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

Started Feb 17, 2014 | Discussions thread
tt321 Forum Pro • Posts: 11,444
Re: At those apertures 0.1ev is significant

Pixnat2 wrote:

tt321 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.

Either way, I wouldn't worry about it. Since Olympus is the cheaper of the two, I just buy it over Panasonic.

You are right.

I looked at a number of FF 50/1.4 lenses (Canon, two versions of Nikkors, and a Sony) and they are all T1.6 so the PL25 is a bit worse in class, but not too radically different.

When we think about the price difference between, a f/1.4 and f/1.2 lens, it's like 500$ for 0.1 EV

More seriously, the claimed difference between f/1.4 and f/1.8 is 0.4ev. It's not a good surprise if it's reduced to 0.3EV is reality (supposing the Oly 25mm will be 2 Tstop)

This also depends on how the manufacturers and DXO do their rounding. If you put the PL25 on a camera and monitor the F stops the camera can set, you will see 1.4, 1.6, 1.8 and 2. So in their terminology 1.4 is 1/3 EV faster than 1.6 which is again 1/3 EV faster than 1.8 which is 1/3 EV faster than 2.0. There is no 1.7 in spite of Panasonic liking that number - and disliking 1.8, on their primes. On the other hand, presumably DXO has no such obligations and puts their T numbers where the nearest approximate is. So T1.7 is better than T1.8, but this 'better' could be between almost 0 and almost 0.2 because of the precision of the reported T numbers.

According to this patent, the PL25 is actually F1.44319, any narrower it should be called F1.5! It's also slightly longer than 25, at 25.6692. So if the Olympus turns out to be 23.5571 and F1.75812 just to throw some arbitrary numbers around there could be only F0.3 in the difference. And this gets translated to T0.3 difference with one at 1.7 and the other at 2, very roughly speaking and massively into the realms of speculation now...

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