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Sony HVR-Z1 / HDR-FX1
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Old January 3rd, 2008, 01:50 PM   #1
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Black stretch, Z1

How do most Z1 users use the black stretch?
I find that with stretch on it brightens the low end... am i right in asumming this?
If i shoot outside on a bright day would i use black stretch?
If i shoot inside shoud i use balck stretch?
Lets say shooting a wedding would i use black stretch?

Would i be better doing this in post production?

Thanks
Simon
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Old January 3rd, 2008, 03:20 PM   #2
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I use black stretch in low light conditions, or when the subject is dark. It changes the gamma curve "so that the graduation of the dark part is reproduced better" (quote from the Z1U manual).

What does that mean? Well, when you record video, brightness information is measured at the sensor, which outputs a voltage as a function of brightness, and that voltage is digitized to a numeric value (with some gain applied in between). This numeric value (let's call it the "measured brightness value") is then mapped to another numeric value, the one that is actually stored as the brightness information on the tape (let's call this one the "recorded brightness value").

Bandwidth is precious, so the possible range of recorded brightness values must be small, in order to save bits on the recording media. That's why the recorded value has fewer bits than the measured value. The mapping from measured values to recorded values is not linear; instead, the "gamma curve" is applied in order to best exploit the available bits for the recorded value such that meaningful increments of brightness can be recorded, meaningful with regard to the performance of the human eye.

What black stretch does in the Z1U is simply a switch to a different gamma curve, one that allocates more of the possible recorded values to the darker range of the brightness scale, at the expense of the number of values left to encode brighter areas.

You can easily change the gamma curve in post, but without black stretch, the image definition that you start with lacks the detailed brightness information for the darker areas. If two pixels are recorded with the same recorded brightness value, the gamma correction in post will assign both of them the same new brightness, whereas black stretch in the camera might have recorded different brightness values, giving you more detail in the recorded image.

I guess an analogy may be an underexposed image - sure, you can change that in post, but you cannot restore the fine detail that could have been captured with proper exposure.

- Martin
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Old January 4th, 2008, 02:05 AM   #3
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Thanks Martin,
Great answer to what i was searching for.

Cheers
Simon
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Old January 4th, 2008, 08:29 AM   #4
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Well done, Martin!

That was a very eloquent explanation of black stretch!

-gb-
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Old January 4th, 2008, 09:03 AM   #5
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Yes, that clears up very well why it's better to enable black stretch in the camera instead of relying on adjusting in post. Something I always wondered about.

I have the same question about gain in camera. Is there any difference between applying it in camera and in post?

Thanks for the great explanation, Martin.
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Old January 4th, 2008, 12:50 PM   #6
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Camera gain seems visually to me much better than post brightening techniques. If I remember right, I heard an explanation that this is due to the fact that it is applying the gain effect before compressing - using more information to perform the level adjustment. Very similar to the black stretch issue - simply more color information at that stage to raise levels without noise.

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Old January 4th, 2008, 12:56 PM   #7
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Thanks, Carl. Seems very logical. I had always wondered why gain in camera could be better.
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Old January 5th, 2008, 02:24 AM   #8
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Black Stretch

Hi Martin,

Do you leave Black Stretch on all the time? If not, when do you turn it off? Great explanation, by the way. Thanks.

Jack
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Old January 6th, 2008, 11:49 AM   #9
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Originally Posted by Jack D. Hubbard View Post
Hi Martin,

Do you leave Black Stretch on all the time?
No, I sometimes turn it off in bright conditions (outside, or with good lighting). It depends on whether I want to emphasize brighter or darker details of the image. If the subject is bright, I want a gamma curve that allows recording the subject with more possible values for the brighter portion of the luminance range, so I turn Black Stretch off.

- Martin
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Old January 6th, 2008, 03:39 PM   #10
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I have just done a shoot on the weekend outdoors in bright sunlight and left the black stretch on and i'm very pleased on the look.
I have found with black stretch off the blacks tend to be crushed.

Love the look of this camera.

Cheers
Simon
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Old January 6th, 2008, 05:14 PM   #11
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Tnx..

Martin, Simon,

Thanks for the imput, guys, very helpful.

Regards,
Jack
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Old January 9th, 2008, 03:19 AM   #12
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Quote:
Originally Posted by Martin Pauly View Post
No, I sometimes turn it off in bright conditions (outside, or with good lighting). It depends on whether I want to emphasize brighter or darker details of the image. If the subject is bright, I want a gamma curve that allows recording the subject with more possible values for the brighter portion of the luminance range, so I turn Black Stretch off.
Shouldn't it be the other way around? Black stretch off when shooting in dark conditions and on when shooting in bright conditions.
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Old January 11th, 2008, 01:18 PM   #13
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Mikko,
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Originally Posted by Mikko Lopponen View Post
Shouldn't it be the other way around? Black stretch off when shooting in dark conditions and on when shooting in bright conditions.
I have been trying to understand what you wrote. When you say "shooting in bright conditions", do you mean a dark subject in an otherwise bright environment? I can see the use of black stretch there, to give you more detail on the dark subject. And if everything is dark, I guess your point is that you'd rather use in-camera gain than just black stretch.

Those are valid points. What I stated earlier has worked well for me, but of course I can only speak for the shooting scenarios that I have experienced. More generally speaking, how about the following recommendation:

Look at the total range of brightness in your frame. Adjust exposure such that this total range can be recorded. Following that, if your subject - whatever is most important in the visible frame - is towards the lower end of this brightness range, then enable black stretch. Otherwise leave it off.

- Martin
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Old January 15th, 2008, 08:26 AM   #14
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Black stretch

I do weddings and events with the Z1, and usually have the black stretch on. I have not seen any adverse affect on the brighter end from doing this. Interpretation depends a lot on how its being viewed.

My HDV editing laptop has a lower contract image. My 52" Sony LCD HDTV is contrastier.

The only time I can see taking it off is when I am trying to get a darker more dramatic film type look, but most of the time, I am just trying to get through the job.
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Old January 17th, 2008, 05:03 PM   #15
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Quote:
Originally Posted by Martin Pauly View Post
When you say "shooting in bright conditions", do you mean a dark subject in an otherwise bright environment? I can see the use of black stretch there, to give you more detail on the dark subject. And if everything is dark, I guess your point is that you'd rather use in-camera gain than just black stretch.
n
Exactly. Black stretch doesn't work in low light because there isn't anything except noise in the blacks to bring out. The result is greyish material. In bright conditions black stretch gives more detail to the blacks and also helps with mpeg macroblocking.
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