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Sunday, December 27, 2015

••◊ 18% Gray Cards and Camera Exposure

With my recent interest in log gamma curves I've been having to do some self-education on the meaning of 18% gray cards.  One of the issues I've run into is that I can't justify the cost of true cinema lenses, so I'm using Zeiss ZF.2 lenses.  I like the sharpness of the glass, but the lack of aperture measurement in T-stops makes getting accurate exposure with S-log a real chore.  Those lenses also make my life difficult because I can't just use a factory calibrated light meter.  See my previous post and tutorial video about why this is.

The first thing I learned is that 18% gray became a reference standard by accident.  The history of it goes something like this...  Kodak originally produced R-27 reference cards in the 1970's with the instructions that you were supposed to take an exposure reading off the card, then open up the aperture 1/2 stop more.  Somehow Kodak's manufacturing organization lost those last instructions for 20-some odd years, so a whole generation grew up believing that the exposure reading was exactly what an R-27 card read in your exposure meter - making everyone's readings 1/2 stop off!  Then it seems video cameras came along and adopted a "standard" that didn't exist.

The "official" ANSI reference is really 12.5% gray, which is 1/2 stop brighter than an 18% gray card.  One thing I forgot to mention is that 18% gray is a rounded number.  An 18% gray card is really 17.7% gray.  The difference between the ANSI/Kodak standard and the vendor specific adoption of a "18% gray standard" is where the confusion lies.  There are much more notable people on the web who have written longer diatribes on why this mythology is confusing...here, here, and here.  I recommend reading all three links to get a more in-depth understanding - after you finish my article!

So as a personal educational experiment I took my Sekonic L-758DR light meter and measured both an incident meter reading of sunlight as well as a reflected reading off an 18% gray card.  They matched exactly.  So my light meter appears to be calibrated to 18% gray.  If it was calibrated to 12.5% gray then the reflected reading would be 1/2 stop more closed than the incident reading.

The good news is that a Kodak R-27 card (18% gray) is still very useful if you use it correctly.  An 18% gray card is also perfect for calibrating your light meter to the particular camera system you own, as vendors shift around the meaning of "middle gray" quite significantly.  Middle gray can mean anywhere from 32-50 IRE in my recent experience, whether you're using rec.709 or a log gamma.  It's camera manufacturer specific.  So while the strict mathematics works out to 44 IRE for 18% gray (assuming 0-108.5 IRE range), the camera manufacturer may choose to put 18% gray at 50 IRE, for instance.  I see a part 2 and 3 of our camera exposure tutorials already!

So here's the other experiment I did.  I took both the Lastolite and R-27 card and measured their actual reflectance using a Gretag Macbeth SpectroEye.  This color science instrument accurately measures color and is used in the print and textile industries.  First, let's look at the R-27 18% gray card.


I know the instrument is measuring in LAB color space, but just stick with me here.  I created a spreadsheet to calculate the reflectance from the L*a*b* numbers.  If a* and b* are 0 this would be an absolutely neutral reference since those are the color coordinates.  So we can see here that the R-27 card is pretty much perfectly neutral.  The L* value of 49 translates to a reflectance of 17.6%, which is almost perfectly the 17.7% reflectance target for an "18% gray" card.  So this inexpensive purchase from Amazon is actually quite good quality, despite being painted on cardboard.  This is a reference chart I can trust to get any future measurements right.

The next experiment I did was with the Lastolite Ezybalance fold out 18% gray/white "card".



This 18% gray "card" is nice to have since it folds up for travel.  Here we can see that the gray side measures an L* value of 45, which translates to a reflectance of about 14.5%.  That's a difference of about 0.3 stop between it and 17.7% gray.  So any reflected light meter exposure measurements will be about 0.3 stop too bright - which is nothing really unforgivable when using a log gamma curve such as S-log3.  It might even be a good thing considering that some of the new Sony cameras have dark noise that could benefit from a bit of over exposure.  If you're really worried about it you can either subtract 0.3 stop from your exposure measurement, or set an offset in your light meter (Sekonic lets you set an offset value).  One other minor issue is that the fabric material appears to be just slightly blue from the a* and b* measurements - but this is just slight.  Probably nothing significant to fret about.  Still, this isn't as dead-on perfect as the R-27 cards.

There's also the option of using the white side of the chart as a reference since camera manufacturers specify both the 18% gray and 90% white targets (at least Sony and Canon do).  We can see here that the white side has an L* value around 90.4.  This translates to a reflectance of 77%, so while we could use this side for white balancing, it appears to be too dark to use as a 90% reflectance chart.

This is all I have for now, but I'm sure we'll be producing some more camera exposure tutorials in the next few months.  I know these items were confusing to me at some point, so there's a need for clear in-depth information.

Tuesday, December 22, 2015

••◊ How to Exposure Your Camera for Log Gamma (part 1) - tutorial video

Last weekend Dominique and I made this short tutorial on how to exposure your camera when using a log gamma picture profile, such as Sony S-log or Canon C-log.  After we finished filming I took the Lastolite "18%" gray target home and measured its reflectance versus the gray cards I bought.  It turns out that the Lastolite product is actually about 15% gray (a bit darker) and the R-27 gray cards I bought are almost exactly 17.7% (what "18% gray" is really supposed to be). So the cards are the more accurate way to go.  The Lastolite target will cause an over exposure of 0.2 stop, which is sort of in the noise when it comes to exposure with log gamma picture profiles.

From a practical point of view you're perfectly OK using either.  In fact, S-log usually likes to be a bit over exposed from a camera noise standpoint.  You're going to color correct the picture either way.  Since S-log3 is so linear all you need to do it make a slight gain adjustment and you're there - easy.


After doing this tutorial I found out a lot more about the history of 18% gray cards.  I'll share that in the next post.

Saturday, October 31, 2015

••◊ Color Correcting Web Videos

I've recently been doing more color correction with Sony's S-log3 and learning as I go.  A problem that keeps creeping up is that Dominique's skin tones always look too dark.  He has very dark skin and tends to wear typical set work black clothes during our blog videos.  I was wondering why in the world does it look so dark when I get the footage into post on my computer(?)  I use a rec.709 (800%) LUT on the monitor and it looks perfect there.  Is there something wrong with my computer monitor?

As with most technical endeavors, I first set out to make a model of the problem in Excel; but first a brief explanation:  Computer monitors use a standard called sRGB.  This color space is pretty much what the web uses today.  Rec.709 is a television and video monitor standard.  Although both standard have the same color primaries, they differ in the encoded gamma curve.  ...and that's where my problem lies.  When I was viewing the footage for rec.709 it wasn't correct for display on an sRGB display, nor web videos.

...so how far off was my video?  That's where Excel comes in. I started by plotting percentage reflectance of an object being recorded versus the output IRE value.  So if you had a test target that gradually increased reflectance from 0% (absolute black) to 100% (absolute white) then this is what the luma waveform monitor would show for each standard.  The biggest take away here is that sRGB produces higher IRE values for any given reflectance (blue curve).  So if you have an 18% gray card in the scene then the standard says it would produce a 44 IRE level with rec.709 and a 50 IRE level with sRGB (assuming data levels 0-255, not broadcast or "legal" levels).



To further describe the relationship of sRGB to rec.709 I plotted the two against one another.  The take away here is that sRGB produces higher values, especially in the dark tones - i.e. where Dominique's skin and clothing are.  The process to fix this in Adobe Premiere is pretty easy.  All we have to do is apply a "luma curve" effect that emulates the blue line in the graph below and we'll have a correction from rec.709 to sRGB - and a much better looking web video.


The question now is how does this look in the real world?  I added the luma curve effect to the project below and set it to use approximately the same curve as the blue line in the previous graph.  Notice how Dominique's skin now has much more detail and his clothes do too?  Much better, I think.


So my learning here is that if you want to output the video for television, use a rec.709 monitor.  If you want to output to the web and you shot in rec.709, you might want to use the luma curve above to correct your footage.  Next time I know.  Don't assume one equals the other.

Sunday, October 25, 2015

••◊ Tutorial: Mounting a Camera on a Car (Matthews Pro Mount Car System)

Dominique and my friend Kevin helped make this tutorial yesterday on how to mount a video camera on a car.  Yes...I know the camera is making a shadow on the car hood in the end shot, but this was a tutorial NOT a film.  I had a hell of a time doing color grading.  The contrast of the outdoor sun with the inner garage bay was a challenge, but the FS7 was up for it.  Also, I had an "adventure" when I found out my ancient CS5 version of Premiere wouldn't import the Prores codec files.  I spent many hours trying to find a work around using free conversion software, which added time and complexity, as well as lowered the video quality.  -- If anyone wants to support the future of these tutorials please consider sending a few bucks my way to pay for Adobe Creative Cloud software!


Sunday, October 11, 2015

••◊ Street Photography (literally): San Diego Festa 2015

Today I tagged along with some friends to Festa in Little Italy. There was the usual smell of garlic, sausage, and tomato in the air with live bands playing at the ends of the blocks.  With the recent heat wave the shaved ice started to look really tempting.

Every year during Festa Little Italy also hosts a chalk art exhibition.  The art only lasts a day.  By the time I write this the streets might already have been washed clean.  So enjoy art while it lasts.






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Saturday, September 26, 2015

Sunday, September 13, 2015

••◊ Tutorial: Creating Soft Beauty Lighting with Double Diffusion

Dominique and I worked on this short tutorial about creating soft beauty lighting over the weekend with my friend and film producer Natalie.  Dom's schedule is being dominated by his senior year school work, so we have to make our tutorial endeavors much shorter and simpler to fit his schedule.

One technical issue I'm having is with color rendering.  When I use Premiere Pro or Windows Media Player the video looks correct.  However, when I play it back online the video looks washed out and magenta.  I hope this is just a Vimeo issue.  The real color grade on this video came out nice, in my humble opinion, so I hope the experience isn't ruined by a technical glitch in Windows 10.


Sunday, June 21, 2015

••◊ How To Setup The Sony FS-7

I know the title sounds a bit overly authoritative, as if there's just one way to setup a camera.  However, this is the way I've found to do it after shooting three projects with the Sony FS-7.  I *love* this little (OK, it's actually kind of big) camera.  With S-log3 and the new color gamuts this camera just rocks.  It's nothing like the bad old days with the FS-700.  With the right settings I feel like I'm looking at a movie...OK, enough with my camera crush, let's get to the reason you're reading this blog entry.

The first thing I like to do is to make sure there is a card placed in each slot.  The FS-7 can quickly change between the two slots by using the "slot select" button right next to the card door.  This way I don't slow down the production when a card runs out of room.  In XAVC-I and UHD the card uses approximately 2GB/min so the 64GB cards that come with the rental kit run out pretty quick.  When the DIT gets handed a card I can continue shooting without a production delay.


One thing you should immediately become familiar with is the button interface on the operator side of the camera body.  Arguably the most important button on this interface is the "user menu" button on the left vertical panel.  This takes you to most (not all) of the settings you'll need to setup the FS-7.

 

The first thing I do is make sure that both cards are formatted.  This operation is pretty obvious, so I won't dwell on it.


As for the base setting, my personal preference is to use the CineEI mode because that takes away a lot of the internal processing, which makes my life easier since I typically turn the internal processing off anyway.  CineEI has some limitations, so read the user manual and understand if you can live with these.  I also like to use the S-Gamut3.Cine color space.  I discussed this in a previous post, which shouldn't be too hard to find, so you can read about it there.



The next thing to setup is the resolution of the camera and the recording codec.  The FS-7 can record in either HD or UHD (which some companies use interchangeably with "4K".  I don't use raw recording and for the budgets I work on I generally don't recommend it either.  S-log3 is about as raw of an image as the producers generally feel comfortable working with.  As for the codec; if I don't know what the producer is going to do with the footage I'll deliver it in XAVC-I.  If it's just a corporate video or a narrative scene it might make more sense to use XAVC-L to use less disk space.  With a sit down interview XAVC-I isn't going to buy you more than XAVC-L.

 


One of the really nice features of this camera is the ability to over- and under-crank footage.  In HD you can record up to 180fps.  In UHD you're limited to 60fps.  Also, be aware that the monitor LUT gets turned off when you go into over-crank mode.  I'll talk about the viewing LUT a bit further down, but this is somewhat inconvenient for setting focus and exposure.  The really nice feature here is that you can set the frame rate in 1pfs increments.
 
To turn on and off the S&Q (slow and quick) mode all you have to do is press the S&Q button on the side of the camera.  The frame rate has to be set in the menu system.





For the default user menu (yes, it's custom configurable) the last item I configure is the viewing LUT.  The "category" setting is set to "LUT".  Then I set the "LUT Select" to "709 (800%)" because rec.709 has much more contrast than S-log3 delivers.  My thinking is that if exposure and white balance look OK with this viewing LUT turned on then my footage should look even better and I probably nailed exposure by eye.  Also, it makes pulling focus feasible.  Pulling focus with the S-log3 look is nearly impossible.  Notice that the "viewfinder" setting is set to "MLUT On"
(monitor LUT = On).  Using a viewing LUT does not affect your recording, it just makes your display look like a post processed image.  The nice thing here is that you can create your own custom LUTs with external software packages and upload them to the camera.

 



A setting that's not in the user menu by default is the setup for ISO/gain.  I always set this configuration to ISO since gain doesn't really make any sense.  What exposure meter reads in gain?  In CineEI mode you use the three settings marked EI.  There's a corresponding H,M,L switch on the side of the camera that allows you to set the camera to the ISO settings you have here.  Each of the EI ISO settings are user configurable.  I always set the M setting to ISO 2000 since that's the native ISO for CineEI mode and what I typically stick to.  Note in the second picture below that the camera shows you the highlight range for each ISO setting.  For ISO 3200 it's 6.7 stops above neutral gray.  For ISO 2000 it's the standard S-log 6 stops above neutral gray.

 
 

The next important setting in the "camera" menu is the "shutter" setting.  This is just my personal preference, but I like to use shutter angle instead of a time setting.  That way when I switch frame frames the shutter speed is always set to the typical 180-degree shutter angle by default.  If you do it by time then you'll have to mess with the shutter speed when setting up an over-cranked or under-cranked shot.  I'm also used to dealing with shutter angle and find it more efficient.  When you press the shutter button on the side of the camera you'll see the second picture below.  Last night I was recording a sports event with a 45-degree shutter angle to give the footage a more "attacking" feel as they often do for action movies.



The camera I have is a rental camera so I just double check the "audio" menu just to make sure that they didn't do anything strange in here.  There's no special settings I use.  I just don't want to be messing around with this on set.


The viewfinder settings allow you to turn on and off the "advertisements" on the LCD.  I prefer to turn as many of these off as possible so I can actually see the image I'm recording.  The FS-7 has a good array of scopes (luma, vectorscope, histogram).  However, when you turn on a viewing LUT, as I did for the viewfinder, you lose the scopes!  So what I do is turn on the scope I want, then disable the viewing LUT when I want to see the scope.  It's much quicker than doing both operations every time I want to turn on a scope.

 



Lastly (at least most of the time...), I setup peaking so I can pull focus.  By default the peaking is set to monochrome.  I typically use the color peaking with red color.  When I want to turn peaking on and off I just use the peaking button on the side of the LCD.

 


Hope this helps.  Leave a comment if you have questions.

Tuesday, June 2, 2015

••◊ New Work: "You're Everything"

Director Tommy Friedman and I shot this music video for Jorandy a few months back, primarily at Jorandy's Mother's home and Belmont Park at Mission Beach.  I used the Canon C100 on a DJI Ronin stabilizer to keep the camera moving at the park and shot on sticks at the house.  By the end of the day my shoulders were completely ruined and I had to give the camera to my AC to do the final shots.  Those handheld stabilizers are amazing, but very strenuous.

One problem we had was that Tommy was super nervous on the roller coaster and kept stopping the camera, which meant the entire cast and crew had to ride the coaster three or four times to get the 3 seconds you see in the video!  Everyone came off that last ride looking a bit green.  I also guess-timated the focal length and rented my friend's Zeiss 15mm ZE lens.  Turns out it was perfect for the roller coaster and a heck of a great lens. (self-pat-on-the-back)

The gaffer and grip, who shall remain anonymous decided to not show up.  So there was a lot of work moving equipment around and keeping track of it all.  We were also chasing the winter sun since the director decided to shoot some of the scenes in places where the setting sun was quickly stealing our light.  Most of the Park scenes were shot using shiny boards or handheld reflectors to provide some "sunny day" contrast to the images.

Stephen Mickelsen of Bad Cat Films did the final color grade and did a really nice job.  The song is a bit bubble gum teeny bopper for my taste, but I think the images sort of match that younger free and fun feel.



Sunday, May 24, 2015

••◊ How To Setup A Lavaliere Microphone

My friend Tommy inspired me to make a tutorial for Video Gear on how to setup a lav mic after he started talking about buying his own audio package.  You know the old saying about "necessity is the mother of all invention.?"  Tommy is s corporate shooter and I knew it was either making a tutorial for Video Gear or taking a call mid-day at work with a last minute panic in his voice as he wonders if he can save the audio from his last gig.  So thanks for the inspiration Tommy.  We tried to keep to the 3 minute format, but this one runs a bit long at 5 minutes just because of so many details you need to know how to get right before pressing record.


Tuesday, May 19, 2015

••◊ Using S-log3 with the Sony FS-7

This last Thursday I received a call from the rental shop I do video blog work for.  Dominique tells me that the brand new FS-7 is suddenly free Saturday and I can shoot with it.  Woohoo!  Breaking in a new camera is a challenge that I readily accept.  The problem is that I get to spend zero time with the camera before the shoot.  So that night I read through (the important parts) of the user manual and study up online.  Lucky for me, there's lots of great user advice out there.

Then in the middle of the night (yes, I know that's weird...I'm a camera guy) I get the idea that I should try out S-log3.  I'm not really a log shooter for blog videos.  I'd rather just shoot with the final look baked in for these short projects that go straight to Vimeo/YouTube.  However, S-log3 is it's own work flow and I figured I'd better learn it sometime.  This was going to be a glorious success or an absolute A-bomb disaster.  What the hell...you only live once.  What could go wrong?

So I got to the shop about 10am on Saturday and had about 30 minutes to do camera prep, which involved setting at least 20 parameters in the camera.  I'm trying to read the PDF of the manual and figure out the menu system at the same time.  Cameras pretty much all do the same fundamental stuff.  It's usually a case of where did the engineers hide it?!

The first problem I needed to resolve is with exposure.  The battery on my Sekonic meter died the morning of the shoot, however I had a DSC ChromaMatch Lt. chart that the kind folks at DSC gave to me during NAB.  As far as I can tell, the chart has an 18% neutral gray background.  S-log3 needs middle gray be exposed at 41 IRE for proper exposure.  So I was in luck.  You can't exposure S-log3 by eye since it will look too dark while recording in camera - see the picture below.  Even while using the rec.709 monitor LUT, the exposure was a bit touchy for "eyeballing-it."  On the Atomos Samurai I had connected I looked at the gray surround and exposed it at exactly 41 IRE using their luma scope.  In the second picture below I cropped the first picture to just the right edge of the card so you can clearly see the exposure level.  The other advantage I had here is that the card has a number of neutral patches, any of which could act as a white balance reference in post.



This brings me to my second point...In S-log3 you only get three white balances: 5600K, 3200K, and 4300K (I think).  So if you need another white balance for a tricky lighting situation or effect, then you'll need to take a few seconds of video with a white balance chart.  The plasma lights I was using were pretty close to 5600K and a good CRI rating, so I chose 5600K and didn't have to do any re-balancing in post.  Even when I tried to white balance using the DSC chart in post, the color wheel barely moved - not enough to bother.

What I still don't completely understand are the SGamut3 and SGamut3.Cine color gamuts.  Did I mention that I only got 30 minutes of camera prep?  Sony has an excellent tutorial posted on their web site.  Essentially SGamut3 is a super wide gamut meant for ultra-HD TV (i.e. UHD rec.2020), and ACES work flows.  SGamut3.Cine is meant for film print emulation and DCI-P3 work flows.  Which works better for rec.709/sRGB displays?  I'm not that far yet.  Sony says they have a .cube file to convert SGamut3.Cine to 709, so that might be the way to go until I learn more.  The Sony Community Forum posts also suggest this from a few users.

So...how about the color grade.  Well, let me tell you - it's a whole different world having a professional 10-bit codec.  No more suffering the limitations of 8-bit banding and artifacts.  You don't have to be gentle during color correction.  The first picture below is of the uncorrected image.  In the second picture I started by raising the recorded middle gray (41 IRE) to 55 IRE for rec.709/sRGB displays,  then I applied the toe and knee to the curve to add back in highlights and shadows.  The third picture below shows that I added about 40% saturation to the image, which is what Sony seems to recommend for S-Gamut3.Cine.  This make the skin tones about perfect for Dominique.


 

You can use S-log3 in "Custom" or "Cine EI" modes on the Sony FS-7.  In custom mode you can tweak the image parameters, but in Cine EI mode the list of parameters you can change decreases, more like a film stock.  Most notably, in Cine EI mode there is no in-camera noise reduction.  For those used to standard video cameras that do noise reduction, having no noise reduction by default can be a bit discerning at first.  Cine-EI is meant for people that will have a definite post production work flow and treats the image more like a digital negative than image processed video.

One thing I noticed is very slight red noise in Dominique's black shirt.  However the noise was so minor that it didn't really warrant any post noise reduction.  Members of the Sony Community forms suggested that you're OK up to about ISO4000 with noise, but until I can test it for myself I won't know for certain.  If you can't see it in the picture below, then it's probably not going to be an issue.  Other members suggested recording at a lower ISO sensitivity to reduce noise further.  At a native ISO of ISO2000 there's certainly room to do so.


Which brings me to my last topic: ISO sensitivity in Cine-EI mode.  By design, S-log3 has six stops above middle gray when you use the native ISO rating.  If you reduce the ISO sensitivity in Cine-EI mode you lose 1 stop of highlight range for every 1 stop decrease in sensitivity.  For instance, if you were wanting to record at ISO1000 to reduce noise in a dark scene you would only have five stops of highlight range.  Conversely, if you increased the sensitivity to ISO4000 you would have seven stops of highlight range, but a hella-ND filter to put in the matte box! 

The sensitivity in Cine-EI mode is just like push and pull processing with film.  The digital negative has a set sensitivity that does not change in Cine-EI mode; in this case ISO2000.  If you set the ISO setting one stop lower in Cine-EI mode (i.e. ISO1000) and expose properly for that, your digital negative will be one stop over-exposed and you'll have to effectively do pull processing in post production.  The good thing about this feature with a digital camera is that the monitors can be compensated look like the ISO sensitivity you choose, which gives you immediate feedback on what you're about to record.  Also, in Cine-EI your dynamic range doesn't change.  It's always 14-stops with S-log3.  You only trade off highlights for shadows and vice versa if you diverge from the camera's native ISO2000.

I personally found that the full six stops of highlight range was more than enough.  When I viewed the white walls at the left of the frame with a rec.709 monitor LUT I was worried that they were going to be greatly over-exposed - and even went to the extent of flagging some light off of them.  However, when I got in post I saw that the walls were nowhere near over-exposed - not even close to nowhere near.

It's been a wonderful camera so far and I look forward to using it again.  Now if only I could afford one!