What are STF (smooth transition focus) lenses? How much do they differ from other fast lenses? What artistic techniques are done with them? For example Sony has an a-mount 135mm F2.8 [T4.5] STF. What is their specialty? Also what does T4.5 mean ?

up vote 9 down vote accepted

STF stands for "Smooth Transition Focus", and is a Sony-specific* term indicating that the lens includes an apodization filter to create smooth bokeh (out-of-focus blur) — and smooth bokeh is generally considered to be the best bokeh. So, you'd generally use it for non-studio portraiture or in other cases where that blur is an important artistic element.

Other lensmakers use different terms; for example, Fujifilm designates their lens with this feature "APD".

Note the t-stop designation of T4.5. This indicates that the actual transmission of light at the widest aperture is that of a theoretical f/4.5 lens — even though the effective maximum aperture is f/2.8. That's because about a stop is lost to the filter. (Fujifilm's f/1.2 APD lens is similarly limited in this way, with a t-stop of 1.7, again about one stop lost.)

Read more about t-stops at What is T-number / T-stop?. This concept is used for any light loss (which all lenses invariably have, since they're made of real-world materials), but normally the difference is small enough that we just ignore it — see Why are DSLR lenses measured in F stops instead of T stops? for more on why. But with the apodization filter, there's enough of a difference that it can be important to be aware. Any through-the-lens autoexposure program mode will take this into account automatically; if you are doing manual metering and calculations, you should figure it in (although of course these digital days it's easier to guess and check than worry about the numbers).


* Previously, a Minolta-specific term, and then Sony bought Minolta's camera business....

Your Answer

 

By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.

Not the answer you're looking for? Browse other questions tagged or ask your own question.