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<blockquote data-quote="spb_stan" data-source="post: 668762" data-attributes="member: 43545"><p>Lots of confusion in this thread as to the meaning of Aperture and Transmission. Texas has it right, the aperture just tells of a ratio of focal length to effective aperture diameter. That is all it tells you, and does not infer anything about light transmission. a complex lens with 23 elements will almost always have less light transmitted through it than a simple 3 element lens made of the same material. But lens elements are not of the same material, they are selected for their ability to compensate for path disturbances by color frequency, scattering, collimating etc. The Transmission factor is measured as a T number. F stops are calculated by simple measuring the dimensions of the lens and T-stops are actual light transmission efficiency. T-Stops are almost always lower in transmission than F-stops suggest since F apertures assume dry air as the medium of transmission and T-Stops ratings are actual lens optical path attenuation measurements. </p><p>DSLRs using through the lens metering and VF compensate so exposure is pretty close to T-stop but that has limits when wide open when no more aperture vane movement is possible, you can see that camera gain has to make up for the lower transmission than ideal. So a f/1.4 lens with a T-stop rating of 1.8 might give a lot less light than a lens with identical physical dimension f/1.4 but T-stop t-1.5. The result in low light will be a lot lower noise and better DR for the latter lens.</p><p>T-stops are not so important for most general photography because there is nothing to compare within individual images, each is a stand-alone visual experience. Where it becomes very important is when images have to match when changing apertures and lighting. A case there is astrophysics and cinema where scene exposure has to be carefully calibrated to match another scene or angle.</p><p>General photography can be sloppy in light gathering attention to detail but movies are another story and astrophysics calibration of the light transmission is critical for optical measurements to mean anything other than just pretty pictures. Actual light intensity falling on a pixel can make the difference between just a pin of light and evidence of an entirely undiscovered galaxy. </p><p>Why don't lens makers list T-stop? It is expensive to measure and to assure a certain specification and not demanded by still photographers so why bother. Cinematographers, however, have the need and budget for it so demand that spec.</p></blockquote><p></p>
[QUOTE="spb_stan, post: 668762, member: 43545"] Lots of confusion in this thread as to the meaning of Aperture and Transmission. Texas has it right, the aperture just tells of a ratio of focal length to effective aperture diameter. That is all it tells you, and does not infer anything about light transmission. a complex lens with 23 elements will almost always have less light transmitted through it than a simple 3 element lens made of the same material. But lens elements are not of the same material, they are selected for their ability to compensate for path disturbances by color frequency, scattering, collimating etc. The Transmission factor is measured as a T number. F stops are calculated by simple measuring the dimensions of the lens and T-stops are actual light transmission efficiency. T-Stops are almost always lower in transmission than F-stops suggest since F apertures assume dry air as the medium of transmission and T-Stops ratings are actual lens optical path attenuation measurements. DSLRs using through the lens metering and VF compensate so exposure is pretty close to T-stop but that has limits when wide open when no more aperture vane movement is possible, you can see that camera gain has to make up for the lower transmission than ideal. So a f/1.4 lens with a T-stop rating of 1.8 might give a lot less light than a lens with identical physical dimension f/1.4 but T-stop t-1.5. The result in low light will be a lot lower noise and better DR for the latter lens. T-stops are not so important for most general photography because there is nothing to compare within individual images, each is a stand-alone visual experience. Where it becomes very important is when images have to match when changing apertures and lighting. A case there is astrophysics and cinema where scene exposure has to be carefully calibrated to match another scene or angle. General photography can be sloppy in light gathering attention to detail but movies are another story and astrophysics calibration of the light transmission is critical for optical measurements to mean anything other than just pretty pictures. Actual light intensity falling on a pixel can make the difference between just a pin of light and evidence of an entirely undiscovered galaxy. Why don't lens makers list T-stop? It is expensive to measure and to assure a certain specification and not demanded by still photographers so why bother. Cinematographers, however, have the need and budget for it so demand that spec. [/QUOTE]
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