Understanding the Histograms...

[pgriz]

It helps to know what your dynamic range (of the camera) actually is. In my case, I've tested my camera and know that I have about three stops of latitude of overexposure, and four stops of latitude underexposure (ie, 7 stops total). If I have my highlights exposed so that they are at most 3-stops overexposed relative to my "proper" exposure, and the scene has 7 stops of dynamic range, then I know that setting an exposure that puts my highlights at 3 stops over, I will still be able to fill the histogram without clipping.

 

[mnmcote]

I am talking about the histogram I see while editing...

I noticed from your baseball photo (different thread) that you have a Canon sx50. That camera can save an RGB JPEG or a CR2 raw file or both. What you do in editing and how you think of the histogram is going to be somewhat different depending on which of those you're working with.

Sparky who hangs out here has a good analogy for the difference. The CR2 file is all the raw ingredients you need and could typically want to cook an awesome burger. The RGB JPEG is a Big Mac handed to you through a window in the side of a building as you drive by. If you don't like the Big Mac tough or you're going to try and fix it? Seriously?! You think you can make it better? With the CR2 file nobody's going to hand it to you as you drive by -- you need to know how to cook.

1. The camera JPEG: It's already been processed. If you don't like the way it looks you should go back to the point where it was processed -- the raw sensor capture -- and get it right. If you're going to edit that JPEG what you're really doing is trying to repair an already damaged product; fix a Big Mac. Most chefs would rate your odds pretty low of ending up with a great burger if you start with a Big Mac. First, scrape off the secret sauce....

If the camera software has already done irreparable damage you start out checkmated. For example if the histogram indicates that the diffuse highlights (white ball players uniforms in the sun) have been smeared all over the right threshold wall of the histogram then the data you need isn't there. If there's no data then there's nothing for you to edit: checkmate.

Hardcore purists will insist that it's always wrong to edit a camera JPEG because you can't avoid ancillary damage created by the compression grid when you manipulate it. It was designed not to be manipulated. Now while I wrote that a million people just did it and before I finish this post millions more will do it. A lot of them will sell the result (McDonald's sells Big Macs). The damage caused by editing a JPEG starts out pretty minor and if you're reasonable about it, the convenience of shooting a JPEG and making a few tweaks later versus processing a raw file can save you considerable time.

Critical: You need a JPEG with as much data as possible. It is therefore essential that you have the camera skills and are familiar with your camera's processing software so that you don't get checkmated from the start (two paragraphs above). You begin by examining the histogram and you're first concern is that it extend over the full range from left corner to right corner without breaking the threshold limits. If it doesn't reach the two corners a Levels correction will repair that. In making the edit you must do as little damage as possible and DO NOT LOSE DATA. Losing the data you already have can't be a good thing -- you're manipulating it not discarding it.

Once the histogram extends over the full range of the graph you assess the tone-response of the photo. Is it too light or too dark? Does it need more contrast. Levels and/or Curves and various contrast controls will allow you to adjust that. (Don't know what software you're using).

2. A camera raw file (CR2 in your case): My favorite analogy here is a ruler and a yardstick. A one foot ruler is your finished RGB photo which would include the camera JPEG. This is the photo that will print. This is the photo in which the histogram is really a graph of your data processed for output where the left corner black really is black ink and the right corner white really is white paper. The blackest ink on the whitest paper is a fixed and limited range. The histogram reflects that and let's think of that as a one foot ruler -- a fixed data set -- 12 inches. The sensor in your camera can record 36 inches. It's actually a pretty close analogy, depending on your camera model, it's sensor records between 2 to 3 times as much data as your output target is limited to. If you're using a camera raw file you've actually got too much data or rather more data than you can stuff onto the ruler. The process of editing a raw file is managing the reduction as you take 30 inches of data and make it fit into that 1 foot histogram.

In this case the software you're using is showing you your final output histogram and it's helping you adjust the data to fit. The difference is that, until you push the commit button you're plugged into the full 30+ inches that the sensor recorded. You have to end up in the same place: a final RGB output file but you're working with a whole lot more original data and the raw conversion software is engineered to assume as much and help you take advantage. You're seeing the same histogram and your ultimate goal is the same. You generally want to see a histogram that extends corner to corner and does not slam up against the right threshold wall or pile up too much against the left threshold wall.

Joe

edit: I got carried away.

Love the 'Fixing a Big Mac' statement.. I totally hear you... I've come to the same conclusion while looking at the histograms of many of my photos... It's really important to get a "good" shot instead of trying to fix a poor shot. I'm still learning... And really.. I'm enjoying the process.. Thanks for taking the time to lend your thoughts... Much appreciated...

 

[rudimaes]

Here you find a document 'Understandig Histograms':

http://www.creativephotobook.co.uk/pdf/histograms.pdf

Rudi

 

[Ysarex]

Very good explanation, Joe. So, in wanting corner to corner coverage, how does "shooting to the right" figure into this? Can images be exposed to the right while still having corner-to-corner coverage?

Yes.

I really wish ETTR wasn't called ETTR. "The right" in the term refers to "the right" of a histogram. The problem is most people think in terms of the processed RGB histogram. So there's massive confusion over what ETTR really means and it derives from misunderstandings about how histograms relate to sensor exposure. Almost every photographer has seen an RGB photo histogram but most of those photographers have never seen a sensor histogram. ETTR should refer to a sensor histogram and not the histogram of a processed RGB photo.

This really creates a mess behind the camera because cameras only display processed RGB photo histograms and yet I could go right now to Google and find a bazillion blog posts and articles explaining how to use ETTR by looking at the camera histogram -- mass confusion. NO! That's not how it works!

The point behind ETTR is that you want to take full advantage of the camera sensor's recording capacity. Think of the camera sensor as having two ends, one end gets no exposure and then exposure increases as you progressively move to the other end until you reach maximum exposure, the sensor is delivering an electronic signal. That signal gets stronger and cleaner -- the data gets less noisy -- as you move from the no exposure end to the max exposure end. We want the data on that max exposure end for best results. It's a waste not to use the full capacity of the sensor and fail to reach that max exposure end.

We also want to use the full capacity of the sensor when the scene has a high contrast range. Note Paul's previous post where he mentions knowing his camera's dynamic range recording capacity. He's talking about the sensor's recording capacity. (While mentioning Paul's post let's also note how he talks about testing his equipment: YES!!!) In this situation we're taking advantage of the sensor's ability to record as much tonal data as possible. This gives us an advantage in processing because we can manipulate all that data to achieve the best possible end result. When making a recording more data is good.

All of this talk about ETTR and sensor recording ranges etc. presupposes that we're shooting and processing raw files. It does not apply if we're shooting and using camera processed JPEGs.

The processed RGB photo histogram is a graph of the processing job and not a graph of the sensor data. A processed RGB histogram can indicate poor or faulty processing when the sensor data is just fine.

There I go getting carried away again. The point: When making ETTR exposure decisions and when assessing an ETTR exposure it's a mistake to reference the processed RGB histogram.

Joe

 

[KmH]

Photo Editing Tutorials

Optimizing Exposure

http://wwwimages.adobe.com/www.adob...e/en/products/photoshop/pdfs/linear_gamma.pdf

http://www.lumita.com/site_media/work/whitepapers/files/pscs3_rendering_image.pdf

 

[snerd]

Photo Editing Tutorials

Optimizing Exposure

http://wwwimages.adobe.com/www.adob...e/en/products/photoshop/pdfs/linear_gamma.pdf

http://www.lumita.com/site_media/work/whitepapers/files/pscs3_rendering_image.pdf

From one of them.......................

......... If you think about it, the complex TTL metering systems that manufacturers have built into cameras for the past 60 years are all but obsolete. A live-view histogram-based auto-exposure system is all that needed to generate the best possible exposure from a technical perspective.....

 

[Ysarex]

Photo Editing Tutorials

Optimizing Exposure

http://wwwimages.adobe.com/www.adob...e/en/products/photoshop/pdfs/linear_gamma.pdf

http://www.lumita.com/site_media/work/whitepapers/files/pscs3_rendering_image.pdf

From one of them.......................

......... If you think about it, the complex TTL metering systems that manufacturers have built into cameras for the past 60 years are all but obsolete. A live-view histogram-based auto-exposure system is all that needed to generate the best possible exposure from a technical perspective.....

That would be great if the camera manufacturers would build it into our cameras. I'd like to have that, but right now it doesn't exist in any modern digital camera. There is a kludge option available called UniWB but it's a pain to implement and use.

Joe

 

[greybeard]

It is basically telling you exposure information about the last frame you shot. I try to keep the "hump" in the middle but, most importantly not letting it pile up on either end. If it is piled up on the far right side then it will be over exposed and will loose detail in the highlights. If it is piled up on the far left side then it is under exposed and will loose detail in the shadows. Back in the "bad old days" of shooting slides, every shot was a crap shoot so I often would shoot a 5 frame bracket of something really important. The histogram eliminates this problem.

 

[Ysarex]

It is basically telling you exposure information about the last frame you shot. I try to keep the "hump" in the middle but, most importantly not letting it pile up on either end. If it is piled up on the far right side then it will be over exposed and will loose detail in the highlights. If it is piled up on the far left side then it is under exposed and will loose detail in the shadows. Back in the "bad old days" of shooting slides, every shot was a crap shoot so I often would shoot a 5 frame bracket of something really important. The histogram eliminates this problem.

This only applies to the camera processed JPEG. It does not apply to the raw capture.

Joe

 

[greybeard]

It is basically telling you exposure information about the last frame you shot. I try to keep the "hump" in the middle but, most importantly not letting it pile up on either end. If it is piled up on the far right side then it will be over exposed and will loose detail in the highlights. If it is piled up on the far left side then it is under exposed and will loose detail in the shadows. Back in the "bad old days" of shooting slides, every shot was a crap shoot so I often would shoot a 5 frame bracket of something really important. The histogram eliminates this problem.

This only applies to the camera processed JPEG. It does not apply to the raw capture.



Joe

I shoot Raw, seems to work OK for me.

 

[table1349]

Doesn't always work that way.
Histogram: photography cheat sheets for achieving perfect exposure | Digital Camera World - page 4

 

[Ysarex]

It is basically telling you exposure information about the last frame you shot. I try to keep the "hump" in the middle but, most importantly not letting it pile up on either end. If it is piled up on the far right side then it will be over exposed and will loose detail in the highlights. If it is piled up on the far left side then it is under exposed and will loose detail in the shadows. Back in the "bad old days" of shooting slides, every shot was a crap shoot so I often would shoot a 5 frame bracket of something really important. The histogram eliminates this problem.

This only applies to the camera processed JPEG. It does not apply to the raw capture.



Joe

I shoot Raw, seems to work OK for me.

Works OK is a good description. Typically your raw file can handle more exposure than the JPEG histogram indicates. So the raw is often better exposed and without clipped highlights when the JPEG histogram is indicating clipped highlights. The point is it's not a histogram of the raw file. How much this really matters is when the discussion gets fun. If you're trying to squeeze as much out of your sensor as you can get in a high contrast scene then it matters, otherwise works OK is OK.

Joe

 

[pgriz]

As Joe alludes to, there's not a direct relation between the histogram showing at the back of the camera, and the RAW data. It took me a while, but after doing some careful testing, I have learned that the RAW dynamic range of my camera is about 4 stops over and about 6 stops under, giving a value of about 10. However, the raw conversion to jpg (in camera) gives an apparent range of only about 7 (3 over, 4 under). It's this converted JPG that we see on the back of the camera, and on which the histogram is based. By shooting within the confines of the histogram (which is based on the JPG conversion, even if I shoot RAW), I know I have, if I need it, another stop of overexposure and 2 stops of underexposure capability buried in the RAW data. Therefore, while I use the histogram to give my self some useful info in terms of overall exposure, I also know that if I needed it, there is still some usable signal in the lower and upper ranges that the conversion effectively throws away.