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

Started Feb 17, 2014 | Discussions thread
tt321 Forum Pro • Posts: 10,829
Re: Effective vs Theoretical way of measuring aperture.

Pixnat2 wrote:

tt321 wrote:

Pixnat2 wrote:

Dheorl wrote:

Pixnat2 wrote:

Dheorl wrote:

Lenses t-stop should never match their f-stop.

The Canon EF 24mm f/2.8 IS USM, Canon EF 35mm f/2 IS USM, Canon EF 28mm f/2.8 IS USM for example have 1:1 t-stop f-stop.

Then as stated elsewhere there is either a measurement error, canon are misrepresenting them or there is a rounding error. (For instance the lenses might actually be F1.95 which canon rounds up to f2 and DXO might measure f2.05 which they round down to f2.

That are suppositions from your side. You may or may not be right. But that's not the important point.

In principle, it is not possible to make a real precise F2 lens which also has a real T2 precisely - This implies 100% efficiency which we have yet to achieve in anything. Specifically for lenses, if you can see the glass when looking at a lens it's not 100% efficient - you will need to look at the lens and see a hollow opening - and only able to find the glass by touch - for that 100% light passing to have any hope to be true.

Of course. But an f/1.95 that reach a t/2.05 (specualtion) and rounded both at f/2 is better than a lens advertised are f/1.4 reaching f/1.7. Canon has made a fine job with those lenses.

Yes in this particular case. Although you may say that for someone who want exactly the DoF characteristics of F2 might be disappointed by results coming back from F1.95

You're absolutely right. An f/1.4 lens has an aperture of f/1.4.

What DXO states is that despite of the f/1.4 aperture, the (for example) Panasonic 25mm behave like a f/1.7 lens in terms of light reaching the sensor.

No, because no true F1.7 lens could achieve true T1.7. However, if the 1.4 lens should have a T number of 2.0 then ...

As the Canon exemples up there, we agree that a f/1.44567 can't reach exactly t/1.44567, but it can achieve, if it's well conceived, a t/1.54897. It will be advertised as an f/1.4 lens with a measured t/1.5.

T1.5 would be nice to have, but seems a very hard thing to achieve for F1.4 lenses, unlike T2 for F2 lenses. There has to be something relating to very fast lenses not having as fast T numbers...

Some video lenses are labelled with T numbers directly. Now if they miss the target that's really cheating.

Fast lens have other caracteristics than allow you to shoot in low light. But the word Nocticron or Noctilux should give us a hint for what they're made for

Even though the PL is labelled Leica, it has none of these other names, perhaps the designers know that the T number is nothing to write home about

By the way it appears the T number for the PL25 is at least 1/3 EV higher than that of the Olympus 1.8/50. The price for this 1/3 EV in the UK seems to be £40, or 10%. That's less than what the government collects on lens purchases...

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