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forgive my ignorance, some questions...
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PostPosted: Fri Feb 28, 2014 1:23 pm    Post subject: forgive my ignorance, some questions... Reply with quote

Why would one want click stops removed from manual lenses for video?

And why do some video lenses have t stops rather than f stops?


PostPosted: Fri Feb 28, 2014 10:09 pm    Post subject: Reply with quote

Because then you can "stop down" (or up) smooth when filming Wink

F-stops are for focal ratio, T-stops are for transmission. F-stops are the theoretical amount of light the lens passes and T-stops measure the actual amount of light the lens passes.


PostPosted: Fri Feb 28, 2014 11:20 pm    Post subject: Reply with quote

In addition to the ability to change aperture while shooting (and not having an obvious change), the ability to set the aperture between stops can also be really valuable. In fact, most cinema lenses have much longer aperture throws than still lenses, so setting it at 4.0 3/10s is actually something you can set pretty accurately, so de-clicking an aperture on a still lens only gives you a bit more play. And while most cinematographers try to "light to a stop", it's not always possible, especially when you're using finicky available light. Since your frame rate and ISO are stuck, being able to open up about 1/3 of a stop can make a big difference


PostPosted: Fri Feb 28, 2014 11:32 pm    Post subject: Reply with quote

I'm asking this because I read on the forum about de-clicking apertures, and so on. I'm interested to learn why. This is a gap in my knowlege that I'm keen to fill. I read many of the old posts here about video and much of it goes over my head.

I am from a time when editing movies involved cutting film and perforated audio tape then splicing it together and synching the tape soundtrack onto the film stripe. Fades were done using the aperture or a chemical which was painted onto the film. Crossfades were done in camera by rewinding the film a few frames and carefully fading out and in again using the aperture.

I make short home movies and edit them in Windows Moviemaker which does for me.

so your answer:

Nordentro wrote:
Because then you can "stop down" (or up) smooth when filming Wink


That is what I assumed. Surely that will cause the gain to increase and degrade the image quality. Why would you want to change aperture when 'filming'?

Nordentro wrote:
F-stops are for focal ratio, T-stops are for transmission. F-stops are the theoretical amount of light the lens passes and T-stops measure the actual amount of light the lens passes.


I understand the idea there, so what's the difference in practical terms, the Kerns and Yvars on my Bolex all use f stops.

Thanks.


PostPosted: Fri Feb 28, 2014 11:48 pm    Post subject: Reply with quote

philslizzy wrote:
I am from a time when editing movies involved cutting film and perforated audio tape then splicing it together and synching the tape soundtrack onto the film stripe. Fades were done using the aperture or a chemical which was painted onto the film. Crossfades were done in camera by rewinding the film a few frames and carefully fading out and in again using the aperture.


actually, many cine lenses from film days also had stopless apertures for the same reasons Nordento and I listed - and when you were shooting on film, fixing something "in post" was a lot more difficult, especially when you were pushing the film to its extremes, so setting a stop exactly was just as important, if not more so...

philslizzy wrote:
Nordentro wrote:
Because then you can "stop down" (or up) smooth when filming Wink


That is what I assumed. Surely that will cause the gain to increase and degrade the image quality. Why would you want to change aperture when 'filming'?


a common reason is that the camera is moving and the lighting situation changes - for example, moving to a window or through a door


PostPosted: Sat Mar 01, 2014 12:43 am    Post subject: Reply with quote

T-stops where adopted by filmmakers because they needed to know exact transmission of light for pre lit scenes so they could mix lenses from different manufactures and different types of lenses (zooms + primes etc.) . F-stops are theoretical calculations and there was a little bit of variations between the lenses and the lens manufactures Wink

A Carl Zeiss employee explains nicely here:
http://vimeo.com/14951435


PostPosted: Sat Mar 01, 2014 9:04 am    Post subject: Reply with quote

strangelove wrote:
...a common reason is that the camera is moving and the lighting situation changes - for example, moving to a window or through a door


So I guess you would have the camera on manual exposure rather than auto - which would adjust the gain automatically - and adjust exposure manually with the aperture. I understand that.

Thanks Nordentro and Strangelove for your answers, I now understand why these are important to advanced amateurs and professionals.

You learn so much on this forum.