7Artisans 35mm f/0.95 -- is it really f/0.95? Reliable way to find that out?

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Tim van der Leeuw Contributing Member • Posts: 517
7Artisans 35mm f/0.95 -- is it really f/0.95? Reliable way to find that out?

Hi,

I watched a video on YouTube by Andrew & Deanae where he plays around with a set of 3 cheap manual-focus lenses, including the TTArtisans 50mm f/1.2 and 7Artisans 35mm f/0.95 which of which I own, and he made a comment somewhere near the start that piqued my interest, about measuring the actual light-gathering abilities of these lenses.

Here's the link to the video:

https://www.youtube.com/watch?v=ac3Qv_NHzTA

So I played around a bit with both lenses on my X-H1, setting the camera in spot-metering and setting a fixed ISO and shutterspeed, and checking the under/over exposure indication on the top plate.

I chose some settings so that with the available light and the chosen target, at f/1.4 both lenses gave a reading of +2 EV and -5EV at f/16 (their narrowest aperture).

I should have chosen a different target and higher overexposure at f/1.4 I think, the EV under/over exposure was not always fully reliable but some observations were repeatable.

Anyway, between f/1.4 and f/16 both lenses seemed to have the same light-gathering ability and although I observed minor repeatable irregularities, that could have all been due to my improvised amateurish test setup.

Also, the exact aperture for the 7Artisans 35mm is hard to determine because it's aperture ring is clickless so I have to guess from reading the markings where I kind of am.

But f/16 is a hard stop, and settings around the f/1.4 marker mostly give the same EV reading on the camera.

So far so good. But now. F/1.4 is not the widest aperture for both these lenses. What happens when opening the lens wider than f/1.4 is what made me scratch my head.

On the TTArtisans 50mm f/1.2, opening the aperture from f/1.4 fully to f/1.2 changes the measured exposure from +2 EV to +2 1/3 EV, exactly what you would expect.

On the 7Artisans 35mm f/0.95 however, turning the aperture ring from around f/1.4 to 0.95 also made a measurable difference in exposure of only 1/3 of a stop... And I had expected the difference to be at least one full stop.

So is the 7Artisans 35mm f/0.95 really an f/0.95 lens? When looking at the lens, or through the lens, I certainly see the aperture blades more all across the range from f/0.95 to f/1.4 and on. But the camera doesn't pick up more light.

I tried different settings for ISO and shutterspeed to get a different starting point at f/1.4, both higher and lower base-exposures.

But the results didn't change.

And mind you, the camera is not aware of the lens aperture opening because both are fully manual lenses. So the camera is not having bugs due to unforeseen extra wide aperture openings or anything like that -- the camera is just measuring the available light coming onto the sensor.

That should have increased with over stop if you go from f/1.4 to something that claims to be f/0.95.

It works when going the other way, from f/1.4 to f/2 etc.

I see the aperture blades moving when turning the aperture ring, and they do not appear to be stuck. I see the aperture narrowing as soon as I twist the ring a teeny bit from its 0.95 setting. And I don't see how they could open any wider, at their widest they appear flush with the housing.

Now I know that f-stop is not an absolute measure of light-gathering ability, that would be T-stop. But I would expect the relative light gathering abilities between f-stops on the same lens to be following full EV stops?

Does anyone know of a more reliable way to test if a lens is really f/0.95?

Do I have some wrong expectations from what an f/0.95 lens could do?

Or is this lens not really what it claims to be?

 Tim van der Leeuw's gear list:Tim van der Leeuw's gear list
Canon EOS M5 Fujifilm X-H1 Fujifilm X-T3 Sigma 2x EX DG Tele Converter Canon EF 24-70mm F4L IS USM +12 more
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