Is the Panasonic 25mm a real f/1.4 lens?

Started Feb 17, 2014 | Discussions thread
OP Pixnat2 Veteran Member • Posts: 5,121
Very interesting!

Steen Bay wrote:

007peter wrote:

I share the same thought as you (for the most part). However, you must keep your Expectation Realistic & Reasonable. A slight 0.3ev lost in T-Stop is to be expected. AFAIK, no lens live up to their T-Stop claim (or very very few do). Most manufacture measure them under the best LAB CONDITION, so in everyday "Real World", a slight 0.3ev lost isn't outrageous.

For example, the DXO Tested T Stops are:

  • 2.0 T-Stop for Panasonic 20mm F1.7 (II) | -0.3ev
  • 2.1 T-Stop for Panasonic 20mm F1.7 (I) | -0.4ev
  • 2.2 T-Stop for Olympus 12mm F2 | -0.2ev
  • 2.0 T-Stop for Olympus 45mm F1.8 | -0.2ev
  • 2.0 T-Stop for Olympus 75mm F1.8 | -0.2ev

Based on DXO results, I would say Olympus is more consistent with narrower gap between Lab Tested "claimed" vs real-world tested results. It would surprise me if the new Olympus 25/1.8 is tested exactly at 2.0 T-Stop.

We don't really know what the t-stop is for the lens itself. DxO's tests show the t-stop for a given camera/lens combination. For example, the t-stop of the 25/1.4 is 1.7 on E-M1 and 1.8 on E-M5. The article below shows how the t-stop of the same lens can vary considerably from camera to camera. The t-stop of the 25/1.4 could theoretically be 1.4 if it was tested on a camera without ' pixel vignetting'.

Thanks Steen Bay for your insight and link. The DXO article is very interesting. Their conclusion should keep us aware :

DxO Mark measurements are based on an assessment of the effective T-stop for every camera-lens combination. T-stop, a measurement widely used in the industry, especially the motion picture industry, is a measurement of the effective quantity of light transmitted by the lens at a certain f-stop. Thus, T-stop takes into account every reflection or absorption due to the lens. It can be considered as an effective (versus theoretical) way of measuring the aperture.

A possible conclusion of DxO Labs’ measurements is that photographers should consider with caution the maximum f-numbers advertised for lenses. Indeed, depending on the performance of their camera body and sensor, they may not effectively benefit as they expect from such wide apertures.


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