Note from Amin: These posts were originally posted in this thread and moved here to their own thread. The speed of the lens is dictated by the circumference of the aperture opening based on the f-stop formula. The purpose of the f number is to show the size of the opening of whole of the lens so to speak to allow light to expose the sensor to form an image. It is this opening that dictates how much light passes through. Which is why f/stop is so important in photography. But there seemed to be a drive to go with T-stops even with professional photographers and that's where the confusion comes in. Somehow you can bend physics by just magically raise or lower T-stops, but reality only dictates that the physical opening (the size of hole of the lens and the length of the lens) dictates the amount of light that hits the sensor. For example. Let's take a close look at 2 lenses. 35mm @ f/2.8 and 200mm @ f/2.8. The 35mm @ f/2.8 has an aperture opening of 12.5mm (35mm divided by 2.8), whereas the 200mm @ f/2.8 has an aperture opening of 71.42mm. But then you wonder, how the heck you say 200mm @ f/2.8 allows the same amount of light as 35mm @ f/2.8 when you have a bigger opening with the 200mm lens?!? 71.42mm hole vs 12.5mm hole. If you look closely, you will notice that the 200mm lens is LONGER than a 35mm and this length acts like a long extension tube which reduces light, so the amount of light that 200mm lens lets through will eventually be the same as the amount of light as a 35mm lens of the same aperture opening. The T-stops rating basically identify the "APERTURE" blades efficiency by how accurately it stops down the lens to that specific aperture. The confusion comes, in which I'm noticing at lot in those so called marketing guys standing around in trade shows, that they use T-stops as though it replaces f/stops. And with this confusion, they don't realize why the lens is designed this way and why is it so big. There's no confusion if a guy actually knows what he is talking about. This stuff is even taught at least in photography classes 2 decades ago. It's common knowledge, it is immediate that the opening of the lens and length of the extension of the tube do not mirror a constant f/2.8. Having said that and just by looking at the lens itself, this is more likely a 35-100 f/3.5 to 5.6 or f/4 to f/5.6 simply by doing the f/stop formula calculation and by knowing Panasonic has the 12-32 pancake version. The openings of 35 @ f/4 and 100 @ f/5.6 requires only a short extension tube like the photo illustrated. Another possibility is a f/2.8 to f/4 lens, but the length of the lens is too short -- hint hint take a look at the Olympus most hated 12-50 to give you an idea about lens length.