What are T aperture and F aperture?

Aperture value F= diameter/focal length of lens entrance aperture.

T value =F value/square root of light transmittance

The length of the lens changes with the focal length of the lens, and the light will be lost when it reaches the negative from the lens. Therefore, even if the focal length of the lens with the same aperture is different, the luminous flux will be different, so the aperture is expressed by the diameter/focal length of the lens entering the aperture, that is, the F-series aperture. When you see this formula, you must think, is it really such an accurate correspondence? The answer is naturally no, this is just a rough aperture.

Therefore, in order to measure the luminous flux more accurately, there is a T-aperture, the square root/transmittance of F value.

The light transmittance is measured according to the illumination of the image plane. Therefore, the aperture value t is measured. Because the T value considers the influence of various factors on the transmittance of lenses, as long as the T values of two lenses are the same, their luminous flux must be the same.

So the value of t is more scientific and accurate than the value of f.

However, due to the development of lens coating technology, the difference between T value and F value has been very small. For general photography, the f value can already meet the needs. T-value is often used in focus plane where exposure requirements are more stringent.

Nevertheless, we still encounter great differences in the exposure of different lenses under the same F value in photography practice, which may be caused by the light transmittance of the lens or other reasons (such as the manufacturing accuracy of the aperture itself, the influence of climate change on the aperture shrinkage efficiency, etc.). ).