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About keeping the same f-stop


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#1 Hugo Miro

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Posted 22 August 2011 - 01:41 PM

Hi everyone and sorry for my poor english...

I worked in a short film where the DoP to maintain constant the f-stop in each scene, he varied the ISO (we did not have ND filter). Is it a right way to do it (without reaching extreme ISOs)?
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#2 K Borowski

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Posted 22 August 2011 - 02:29 PM

No, that is a HORRIBLE idea. If you have even the latest, greatest DSLR, there is a SIGNIFICANT difference between ISO 3200 and ISO 100 in terms of noise.

If you want to maintain a constant depth of field, why not stack NDs (0.3ND) (0.6ND) (0.9ND) and try to set a standard exposure for your dimmest, least illuminated scene, and work from that to how much ND you need for your brightest scene?

In other words, I would SET your ISO for the dimmest lighting of the movie, then build up ND filters to your brightest scene (although past three stacked filters - still 9 full F/stops - you get noticeable softening) before I would change ISO or lens aperture.



EDIT: I see you have no ND filters - buy some! You will be rewarded with a consistent professional look. Only in a music video calling for bizarre changes in noise amounts would I do what you are suggesting. Compression noise is not aesthetically pleasing, just like extreme chunky, muddy film grain.

Edited by K Borowski, 22 August 2011 - 02:31 PM.

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#3 Adrian Sierkowski

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Posted 22 August 2011 - 02:40 PM

It would be much wiser, as Karl suggests to control your lighting by, well lighting, and, when all else fails (during the day, for example) by NDs before changing ISOs. That being said, it is OK to vary F Stop slightly, within a stop or so +/- sometimes more before throwing on ND.
It would also be appropriate to change ISO when changing locations (For example, you're inside at say 400, and then outside @ 100.)
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#4 David Mullen ASC

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Posted 22 August 2011 - 03:58 PM

Within narrow limits you can adjust the ISO, but only by about a stop is safe on many video cameras, like 250 ISO outside and 500 ISO inside for lower light levels, for example (not saying that those particular ratings are good for your camera however.) I certainly wouldn't hesitate to vary the f-stop within limits (let's say, f/2.8 to f/5.6 for example), and maybe even shutter speed a little... but you need to also use ND filters outdoors, it's standard operating procedure.
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