Maximum aperture in video C4k full frame mode? 1.7 with a 1.4 lens?

[bpLeica]

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Folks I just realized that my SL 50/1.4 cannot be set to less than 1.7 when shooting MOV video in C4k... ditto with 24-90?? is this normal or my settings are wrong?

[frame-it]

at minimum focus distance?

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[bpLeica]

28 minutes ago, frame-it said:

at minimum focus distance?

Focusi distance doesn’t change anything. Basically in video mode maximum aperture is limited to 1.7 on the 50/1.4 and 3.2 om]n the 24-90 @28mm

Edited June 23, 2022 by bpLeica

[hansvons]

Interesting. T stop vs F stop might be the reason, as in "proper" filmmaking cinematographers think in T stops, which take the ND factor of the lens‘ glass in account. Depending on the lens design, a f 1,4 lens can be a t 1,7 lens (which is at the "dark" end of the T vs F delta). 
 

All that is moot of course, if your lens cannot open up fully in terms of physical aperture and not nomenclature. If that’s the case, there might be a technical reason, which I‘d like to know.

[frame-it]

11 minutes ago, hansvons said:

there might be a technical reason, which I‘d like to know.

there was something in the Q threads about similar behavior

[Slender]

14 hours ago, bpLeica said:

Folks I just realized that my SL 50/1.4 cannot be set to less than 1.7 when shooting MOV video in C4k... ditto with 24-90?? is this normal or my settings are wrong?

Its the "T" rating of the lens, available since the last update. The number can seem heartbreaking with our beloved 24/90 where the "geometrical" F2.8-4 aperture becomes a T3.2-4.5.

 

The f-number ("relative aperture"), N, is defined by N = f/E_N, where f is the focal length and E_N is the diameter of the entrance pupil (rather than the actual aperture system inside the lens).

"T" standing for "Transmission" (of light) of light rays accross the whole frame covered by the image lens circle. Put simply, you substract the vignetting from your theoretical "F" number to get an idea of how much light the lens is able to transmit from the front element AKA entrance pupil back at the rear exit onto your sensitive surface. You can calculate it by dividing the f-stop of the lens by the square root of light transmittance. So, let's suppose we have a lens with a maximum aperture of 2.8, and 90% of the light goes through. We divide 2.8 by √0.90. The resulting value is 2.953, which we can round up to T/3.0

The more reflections you can observe by looking the front element of a lens, the more light that is reflected back out and that won't make its way down the lens.

Summicron SL are said by DXO test to be T2.0 lenses, which suggest that their actual F number in design would be closer to a F1.8 than F2.0. I havent seen what Leica rating for that is, all one has to do is try it on their SL2(s) with latest software.



 

Edited June 23, 2022 by Slender

[Photoworks]

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46 minutes ago, Slender said:

Summicron SL are said by DXO test to be T2.0 lenses,

The summicron-SL read T2.3. 

 the zoom 90-280 starts at T3.5

[bpLeica]

1 hour ago, Slender said:

Its the "T" rating of the lens, available since the last update. The number can seem heartbreaking with our beloved 24/90 where the "geometrical" F2.8-4 aperture becomes a T3.2-4.5.

 

The f-number ("relative aperture"), N, is defined by N = f/E_N, where f is the focal length and E_N is the diameter of the entrance pupil (rather than the actual aperture system inside the lens).

"T" standing for "Transmission" (of light) of light rays accross the whole frame covered by the image lens circle. Put simply, you substract the vignetting from your theoretical "F" number to get an idea of how much light the lens is able to transmit from the front element AKA entrance pupil back at the rear exit onto your sensitive surface. You can calculate it by dividing the f-stop of the lens by the square root of light transmittance. So, let's suppose we have a lens with a maximum aperture of 2.8, and 90% of the light goes through. We divide 2.8 by √0.90. The resulting value is 2.953, which we can round up to T/3.0

The more reflections you can observe by looking the front element of a lens, the more light that is reflected back out and that won't make its way down the lens.

Summicron SL are said by DXO test to be T2.0 lenses, which suggest that their actual F number in design would be closer to a F1.8 than F2.0. I havent seen what Leica rating for that is, all one has to do is try it on their SL2(s) with latest software.



 

Thank you this clears it up. I didn’t notice in the FW update notes about this change which is good when shooting video.

[Slender]

17 hours ago, bpLeica said:

Thank you this clears it up. I didn’t notice in the FW update notes about this change which is good when shooting video.

It was promised from launch, and then quietly forgotten for a while. The more lens elements the better the quality of the optic, but yeah it becomes harder to channel of the "perfected" light rays through so many layers of glass and air (18 elements in 24/90.... and like 23 from memory? in 90-280)

Edited June 24, 2022 by Slender