Correct exposure under various strengths of light

[Gelber]

Evening all,

I have a quick question about setting exposure in general. I understand all the usual information - set white at 90% IRE, 70 for caucasian skin etc and the changes that have to be made when shooting in the various Logs. But something that I've never been entirely clear on and haven't had much luck tracking down a simple answer is how these values change under varying strengths of light.

For example, if shooting a non Log gamma (say a 709 equivalent) in clear sunlight I would expose by setting a bright white subject to 90% on my waveform or revealing zebras set at 90.

But what do I do when its overcast? What about when its under twilight? WHAT ABOUT CANDLELIGHT?!

Hitting 90% for a white shirt under the dated, but always erotic, ambience of a lava lamp is going to be quite the mood killer, no? What's the rule of thumb?

Any advice will be much appreciated as always.

 

[j1clark@ucsd.edu]

Evening all,

I have a quick question about setting exposure in general. I understand all the usual information - set white at 90% IRE, 70 for caucasian skin etc and the changes that have to be made when shooting in the various Logs. But something that I've never been entirely clear on and haven't had much luck tracking down a simple answer is how these values change under varying strengths of light.

The percentages that you are using don't change under varying 'strengths' of light... You may change the f-stop or ISO value to achieve the listed captured image values... but if those are your working parameters, they don't change.

There may be 'mood' reasons to have caucasion skin at less than 70%, dark night, limited light... but that is a creative selection.

 

[Bruce Watson]

...if shooting a non Log gamma (say a 709 equivalent) in clear sunlight I would expose by setting a bright white subject to 90% on my waveform or revealing zebras set at 90.

But what do I do when its overcast? What about when its under twilight? WHAT ABOUT CANDLELIGHT?!

Hitting 90% for a white shirt under the dated, but always erotic, ambience of a lava lamp is going to be quite the mood killer, no? What's the rule of thumb?

You're confusing exposure for capture with the final look of the film which is obtained in the color grading step of post production. You're job at capture time is to properly expose, which is what setting your zebras and hitting their marks is all about. In any lighting.

Once the footage gets to post production, you color correct and color grade. Color grading in particular is about giving the footage that artistic look you want. This can mean crushing the blacks and applying an orange color cast to that lava lamp scene for example. But the foundation to successful color grading is a good exposure.

 

[Greg_E]

90% is dirty for a lot of subjects materials, depends on the material in question not just a hard and fast rule in the digital age. Only back in analog did 100% cause a buzz in the audio, and only when transmit, and only when the first audio subcarrier was in the lower portion. Push the audio up to 5Mhz or so and the problem was gone.

 

[Gelber]

Thanks very much.

I'll add that the lava lamp is a mainstay of my filming setup.

 

[Tim Sewell]

I'll defer from offering my opinions on lava lamps save to comment that it's my 6 year old son's favourite practical.

On the subject of exposure - and I'm still learning myself so may well be shot down in flames by the pros on this board - my interpretation is that the key task is to make sure that one places the elements of the scene that need to appear to the end viewer flawlessly in the part of the digital capture to which the most bits have been allocated in the gamma employed. It may well be that that demands you place your whites in the 90% region, or it may be that you're shooting log and want to elevate your shadows to the lower end of the bit-rich section of the gamma so you place your whites at 100 or even 105%.

When shooting a roll of HP5 in my old Olympus OM4 I might set the ISO to 1600, thus elevating my shadows in order to preserve their detail. To yield a usable negative I would increase the development time in order to access that highlight detail I had acquired - for a 'normal' development would leave such detail undeveloped. This is analogous to the digital workflow.

 

[Run&Gun]

And throw 70% for skin tones out.

 

[Bern Caughey]

When shooting a roll of HP5 in my old Olympus OM4 I might set the ISO to 1600, thus elevating my shadows in order to preserve their detail.

In your example your under-exposing the HP5 (ISO 400) by two stops, & starving the shadows of detail.

With B&W negative film the basic rule is exposure controls the shadows, & developing controls the highlights. The Zone System is a precise method for determining both these parameters, but most Pros simply overexposed by one stop to dig into the shadows, & pulled the development by one stop to wrangle the highlights.