Let's sort this out... Ever since an article went live over at www.newsshooter.com
I've had an email or a phone call almost once a day asking me if the new Sigma Cine Zooms
were parfocal. Or as many of the calls/emails referred to it: "parafocal, paranormal, parafocusing, paralegal", etc. etc. It's parfocal (par·fo·cal), okay?! It seems like the entire industry suddenly became hyper aware of this term which has been around for a very long time and overnight, everyone's a lens expert. Let's make sure we're all on the same page...
What does it mean for a lens to be parfocal? Basically, it means that the lens holds it's focus while zooming. If you set your zoom at the wide end, focus on your subject, then zoom in - the image should stay focused throughout the entire zoom range. This is something that cinema lenses take into consideration during the design phase and will compensate for and sometimes even have adjustments for after they're assembled. Still photography lenses are generally NOT parfocal, despite the occasional "well I got a good copy that IS parfocal"... Manufacturers can save a lot of money and assembly time by not including measures to keep a lens parfocal. Besides, with a still photo lens, you press the shutter button half way and the lens focuses instantly, regardless of where you have the zoom set. So it really doesn't matter in the still photo world. This brings us to the new Sigma Cine zooms. They ARE based on still photo lenses which would imply that they were not designed to be parfocal.
First we'll take a look at how we test for this. We use a simple chart to illustrate what we call zoom curve. Zoom curve is something that every zoom lens exhibits to a degree. Observing this curve will depend on how discriminating we are in the evaluation process. We use different tolerances for different lenses. For example, wide angle lenses have a much tighter tolerance than telephoto lenses, simply because of the sensitivity of flange depth on wide angle zooms. The unit of measurement is usually hundredths of a millimeter or thousandths of an inch and is obtained on our auto-collimator bench. This allows us to evaluate and record the discrepancy in zoom curve and adjust accordingly. Every lens that comes through Duclos Lenses for sale is subjected to this test, among many others, to ensure the lens is performing the best it can. Performing quality assurance tests at this stage gives us the opportunity to make any adjustments that are needed before delivering a final product from Sigma, or any other brand for that matter, to our customer. So let's see how the Sigma stacks up!
Pretty damn good! I picked a random 18-35mm from our stock and another popular $20,000+ zoom lens which will remain nameless as to not bash any brand or model (I also truncated the zoom range values to further hide it's identity). As I mentioned earlier, depending on the specific zoom range, we can alter the parameters to be more or less discerning. In this case, we're using a range of +/- 0.1mm. At wide focal lengths, if a lens exceeds the 0.1mm range, something is really wrong... Let's look at the Zoom Curve Charts below:
[gallery type="square" columns="2" size="medium" link="none" ids="5777,5778"]
If you still aren't quite clear as to how Zoom Curve works: flat=good, curvy=bad. So you can immediately see that YES, the Sigma Cine 18-35mm exhibits some Zoom Curve. This is why the phrase "nearly parfocal" has spread like wildfire. If all lens manufacturers were being honest, they would label every lens as "nearly parfocal". While this particular Mystery $20k zoom is obviously performing much worse than the Sigma, it's still not bad and would most likely go unnoticed, especially at smaller apertures. Moving on to some more details: our measurements give the Sigma a curve of +0.019 to - 0.009, a range of +/- 0.028mm. Not bad at all - in fact, quite respectable.
So why did Sigma call their new cine zooms "nearly parfocal"? I have no idea! My best guess is that Sigma, being new to the cinema lens market, is being honest to a fault. I'll continue to test additional lenses to monitor copy variation, but so far, the results shown above are consistent, even from the earliest models we received from the factory. I really don't have any motivation to prove that Sigma Cine zooms are or are not parfocal, but I felt obligated as a result of the influx of questions in regards to this particular feature, on these particular zooms. I could spend days comparing lens A to lens B and see who comes out on top, but then I'd have a bunch of enemies at the factory. Kinda like the T-stop Mega-Test that I scrapped, or the moral dilemma I'm currently facing in regards to the Celere tests... To answer the question "are the Sigma Cine zooms parfocal?"... No. No they're not. And neither are any other cine lenses if you look hard enough.