Lens Turbo: Don't get 1-stop advantage at f/1.4?

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pacolucia
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Lens Turbo: Don't get 1-stop advantage at f/1.4?
4 months ago

I purchase a LensTurbo (MD to E-Mount) to use with my A6000 and some legacy, manual-focus lenses. One of the main attractions of the LensTurbo for me was the 1-stop gain in light gathering ability. So I hear a f/1.4 lens becomes effectively a f1.0 lens, for example. So I decided to test this with a Super Takumar 50mm f/1.4 (with MD-M42 adapter and Lensturbo) and compare the results against a regular adapter.

To avoid metering issues, I fixed the A6000 ISO at 800 (Auto ISO OFF) and recomposed the scene to make sure I was metering an identical scene (the LensTurbo does change the perspective back to what you expect in a 50mm FF equivalent). Here's what I got for shutter speed at each aperture (as shown in the lens):

Aperture   No LT      With LT

F1.4           1/25s       1/25s

F2.8           1/8s         1/13s

F4              1/4s        1/8s

F5.6           0.4s        1/4s

F8             0.8s         0.5s

As you can see from the table above, there is a (roughly) 1-stop advantage when using the LT for apertures of f/2.8 or smaller. That is to say, I get a shutter speed about twice as fast. However, at f/1.4 there is no difference! This is pretty disappointing since my main purpose was to use this setup for low-light shooting. Thinking there may be a setup issue, I tried to change the metering to center-weighted instead of "Multi" as maybe the vignetting at f/1.4 influences the camera's meter. There was no change in behavior. Furthermore, I also tested a different lens, a Mamiya 50mm f/1.4 and saw the same thing.

Any thoughts on why I see this behavior? If anyone here that owns a LT could do a similar test for comparison it would be appreciated.

Thanks,

Paul

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