SIGMA threw out the rulebook with the Classics. In their effort to deliver a look reminiscent of vintage lenses, they built a limited number of lens sets based on their excellent FF High Speed primes, but with the Classics, they took a completely different approach to the coatings.
Whereas the "standard" SIGMA High Speed FF primes have modern multi-layer optical coatings, the SIGMA Classics are only coated on the front and rear element, and the rest of the elements are left uncoated. This results in lenses that have extremely low contrast, and are highly sensitive to flaring and ghosts. The lack of coatings also affect how much light the lenses transmit to the sensor. So instead of most of the lenses being T1.5 like their SIGMA FF High Speed cousins, the Classics are all T2.5 or slower.
Overall milky image.
Overall milky image.
No lenses are better suited to explain the difference between f stop and T stop as the SIGMA FF High Speed primes, and their siblings, the SIGMA Classic primes. Both sets of lenses start out as the same SIGMA optical designs, and both are f1.4. Both deliver images with the same depth of field and background blur. So why are the Classics so slow? The reason is that most of the elements in the Classics are uncoated. In fact, from what I have heard, only the front and rear elements are coated, and the rest of the optical surfaces are uncoated. Lens coatings allow more light to transmit though the lens to the sensor. Elements with no lens coatings, reflect more light, hence the slower T stop.
As far as their image is concerned, the Classics look nothing like the "standard" version of SIGMA cine lenses. They have extremely low contrast, and are unbelievably susceptible to flare, ghosts, and reflections. Even our white slate, caused the lens to completely bloom. It's a very strong effect. There is nothing subtle about it. If you are considering these lenses, test them for sure. The overall look is a bit too much for me, but where I think the lenses have something to offer, is the way they flare. As you can see from our tests, they flare BIG, and they produce white ghosts. It's a unique look, and it's important to remember that the ghosts will take on the color of your light source. So if you are shooting a deep amber sunset, you will get deep amber flares/ghosts. If you gel a flashlight cyan, you will get cyan ghosts.
The SIGMA Cine FF Classics provided a whole-lot of flavor. These particular lenses have no coating on the glass so almost anything flares them! Even our dimly lit light bulbs just behind our model presented some interesting flares. If you're looking for something solid and sharp yet with a lot of character and flare artifacts for a dream sequence, party-scene and so-on, these are worth a look!
The SIGMA Classic Primes are also a personal favorite of mine. I had long wanted to properly produced set of primes with little to no AR coatings. SIGMA delivered! Only the front and rear outward facing surfaces are coated (for protection) and the look these primes provide is very, very unique. The trend of polishing the coatings of one or two elements was a waste of time in my opinion.
Going full tilt like SIGMA did was a huge risk that I believe paid off. The images that these lenses produce are very unique and can be a powerful storytelling tool in the hands of the right cinematographer.