Zuiko 14-42mm ED EZ off by 1 stop

[MJ2020]

Hi,

I am new to this forum.

I recently bought an Olympus E-PL6 to use as a light meter for my medium format camera: a Bronica SQ-A. The reason I chose this system is because its portability, the ability to set a 1:1 aspect ratio on the viewfinder, live histogram and the articulating screen. I have an 80mm lens on the Bronica which is equivalent to a 25mm in MFT. So I went and got a 7artisans 25mm manual lens. All things good so far.

Recently, I acquired a Zuiko 14-42mm ED EZ because I want to have a lens with auto focus. In order to get the same histogram with both lenses (at same aperture, ISO and focal length) I need to set the Zuiko to a lower speed (1 stop). I tested with a light meter under controlled light and the manual lens is correct. (Also tested with a Canon 5DmkIII)

What is going on? Bad lens? Hope someone can explain.

 

[Derrel]

Camera ISO off a bit? Lens T-stop not aligned too well with fstop, or maybe a little bit of both?

 

[MJ2020]

Thanks for the reply Derrel.
ISO is fixed at 200. Lens mounts perfectly so I looked at T-stops which is something I was not familiar with. Maybe the answer is here:

Olympus M.ZUIKO DIGITAL ED 14-42mm f3.5-5.6 - DxOMark

If I am interpreting the data correctly, this lens has less transmission at 25mm which is the focal lenght I was testing. Is this correct?

 

[VidThreeNorth]

The chart shows the nominal F-stop at 25mm as 4.8 and the T-stop is a perfect match (4.8). Ironically, I think 25mm is the only focal length that has a perfect match. But the T-stops reported were all very close to their nominal F-stops, and I would call them "acceptable" performance". I don't know what's going on. . . .

 

[MJ2020]

Thanks.
This is the chart (attached) I looked at. I will test again at different focal lengths.