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

Started Feb 17, 2014 | Discussions thread
Dheorl Veteran Member • Posts: 3,786
Re: DXO : t-stop is the effective way of measuring aperture.

Pixnat2 wrote:

Dheorl wrote:

Pixnat2 wrote:

I'm interested on a 25mm for my E-M1. We have two native 25mm now : a f/1.4 and a f/1.8 lens.

But there's some questions about the fact the Panasonic 25mm is a real f/1.4 lens :

That was in a very uncontrolled environment making it hard to tell how much was down to the camera making correction or just metering slightly differently. For example is the olympus does stronger vignetting correction on it's own lenses than the panny the metering in total will measure the scene differently.

True, but Robin takes real life pictures and they are much more important to a photographer than a wall or ruler picture to assess what a lens can do.

Yes, but also a very bad way of making claims about a very technical aspect of a lens. I don't disagree that real life photos are a very good way of getting a feel for how a lens renders an image, I'm just saying they're not a good basis for those sort of claims.

I'm very doubtful of that test. For instance different looking bokeh can cause the DoF to look different. If someone took a photo of a target with a front/back ruler like lenstip (and probably other review sites use) then we might actually be able to compare them accurately.

You're probably right about it.

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.

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.

So I think it's worth watching their findings for each lens-sensor combination

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