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How can I compare low light performance for different formats? Are there some standard equations to do this?

For example, let's say I want to compare these three camera-lens combinations:

  • Nikon NIKKOR Z 58mm f/0.95 S Noct Lens on a Nikon D850
  • Fujifilm GFX 50R Medium Format Mirrorless Camera with the bundled 45mm lens
  • Olympus OM-D EM1 III with a Voigtlaender Nokton 0.95 42.5 mm lens

When the formats are the same, then F-stop gives a low light comparison, but if the formats are different, then it gets a lot more complicated obviously.

Note that when I speak of "formats" I am including sensor characteristics. So, the variables are: sensor sensivity, sensor size, and lens brightness.

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The f-stop is independent of the format. Same f-stop means that with a given light, you will have the same exposure time on equally sensitive film/sensors (same ISO). The f-stop already factors in the focal length...

This said, the F-stop spec is not very accurate, and the movie industry seems to use T-stop. Actual focal length (that can vary with focusing in some lenses), and actual ISO sensitivity of your film/sensor could also be taken in account.

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  • \$\begingroup\$ Probably worth noting though that most of the exposure settings, including f/stop, are often somewhat lacking in precision - f/2 on one camera/lens might be f/2.05 or f/1.95 on a different rig.... Especially since a 100mm lens might actually be 97mm or 103mm or something... \$\endgroup\$
    – twalberg
    Jun 1, 2020 at 15:03
  • \$\begingroup\$ @twalberg Not speaking of actual sensor sensitivity \$\endgroup\$
    – xenoid
    Jun 1, 2020 at 15:35
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    \$\begingroup\$ Close, but not cigar. F-stop is dependent on the format. A full frame f/4 lens has better low light performance than a crop f/2.8 lens, because the crop f/2.8 lens is equivalent to f/4.48, assuming the crop system has 1.6x crop factor. \$\endgroup\$
    – juhist
    Jun 1, 2020 at 17:29
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    \$\begingroup\$ It's probably not worth confusing things by bringing in T-stops. T-stop is nothing more than f-stop + a number to account for the light loss through a lens (i.e., sort of a low-value built-in ND filter amount). The reason it matters in videography is because you can't just compensate with more shutter duration in videography like you can in photography. So photography only cares about the aperture diameter aspect of f-stop (and related effects, such as DoF, etc.), because the shutter can just be held open longer to account for a lossy lens. That's all T-stop is about. \$\endgroup\$
    – scottbb ♦
    Jun 1, 2020 at 18:10
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How can I compare low light performance for different formats?

Easy. By calculating the 35mm equivalent f-stop.

The 35mm equivalent f-stop divisor is the divisor in the f-stop multiplied by crop factor.

An example:

  • You have 17-55mm f/2.8 lens for a system with crop factor 1.6x and 24-105mm f/4 lens for full frame.
  • The 24-105mm f/4 lens for full frame is simply f/4 equivalent.
  • The 17-55mm f/2.8 lens is f/4.48 equivalent because 2.8*1.6 = 4.48

Thus, this example shows that a full frame 24-105mm f/4 lens has better low-light performance than a 17-55mm f/2.8 crop lens.

Someone could complain that the same true f-stop, same shutter speed and same ISO gives the same exposure, and therefore, because shutter speed is limited by motion blur and ISO is limited by noise, the true f-stop (and not the 35mm equivalent f-stop) would tell the low-light performance.

Such a complainer would ignore the fact that on full frame, you can use higher ISO than on crop. For example, if you have a 1.6x crop camera where ISO 800 is the highest acceptable ISO and higher ISOs have too much noise, a full frame camera with equivalent generation of technology would allow you to use 1.6^2 * 800 = 2048 as the ISO.

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  • \$\begingroup\$ I don't follow your calculations. So, you are saying that the NIKKOR 58mm f/0.95 S Noct lens is f/2.8? I don't get that. You need to be more explicit in your answer how you are doing your calculations. \$\endgroup\$
    – Clickety Ricket
    Jun 1, 2020 at 18:11
  • \$\begingroup\$ So you are saying that for a given light, with the same ISO setting and the same exposure duration, an APS-C camera will shoot at f/2.8 and the FF one at f/4? \$\endgroup\$
    – xenoid
    Jun 1, 2020 at 20:20
  • \$\begingroup\$ @xenoid No. I'm saying that for a given light, with the same noise level and the same exposure duration, an APS-C camera will shoot at f/2.8 and the FF one at f/4.48, because a full frame camera can use 1.6 * 1.6 = 2.56 times larger ISO giving the same level of noise. The FF lens having f/4 thus gathers more light because 4 is smaller than 4.48. \$\endgroup\$
    – juhist
    Jun 1, 2020 at 20:32
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    \$\begingroup\$ So what has the F-stop got to do with this? You are just saying, ithat in general, the sensor of a full-frame is less noisy/has higher ISO than an APS-C sensor. Nothing to do with the F-stop, and this "general" statement, while true, has to be checked when comparing two cameras, a modern APS-C may have a better sensor than an ancient FF. Your crop-factor F-top equivalence is about DoF, not about light gathering ability. \$\endgroup\$
    – xenoid
    Jun 1, 2020 at 20:38

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