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the lower the t-stop, the less focused image?


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#1 kristian andersen

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Posted 01 January 2007 - 01:55 PM

hi.

is it true that a low t-stop (say 2.8) is not as sharp as higher stops like 5.6? why is this? are most films shot at one particular t-stop and lit accordingly to keep sharpness?

thanks!
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#2 Daniel Smith

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Posted 01 January 2007 - 02:05 PM

hi.

is it true that a low t-stop (say 2.8) is not as sharp as higher stops like 5.6? why is this? are most films shot at one particular t-stop and lit accordingly to keep sharpness?

thanks!

No. Well, not exactly. A T stop refers to the aperture of the lens. A low aperture like T1.8 with give a very shallow depth of field. Whereas a higher aperture like T22 will give a wide depth of field.

To understand it properly you need to know how depth of field works:

http://www.cambridge...th-of-field.htm


The T stop in films alternate from shot to shot to control the depth of field, to ultimately, tell a story.

Edited by Daniel Ashley-Smith, 01 January 2007 - 02:09 PM.

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#3 K Borowski

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Posted 01 January 2007 - 02:33 PM

There is some truth in this, although I wouldn't term it as "less focused". Shooting a lens "wide open", i.e at its lowest-numbered, biggest stop, is going to be most telling to image aberration. You'll see lens flaws, such as barrel distortion, most clearly at this aperture.

Rule of thumb, when possible, I stop down two or three stops, from wide open when I'm shooting, and I can afford to light to that stop with my film and lighting combination, as this is the setting that produces optimal sharpness on primes and on most zooms. Past three stops down from full open, you run into other loss-of-sharpness issues due to the f-stop iris.

In general, you want to check with the lens manufacturer. I've never shot with Zeiss glass, as I have little to no money, but I know that their lenses, and the other more expensive glass makers, they are designed to be as optimally sharp wide-open as possible. Other lenses, shot wide open aren't designed with those apertures in mind at all. I.e., I have an Angenieux 12-120mm Zoom that is a T/2.8 (F 2.2) and I've heard several people say that it shouldn't be shot below T-4. I generally shoot it at T 5.6 or smaller. I don't know how much modern cine zooms have improved (I'd assume there's been rather significant improvements), but I'd still generally try to shoot closed down at least one stop from the fullest aperture if possible with modern zooms. Primes I wouldn't be nearly as worried about shooting wide open.

Regards,

~Karl
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