[noor e alam]

hi everyone.
like to hear from T-Stop matters for pentax lenses.

as far i know -
T-stop being the light transmission level of a lens -- which may differ from the aperture.like a lens with f stop 1.8 may not work as i.8 because its t stop, that may be 2.
so the light comes to the sensor for that particular lens will be act as f stop 2.

i think, we all pentax users are eagerly waiting for response from pentax side about this T stop matter.
we actually want to know about the facts of our lenses.
and i ask other users to raise this matter to pantex forum to know the truth.

i give u all a link for further clarification.

You Tube

You Tube

regards
alam

[Adam]

I think this is one of those things that applies to just about each lens, so we ignore it, just like the quoted focal length isn't always 100% accurate.

[narual]

I can say with confidence that I am not eagerly awaiting a response from anyone on this topic.

[Bramela]

T stop?? What?

[blackcloudbrew]

I can say with confidence that I am not eagerly awaiting a response from anyone on this topic.

I could not have said it better!

[fuent104]

I wouldn't expect that information to be divulged, necessarily. Is that something most manufacturers disclose?

[kiwi_jono]

I'm interested from a technical interest standpoint (only). It factors in light transmission efficiency and expresses it as an equivalent aperture.

It would matter if you looking for a lens with maximum transmission for low light situations and need every photon you can get but given that most lenses are fairly good (versus aperture) its unimportant for most people.

DXO reports in Tstops and sometimes Lenstip mentions tstop / transmission efficiency.

[nater]

This is pretty straightforward, let me quote wikipedia: "f-stop is the ratio of the lens's focal length to the diameter of the entrance pupil." T-stops are adjusted f-stops: "A lens with a T-stop of N projects an image of the same brightness as an ideal lens with 100% transmittance and an f-number of N." Lens coatings and every air/glass surface will decrease transmitance. Zooms with lots of elements transmit less light than primes with fewer. In film/video, where they'll mount 3 lenses on a turret, they need to switch lenses quickly without having to recalculate exposure, so they use t-stops. While I think it would be nice if still camera makers did the same (and in the digital era still/motion photography is blending), that would be like asking hard drive makers to use powers of 2 (1024) instead of powers of 10 (1000) to measure drive capacity.

[drugal]

Not that it's the end all and be all, but I take DxOMark's calculations for T-Stops to be accurate enough to satisfy my technical curiosity.

[maxfield_photo]

If you want the equivalent f number to give you a particular T value, you could always buy a Sekonic 478 or 758. Both meters can be calibrated so that the value displayed is the compensated f-stop. The test targets are not cheap though, and they are not included with the meter, but if you keep an eye on e-bay you may find a used one for a good price. Make sure it's free or dirt and oils though before you buy.

For instance, I just set up a light and took a reading with my 758, first using the generic meter profile which gave me f/8 and 3 tenths, I then switched the profile to that of my DA* 50-135 f/2.8 and took another reading from the same position. This time it gave me f/5.6 and 8 tenths. Now I'm not 100% sure that I've nailed the calibration profile, it's devilishly tricky to get it perfect, but that number would suggest that the DA* 50-135 has a T value of 3.5. That's pretty typical of a well-corrected zoom I think, and really nothing to worry about. Primes usually have T values closer to that of the f value.

No lens is 100% efficient at transmitting light, and I suspect that the Tamrons Matt is talking about in the video may actually have "fudged" f values. It's actually pretty clever, they know nobody is actually going to open up the lens and measure the diameter of the aperture blades. [edit] One other thing to bear in mind is that if you're using an auto exposure mode such as Av or Tv or even Program, the camera is basing it's calculations on the actual light it is receiving, so the whole question of a lens' T value is pretty moot. It's really only when you're using some other method such as an incident meter to calculate exposure do T-stops really even begin to become important.

---------- Post added 02-11-14 at 07:16 PM ----------

Not that it's the end all and be all, but I take DxOMark's calculations for T-Stops to be accurate enough to satisfy my technical curiosity.

When DxO publishes their testing methods and criteria I will take their calculations as accurate, but not before then.

[Na Horuk]

about this T stop matter.

Um, what? Since when is this an issue? It is a well known thing. If you want to know T stop, buy video lenses (Cine). T stop is always slightly higher than f-stop (as in, some light is always lost), but that isn't as important for still photographers as aperture, because aperture gives us exposure values and DoF for that specific focal length, and it tells about the lens' size. For brightness, its close enough.
Its not a "matter", at least not as long as T-stop is reasonably near the F-stop. It would be a problem is an f1.8 lens gave you the brightness of a f2.8 lens, but that doesn't happen. You're not "cheated" out of anything, the difference between F and T stop is essentially insignificant

Reason why cinematographers want T-stop is because its a different medium with different techniques and different priorities. They also have stop-less aperture and other things on their lenses that still photographers don't want.

pantex

The general tone and spelling of the OP makes me wonder if this is a troll post, trying to cook up "controversy" and get a reaction over nothing. Sorry if I am being presumptuous.

[stevebrot]

For brightness, its close enough.

I don't know if it has been brought up, but we use TTL metering for a reason.


Steve

[narual]

The general tone and spelling of the OP makes me wonder if this is a troll post, trying to cook up "controversy" and get a reaction over nothing. Sorry if I am being presumptuous.[/SIZE]

He's from Bangladesh. I'll go out on a limb and guess that English isn't his first language. He probably read some article online that made it sound important or has a friend into video who is trying to explain stuff to him. It doesn't smell so much of troll as it does of someone who's been misled/misinformed.