Gamma

[Jase Ryan]

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.

[Scott Bryant]

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?

[John Holland]

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:

[John Sprung]

Here's a good place to start:

 

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

 

 

 

 

-- J.S.

[Michael Nash]

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/articles/filmlook/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.

[Scott Bryant]

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.

[Andreas Nicholas]

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.

[alan smith]

 

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

[Dominic Case]

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.

[Scott Bryant]

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?

[Jase Ryan]

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

[Dominic Case]

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

Exactly.

[Scott Bryant]

Exactly.

 

Thanks a lot Dominic.

[DJ Joofa]

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.