Our Friend The Histogram

[another view]

One of the most powerful tools with digital photography is the histogram, which can be looked at as soon as an image is recorded. This is true on DSLR's and many compacts, but some compacts don't have this feature. If you've got it, maybe you've heard it and wondered about it but you're not really sure what to do with the information. This was discussed a little bit in this thread, and I thought it would be a good subject to cover a little more in-depth. I'll explain how I do it, which may be different than how others do it - but feel free to add your own techniques.

I've got three different shots; two of them right out of the camera other than RAW conversion with no exposure adjustment but slight WB correction. The images were sized for viewing here and it was all done in Adobe Photoshop CS. Other image editing software may look slightly different. To keep it organized, I'll put the three shots in different posts.

The histogram is a great tool, and it doesn't take a lot more than reading this thread (hopefully!) to get an understanding of how to use it. The left side is the shadow, the right side is the highlight. The center could be thought of as "middle grey". If you have a histogram that has information cut off at either side (like I do in my dark shot) then there's no detail in a some of those areas. This can happen in either side of the histogram - or both in really contrasty situations.

This first shot was taken in my front yard right after a snowfall. What do you see in this shot? Snow! What color should it be? White! So, before I even took the shot I knew that I wanted most of the information on the histogram to be very close to the right side but not touching it. Maybe a little info could (slight blown highlights) but that didn't happen here because it was overcast. Since the built-in light meter in the camera assumes that everything is grey (yes, matrix meters do this too) I probably put about +1 or so of exposure compensation at the time I shot it. Had I put in 1/2 stop more, I'd have too many blown highlights, but 1/2 stop less would be too dark of an image for what I was hoping to capture. If the histogram wasn't what I wanted, then I'd adjust the exposure and shoot it again, just like bracketing exposure with film but knowing what the outcome will be. Almost like cheating. Again, this shot is right out of the camera and I might do a little PS work to it for a "finished" image. Here's the shot, and also a screenshot of it in PS showing the histogram.

[another view]

Here's another shot that's more "middle grey". Actually very grey - the trees were getting a lot of rain and it was headed my way. I pulled the car over quickly and had time for a couple of shots before I got soaked. Again, it's a RAW file right out of the camera with no corrections other than slight WB and a re-size.

Slightly off-topic, but there is a technique I learned from Luminous Landscape called something like "Exposing to the Right" which would mean that I'd overexpose this a little bit. That way I may be able to lower the noise in my shot but I was at ISO100 anyway - plus I didn't have time to think about that. The histogram has no highlight or shadow because it was such a grey day. Since I shot in RAW which has a 12-bit color depth, I could spread that info out in Levels to give it a bit more contrast. With a histogram that small in an 8-bit jpeg, you'd be limiting yourself in print quality. This is kind of an extreme example, but that's what I was looking for.

[another view]

Finally, my dark shot. This is a friend's band shot in a club with very difficult lighting to deal with - photographically at least. I decided to try a couple of dark shots just to see what I'd get, in this case 86mm, 1/30 sec at f2.8 and ISO800. A more "regular" shot with a more centered histogram would have been probably a stop and a half or two more stops of exposure. That means maybe 1/8 sec - not going to work too well with him moving. I didn't have a tripod but shot this from the balcony with the camera resting on the railing (use what you've got!).

As you can see, it is a very dark shot so the histogram should have all the info on the right side. It's also very cut off meaning that there are a lot of shadow areas that have no detail. This isn't necessarily a bad thing; it depends on what shot you want. That more "regular" shot with the centered histogram would have given me a totally different shot.

Hope this helps. These aren't really favorite shots of mine but I think they do a good job of illustrating what I'm talking about. Feel free to ask questions and contribute your techniques!

[drg]

Interesting so far. I'll add some images later for illustration.

There is at least one important use of the Histogram that you haven't touched on yet-
Dynamic Range.

The overall brightness in a Bayer sensor chip is what you really want to examine in the camera's display. The more appropriate the display to the image the better the exposure. That really means that if you have a scene of a field in good light and all you see on the display is a big spike, something went very wrong (extreme example).

The distance between the left and right terminus of the histogram is the dynamic range. (I'll post images later as I've just sat down in front of the computer). This can also be thought of as the dark to light or Contrast range. This is where the Expose to the Right as a rule falls down. If you have dark subject, like that displayed, to get the images histogram very far right toward the light or "white point terminus" or blown highlight, will over expose the photo.

Some very eye pleasing pictures from nature will display a standard bell curve that come no where neat either the left or right of the graphic representation as all the information in the photo is a midtone. Hmmmmm.....

When editing there are different portions of the histogram that look different from the Luminosity or composite, and the most common availble are the R-G-B components.

Something else that I just thought of is that there are cameras starting to appear with live histograms that let you see the light distribution prior to capturing the image. I can't wait to get my hands on one of these in a high end DSLR other than Oly.

Finally for the moment,

There is NO inherently bad histogram. They reflect the what the camera sees.

[walterick]

Great job Steve! You've given me something to think about next time I'm out shooting digis!

I have a request: I would love to see a "before and after" representation of the effect of monitoring your histogram. For instance, your snow scene shooting normally, and then shooting with an exposure adjustment based upon the histogram.

I stickied this for you, I assumed you are too humble to do so

[another view]

The distance between the left and right terminus of the histogram is the dynamic range. (I'll post images later as I've just sat down in front of the computer). This can also be thought of as the dark to light or Contrast range. This is where the Expose to the Right as a rule falls down. If you have dark subject, like that displayed, to get the images histogram very far right toward the light or "white point terminus" or blown highlight, will over expose the photo.
******
There is NO inherently bad histogram. They reflect the what the camera sees.

I completely agree. My mid-tone shot of the trees could be a candidate for "Expose to the Right" but that's not always the case in real life. Often not the case.

Here's a more normal light situation, and a histogram that nearly goes from end to end. Exposing to the Right would have lost the highlights. Matter of fact, if you look close at the histogram there is a little bit of blown highlights in it already. It's probably the map in front of him (next to the water bottle) and some specular highlights off the water. Not a problem IMO.

[another view]

I have a request: I would love to see a "before and after" representation of the effect of monitoring your histogram. For instance, your snow scene shooting normally, and then shooting with an exposure adjustment based upon the histogram.

I stickied this for you, I assumed you are too humble to do so

Thanks Rick - I'm not worthy!

Actually with the snow scene the first shot was pretty good - this goes back to shooting a lot of slide film before I went to digital. A +1 compensation is pretty much standard for this, so based on this it's where I started and the histogram was where I wanted it. So - I cheated here. In the RAW converter screen, I put in exposure correction just to show where the histogram would go. This is that screen in Photoshop - first is at -1 and second at -2. This histogram is a little different - it shows individual red, green and blue values instead of a "master" that you saw above. But see how the whole thing moves to the left with the underexposure? Looking at it now, I might have used a +1.5 exposure compensation. That might make a little more sense - it was obviously a while ago...

[photophorous]

AV,

Thanks for the excellent explanation. I will definitely start using my histogram now. This has helped my understanding.

I noticed the histogram on my camera is divided into four vertical segments. Do you know if that is meaningful, or is it just there as a point of reference? Seems like you'd want to divide it into segments that correspond with f-stops, so you could just look and quickly tell how much to change your exposure...but, I'm hoping my camera has more than a four stop range.

Paul

[another view]

Thanks Paul, glad to help. I'm really not sure about the vertical lines. It may be close to a one stop difference between a pair of lines, but I doubt it's exact. At high ISO's, images seem to be have a lot more contrast than low ISO (compare 100 or 200 with 1600 in the same light conditions), so maybe they're accurate at low ISO's but not quite at high ISO's... Just thinking and typing...

[Old Timer]

Great post Steve. You have information here a lot of us can use. Thanks for stepping up.

[SmartWombat]

Three more examples

(top) no compensation
(middle) exposed to the right
(bottom) fill flash added

Note how the colour shifts (using auto WB) with exposure, and with flash.
On this camera (the Casio 3200) the white balance is sensitive to proper exposure.

[drg]

These three examples were created with camera driven Auto Bracketing. The exposure compensation is as noted -1, 0, +1. They were taken in a different order than displayed. Extracted EXIF data shows the order in which they were created.

The images were created to be similar to a camera display and to show how the Histogram shifts in relation to the Brightness of the recorded image.

[drg]

These three examples were created with camera driven Auto Bracketing. The exposure compensation is as noted -1/3, 0, +1/3. Spot meter was on an area of the trellis just below half way from top to bottom of the image and closer to the left than the right. I locked the exposure and let auto bracketing do the rest.

At the very small change of 1/3 a stop, the difference is pronounced even at this compressed size. The white painted cinder blocks already are losing definition at 1/3 stop over.

Dynamic Range is illustrated in the histogram with there being data from left to right. In the area not over exposed (the far right touchs top and right sides), there is a fair representation of even distribution of information with a bell type curve. The -1/3 setting allows for the best overall image of these three.

The images were created to be similar to a camera display and to display Dynamic Range in a Histogram.

[another view]

Another example. The first shot is the exposure as captured. Concert lighting is very contrasty and if you look closely at the histogram you can see that both ends are clipped. There's a high but narrow peak on the right indicating blown out highlights, and the graph cuts off on the right - all in the same shot. It happens sometimes and you have to either decide what part of the shot most important and expose for it, or do a composite in Photoshop which really isn't an option in a case like this.

However, even with the histogram stretched from end to end, it lacks contrast IMO. In levels, I slid the left point over to the left a ways to lose even more of the shadows. Looks better to me although it would probably be better in Curves (whole 'nother topic!). Notice how the histogram has changed between the first and second shots, but also notice the spaces in the second histogram that make it look like a picket fence. Since this was an 8-bit jpeg instead of 12-bit RAW, there's just that much less color depth in the file. I start spreading out the histogram (by moving the left point over to the right which stretches it out) and I have these empty spaces. There isn't enough color information to keep the histogram "full". This can show up on a print as banding, or lines between two similar colors rather than continuous tone. The advantage of RAW is that there is much less chance of this happening.

[Ronnoco]

This first shot was taken in my front yard right after a snowfall. What do you see in this shot? Snow! What color should it be? White! So, before I even took the shot I knew that I wanted most of the information on the histogram to be very close to the right side but not touching it. Maybe a little info could (slight blown highlights) but that didn't happen here because it was overcast. Since the built-in light meter in the camera assumes that everything is grey (yes, matrix meters do this too) I probably put about +1 or so of exposure compensation at the time I shot it. Had I put in 1/2 stop more, I'd have too many blown highlights, but 1/2 stop less would be too dark of an image for what I was hoping to capture. If the histogram wasn't what I wanted, then I'd adjust the exposure and shoot it again, just like bracketing exposure with film but knowing what the outcome will be. Almost like cheating. Again, this shot is right out of the camera and I might do a little PS work to it for a "finished" image. Here's the shot, and also a screenshot of it in PS showing the histogram.

Not a very good example, since you still have slightly grey snow but at the same time, you have no detail, texture or shadow in the snow to give it a surface. Tonal quality is poor as well even in the birch trees. Your histogram as a matter of fact demonstrates that it is a poorly lit photo.

Ronnoco

[Ronnoco]

[QUOTE=another view]Another example. The first shot is the exposure as captured. Concert lighting is very contrasty and if you look closely at the histogram you can see that both ends are clipped. There's a high but narrow peak on the right indicating blown out highlights, and the graph cuts off on the right - all in the same shot. It happens sometimes and you have to either decide what part of the shot most important and expose for it, or do a composite in Photoshop which really isn't an option in a case like this. QUOTE]

The problem with this example is colour. You should be looking at histograms of the colour channels and adjusting them to produce a more natural skin colour, particularly eliminating the yellow.

Ronnoco

[Photo-John]

The problem with this example is colour. You should be looking at histograms of the colour channels and adjusting them to produce a more natural skin colour, particularly eliminating the yellow.

Only if he wants more natural skin color. He might not.

[Ronnoco]

These three examples were created with camera driven Auto Bracketing. The exposure compensation is as noted -1, 0, +1. They were taken in a different order than displayed. Extracted EXIF data shows the order in which they were created.

The images were created to be similar to a camera display and to show how the Histogram shifts in relation to the Brightness of the recorded image.

These shots may illustrate the histogram, but none of them constitute the ideal, well lit shot, so the histogram certainly was not used to create the perfectly exposed shot. So, you did not illustrate good use of the histogram at all.

Ronnoco

[Ronnoco]

Only if he wants more natural skin color. He might not.

Sure, John and all good photographers want to occasionally produce garbage shots.

Ronnoco

[Photo-John]

Not a very good example, since you still have slightly grey snow but at the same time, you have no detail, texture or shadow in the snow to give it a surface. Tonal quality is poor as well even in the birch trees. Your histogram as a matter of fact demonstrates that it is a poorly lit photo.

It's a fine example for the purpose it was posted. The histogram helped him make a better exposure decision than he might have without it. It is a tough lighting situation, though. But sometimes that's how it goes. This image probably needs an aggressive curve adjustment to look good.

[Ronnoco]

Three more examples

(top) no compensation
(middle) exposed to the right
(bottom) fill flash added

Note how the colour shifts (using auto WB) with exposure, and with flash.
On this camera (the Casio 3200) the white balance is sensitive to proper exposure.

The bottom one has a colour problem as in the greens and red are too pale. Using the channel histogram and adjusting red and green might solve the problem.

Ronnoco

[Ronnoco]

It's a fine example for the purpose it was posted. The histogram helped him make a better exposure decision than he might have without it. It is a tough lighting situation, though. But sometimes that's how it goes. This image probably needs an aggressive curve adjustment to look good.

No, John, it really isn't a fine example. The point is using the histogram to make a well composed photo that is off in exposure, much better through using the histogram. A "tough lighting situation" that is minimally improved through using the histogram still produces a less than ideal photo and I don't think that is the purpose of the post.

Ronnoco

[mdmc]

No, John, it really isn't a fine example. The point is using the histogram to make a well composed photo that is off in exposure, much better through using the histogram. A "tough lighting situation" that is minimally improved through using the histogram still produces a less than ideal photo and I don't think that is the purpose of the post.

Ronnoco

. Ronnoco, this post wuold be usefull to many people for exploring the use of the histogram to help with getting better exposures.
It's not about color corrections, which confuses the issue, but about how to use the histogram. Espeacially in tricky conditions.
All of your negetive comments are a great distraction for me.
Could you please refrain from being so negative. Or simply refrain from posting.
This would would be a great help to me.
Mark.