Color Rendition...T-stops?

[Allen Achterberg]

well I was recently informed that changing your stop throught a scene is not generally a good thing. Something I never payed attention to but usually did shoot at the same stop (usually wide open) and never noticed anything with color. well I was informed by a student DP that your stop has a lot to do with the color rendition on the film. I guess what I'd really like to know is why is this? I am looking for a scientific explanation for this, it would help me a lot. any thoughts? any reasons? anything you got I'd like.

 

thanks,

Allen Achterberg

[David Mullen ASC]

At the extreme ends (wide open or stopped far down) there may be some odd optical problems like abberation, diffraction, etc. but the notion that color or contrast changes as you change the f-stop is somewhat mistaken.

 

It's only the PERCEPTION because at wider f-stops, more of the image is out-of-focus, and in those blurred areas, color and contrast get blended and somewhat muddied, softened -- while in the in-focus areas, the color and contrast are the same. Now wide-open, you may get a little more flaring from lights in the shot, which may reduce contrast.

 

The main reason you use the same f-stop is consistency of depth of field and optical quality of the lens, although in truth, the focal length and distance focused will affect depth of field characteristics, and you may want less sharpness for a close-up than you would for a wide shot.

[Allen Achterberg]

David, big help. thanks

[Glenn Hanns]

I agree with David, I'd like to add that changing your stop during a shot is but another tool/technique and not necessarily a "bad" thing to do. An example might be a dolly into a shaded porch from a sunny exterior. You could cheat the porch exposure a little with a slight iris open if getting enough light into the shaded area is a problem.

 

Cheers

Glenn Hanns

[Allen Achterberg]

I agree with David, I'd like to add that changing your stop during a shot is but another tool/technique and not necessarily a "bad" thing to do. An example might be a dolly into a shaded porch from a sunny exterior. You could cheat the porch exposure a little with a slight iris open if getting enough light into the shaded area is a problem.

 

Cheers

Glenn Hanns

<{POST_SNAPBACK}>

 

thanks glenn, I agree too it makes more sense. it kind of freaked me out when two student DP's told me this because I started thinking of your above technique and also ramping iris while ramping filmspeed. god I love these forums. you guys are great.

 

Allen Achterberg,