Jump to content


Photo

Gamma


  • Please log in to reply
13 replies to this topic

#1 Jase Ryan

Jase Ryan
  • Basic Members
  • PipPipPip
  • 119 posts
  • Cinematographer

Posted 04 April 2008 - 01:07 PM

can anyone help explain what Gamma is when talking about photography? And from the little I know about it, why do we as cinematographers need to know this and be aware of it?


Sorry about the novice question... just something I never fully understood.

Thanks.
  • 0

#2 Scott Bryant

Scott Bryant
  • Basic Members
  • PipPipPip
  • 151 posts
  • Other
  • Nevada

Posted 04 April 2008 - 01:16 PM

I don't have an answer for your question as I am somewhat confused as well even after reading wiki.

but I also have a question to tack onto yours. When shooting film, how do you achieve the deep blacks in film without darkening everything else in the frame? I've attempted to adjust gamma and contrast but nothing allows me to deepen the blacks while keeping the mids and whites from disappearing?
  • 0

#3 John Holland

John Holland
  • Basic Members
  • PipPipPipPip
  • 2248 posts
  • Cinematographer
  • London England

Posted 04 April 2008 - 01:20 PM

Suggest you go to your local library or if that to hard just Google Gamma . think you are being lazy its not worth the time on here trying to explaining . Sorry . :rolleyes:
  • 0

#4 John Sprung

John Sprung
  • Sustaining Members
  • 4635 posts
  • Other

Posted 04 April 2008 - 02:04 PM

Here's a good place to start:

http://www.poynton.com/GammaFAQ.html




-- J.S.
  • 0

#5 Michael Nash

Michael Nash
  • Sustaining Members
  • 3330 posts
  • Cinematographer
  • Pasadena, CA

Posted 04 April 2008 - 03:10 PM

It's a pretty simple concept but a little difficult to explain. It really helps to illustrate it visually.

In very simple terms, gamma refers to the distribution of brightness throughout the tonal range, particularly, the way that input values relate to output values. In other words, the way the contrast and brightness of a real-world image compares to the way it's captured in camera, or displayed on a screen.

This article can help show you what's going on: http://dvinfo.net/ar...k/broadway2.php

In video cameras you can often adjust the master gamma, which makes the midtones brighter or darker relative to your exposure, and in turn makes the adjacent highlights and shadows appear higher or lower in contrast, depending on which direction you go. For example if you raise the gamma and make the midtones brighter, the highlights get "squeezed" or lowered in contrast while the shadows get stretched to a higher contrast (and some brightness). The opposite gamma adjustment has the opposite effect.

The gamma curve refers to the distribution of brightness through the whole range, not just the mids but also including the shadows and highlights. Video cameras that offer "cinelike" or "filmlike" gamma presets are pushing the shadows, mids, and highlights independently brighter or darker to obtain a different distribution of brightness throughout the range.

Different film stocks also have their own characteristic curve, which is the way it responds to light throughout the exposure range. Some film stocks "see deeper into the shadows" than others, meaning that their characteristic curve is flatter (less steep) in the shadow region.

This all becomes important to us cinematographers because we're "painting with light," and the gamma curve of the system we're using is part of our palette. We want to know if the dark shadow we're looking at on set is going to come out detailed brown, or completely black. By understanding gamma we can choose to exploit, or compensate for, the characteristics of the system we're using to get what we want up on the screen.
  • 0

#6 Scott Bryant

Scott Bryant
  • Basic Members
  • PipPipPip
  • 151 posts
  • Other
  • Nevada

Posted 04 April 2008 - 04:24 PM

Suggest you go to your local library or if that to hard just Google Gamma . think you are being lazy its not worth the time on here trying to explaining . Sorry .


I'm glad it is worth the time to comment on the inadequacy of someone's post rather than answering the question or just not responding altogether. I hope part of that wasn't directed at me as I explained I already read the wikipedia article and still did not understand.

Thanks for the answers from others though. I didn't start the topic but it helped clear up some of my misunderstandings.
  • 0

#7 Andreas Nicholas

Andreas Nicholas

    New

  • Basic Members
  • Pip
  • 1 posts
  • Student
  • New York, NY

Posted 05 April 2008 - 02:03 PM

Suggest you go to your local library or if that to hard just Google Gamma . think you are being lazy its not worth the time on here trying to explaining . Sorry . :rolleyes:


No Need to be a bloke about it. These forums exist so that people can ask questions. Your post helped no one, not responding would be more helpful next time. Cheers.
  • 0

#8 alan smith

alan smith
  • Basic Members
  • PipPip
  • 29 posts
  • Other

Posted 06 April 2008 - 01:29 PM

[quote name='Michael Nash' date='Apr 4 2008, 12:10 PM' post='225613']

Different film stocks also have their own characteristic curve, which is the way it responds to light throughout the exposure range. Some film stocks "see deeper into the shadows" than others, meaning that their characteristic curve is flatter (less steep) in the shadow region.

An observation of mine own, pertinent to the remark re. "Different film stocks..."

I once tried 'push' processing Plus-X 35mm still film stock hopeing to achieve a Tri-X like gamma or 'look': deeper blacks, whiter whites, better tonal seperation O.A. but with the finer grain of Plus-X. This was pre-TMAX-100.

No luck - couldn't be done. The Plus-X neg. (qualified: "As I processed it") was simply a flatter stock, meaning it "sees" and looks flatter - with a long flat curve with lots of steps in its range of tones - in a different class than Tri-X.

Conclusion? Trial and error.

Pertinent to anything? - impertinent? - don't know - likely just wasteing bandwith.

Regards
  • 0

#9 Dominic Case

Dominic Case
  • Basic Members
  • PipPipPipPip
  • 1357 posts
  • Other
  • Sydney Australia

Posted 06 April 2008 - 06:45 PM

The Poynter link is probably excellent - haven't followed it, but he has the best way I ever cam across of dealing with gamma.

Essentially it is a measure of the relationship between the input scale of brightness and the output scale of brightness in any component of any imaging system.

That's the high-level overall view of it. In application, it turns out looking a little different in film or in video as a result of the different maths used to describe each system.

In film, gamma is defined as the slope of the characteristic curve - that is, the graph of Density versus logE (the scene brightness). Essentially that defines the contrast of the image as compared with the original. A lower gamma will give a lower contrast image.

In video, gamma is - mathematically - a very similar thing, but is expressed as the power factor in a transfer function. Since black and white levels are fixed, altering gamma has a rather different effect: simply, it shifts the mid tones up or down, thereby stretchng the shadows and compressing the highlights or vice versa.

For Scott, one approach to your problem ("When shooting film, how do you achieve the deep blacks in film without darkening everything else in the frame?") lies in the question itself. "When shooting film". The solutions you've tried ("I've attempted to adjust gamma and contrast ") are too late in the process. If you want greater separation between the blacks and the midtones, you need to light the scene more. Also, make sure you have a full exposure, as this will move the entire scene

up

the characteristic curve a little, meaning the shadows won't roll off so quickly.
  • 0

#10 Scott Bryant

Scott Bryant
  • Basic Members
  • PipPipPip
  • 151 posts
  • Other
  • Nevada

Posted 06 April 2008 - 10:50 PM

For Scott, one approach to your problem ("When shooting film, how do you achieve the deep blacks in film without darkening everything else in the frame?") lies in the question itself. "When shooting film". The solutions you've tried ("I've attempted to adjust gamma and contrast ") are too late in the process. If you want greater separation between the blacks and the midtones, you need to light the scene more. Also, make sure you have a full exposure, as this will move the entire scene


Thanks alot. So by lighting more, do you mean i would be essentially lighting the darks less and the mids more?
  • 0

#11 Jase Ryan

Jase Ryan
  • Basic Members
  • PipPipPip
  • 119 posts
  • Cinematographer

Posted 07 April 2008 - 07:32 PM

Great... thats simple enough to understand. thanks!
  • 0

#12 Dominic Case

Dominic Case
  • Basic Members
  • PipPipPipPip
  • 1357 posts
  • Other
  • Sydney Australia

Posted 07 April 2008 - 08:40 PM

So by lighting more, do you mean i would be essentially lighting the darks less and the mids more?

Exactly.
  • 0

#13 Scott Bryant

Scott Bryant
  • Basic Members
  • PipPipPip
  • 151 posts
  • Other
  • Nevada

Posted 07 April 2008 - 11:06 PM

Exactly.


Thanks a lot Dominic.
  • 0

#14 DJ Joofa

DJ Joofa
  • Basic Members
  • PipPipPip
  • 149 posts
  • Other

Posted 08 April 2008 - 01:32 AM

Gamma has some serious implications based upon is it applied to:

(1) relative luminance obtained from linear light R,G,B; or,
(2) individual R,G,B and then deriving luma from such non-linear R*, G*, B*.

(1) results in the better reproduction of luminance detail, where as, (2) results in better color fidelity near saturated primary colors.
  • 0


Abel Cine

Broadcast Solutions Inc

Aerial Filmworks

Wooden Camera

Visual Products

Ritter Battery

Media Blackout - Custom Cables and AKS

Gamma Ray Digital Inc

Rig Wheels Passport

The Slider

rebotnix Technologies

CineTape

Paralinx LLC

Metropolis Post

Tai Audio

CineLab

Willys Widgets

Glidecam

FJS International, LLC

Technodolly

Opal

Technodolly

rebotnix Technologies

Rig Wheels Passport

Opal

Gamma Ray Digital Inc

Ritter Battery

Paralinx LLC

CineLab

Visual Products

Aerial Filmworks

Willys Widgets

Glidecam

Media Blackout - Custom Cables and AKS

Wooden Camera

The Slider

Tai Audio

CineTape

FJS International, LLC

Broadcast Solutions Inc

Abel Cine

Metropolis Post