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Photo Equipment Standards

An enthusiast interested in detailed specifications of photography gear should consider the following as a warning: Even though "ISO" is named after the International Organization for Standardization, the ISO setting reported by digital cameras is often significantly different[1][2] from the sensor's true sensitivity defined by ISO Standard 12232.

But professional photographers are unlikely to worry about this. As online forums say, "get out and shoot."

Videographers do care about precisely controlling exposure, which is why cine lenses are rated by t-stop rather than f-stop. But although photography enthusiasts may want to know whether one wildlife lens transmits more light than another, manufactures do not usually publish t-stops for these lenses.

Even published f-stop numbers are rounded, as are focal lengths. Every lens sold as a 600mm f/6.3 is likely to have a 95mm filter thread diameter, but the nominal specs imply that's impossible: The entrance pupil is nominally 600/6.3 = 95.24mm in diameter. The real discrepancy is often far larger than that: The Sigma 150-600mm f/5-6.3 Contemporary is really 155mm-579mm f/5.15-6.5, so the long end is more than 1/3rd EV worse than a true 600mm f/6.3.

Some specifications aren't published by manufacturers at all, like sensors' dynamic range and readout speed. And although manufacturers do publish MTF charts to describe lens sharpness, these are not comparable across manufacturers.

Resources:

• t-stop: DxoMark measures this. Their method appears to be based on nominal focal length, and I believe it penalizes lenses whose true focal length is shorter than nominal (they do not measure true focal length). Unfortunately, they have not tested many recently released lenses, and appear to have shifted their focus to smartphones. Thom Hogan randomly lists "effective t/stop" for at least one lens, but I haven't found another source or test details for that number. A dpreview forum user did some testing of Nikon lenses in 2009.
• Focal length and f-number: Photons To Photos is an excellent resource, but it is based on patents, and cautions that "there is no guarantee that a production lens followed the patent optical prescription precisely. And in some cases the patent optical prescription below doesn't line up perfectly with the production lens." I noticed that the aperture numbers for Nikon Z 100-400 f/4.5-5.6 seem hard to believe, since they are no brighter on the short end than the long end. Outside of Photons to Photos, I found little data aside from miscellaneous forum posts. It is possible to calculate focal length from pictures of the Moon or stars, using plate solving, so if more photographers could post pictures of the moon with accurate dates, that would be great.
• Sensors' dynamic range: Photons to Photos is again an excellent resource, but it cautions that "the x-axis is ISO Setting and not a 'measured' value," so manufacturers' pesky habit of misstating ISOs complicates the interpretation of these dynamic range numbers. Additionally, manufacturers "cooking" RAW files with added noise reduction may affect the results. Still, this is a great resource for checking whether your camera has a dual-gain sensor like the Nikon Z8, since image quality will be best when shot with ISO on the correct side of the "bump."
• Readout speed: https://horshack-dpreview.github.io/RollingShutter/
• MTF charts: Photography Life claims that the sharpness testing they include in lens reviews is valid across manufacturers. If you are only interested in comparing two lenses from the same manufacturer, then you can use the chart from the manufacturer. If they are Canon lenses, then you'll want to check the Canon Japan website, since the English websites do not appear to have MTF charts.