Transmission, Aperture, and Focal Length in Low Light Situations

Question

I've been looking for an affordable lens I can use for nighttime shooting, including astrophotography. The two lenses I have found that seem best suited for this are the Samyang 16mm f/2 and the Samyang 14mm f/2.8.

Simply looking at the aperture it seems I would get another full stop of light from the 16mm f/2, however looking at the charts on DXOmark I see that the 16mm has a transmission of 2.4TStop while the 14mm has a total Transmission of 3.4TStop.

I don't know much about transmission, is this simply because of the wider field of view of the 14mm, with the 16mm still providing more light considering only what's within frame of it. Or would the 14mm actually provide more light overall, perhaps due to the quality of a higher quality of the elements within it despite the slower aperture?

Succinctly : If I where to shoot the same image with both lenses at the exact same settings and crop the 14mm image to match the 16mm, which would be brighter? Why?

I wonder if it might also make a difference that the 14mm is full frame whilst the 16mm is specifically designed for APS-C. (I would be using an APS-C format camera for reference, 700D)

Answer 1

Brightness in terms of exposure is always about light per unit area. If you are using both lenses on an APS-C camera the larger size of the image circle projected by the 14mm lens is totally irrelevant.

T-stops are similar to f-stops: the lower the number the brighter the lens. A lens with a transmission of T3.4 is dimmer, by about one stop, than a lens with a transmission of T2.4. Please note the transmission of each lens is measured with the aperture wide open. When stopped down, the transmission will change. For most lenses (but certainly not all), the transmission will be closer to the actual f-number at apertures narrower than the lens' maximum because most manufacturers fudge the number of the widest aperture by rounding it down (brighter) to the nearest 1/3 or 1/2 stop. Some will even round it to the next brightest whole stop!

If you shoot with both lenses on the same camera with the same ISO and shutter time and the apertures of both lenses are left wide open, the 16mm f/2 lens would be one stop brighter than the 14mm f/2.8 lens. This is assuming the slightly wider parts of the field of view for the 14mm lens are equally bright as compared to the field of view shared by both lenses.

If you shoot with both lenses at the same aperture setting, you would expect the two images to be equal in brightness. What minor differences there may be would be due to the accuracy of the aperture diaphragm of each lens.

Answer 2

DXOMark defines and reports Transmission as:

Do lenses affect light transmission? Photographic lenses are made of several elements of very pure glass, each element being carefully coated to limit reflection. However, a portion of the light is lost in the optical system due to residual reflection and absorption. The transmission factor (the proportion of light that actually makes its way to the sensor) depends on the optical formula (the number of lenses, the glass and coating formulas) and to a lesser degree on the shooting parameters.

Reference: https://www.dxomark.com/About/In-depth-measurements/Measurements/Light-transmission

If I where to shoot the same image with both lenses at the exact same settings and crop the 14mm image to match the 16mm, which would be brighter? Why?

The 14mm would be brighter, not just because of the difference in transmission, but also because the 16mm would be stopped down to f/2.8 to achieve the same settings for both. Therefore, in addition to the difference in transmission, an additional stop of light would be lost due to the change in aperture.