Is the Panasonic 25mm a real f/1.4 lens?

Started 10 months ago | Discussions thread
Managarm
Regular MemberPosts: 308
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F-Stop = theoretical value, T-Stop = practical value
In reply to 007peter, 10 months ago

007peter wrote:

A slight 0.3ev lost in T-Stop is to be expected. AFAIK, no lens live up to their T-Stop claim (or very very few do). Most manufacture measure them under the best LAB CONDITION, so in everyday "Real World", a slight 0.3ev lost isn't outrageous.

Doesn't have to do with different testing conditions or manufacturers cheating. F- and T-Stops are simply the results of two different kinds of methods.

F-Stops are just calculated from the diameter of the entrance pupil and the focal length of the lens. F = focal length / diameter of entrance pupil

T-Stops are actually measured values of the transmission of light trough the lens.

Since you will always lose some light when it travels through lenses (due to reflection & absorbtion & vignetting) the F-Stop is at best equal to the T-Stop. The more complex the lens design (more elements) and the greater the aperture, the higher the loss of light and hence the difference between F-Stop and T-Stop.

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