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Sony HVR-Z1 / HDR-FX1
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Old June 15th, 2005, 10:56 AM   #1
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Some thoughts on exposure

Out of curiosity and trying to understand my FX1 better I dove into some experimenting with exposure today.

The iris covers almost 6 stops from F1.6 to F11:
1.6, 1.7, 1.8, 2.0*, 2.2, 2.4, 2.6, 2.8*, 3.1, 3.4, 3.7, 4.0*, 4.4, 4.8, 5.2, 5.6*, 6.2, 6.8, 7.3, 8.0*, 8.7, 9.6, 10, 11*

The star* marks standard full stops (each full stop doubles the light volume through the lens).

The gain is measured in dB with 9 dB and 18 dB being standard settings for medium and high. An amplification of 6 dB is a double, so in theory the 9 dB would equal 1.5 stop and 18 dB would equal 3 stops. So I hooked up the FX1 and recorded three segments with each gain setting (L/M/H) and compensating with iris to make them even in exposure. They were almost identical when watched in succession on a monitor, evidence that gain works as I thought.

The exposure range of the CCD is harder to measure. I set up the FX1 to shoot an evenly lit concrete wall and changed the shutter speed and tried to notice on a monitor the two ends: A) when texture is completely gone due to over-exposure (=all white), B) when texture is completely gone due to under-exposure (=all black). I tried this with different iris values for error checks. It resulted in about 6 stops of exposure range. I don't know if this is a proper way to measure the CCD.

To sum up, the CCD accepts an image with contrast of maximum 6 stops, and this exposure "window" can be met by adjusting iris almost 6 stops and gain 3 stops. So at our disposal we can cover about 15 stops of light dynamics, but only 6 stops at once.

The exposure range is actually less than I expected, and this makes it even more important to adjust the exposure right while shooting. As a rookie I have been very careful about not getting over-exposed areas in my shots (watch out for those zebras!). Sometimes this has been on the expense of getting the overall lightness I want. But after a lot of practical shooting and this small exercise, my conclusion is: you are always going to get over- and under-exposed parts in your shot, because the contrast range of the CCD is hugely out of league of most shiny outdoor scenes. So it's all a matter of setting the exposure "window" according to what subjectively is the most important parts in the scene. Do I care enough about that well lit side of the boat so I want texture in it? Do I really need texture in that dark area? What overall lightness do I want?

What do the pros have to say about this?
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Old June 15th, 2005, 11:31 AM   #2
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I actually like a few zebras in my shots, otherwise the whole scene is often to dark unless lighting is being tightly controlled.
For outdoors, I opten let the clouds just start to zebra and they come out looking just right.
For me, I worry about how the camera creates noise in dark areas that are lightened up in post, so I overexpose a little to compensate.
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Old June 15th, 2005, 11:59 AM   #3
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I'm hardly a pro, but your report is consistent with what I've experienced shooting with an FX1.

I usually try to get as much lattitude as I can out of the scene of choice, trying to get what colour is available in the image going from fully saturated to black to maximize the dynamic range of the stored image. If I find the image has a lot of blown out highlights I experiment with the Cinematone Gamma and polarizers. In general, I try to open the aperature 1/2 stop past when I first see 100% zebras. When I first started shooting I was afraid of blowing out the image, and as a result I shot a lot of underexposed footage.

It does take some fine tuning though. A friend of mine (unfamiliar with the camera, and thus using auto-exposure) managed to ignore the ND filter warnings while shooting white plastic against bright concrete and pretty much blew out all the interesting texture to be had.

-Steve
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Old June 15th, 2005, 12:10 PM   #4
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Interesting to hear about it Jeff. I was under the impression that zebras was a big no-no, because there's nothing you can do about it in post (overexposed = no texture left). I've been editing some footage of boats recently that was overexposed, and I was really annoyed about how I shot it.

But I've been a bit stupid and overdone the zebra thing, since clouds, wave reflections and solid white areas dont contain much texture and can cope with a slight overexposure.
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Old June 15th, 2005, 12:20 PM   #5
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Quote:
Originally Posted by Steven White
I usually try to get as much lattitude as I can out of the scene of choice, trying to get what colour is available in the image going from fully saturated to black to maximize the dynamic range of the stored image. If I find the image has a lot of blown out highlights I experiment with the Cinematone Gamma and polarizers. In general, I try to open the aperature 1/2 stop past when I first see 100% zebras. When I first started shooting I was afraid of blowing out the image, and as a result I shot a lot of underexposed footage.
Steve, this is exactly where I'm at right now, after a lot of tests and annoyances... ;-) I have too much underexposed footage. But on the other hand, it is not completely lost, a lot of it can be saved in post.

I've been testing Cinematone some, and I like it too. I've moved away from thinking it has to do with creating a "film" look, but instead use it for the same purposes as you do.
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Old June 16th, 2005, 12:48 PM   #6
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Most of my stuff is in the theatre and my rule is no zebras on faces but a little zebra on white shirts or costumes is OK for the same reasons as mentioned before--the dark areas will suffer if the bright spots aren't right on the limit. Distance shots with full stage camera is even more biased for stage depth rather than faces because you can't see them at that distance anyway, but viewers like to know what the stage looked like. My FX1 settings recently have been 1/60, 9db at between F2.8 and F5.2 average at F3.1.

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Old June 16th, 2005, 03:20 PM   #7
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However, it all depends where those zebras are ? Are they at 100, 90, 80, 75, 70 IRE, etc. What kind of zebras are you talking about? Of course, if zebras are at 70 and you have zebras on the sky, that means everything is way overexposed. SO I am assuming that your zebras are at 100 unless you are measuring the blue part of the sky where you could have zebras if they are much lower than 100.

Lets be more specific ...

Quote:
Originally Posted by Jeff Baker
I actually like a few zebras in my shots, otherwise the whole scene is often to dark unless lighting is being tightly controlled.
For outdoors, I opten let the clouds just start to zebra and they come out looking just right.
For me, I worry about how the camera creates noise in dark areas that are lightened up in post, so I overexpose a little to compensate.
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Old June 17th, 2005, 02:13 AM   #8
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Quote:
Originally Posted by Augusto Manuel
However, it all depends where those zebras are ? Are they at 100, 90, 80, 75, 70 IRE, etc. What kind of zebras are you talking about? Of course, if zebras are at 70 and you have zebras on the sky, that means everything is way overexposed. SO I am assuming that your zebras are at 100 unless you are measuring the blue part of the sky where you could have zebras if they are much lower than 100.

Lets be more specific ...
I've only used the 100+ setting for zebras so far. I only want to know what parts of the scene will be overexposed.

If zebras are set to 70 and you have zebras on the sky that could mean that the sky wont be overexposed, since the zebras only mark the 70 level, nothing above and nothing beyond. Then I have to judge for myself in the viewfinder/LCD if I can see texture well enough in parts that are lighter. For me that is a more difficult way to work.

A zebra setting of 50 (not available on my FX1) could perhaps be useful for identifying where the center of the exposure range will be in the scene.
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Old June 17th, 2005, 06:08 AM   #9
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I set zebras at 90 -100 to know when I am at clipping. I had to use a DigiBeta last week with zebras set at 70 and it was a pain. Tried to reset to 90 in the menu, but couldn't get the settings to stick.
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Old June 17th, 2005, 07:51 AM   #10
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Good post Bjorn. Although your diaphragm will stop down to f11 it may well shoot at f/16, f/22 and even f/32 if it's anything like the exposure control of the PD170. These apertures are there for those that have the auto shutter set to off and who ignore the silent scream for the NDs to be inserted.

Thing is that at these tiny apertures the exposure may be correct but the pictures get decidedly soft due to difraction losses. With 1/3" chipped camcorders it's best to consider f8 as your smalest aperture, and don't go smaller.

tom.
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Old June 17th, 2005, 09:23 AM   #11
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Actually, both ZEBRAS at 70 and 100 are both quite useful and in more expensive camcorders, both can be on at the same time. 70 zebras will appear as the usual striped lines and 100 as a squarish type of pattern. Wish Sony had provided this type of zebras for the Z1. In the Z1 you can adjust the zebras but can have only one type all the time.

To me zebras at 70 are as useful as 100 if not more important than 100 zebras. Actually in DV, your overexposure really start at 107 and not at 100. Zebras at 70 basically are equivalent to the 18% gray card in film and it is designed to set exposure on a person's face. Once you see a bit of zebras set at 70 on a person's face, then you set your iris aperture.

If the subject is speaking in front of an audience and has large bright windows behind him, I dont care if the windows are blown out more than 107. I read the person's face with zebras set at 70 and I let the windows overexpose, otherwise, you have an underexposed subject. For interviews is nice to read the face with zebras at 70 and overall highlights within the frame at 100. Though nothing beats a waveform.

For a scene that will placing the subject in a shadow area in which parts of the frame show sunlight, you have to determine if you want the subject appear as part of the shadow area, slightly underexposed or disregard the shadow area and expose correctly with zebras at 70 on his/her face. If he/she needs to appear as part of the shadow area, I let the 70 zebras dissapear and expose him/her just above the shadow area.

But another thing you need to do regarding exposure since we cannot have access to waveforms all the time, in addition to the help of zebras is to build a "rapport" with the viwefinder of your camera. This is somewhat hard because if you donot have a feel for you viewfinder or you are not sure if you have a correctly adjusted B & W viewfinder, the you should not do it. But in addition to metering with zebras, with my correctly adjusted viewfinder, I look for the tonality of the image. If I see zebras on a person on a wide shot and the sky blooming a bit too much, I back off just until I have a nice balance and the zebras have just dissapeared on a person's face and the sky or parts of overexposure are no longer blooming. This is hard to do sometimes in very bright sunny days. In any case, it is nice right in those situations to have the type of cameras that display more than one type of zebras.





Quote:
Originally Posted by Bjorn Moren
I've only used the 100+ setting for zebras so far. I only want to know what parts of the scene will be overexposed.

If zebras are set to 70 and you have zebras on the sky that could mean that the sky wont be overexposed, since the zebras only mark the 70 level, nothing above and nothing beyond. Then I have to judge for myself in the viewfinder/LCD if I can see texture well enough in parts that are lighter. For me that is a more difficult way to work.

A zebra setting of 50 (not available on my FX1) could perhaps be useful for identifying where the center of the exposure range will be in the scene.
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Old June 17th, 2005, 10:53 AM   #12
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Tom, thank you for this valuable information. Although I rarely use AE, I will in my manual shots now stay away from anything smaller than f8. It's quite easy to kick in one of the two ND filters when needed. They seem to block 2,5 stops and 5 stops respectively.

Augusto: really good pactical info. Thanks for sharing. From looking at my footage I got a sense that the FX1 is slightly forgiving about overexposure, and your 107 range info explained that. Since I've gone trough a change of mind in how I measure exposure, I will try your 70 zebra tips in the future. I will try to develop a sense for what in a scene should be at 70 zebra (faces and other stuff) to get the look I'm after. In that respect the 100+ setting is not very useful.

My conclusion: In most shots (w/o controlled light) it is almost impossible to get no parts at the overexposure end of the range. With a little shooting experience those areas stand out even without zebras on the viewfinder. They are really bright and textureless. One can double check for them with a 100+ zebra setting. But more important is getting the 70 zebra stripes at the correct areas of the shot (depending on what look your after), because that is an easier way to control exposure range "window", ie get max tonal range and correct brightness for parts that count in the scene.

Interesting what you mention about the viewfinder. I know that the (internal)viewfinder would probably give me the best representation of the exposure, but I use mostly the external LCD one because its most convenient. However, there's no way at all to get an indication of exposure just by looking at the overall lightness of the LCD. There's just too much stray light all around that interfere. So zebras is the only way to go for me. Also I have to set LCD color to max to get anything that resembles what goes into the tape.

My usual procedure before a shot is to bump into AE for a second and then back to manual to get a coarse setting fast. Then by guide of zebras adjust exposure to the specific look I want for the shot.
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Old June 18th, 2005, 09:46 AM   #13
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Underexposure in DV - YES ! Overexposure - A big NO! NO!

I keep repeating the following to everyone. In video and particularly in DV, the footage looks its best, specially in daylight, if we underexpose by 1/2 stop or even by a full stop. No matter how your subject looks like or the type of scene, the best looking footage I have always gotten is when I have applied that basic rule of thumb (should I call it axiom?) .

It is amazing to see how much footage shot out there in DV blooms with blownout highlights. The trick is to get rid of those unwanted extreme highlights because that is when your video screems "I WAS SHOT ON DV !!!!! ".

We must always remember, and we all seem to know this, that the best looking footage is the one that shows detail in the dark areas and detail in the highlight areas with good solid midtones. In DV this is hard because even the Z1/FX1 have a limited latitude even though Sony has blessed that camera with Cinematone 1 & 2 and BlackStretch which helps alleviate somewhat the problem. I guess the problem also lies on the still small 1/3 inch chips.

But I repeat, underexposing just a bit to get rid of highlights is the way to go if you want your footage to look "more cinematic". (phew! that word is kind of a buzzword these days ...)

It is really amazing also to see footage shot in 24p which has peeking highlights. In a way, it is kind of contradictory. Here you have footage where the videographer is trying to emulate footage shot on film, and on the other hand, he/she is blowing the highlights like crazy. Come on ! Give me a break! That 24p footage s going to look more videoish -and ugly video by the way- with blown out highlights no matter if it was shot on 24 or 30, with any type of pull-down or not.

Get rid of those highlights and show detail in your blacks, that's my humble advice ...
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Old June 18th, 2005, 12:30 PM   #14
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Quote:
Originally Posted by Augusto Manuel
I keep repeating the following to everyone. In video and particularly in DV, the footage looks its best, specially in daylight, if we underexpose by 1/2 stop or even by a full stop. No matter how your subject looks like or the type of scene, the best looking footage I have always gotten is when I have applied that basic rule of thumb (should I call it axiom?.
Same rule I followed with 8mm, Super 8, and 16mm. And, of course, DV. Not only does it prevent blown-highlights (which are horrible when HD is projected to a big screen because projectors can also be set too hot as folks try to get max brightness) it brings the colors out.

By setting FX1/Z1 to 100 you know that the signal is AT the clipping point for RGB-based NLE's like Premiere.

If you keep only tiny areas -- never any part of a face -- at zebra, you will likely find these peaks to be under 110 IRE which is the maximum that can be recorded to tape. (YUV-based NLE's allow super whites up to 120IRE during calculations.) In other words you are allowing the camera+NLE to reach their maximum point.

Now does this result in under-exposed video? If it does -- so what! The limited latitude of these cameras forces you to choose between great highlights and some under-exposure or no under-exposed video and blown highlights.

Now if you simply use AE, the FX1 will solve the problem for you PERFECTLY. The DXP calculates -- using its auto-keee and iris setting -- exactly what is best. (No more need to bias the exposure as in the old days.)

However, if you turn on CineTone you will kill shadow detail -- never use it.

On the Z1 use Black Stretch to give you more shadow detail.

Likewise, AF is always perfect on these cameras -- if you center the subject during the focus time. Simply let the DXP do its work and then lock both before shooting.

It seems folks still don't trust modern camera logic to control audio and video levels. They still think they must do things by eye and ear that computers no can do far faster and better. (It's like when Anti-Lock brakes came-out -- pro drives thought they could out-think a computer. And, race cars now use AT because a computer can calculate shift points more accurately than can a human. Likewise Fly-by-Wire.)
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Old June 18th, 2005, 01:02 PM   #15
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I'm with Augusto and Steve. Watch the clipping. That is why I like my zebras set high, just to be sure. With a LCD viewfinder/ display it is hard to tell.
I have a plan to take an old Ikegami B&W viewfinder and feed the video out-- with a high rez Black and white finder you can really tell when the whites lose details.

My $.02
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