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Day one camera evaluations

Geoff Boyle
 

Well, today was a day with a lot of slop in it to allow for "discoveries"

First i found thst I'd madexa basic mistake inbthe footcandle level and stop calculations. We needed twice as much level as I thought we did.

Had to give up some bright ideas about HDR and colour saturation.

That was OK, we shot the tungsten having finally sorted the reflection problems with bloody shiny chart's.

When we switched to led daylight and balanced to the same stop as we used for tungsten everyone said I had made a mistake and that I'd lit the daylight a stop brighter than the daylight.

Lot's of toing and froing and eventually i loaded the tungsten material into prelight and i had shot it at the stop i said i had.

So what was happening? To the eye it looked much brighter now. WTF! Just shoot it. We did and then loaded it in to prelight and it was 2/3rds to a stop brighter than the tungsten.

Go back and remeasure with 2 minoltas and 2 sekonics, 3 of the meters agreed exactly, one was a little different but in the other direction!

We checked again, same, so we checked on the odyssey waveform and sure enough, the levels were different but the meters said they were the same.

Now this is an issue for me because in the real world you cant use a camera and waveform to prelight or match location and studio or lots of things. We need to be able to rely on our meters.

I'm not sure how I'm going to go ahead.

This has thrown me big time.

cheers
Geoff Boyle NSC
EU based cinematographer
+31 637155076
www.gboyle.co.uk

alister@...
 

Could this be due to gaps in the spectrum of the LED lights? If the light meter takes an average across all wavelengths, any gaps will result in a lower average reading. Or the lightmeter responds to a particular wavelength that is weak from the LEDs. So the light meter thinks there is less light and that will offset your exposure upwards. Then depending on how the camera responds to the same spectrum, you will have a different exposure, which may vary from camera to camera.

Have to say I that on the whole I have found my Sekonic’s to be well within the right ball park for most cameras under tungsten or real daylight, but tests done under LED’s and Florries have often shown a much greater margin of error.

It’s an interesting result. Anyway you can test under real sunlight, even if it’s just one camera to see if it’s the lights creating the exposure anomaly.


Alister Chapman

DoP - Stereographer
UK Mobile +44 7711 152226
US Mobile +1(216)298-1977


www.xdcam-user.com    1.5 million hits, 100,000 visits from over 45,000 unique visitors every month!  Film and Video production techniques, reviews and news.



Art Adams
 

What Alister said. Light meters primarily read green, so if an LED punches that hard or has a spike somewhere else then it might throw things off. Meters don't care about metamerism, they're just counting photons.

I'd look at daylight as a comparison, or gel tungsten blue (not ideal as it passes more IR than daylight, but still informative).

 There are certainly variances in what light meters do under daylight vs. tungsten, and the same with cameras. I've seen discussions of that on CML in the past.

Which LED lights are you using?
--
Art Adams
Director of Photography
San Francisco Bay Area

Geoff Boyle
 

2 Minolta meters, a 4 & a 6 and 2 different Sekonics.

It can only be spectrum issues but if so this is a bloody nightmare.

Im really glad that Richard Lewis was there, he was a huge help, we tried presets, we tried autobalance, we tried Alexa, we tried Venice.

I'll try real daylight but we can't do that for all the camera's.

I think I'm going to change to real tungsten V led tungsten so we have a pure like for like.

cheers
Geoff Boyle NSC
EU based cinematographer
+31 637155076
www.gboyle.co.uk


On Mon, 9 Jul 2018, 18:19 , <alister@...> wrote:
Could this be due to gaps in the spectrum of the LED lights? If the light meter takes an average across all wavelengths, any gaps will result in a lower average reading. Or the lightmeter responds to a particular wavelength that is weak from the LEDs. So the light meter thinks there is less light and that will offset your exposure upwards. Then depending on how the camera responds to the same spectrum, you will have a different exposure, which may vary from camera to camera.

Have to say I that on the whole I have found my Sekonic’s to be well within the right ball park for most cameras under tungsten or real daylight, but tests done under LED’s and Florries have often shown a much greater margin of error.

It’s an interesting result. Anyway you can test under real sunlight, even if it’s just one camera to see if it’s the lights creating the exposure anomaly.


Alister Chapman

DoP - Stereographer
UK Mobile +44 7711 152226
US Mobile +1(216)298-1977


www.xdcam-user.com    1.5 million hits, 100,000 visits from over 45,000 unique visitors every month!  Film and Video production techniques, reviews and news.



Geoff Boyle
 

Sky panels as previously discussed.

They are prolific in our business.

cheers
Geoff Boyle NSC
EU based cinematographer
+31 637155076
www.gboyle.co.uk


On Mon, 9 Jul 2018, 18:25 Art Adams, <art.cml.only@...> wrote:
What Alister said. Light meters primarily read green, so if an LED punches that hard or has a spike somewhere else then it might throw things off. Meters don't care about metamerism, they're just counting photons.

I'd look at daylight as a comparison, or gel tungsten blue (not ideal as it passes more IR than daylight, but still informative).

 There are certainly variances in what light meters do under daylight vs. tungsten, and the same with cameras. I've seen discussions of that on CML in the past.

Which LED lights are you using?
--
Art Adams
Director of Photography
San Francisco Bay Area

Daniel Rozsnyó
 

I would say the meters work with monochrome sensors to measure the total energy it receives, whereas the cameras do have colour filters.

The (poor CRI) LEDs exhibit a narrow band of wavelengths where the energy is radiated, and with the same peak of sensitivity in the camera filter, it will produce a well exposed image. The meter will have no energy to count in from the walleys between the LEDs peaks, so it will show you a number as
if the scene was darker.

I doubt there would be a good way out, as every camera may use different colour filter dies, that are specifically wide with a peak at wavelength other than the next camera.

That makes up the usual knowledge, that you get hard time to colour-match multiple-vendors under such a poor quality light (same is valid not just for LEDs, but also for fluorescent, that has random peaks that add up in different ratio based on the camera filter die).

On the other side, such narrow band lighting will give you more colour separation and may lead to a wider gamut :-)


Daniel Rozsnyo
camera developer
Prague, Czech Republic



On 07/09/2018 07:04 PM, Geoff Boyle wrote:
So what was happening? To the eye it looked much brighter now. WTF! Just shoot it. We did and then loaded it in to prelight and it was 2/3rds to a stop brighter than the tungsten.

Go back and remeasure with 2 minoltas and 2 sekonics, 3 of the meters agreed exactly, one was a little different but in the other direction!

Video Assist Hungary
 

wow. I am jost a noob here, but that sounds frigthening...Geoff, I am in Hungary and happy to fedex you a brand nex Sekonic 858D. It just came in a few days ago.
But what would be very interesting is to see it with a C-700 or C-7000 color meter to see the spectrum...

-----------------------------

Balazs Rozgonyi IMDb
CEO - technical director / VA - DIT -  3D

0-24 central phone: +36 70 626 2354

Video Assist Hungary - Technology for your vision IMDb IMDbPro
The only video rental place in Hungary!





On 2018. Jul 9., at 19:24, Art Adams <art.cml.only@...> wrote:

What Alister said. Light meters primarily read green, so if an LED punches that hard or has a spike somewhere else then it might throw things off. Meters don't care about metamerism, they're just counting photons.

I'd look at daylight as a comparison, or gel tungsten blue (not ideal as it passes more IR than daylight, but still informative).

 There are certainly variances in what light meters do under daylight vs. tungsten, and the same with cameras. I've seen discussions of that on CML in the past.

Which LED lights are you using?
--
Art Adams
Director of Photography
San Francisco Bay Area

Mark Kenfield
 

That's both strange and a little scary Geoff. 

How were you using the meter? Reading footcandles? Reflected meterings? Or incident readings? 

Cheers,

Mark Kenfield 
Cinematographer 
Melbourne 

0400 044 500

Geoff Boyle
 

Thanks for the offer of the meter but we already have four, and two of them were recently calibrated. Not the same meters or companies doing the calibration!


cheers
Geoff Boyle NSC
EU based cinematographer
+31 637155076
www.gboyle.co.uk


On Mon, 9 Jul 2018, 18:34 Video Assist Hungary, <info@...> wrote:
wow. I am jost a noob here, but that sounds frigthening...Geoff, I am in Hungary and happy to fedex you a brand nex Sekonic 858D. It just came in a few days ago.
But what would be very interesting is to see it with a C-700 or C-7000 color meter to see the spectrum...

-----------------------------

Balazs Rozgonyi IMDb
CEO - technical director / VA - DIT -  3D

0-24 central phone: +36 70 626 2354

Video Assist Hungary - Technology for your vision IMDb IMDbPro
The only video rental place in Hungary!





On 2018. Jul 9., at 19:24, Art Adams <art.cml.only@...> wrote:

What Alister said. Light meters primarily read green, so if an LED punches that hard or has a spike somewhere else then it might throw things off. Meters don't care about metamerism, they're just counting photons.

I'd look at daylight as a comparison, or gel tungsten blue (not ideal as it passes more IR than daylight, but still informative).

 There are certainly variances in what light meters do under daylight vs. tungsten, and the same with cameras. I've seen discussions of that on CML in the past.

Which LED lights are you using?
--
Art Adams
Director of Photography
San Francisco Bay Area


Mako Koiwai
 

Sunny 16 Rule is always a good quick test …

makofoto, s. pasadena, ca

Art Adams
 

I would say the meters work with monochrome sensors to measure the total energy it receives, whereas the cameras do have colour filters.

The sensors in exposure meters have spectral shaping filters so they only read the visible spectrum and not UV or IR. They peak around 525 ("green") and sensitivity tends to roll off toward the upper and lower ends of the visible spectrum.

This usually isn't a problem, because white light always has green in it. What changes in CCT is the ratio of red (the range above green) and blue (the range below green). Once you start working with very pure hues (deep red or full CTB) modern meters will read as much as a stop less for the same quantity of light.

You can see this in how meters respond differently under tungsten vs. daylight.

If someone repeats these camera tests exactly except they use Cineos, or Lightpanels, or LiteRibbon, or Area 48s, or Nilas—or any of the other LED lights commonly being used in the industry—they are going to get different results. That's why I said, a couple of weeks ago, that unless this test uses broad spectrum lights that are commonly used and understood in the industry (tungsten and HMIs) it's not testing cameras but lights. And light meters will respond to those lights in different ways depending on the LED technologies employed. (As will cameras.)

There'll still be some variation between tungsten and daylight sources when using a light meter, but at this point you don't know if it's the normal variation or an exaggeration due to the interaction between the meter, the LED light and the camera.
 
--
Art Adams
Director of Photography
San Francisco Bay Area

Lars Borg
 

AFAIK both AMPAS and EBU have methods for evaluating the color quality of luminaries used with camera capture.
(I’m familiar with the EBU method only)
Is anyone using those methods?
Would be helpful to know how well the lights perform when doing a camera test.

Thanks,

Lars Borg  |  Principal Scientist  |  Adobe 

_._,_._,_

Bob Kertesz
 

On the other side, such narrow band lighting will give you more colour
separation and may lead to a wider gamut :-)
Love your optimistic outlook.

-Bob

Bob Kertesz
BlueScreen LLC
Hollywood, California

DIT, Video Controller, and live compositor extraordinaire.

High quality images for more than four decades - whether you've wanted
them or not.©

* * * * * * * * * *

Geoff Boyle
 

I know we discussed this Art but its a real world test and has already provided very useful info for a real world situation.

We don't get to shoot with theoretical black nody radiators, we have to use what we have.

Yes i know it will vary from lsmp to lamp which is why we're using the most common.

Can not do this again, again, again.

cheers
Geoff Boyle NSC
EU based cinematographer
+31 637155076
www.gboyle.co.uk


On Mon, 9 Jul 2018, 20:34 Art Adams, <art.cml.only@...> wrote:
I would say the meters work with monochrome sensors to measure the total energy it receives, whereas the cameras do have colour filters.

The sensors in exposure meters have spectral shaping filters so they only read the visible spectrum and not UV or IR. They peak around 525 ("green") and sensitivity tends to roll off toward the upper and lower ends of the visible spectrum.

This usually isn't a problem, because white light always has green in it. What changes in CCT is the ratio of red (the range above green) and blue (the range below green). Once you start working with very pure hues (deep red or full CTB) modern meters will read as much as a stop less for the same quantity of light.

You can see this in how meters respond differently under tungsten vs. daylight.

If someone repeats these camera tests exactly except they use Cineos, or Lightpanels, or LiteRibbon, or Area 48s, or Nilas—or any of the other LED lights commonly being used in the industry—they are going to get different results. That's why I said, a couple of weeks ago, that unless this test uses broad spectrum lights that are commonly used and understood in the industry (tungsten and HMIs) it's not testing cameras but lights. And light meters will respond to those lights in different ways depending on the LED technologies employed. (As will cameras.)

There'll still be some variation between tungsten and daylight sources when using a light meter, but at this point you don't know if it's the normal variation or an exaggeration due to the interaction between the meter, the LED light and the camera.
 
--
Art Adams
Director of Photography
San Francisco Bay Area

Daniel Drasin
 

Geoff writes:  this is an issue for me because in the real world you cant use a camera and waveform to prelight or match location and studio or lots of things. We need to be able to rely on our meters.

-----

Two questions here:

Why has such an obvious discrepancy not been noticed before? We've been working with discontinuous-spectrum lighting and single-sensor cameras for how long, now?

Can your tests produce a ballpark compensation factor that can apply broadly to measuring daylight LED sources with existing meters? If we're talking about a roughly one-stop discrepancy, then at worst the result of such compensation should be a fraction of a stop, and in most cases well within correctable limits, no?

Dan Drasin
Producer/DP
Marin County, CA



Adrian Jebef
 

The camera is the meter. It is also the meter that records what illumination it is measuring. A well executed camera system will give you a reading through its video out that will exactly match the recorded file later on in Post. The camera is the only meter that actually matters IMHO. I think of the camera as a pixel spot meter. There is nothing as exact as the camera itself. All cameras are different. Just as all incident and reflective spot meters are different. They can all be “calibrated” but they will all still produce slightly different results.

Personally when I have tested multiple camera systems, lights, lenses etc I prefer to set exposure based on the camera itself. A simple grey card will allow different cameras to align at the same base code value (ie middle grey or whatever your intended exposure is). Note the actual exposure will be different to each camera as each camera is it’s own meter. One may be at a 4, another at 4 1/4, the last at a 2.8+9/10ths. Although counterintuitive this is the way I believe camera comparison tests should be carried out.

Adrian Jebef
Digital Imaging Technician

Geoff Boyle
 

Once again.

The camera cannot be the meter, I am a cinematographer not a DiT.

I don't always have a camera when in pre-lighting or on s recce.

cheers
Geoff Boyle NSC
EU based cinematographer
+31 637155076
www.gboyle.co.uk


On Tue, 10 Jul 2018, 05:51 Adrian Jebef via Cml.News, <adrianjebef=yahoo.com@...> wrote:
The camera is the meter. It is also the meter that records what illumination it is measuring. A well executed camera system will give you a reading through its video out that will exactly match the recorded file later on in Post. The camera is the only meter that actually matters IMHO. I think of the camera as a pixel spot meter. There is nothing as exact as the camera itself. All cameras are different. Just as all incident and reflective spot meters are different. They can all be “calibrated” but they will all still produce slightly different results.

Personally when I have tested multiple camera systems, lights, lenses etc I prefer to set exposure based on the camera itself. A simple grey card will allow different cameras to align at the same base code value (ie middle grey or whatever your intended exposure is). Note the actual exposure will be different to each camera as each camera is it’s own meter. One may be at a 4, another at 4 1/4, the last at a 2.8+9/10ths. Although counterintuitive this is the way I believe camera comparison tests should be carried out.

Adrian Jebef
Digital Imaging Technician

Adrian Jebef
 

Sorry but the camera is the best meter available. Always. It is the only constant that actually matters. I understand your point on prelights and prep. It is valid. An analogue incident or spot meter is a fine tool to use but it will always be incorrect when compared against the specific camera you use to record your images. Personally I would not put much faith in an analogue light meter when shooting digitally. By current standards it is a blunt imperfect instrument. At the end of the day you must decide which instrument is more important and precise when recording your images.

And I don’t think being a DIT has much to do about it. It’s a simple matter of choosing the right tool for the job.

Cheers,

Adrian Jebef
Digital Imaging Technician

Geoff Boyle
 

This is the kind of reply that drives me crazy.

It has everything to do with you being a DIT and having a very narrow view.

A cinematographer doesn't always have a camera and a monitor, in fact we often have to make lighting and exposure decisions without a full kit and only the meter in hand.

This is the reason i use a meter, or several meters for these tests.

Its not about testing a camera in isolation, its how it works as part of a chain of kit used by a cinematographer.

That chain includes lights and meters as well as post.

cheers
Geoff Boyle NSC
EU based cinematographer
+31 637155076
www.gboyle.co.uk


On Tue, 10 Jul 2018, 07:18 Adrian Jebef via Cml.News, <adrianjebef=yahoo.com@...> wrote:
Sorry but the camera is the best meter available. Always. It is the only constant that actually matters. I understand your point on prelights and prep. It is valid. An analogue incident or spot meter is a fine tool to use but it will always be incorrect when compared against the specific camera you use to record your images. Personally I would not put much faith in an analogue light meter when shooting digitally. By current standards it is a blunt imperfect instrument. At the end of the day you must decide which instrument is more important and precise when recording your images.

And I don’t think being a DIT has much to do about it. It’s a simple matter of choosing the right tool for the job.

Cheers,

Adrian Jebef
Digital Imaging Technician

Christophe Persoz
 
Edited

Geoff,

Have you the possibility to make a full spectrum analysis of the different sources (meaning the ones that have a CCT match, i.e. Tungsten+CCT and LED in daylight). 
Maybe that could reveal the discontinuity of the LED light spectrum you are using and hence, it could explain the difference between what you are getting on the light meter and what you shoot. 
It's a quite longer process, but it could helps. 

Those spikes are barely visible or sometime invisible to human where as some cameras can enhance and reveal them a lot. 

As already suggested, Sekonic C700 or Asensetek Light Passport can do that. 

Then, how to workaround this with our tools on the case LED fixture market... that's a nightmare.

Good luck and thanks for the tests!

Christophe Persoz
Cinematographer
Motion Control operator

France  and Switzerland based
+33 625 71 00 61
www.christophepersoz.com