300mm f/2.8 EX APO IF HSM AF "they claim that the 'tested' focal length is 123.60mm - 293.86mm with a 'tested' maximum aperture of f/2.88 !! Pop Photo notes that the zoom ring turns 80 degrees clockwise.

To make it more interesting, Pop Photo has no test for the "Canon EF 300mm f/2.8L IS USM" (or even the non IS). They do have a test fro the "Canon EF 70-200mm f/2.8L IS USM". The 'tested' focal lenth is 72.89mm - 190.22mm with a 'tested' maximum aperture of f/2.94 .

It is always nice to have real world comparisons to compare to published reports.

Thanks again E.J. !!

Hey EJ,

Great test and report. I find your conclusion somewhat startling though .. in every test, other than focal length, the differences were 'neglible' (other than flare, as expected on a zoom I suppose - and something which is rarely problematic under normal use) .. but I was kinda blown away when I read that you said the Canon lens is "far superior" overall, due to its longer focal length, when your own tests showed neglible differences across the board. Further, you apparently give no weight to the versatility of a lens that ranges from 120mm to 270mm and say the prime is far superior' overall. My opinion would be that if a zoom lens comes up as practically as sharp as a prime (particularly one so highly touted as the Canon 300mm f2., and with contrast, chromatic abberation and distortion almost the same, that the zoom would be a superior lens, but that's just me. Further, the Sigma lens retails for exactly half the cost of the Canon . I'm not sure why you would then say the Canon is a far superior product.

For example, you use Canon zoom lenses under 200mm, correct? But I'm sure there are prime lenses available in almost all standard focal lengths which are sharper, and outperform the zooms in every category. Yet the zooms are so versatile and handy to have, that this, for most people, more than offsets other differences.

Like I said, I appreciated the test, and it seems to be, as expected, very thoroughly done. I was completely surprised when you then concluded the prime to be 'far superior', as measured by the results of your test.

Cheers

Carl

You may want to reread that last paragraph - I clearly say that because the Sigma's focal length is nowhere near a 300mm lens so if you want a 300mm lens, the Canon is far superior - its a simple as that.

Hey EJ,

I re-read your paragraph, and I still don't follow. At that distance, the focal length difference is clearly discernible, and certainly unreasonable for an item marketed as a 30mm lens ( and I agree Sigma should be called to task for deceit if the focal length is so unreasonably incorrect - but that's a separate issue) However, think of it this way. Would you pay 8 grand for a 340mm f2.8 prime lens if a 150-300mm f2.8 of similar optical quality is 4 grand? I wouldn't. I certainly wouldn't say that 30mm or 40mm at this range is critical, and, weighed against the benefits of the zoom range, more than reasonably offset in its benefit. Focal length is critical for wildlife photography, we agree on that, I expect. But there's nothing magical about the 300mm focal length that makes it far superior to the 260mm focal length.

If I can get a lens that's 270mm, or even 260mm, and delivers almost equivalent optical performance, costs half as much and weighs about the same, and on top of that, I get a focal range of 120mm to 160 or 170mm, then it's a no brainer for me. I might say that the prime is marginally better, and certainly longer, but I wouldn't claim it to be far superior. The IS is certainly a factor, but not like it is in a 500mm or 600mm.

As I said, I appreciate the testing methodology you engaged, and I have complete trust in your results. I just was amazed that your conclusions were what they were.

Cheers

Carl

Chris Fagyal wrote:Thats disappointing to see. You'd expect it to at least be a 285 or 290..not 260...

I think you're being very generous there Chris... I'd kinda be expecting 300mm!

Thanks for doing this EJ... but at the same time you get no thanks for making decisions more complicated than they seemed on the surface! Just to make life more interesting... any chace of you getting your hands on a Sigma 300 f2.8? I'd be interested to see what the marketing department has done with it!

I certainly wouldn't say that 30mm or 40mm at this range is critical

You would be absolutely amazed by the difference 40mm makes in image size in the viewfinder. I was using a resolution target which filled the frame for the Canon lens, with the Sigma, there was significant space around the target - I mean its really noticeable. Think of it this way, the image is 12.5% smaller linearly which comes out to 26.6% less area from a lens that is supposed to have the same focal length

Thanks for the test. I was considering this lens. No way now. The long end of a tele zoom is the most important.

GeneC wrote:I was considering this lens. No way now.

Its actually a great lens, one of the best long zooms available but it is misrepresented

thanks for the test.. I was actually considering this lens and decided to do a search here for it when I found this post.
The problem is that there seems to be no other alternative for a 300 2.8 other than the Canon and Sigma primes (even though the zoom is only 270).

Did you happen to test them both with the 2x converter?

yuri wrote:For the "Sigma 120-300mm f/2.8 EX APO IF HSM AF "they claim that the 'tested' focal length is 123.60mm - 293.86mm with a 'tested' maximum aperture of f/2.88 !!

It is always nice to have real world comparisons to compare to published reports.

Thanks again E.J. !!

For the Poop Photo measurements to be accurate the front element would have to be at least 102 mm in diameter. E.J. measured the front element at 96 mm which means either E.J.s measurement of the diameter was wrong or one or both of the Poop Photo measurements must be wrong.

I trust E.J.s measurement as it's not difficult to measure the diameter of something.

Needless to say, I was surprised by E.J.'s finding that the 120-300 wasn't really "120-300". The strange thing is that when I look at my EXIF data taken with that lens and a Nikon D2H, the data reports the lens to be "122-300". Of course, I have no idea how accurate EXIF data is. Either way, I will continue to consider this Sigma lens to be among the best lenses I have used as far as versatility in the field and sharpness of images captured.

Keep in mind there are also other important items not covered in this set of tests, such as IS (or lack of IS) and autofocus tracking ability, with and without extenders.

Eric