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Andrew McKee
Lighting
on Sep 15, 2010 at 8:43:31 am

Hello.

I mainly work as an editor and colourist but like to learn about all the areas of production to help me do my job better. One of the areas that interests me the most is lighting as it effects my job as colourist dramatically, so occasionally I take a job as gaffer or DOP on short no budget films. Recently I have been trying improve my consistency by deciding on a fixed aperture for each scene across all the lenses I am going to use and then lighting accordingly, sometimes using a light meter, sometimes resorting to using zebra's or a waveform to gauge the right level of light for that aperture. This seems to have improved my work dramatically but I have one thing that is niggling me. If you move a light twice as close to a subject then the light will be twice as bright, so what effect does moving the camera closer and further from the subject have? If I am twice the distance from my subject, will my exposure be half as much? This doesn't seem to be the case but it does seem to make a bit of a difference, although I have yet to do any tests and am just basing this on what I have seen on set. How should I compensate for this, if this is the case?

Andy


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Todd Terry
Re: Lighting
on Sep 15, 2010 at 2:24:47 pm

You are absolutely right, moving a lighting instrument half as close to a subject does double the light (and would change exposure by one stop).

However, moving the camera to and from a subject has no effect on the exposure at all. If, for example, you were 10 feet from a subject and the proper exposure was f/5... if you moved to five free from the subject the exposure would still be f/5. No change.

Just curious, why are you (in advance) deciding on a fixed aperture for all your primes? That's a fairly unusual tack to take. Often one might want a very different f-stop for, say, a 100mm lens compared to a 28mm lens... depending on the depth of field you need for each shot.

T2

__________________________________
Todd Terry
Creative Director
Fantastic Plastic Entertainment, Inc.
fantasticplastic.com



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Rick Wise
Re: Lighting
on Sep 15, 2010 at 7:34:36 pm

Todd, actually moving the light closer by 1/2 the distance it was increases the light intensity by TWO stops. The "Inverse Square Law" applies here. When you move the light 1/2 the distance it was you invert 1/2 to get 2. Square that, and you get four: you now have four times the amount of light, which is a change of two f/stops. (Each time you double the light, the change is 1 f/stop.)

Equally, when you double the distance of the light, you take the inverse of 2 which is 1/2. Square that to get 1/4. You have cut the amount of light by a factor of 4 which translates to two stops less light.

The math is a tad harder when you measure by 1/3, or 3 times the distance, etc., but it works just fine.

For more info: http://en.wikipedia.org/wiki/Inverse-square_law

Rick Wise
director of photography
San Francisco Bay Area
and part-time instructor lighting and camera
grad school, SF Academy of Art University/Film and Video
http://www.RickWiseDP.com
http://www.linkedin.com/in/rwise
email: Rick@RickWiseDP.com


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Todd Terry
Re: Lighting
on Sep 15, 2010 at 9:00:20 pm

Duh, you are right, Rick.... I know that of course, but writing fast and brain in a fog this morning.

Not sure what I was thinking. Or rather, wasn't thinking.

T2

__________________________________
Todd Terry
Creative Director
Fantastic Plastic Entertainment, Inc.
fantasticplastic.com



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Rick Wise
Re: Lighting
on Sep 15, 2010 at 9:19:17 pm

I missed the part of the original question about using the same f/stop on all lenses in the scene. It is common on feature sets for the DP to work at the same f/stop regardless of lens. It's a good way to control the look of the scene. When you switch from a wider lens to a longer one you do get a change in depth of field when the camera remains at the same distance. However, overall, it's a good practice. It just takes more care and time than adjusting iris to lights. Here you adjust lights to iris. Having a light meter makes doing so a whole lot easier -- provided you know how to use it.

Highly recommended: learn the Zone system, and buy a Pentax Digital Spot Meter. They are no longer made but available on eBay and at some vendors. The Pentax is the only one that you can use easily with the Zone system. You just need to add a do-it-yourself Zone sticker in the right place. (I have a print-out file I can send anyone who needs it -- just don't ask for it out of curiosity, please.) Prices for the meter seem to range from $300-$750. Do NOT buy one modified by Zone Vl. However, Zone Vl's leather holster for the Pentax is terrific and very hard to find.

(The email below is no longer in use. Go to my web site and click on the email link.)

Rick Wise
director of photography
San Francisco Bay Area
and part-time instructor lighting and camera
grad school, SF Academy of Art University/Film and Video
http://www.RickWiseDP.com
http://www.linkedin.com/in/rwise
email: Rick@RickWiseDP.com


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Andrew McKee
Re: Lighting
on Sep 19, 2010 at 9:44:51 am

Thanks for your comments guys. I really like the look of using a fixed stop for dramatic scenes. As an editor it makes sense to me as when you start cutting in to tighter shots you are trying to make your audience more involved in the scene and so it makes sense to me that the tighter you are, the shallower the DOF. Sometimes I may vary the stop for a two shot if I need to get both subjects in, but I'm still always trying to light for the stop I want, rather than letting the lighting dictate it. This brings another question, if I want to stop down to increase the depth of field but still want the same level of exposure do I have to relight the scene? Using a DSLR I have tended to just raise the ISO, but I've never really enjoyed the change in grain that results, what do you do with a less flexible medium? Is there any other solution other than increasing your lighting but trying to match what you had before? What if one of your light sources is natural and unchangeable?

Andy

Andy


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Rick Wise
Re: Lighting
on Sep 19, 2010 at 6:50:32 pm

There is no way you can stop down other than to have more light, or else push the camera to a higher relative ISO. If you anticipate you will need more depth of field at some point in the scene, your best bet is to light the scene from the start with sufficient light for that f/stop. For those shots for which you want a more shallow depth of field, you can add an ND filter on the front sufficiently dense to allow you to open the iris; ND3 = 1 stop, ND6 = 2 stops, ND9 = 3 stops. However, most NDs add a bit of color since few are genuinely "neutral." That makes matching the shots between those shot with an ND and those without a bit difficult.

It may make more sense to stick to one f/stop all the way through. Part of the DP's job is to anticipate the demands of the entire scene. Does a key prop need some light later in the scene? then make sure it has that light in the master, etc. Ditto for depth of field.

Shallow depth of field is a function of three things: f/stop, distance (field of view), and focal length. At a given f/stop, and the same field of view of the key subjects you do not gain depth of field by going to a wider lens or gain a more shallow depth of field by going to a longer lens. When you keep the same field of view and go to a longer lens then you have to back the camera up, effectively returning the depth of field to what it was before. Equally, when you go to a wider lens, you now have to push the camera closer to keep the same field of view, effectively returning the depth of field to what it was before. (However, you will see a wider view of the background than you saw with the long lens.)

At a given f/stop if you keep the camera at the same distance and switch to a longer lens, you will now have a more narrow field of view and also a more shallow depth of field. You can make the depth of field even more shallow by moving the camera closer to the principle action, opting for a smaller field of view.

Finally, at a given distance and a given focal length, opening the iris makes the depth of field more shallow and of course stopping down makes the depth of field deeper.

Rick Wise
director of photography
San Francisco Bay Area
part-time instructor lighting/camera
Academy of Art University/Film and Video (grad school)
http://www.RickWiseDP.com


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janusz sikora
Re: Lighting
on Sep 20, 2010 at 6:36:04 pm

Hello Andy...

In reference to holding your F-stop *steady* through out the Sequence ...
this is the only way not only to maintain Lighting Continuity but, most importantly - the only way to engage yourself within a real nature of creative process... Meaning - Lighting from Previsualization.
One has to light *for something"... this *something" is your predetermined F-stop (which by the way is your reference to 15% Grey Card, which is the thing *you dance around* in creative lighting)... - Last nail in the coffin - Do not change it and dance around it.

Second part of your issue pertaining to Intensity of light as you move-on the subject with camera :

Amount of light on the subject does not change with you coming close or moving away... what changes (when you move in) is the apparent contrast ratio and/or lighting ratio... when you move-in - lets say - on a Dark Subject, with your own eyes ... the pupil/aperture of your eye accommodates (opens up to to compensate for darkness and lets more light in (that is why - naturally when you want to see detail you move in closer).
Of course - distance and apparent "subject magnification" is one thing but, without eye accommodation that very detail would seem too contrasty.
Well, professional Motion Picture lenses are - of course - not an Autoexposure lenses that compensate. We're talking about an as Old as Filmmaking itself - Compensation for CU (closeup)... which is about reducing contrast between Key and Fill on CU by increasing the Fill.

Hope this helps :)

janusz
http://www.lightextreme.com


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Rick Wise
Re: Lighting
on Sep 21, 2010 at 12:01:08 am

Whether or not one wants to reduce contrast for a close-up is a judgment call. One might want to increase contrast, leave it the same, or reduce it. If the wide shot has a lot of contrast, I'd leave it the same. Depending on the shot and angle of light, I might slightly close down the iris or slightly scrim the key and maybe also the fill -- or not. More judgment.

Rick Wise
director of photography
San Francisco Bay Area
part-time instructor lighting/camera
Academy of Art University/Film and Video (grad school)
http://www.RickWiseDP.com


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janusz sikora
Re: Lighting
on Sep 21, 2010 at 2:48:08 am

Golden words Rick...

Its always about judgment call...

Did not mean my geeralization come across as radical :)

I do not think there is enough space here for relativity :)

janusz
http://www.lightextreme.com

*It,s quite about Light*


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Andrew McKee
Re: Lighting
on Sep 21, 2010 at 10:07:35 am

Thanks for your comments Janusz. Let me just make sure I've got my head around everything correctly. Please correct me if I say anything wrong. I set my light meter so it knows the sensitivity of my medium and the shutter speed. I then decide on my stop (say F2.8 for instance). I then use a keylight that will gives me a reading of F2.8 on an incident meter at the point my subject will be. Now, something that was middle grey would expose bang in the middle of my dynamic range? As I'm a colourist I tend to think in IRE rather than stops, so this would produce an IRE of 50 on a digital image or a scanned version of the film? However, skin is brighter than middle grey and so I'll get more exposure. Something in the 60-70 range depending on skin tone?

The alternative is to use a spot meter which measures the reflection off my subject. So in order to get that 60-70 exposure on the skin I should be expecting readings a couple of stops over my aperture?

Obviously then there is everything else in the image to consider, I'm just using a person as an example.

Andy


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janusz sikora
Re: Lighting
on Sep 21, 2010 at 2:24:55 pm

Andy...

You are correct ! :)

Provided your video camera is calibrated for correct value of the ASA/ISO equivalent (there are methods to obtain that value).

With your hypothetical example at given ASA value, Incident at 2.8 and Lens at 2.8 will/should produce Middle Grey - IRA 50. Everything else will show like what it is in terms of IRA... so the face of the subject (depending on skin tone) - you can preasume - will show at around IRA 60 (average caucassian - Zone VI/IRA 65/70). And yes... if you want to know exact value of the face, you need to Spotread it :)
So you can easily imagine that creative approach is about "dancing around" F.28 where you set and control values aroud it (IRA 50)to your liking, keeping in mind latitude of the medium )

janusz
http://www.lightextreme.com

*It's quite about Light*


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janusz sikora
Re: Lighting
on Sep 21, 2010 at 2:57:19 pm

Andy, you are correct :)

Provided your Video Cam is calibrated for correct ASA equivalent (there are methods to obtain this value) - your Incident 2.8 with the cam lens at 2.8 will maintain Ira 50 at the middle of the scale.
Everything else will reproduce as what it is... Caucassian face will show at around Zone VI - 60/70 IRA (depending on the skin tone).
Yes, if you want to know the exact value, you will need to Spot read it. You can easily see how "dancing around Grey" will give you creative control over the look/mood you intend to create...
What remaims is to watch you staying within latitude of the medium (Spot Meter).

janusz
http://www.lightextreme.com

*It's quite about Light*


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