Jump to content

Photometric Help


Recommended Posts

I've read a few forum post about foot candles. Looking for a little help to see if I'm on the correct path. 

Quote from David Mullen ASC different post. 

100  foot-candles = f/2.8 (at 24 fps / 180 degree shutter) at 100 ASA.  Remembering that will make it easier to break down photometric data on lamps.

Every f-stop number is the double or the half the number two stops over, i.e. f/1.4 to f/2.8 is a two-stop jump. f/2 to f/4 is a two-stop jump.

Sunny 16 rule, that the exposure is f/16 when the shutter time value under 1/ is the same as the ASA, so 1/50 at 50 ASA = f/16 if shooting under direct sun on a clear day.

 

My Question

400 Foot candles at 100 ASA = F5.6, this would include any diffusion place into front of the light. No matter if its 2ft or 20ft, F5.6  needs 400 foot candles from the light source? A fill light would require 200 foot candles? 

Thanks

 

Link to comment
Share on other sites

  • Premium Member
2 hours ago, Larry Stone said:

My Question

400 Foot candles at 100 ASA = F5.6, this would include any diffusion place into front of the light. No matter if its 2ft or 20ft, F5.6  needs 400 foot candles from the light source?

That sounds right, if you want a technically accurate exposure (i.e. an 18% grey card rendered at 18% grey on film/digital).

It’s unusual to work at those light levels these days on interiors, but for exteriors with ambient daylight and sunlight it’s not a big deal due to the inverse square law of light.

Re: fill levels 

If you want a 2:1 ratio of key to fill, then yes you would need 200fc for fill. That’s a very flat-looking image though, most people would fill at a much higher ratio.

  • Like 1
Link to comment
Share on other sites

Thank You Satsuki,

I was checking to see if I my calculations were correct. I'm looking at 3 different soft light open face light panels.

The smaller version has 510 foot candles at 2ft,  with a diffusion panel the light lost would be 1 to 2 stops. And I want the ability to keep my Fstop at 5.6, after doing the math I think the larger version at 1058 foot candles at 2ft would be a better fit. Would I end up around 400 foot candles, if I was using the larger version? Using this for talking head interviews. 

Thanks 

  • Like 1
Link to comment
Share on other sites

4 hours ago, Larry Stone said:

Thank You Satsuki,

I was checking to see if I my calculations were correct. I'm looking at 3 different soft light open face light panels.

The smaller version has 510 foot candles at 2ft,  with a diffusion panel the light lost would be 1 to 2 stops. And I want the ability to keep my Fstop at 5.6, after doing the math I think the larger version at 1058 foot candles at 2ft would be a better fit. Would I end up around 400 foot candles, if I was using the larger version? Using this for talking head interviews. 

Thanks 

Hey Larry,

if im correct your diffusion panel emitting 510fc at 2ft with a -1 or 2 stop diffusion would be quite a fair bit below what your after. Even your 1058fc panel (does that output include the light cut from the diffusion. I would hope so? If not that means it’ll only output 250 fc at 2ft with it?) may be a little weak. Keep in mind 400  is for zone 5 (middle gray) if your exposing for Caucasian skin I imagine you’d want to go a stop over so that’s 800fc. That’s with the light being 2ft away from the persons face.

Could you rate the camera a little higher than 100ASA? As at 400 ASA at 5.6 you only need 100FC or 200 for Caucasian skin (stop over) which is a fair bit easier to achieve. You’d have the ability to place the panel a little further away and even with the diffusion cut of 2 stops (if that isn’t included in the original output) your still safe. 

I hope this is helpful and I haven’t just barked up the wrong tree.

  • Like 2
Link to comment
Share on other sites

  • Premium Member
9 hours ago, Larry Stone said:

Thank You Satsuki,

I was checking to see if I my calculations were correct. I'm looking at 3 different soft light open face light panels.

The smaller version has 510 foot candles at 2ft,  with a diffusion panel the light lost would be 1 to 2 stops. And I want the ability to keep my Fstop at 5.6, after doing the math I think the larger version at 1058 foot candles at 2ft would be a better fit. Would I end up around 400 foot candles, if I was using the larger version? Using this for talking head interviews. 

Thanks 

No problem. Most interior interviews these days are shot in the range of 400 ~ 1250 ISO on the camera side. So key lights can be in the range of 25 ~ 100fc. Or less, since most people shoot wider open than f/5.6.

Also, it’s better to keep the key light further away than 2’ so it doesn’t crowd the subject and also lights their body evenly. Somewhere between 4-8’, depending on the physical size of the diffusion source. The larger the diffusion frame, the further you can move it back and maintain the same softness.

Not sure what camera system you are using, but you might consider working at a higher ISO before going to a bigger and more expensive lighting unit. If you are working outside and fighting the sun, then you either need big HMI lights, or you need to take away light by using overhead diffusion frames and negative fill. 

Keep in mind that for older productions shot on slow film stock, hard lights like fresnels were used to key actors from longer distances. Due to the inverse square law (which doesn’t apply to soft sources like the light panels you are using), you could still get a workable exposure. But of course, the quality of the light is much different with a hard light source. 

 

  • Like 1
Link to comment
Share on other sites

  • Premium Member

I suppose when working outside, you could also use a big LED source like a Skypanel 360 or a Litemat 8. But I’m guessing those are out of your budget.

I’m also not sure how much a Litemat 8 would do in direct sun. I worked on a commercial where we used the Litemat 8 as a key light in an almond orchard. It worked, but it was quite overcast and dark to begin with.

  • Like 1
Link to comment
Share on other sites

Thank You @Gabriel Devereux

I appreciate you taking the time to provide explain the light lost with the diffusion. That really helped me understand light output. 

Thank You again @Satsuki Murashige for your time and in-depth answers.  No go on the Skypanel. 

I don't own any Lights at the moment. Using a DSLR Nikon D750, I think I can go up to 800 ASA/ISO(still researching that). I was comparing different lights, looking to see what would be a good fit. I pick a Fstop of 5.6, cause I want to avoid purchasing anything that was limited to just Fstop 2.8, better yet 100 Foot Candle. The photometric are from the Kino Flo freestyle 31 510 foot candles at 2ft and the Kino Flo Celeb 401 858 and 401Q 1058 Foot candles at 2ft. All these models have front diffusion built in. I've never worked with any kind of panel light, I guessing you don't need the extra diffusion. At 400ASA/ISO that the two of you have recommended the Freestyle would be a good fit for interviews and the Celeb's would be over kill. And any lights I end up purchasing need to output at least 100 foot candles at 2ft, to be useable in any interviewing set up.

Thank You Both.

 

Link to comment
Share on other sites

1 hour ago, Larry Stone said:

Thank You @Gabriel Devereux

I appreciate you taking the time to provide explain the light lost with the diffusion. That really helped me understand light output. 

Thank You again @Satsuki Murashige for your time and in-depth answers.  No go on the Skypanel. 

I don't own any Lights at the moment. Using a DSLR Nikon D750, I think I can go up to 800 ASA/ISO(still researching that). I was comparing different lights, looking to see what would be a good fit. I pick a Fstop of 5.6, cause I want to avoid purchasing anything that was limited to just Fstop 2.8, better yet 100 Foot Candle. The photometric are from the Kino Flo freestyle 31 510 foot candles at 2ft and the Kino Flo Celeb 401 858 and 401Q 1058 Foot candles at 2ft. All these models have front diffusion built in. I've never worked with any kind of panel light, I guessing you don't need the extra diffusion. At 400ASA/ISO that the two of you have recommended the Freestyle would be a good fit for interviews and the Celeb's would be over kill. And any lights I end up purchasing need to output at least 100 foot candles at 2ft, to be useable in any interviewing set up.

Thank You Both.

 

"need to output at least 100 foot candles at 2ft, to be useable in any interviewing set up."

I'd recommend something a little higher. With your Nikon be sure to test! I say this as when I owned (now) relatively old DSLR's anything over 200 ISO I considered too noisy. Do some tests rating your camera at different ISO's and choose one you consider acceptable. If you where to shoot at 400ASA 5.6 you would at least 200 for caucasian skin. 

So test and it's always a good idea to go with more than less. It's always easier to cut a stop than to add one. 

  • Like 1
Link to comment
Share on other sites

  • Premium Member

Well again, I wouldn't use any interview light at 2' from the subject and wouldn't recommend that you base your calculations on that, as it's just not a realistic working distance for most interviews. You might want to check the photometric tables for those lights at 6' and see if they'll still work for you.

The Kino Flo Celeb is a good light. I would still put at least a 4x4' diffusion frame in front of it though, if not something larger. The diffusion can be closer to the subject than the light. I used the Celeb as a key on these interview frames. The 'Cisco' frame used more diffusion, hence the softer quality to the light.

CROP-2019_OCM_001.thumb.jpg.60e639c6ca400866c8f1b773269c73e6.jpg

CROP-2019_OCM_002.thumb.jpg.348cbc923363ba5c0be087e5d91d95d0.jpg

 

Link to comment
Share on other sites

@Gabriel Devereux

"need to output at least 100 foot candles at 2ft, to be useable in any interviewing set up."

I'd recommend something a little higher. With your Nikon be sure to test! I say this as when I owned (now) relatively old DSLR's anything over 200 ISO I considered too noisy. Do some tests rating your camera at different ISO's and choose one you consider acceptable.

I'm just using the rule of 100 foot candles at 2ft 100 ASA as a tool for researching. Testing the ISO is a good idea! Thank You Again for your answers.

Link to comment
Share on other sites

@Satsuki Murashige

Well again, I wouldn't use any interview light at 2' from the subject and wouldn't recommend that you base your calculations on that, as it's just not a realistic working distance for most interviews. You might want to check the photometric tables for those lights at 6' and see if they'll still work for you.

Thank You for the reference images. I going to check the photometric data, and do a little more research. Again Thank  You.

Edited by Larry Stone
Add "Thank You"
Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
×
×
  • Create New...