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

Started Feb 17, 2014 | Discussions thread
OP Pixnat2 Veteran Member • Posts: 5,103
Effective vs Theoretical way of measuring aperture.

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.

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.

That's important to know when you buy a fast lens to shoot in low light, as it will affect your shutter speed and/or ISO (depends on which mode you shoot).

That's what DXO calls theoretical (f-stop) vs effective (t-stop) "aperture". I agree that the term "apreture" is not very well choosed, but I'm sure you understand what they mean!

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