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over/under exposure versus changing brightness level in raw
Hello,.

I'm learning to use my new camera. It allows me to over/under exposure when taking pictures. But why should I do this? I can always change the brightness level after the picture is taken..

Could anybody tell me, if there's a difference between changing the exposure settings of the camera and changing the brightness level in post processing..

ThxGLT..

Comments (10)

Yes, a lot..

Assume that you are taking pictures as JPEGs. Every pixel will have an output that can be represented as a combination of red, green, blue (RGB) each on a scale of 0-255. So 0,0,0 is black and 255,255,255 is white. This gives a total of about 17 million possible colour values which is plenty for the human eye and you get a nice picture..

Now suppose you underexpose the picture badly so that you only use one quarter of the scope of your sensor, so the brightest parts of the picture register (say) 64, 64, 64 for R/G/B. No problem: you just take this out put and multiply it by four, right? You now have a picture covering the full range of brightness..

BUT... the gaps between the increments are now four times as big. Instead of having values for red (for example) of 0, 1, 2, 3, 4, 5...255, you have 0, 4, 8, 12, 16... with big jumps between them. So your picture how has a total of about 260,000 colour shades in it (64 x 64 x 64) rather than 17 million (256 x 256 x 256). This looks poor: big jumps in colour rather than gradual transitions, blotchiness, loss of detail, etc.



In addition, any random noise (which is always present) will be multiplied by a factor of four abd become much more intrusive..

Conversely, if you overexpose, areas of your picture that should have had detail in them will end up bright white, and you can't recover the detail by dimming the picture: once it is lost it cannot be recovered..

A simpler analogy: if you take a small radio, which produces a small number of decibels, and magnify the sound it produces, will you get something as good as you would get from a more powerful hi-fi? No: because in amplifying the wqeak signal you are smplifying all the noise and distortions too. Same principle..

Best wishesMike..

Comment #1

The non-technical aswer is that yes, there's a difference. You might well be able to rescue your badly exposed photos a little bit in software, but there's no substitute for an image that was exposed properly in the camera, so do your best when taking the photo..

If you try too hard to rescue a badly exposed image you'll potentially get more noise and suffer blown highlight/clipping problems..

If you shoot RAW, you'll have a little more leeway than if you shoot jpeg..

Androohttp://Androo.smugmug.com..

Comment #2

Hey Mike. Do you write manuals for the Civil Service in your spare time?jules.

Mike703 wrote:.

Yes, a lot..

Assume that you are taking pictures as JPEGs. Every pixel will havean output that can be represented as a combination of red, green,blue (RGB) each on a scale of 0-255. So 0,0,0 is black and255,255,255 is white. This gives a total of about 17 millionpossible colour values which is plenty for the human eye and you geta nice picture..

Now suppose you underexpose the picture badly so that you only useone quarter of the scope of your sensor, so the brightest parts ofthe picture register (say) 64, 64, 64 for R/G/B. No problem: youjust take this out put and multiply it by four, right? You now havea picture covering the full range of brightness..

BUT... the gaps between the increments are now four times as big.Instead of having values for red (for example) of 0, 1, 2, 3, 4,5...255, you have 0, 4, 8, 12, 16... with big jumps between them. Soyour picture how has a total of about 260,000 colour shades in it (64x 64 x 64) rather than 17 million (256 x 256 x 256). This lookspoor: big jumps in colour rather than gradual transitions,blotchiness, loss of detail, etc. if you want to see what this lookslike, set your computer monitor to display a much smaller number ofcolours than usual..

In addition, any random noise (which is always present) will bemultiplied by a factor of four abd become much more intrusive..

Conversely, if you overexpose, areas of your picture that should havehad detail in them will end up bright white, and you can't recoverthe detail by dimming the picture: once it is lost it cannot berecovered..

A simpler analogy: if you take a small radio, which produces a smallnumber of decibels, and magnify the sound it produces, will you getsomething as good as you would get from a more powerful hi-fi? No:because in amplifying the wqeak signal you are smplifying all thenoise and distortions too. Same principle..

Best wishesMike.

Why can't you blow bubbles with chewing gum?..

Comment #3

JulesJ wrote:.

Hey Mike. Do you write manuals for the Civil Service in your spare time?jules.

LOL...No... but I do teach, often to students who are slow on the uptake... does it show too much  ? .

Best wishesMike..

Comment #4

Lol.Jules.

Mike703 wrote:.

JulesJ wrote:.

Hey Mike. Do you write manuals for the Civil Service in your spare time?jules.

LOL...No... but I do teach, often to students who are slow on theuptake... does it show too much  ? .

Best wishesMike.

Why can't you blow bubbles with chewing gum?..

Comment #5

From a photography point of view being able to under and over expose a photo is due to the fact that the exposure meter is calibrated for middle gray. If you make up a chart that has 10 steps from solid black thru grey to pure white you can visulize what I am saying. Middle grey (Defined by manufacturer 18% grey in the old days but can vary today.) was used to judge the luminosity of a scene..

Say a picture of snow, the camera would take away light to make it 18% grey, therefore you would have to add a stop + to get back to white. Next, a picture of a black bear, the camera would add light to make it grey, you would have to take away light one + stops to make it black..

Yes it is all depicted in the 0-255 steps in the histogram and it is IMHO the greatest thing that has happened to photography it show you exactly what is happening. All the years I spent with film changed the day I got my first digital camera. It gave me the ability to see what was happening and what I was doing wrong or right with imediate feedback..

All and all IMHO it is always from the photographers point of view or the technical point of view to expose and crop in camera. As described extreamly well in earlier posts doing exposure or crop changes in post- processing just takes informantion away.Let the light in!.

Comment #6

Thanks for all the input,.

And I was so happy that I coud do a lot in pp. Seems that I've still a lot to learn!GLT..

Comment #7

In JPG mode you want to adjust the exposure to have the histogram somewhat centered and look properly exposed without adjusting..

In RAW mode, you want to adjust exposure to push the histogram as far to the right as possible WITHOUT having values in the 255 level. This will give an image that looks a little over exposed intiially. Then in post, adjust it back down to the desired exposure. This maximizes the use of the 255 levels of each pixel...

Comment #8

Wgc wrote:.

Say a picture of snow, the camera would take away light to make it18% grey, therefore you would have to add a stop + to get back towhite. Next, a picture of a black bear, the camera would add light tomake it grey, you would have to take away light one + stops to makeit black..

Thank you, thank you. When I bought my new 560, every thing was snow.Every picture I took was "dingy"I was very disapointed in the camera.Now I know why. Since then I have learned to use the histogram.Kenny..

Comment #9

You are welcome. Glad I could help.Let the light in!.

Comment #10

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This question was taken from a support group/message board and re-posted here so others can learn from it.

 

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