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

Started Feb 17, 2014 | Discussions thread
OP Pixnat2 Veteran Member • Posts: 4,767
Re: Effective vs Theoretical way of measuring aperture.

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.

You may be right on its importance but we need to understand there are invariants.


DXO defines t stop well in layman's terms :

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.

But the t-stop is not measuring the aperture, it's measuring the transmission so tbh I don't know what on earth they're talking about. I mean yes, if you're using it as part of an exposure calculation then it makes more sense to use t-stop, that however doesn't mean it's in any way measuring the aperture. You could get a lens and spray they front with a semi transparent material... it would still have exactly the same aperture and therefore f-stop but the t-stop would obviously be terrible.

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.

A lens avertised at f/1.4 can have a Tstop of 1.5 to 1.7 or more (numerous examples at DXOmark). The closest to the aperture, the better.

That's why DXO called effective "aperture", with the term "apreture" leading to confusion.

For example, the Panasonic 25mm is advertised at f/1.4 and have a t/1.7. The Canon 50mm f/1.4 has the same advertised aperture, but have a t/1.5.

Both have a theoretical "aperture" of f/1.4 on paper, but the Panasonic have an effective "aperture" of t/1.7 and the Canon of t/1.5.

That's not too hard to understand I think.

Also, when you buy a certain aperture lens you are probably hoping to use the widest aperture at least some time, and at least sometimes for DoF reasons. If it's only the light gathering it's probably more economical to get 1. some kind of stabilization and 2. better high ISO behaviour.

Of course!

Before AF, there was one other reason to buy fast glass. Faster glass allow faster and more accurate MF on MF cameras. Now that reason is less prevalent of course, or is it? Maybe a lot of people are using those fast CV lenses at stopped down apertures and the fast wide open gives them an edge in focusing (I don't know. Speculating again.).

Very true.

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

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