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

Started Feb 17, 2014 | Discussions thread
tt321 Forum Pro • Posts: 11,596
Re: Effective vs Theoretical way of measuring aperture.

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.

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 ...

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.

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.).

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