Tuesday, December 29, 2009

Histograms Part 4

Three Color Histograms
To really use histograms effectively when evaluating exposure, you need to use three-color-histograms. If you understand what I've explained in Part 1, Part 2, and Part 3, this isn't too complicated. We must simply realize that our cameras aren't recording different levels of grey, but actually different levels of red light, green light and blue light. So instead of thinking of brightness levels as the variation between black and white, we must think of them as the variations between dark red and light red, dark green and light green, and dark blue and light blue. If that doesn't make sense, come back a read this paragraph again after you read the rest of this post.

When we look at the three color histogram from the picture in Part 3, we see this:

from this picture

So what you are looking at in the graph above is the variation in brightness of red light, green light, and blue light in the picture. So that's how digital cameras work - they record the variation in red, green, and blue light and then overlay the three one-color images to produce color images. So when we take a picture, we need to make sure that all three graphs stay away from the edges.
Next time we will take all the theory and make it practical and usable.

Read Part 5 next.

Monday, December 21, 2009

Christmas Ideas

I read an interesting post on ideas for taking pictures during Christmas that I thought you all might enjoy. It definitely has some creative tips. Check out 16 Digital Photography Tips for Christmas if you are interested.

Picture of the Week

  • Shutter Speed: 1/50
  • Aperture: f1.8
  • ISO: 800
  • White Balance: Incandescent
  • Focal Length: 35mm
  • Exposure Comp: +.3

Saturday, December 19, 2009

Histograms Part 3

Read Part 1 and Part 2 first.

So today we actually get to it. Hopefully, after the last two posts, this principle has become clear: the sensor in a DSLR can only record a limited range of brightness levels in any given scene. Because of that, it would be nice to know how well we are utilizing the different brightness levels that our sensor is able to record. Our effectiveness in using our sensor in this way is all about proper exposure. And the best tool we have to know how well we have exposed a picture is a histogram. So here we go…

As I said in Part 1, a histogram is just a graph that plots data. In digital photography, the data that it plots has to do with brightness levels. The graph basically tells us how many pixels were recorded at each available brightness level. So here's what a histogram looks like:

What you are looking at is a graph with all the different brightness levels the camera could record across the bottom and the number of pixels recorded at each level up the side. Here is the image that this histogram came from:

In this image the available brightness levels are used quite well - this is a well exposed image. The hump on the left of the histogram represents most of the wood and hair while the hump on the right of the histogram represents most of the skin and dress. Here is a color-coded comparison of where the brightness levels are actually distributed:

So you can see the large blue hump represents lots of pixels in the image. Similarly, the green/yellow hump represents a lot of pixels too, but fewer than the blue hump. Obviously there aren't a lot of pixels in the red area, which is why that part of the graph is so low.

When you look at a histogram, a well exposed image will have most of the pixels distributed with the humps away from the edges. If there are humps in the graph up against either edge, you will be losing information. Consider these two pictures from  Part 1 with their corresponding histograms:

Notice how this histogram has a hump on the left representing the blackest parts of the pipe. A wide range of brightness levels that fell below the recordable range are getting assigned the lowest brightness levels. In other words, parts of the pipe that should have been recorded at levels like 0, -1, -2… were all recorded at level 1 since that is the darkest level available. The result is that there is no separation between what differences in brightness there were in real life. So instead of seeing texture in those very dark parts of the pipe, we just see black patches.

In this histogram, notice the spike on the very right edge. This spike represents every pixel in the sky of the image. Even though there were differences in the level of brightness in the sky in real life, they all fell above the range that was recorded and therefore were all assigned the highest brightness level. So in this image, let's say the brightest level is ten. The problem is that the sky has levels 11-15 in it. But since ten is the brightest level that can be recorded, all those pixels were recorded at ten. Again, because there is no difference in the recorded brightness level, there is no detail in that part of the picture.

Ok, so we covered a lot today and actually started talking about histograms. But there is still a lot to cover including how to use this information when you are taking pictures and the importance of three-color-histograms. Like I said in the beginning, this is a complicated subject, but a very, very valuable tool. I hope you stick with it. Let me know if you have questions so far.

Read Part 4 next.

Friday, December 18, 2009

Histograms Part 2

Read Histograms Part 1 first.

As I explained yesterday, our cameras' digital sensors can only record a limited range of brightness values at a given time. Anything that does not fit into that range will show up as either white, if it is brighter than the range, or black, if it is darker than the range. Here is an illustration that I hope will help you to understand this principle.

Imagine that you are working on a paint-by-number picture in gray-scale. You have eight shades from black to white to paint with. When you look at the picture you notice that there are sections numbered from one to ten. Now, you only have eight shades, so you can't paint all ten different shades that are in the picture. You have two choices.
  1. You can use your darkest shade as #1 and let anything marked with #8, #9 or #10 all be filled in with your lightest shade.
  2. You can use your lightest shade as #10 and let anything marked with #1, #2 or #3 all be filled in with your darkest shade.
Either way, you are going to lose some detail that is supposed to be in your picture. Either the three lightest shades will all be one shade, or the three darkest shades will all be one shade. Look and see . . .

Here is your paint by number picture:

If you had ten shades, it would look like this:

But since you have only eight, you must choose one of the following options:

In the first option, the lighter sky shades are combined so you can't distinguish between the sky, clouds and birds. In the second option, the darker shades are combined so the middle of the flower runs together.

I simplified this picture from yesterday to illustrate this point in a real image:

If my D40 sensor could have recorded more levels, there could have been more detail in the pipe. But anything darker than "1" was all recorded as "1."

I hope this is helping you understand a little bit more about how your camera is working. Please let me know if you have any questions. Ultimately, this knowledge will enable you to judge your exposure much more quickly and accurately. Read Part 3 next.

Thursday, December 17, 2009

Histograms Part 1

With this post, I'm in danger of covering a subject that is overly complicated and beyond what photomoms want to know. But I'm going to give it a try anyway, so let me know what you think. Hopefully I can explain it simply enough to be practical and useful.

Histograms are graphs that plot data. When used in digital photography these graphs can provide valuable feedback regarding exposure. This feedback is particularly useful because it is objective. It takes a little effort to understand histograms, but once you do, they are quick and easy.

Important Background Information
The sensor in your camera can only record a certain range of brightness values at a given time. That means that if there is too much contrast in a scene, your camera cannot record it all.

It may help to think about your sensor as the paddle in pong. You can move it back and forth from right to left in order to capture a different range of the brightness spectrum, but the paddle can't cover it all. Thankfully, most scenes don't have a "full spectrum" of brightness in this sense, but often the variation in brightness is greater than our sensors can record. Consider these two examples:

In this image I slid my pong-paddle-sensor toward the brighter side of the spectrum so that I could capture my wife and daughter at the other end of this pipe. In doing this, I sacrificed a lot in the darker part of the spectrum. As a result, much of the pipe appears as solid black with no texture.

In this image I sacrificed some of the brighter part of the spectrum so that my daughter wouldn't be too dark. The result is that the sky is pure white without any detail.

If you understand this principle, you are on your way to understanding histograms and using them to your advantage. Read Part 2 next..

Wednesday, December 16, 2009

Judging Exposure

In a recent comment, one reader wrote:

"A lot of my pictures look well exposed on the screen of my camera, but when downloaded to the computer - they are way too dark!!!???"

Let me encourage all of you to trust your DSLR more than your computer screen when it comes to exposure. Your camera might not be perfect, but newer cameras (including the Nikon D40) have great displays. Your computer screen, on the other hand, probably won't be accurate. It may be older and dim, making your pictures look dark. Or the brightness value may be adjusted either way to change how your pictures look. Have you ever seen one of your images on someone else's computer and thought, "That's not what it looked like on my screen." Who's screen is right? Probably neither. Unless you have an expensive display that is calibrated regularly your computer screen is giving you a pretty arbitrary view of your pictures. It's not a good way to judge exposure or color. Sorry.
I'll explain how to judge your pictures' exposure from an objective point of view next time. In the meantime, don't trust your computer display.

Sunday, December 13, 2009

Picture of the Week

  • Shutter Speed: 1/25
  • Aperture: f2.8
  • ISO: 400
  • White Balance: Auto
  • Focal Length: 35mm
  • Exposure Comp: +.7

First Week with a DSLR: Part 7

Experiment Results (Part 3)
Read about the experiment here.

I showed you these pictures yesterday and talked about how the shutter speed is changing the way the moving hook appears in each image. 

Today I want to explain another effect shutter speed has on your pictures. Consider these close-ups form the images above:

So, what's happening here? A couple of things. In the first image, the label is slightly blurry because of the aperture, just like the Christmas lights in the background - read here for more about this. In the second image, the label is sharp. In the third image, the label is blurry, but not because of the aperture. It is blurry because the shutter speed is too slow. The shutter speed is so slow that while the picture was being taken, the camera moved around enough to make everything in the image blurry.

So what do we take out of this? Well, what I said yesterday is still true. "If there is something moving in your picture, you can change the way it looks by controlling your shutter speed. If you want to "freeze" the action, use a larger number, if you want a motion blur, use a smaller number." But we need to add something. If you make the shutter speed number too small, it will cause everything in your image to be blurry.

So if there are things in your picture that weren't moving, but are blurry when they should be in focus, it is probably because your shutter speed was to slow (the number was too small.) This is why most pictures taken inside without a flash end up being blurry (unless you have huge windows or super bright lights.) So when you are making your shutter speed number smaller and smaller, be careful you don't go so far that everything in your picture gets blurry.

Saturday, December 12, 2009

First Week with a DSLR: Part 6

Experiment Results (Part 2)
Read about the experiment here.

I took these pictures of our mixer with our  Nikon D40 and  Nikon 35mm f/1.8 DX Lens. I used the mixer so I could get some consistent movement to provide a good example, not because these are compelling pictures. I did the whole experiment, but I'm only showing you three examples-the two extremes, and one in the middle.

In these pictures, we can see something similar to what I talked about in Part 1 of the results. If you look at the lights on our Christmas tree in the background, you can notice the difference in blurriness when comparing the pictures. As I said last time, this is a result of the changing aperture (the number next to the "f" in your viewfinder.)

Obviously, there is something else happening in these pictures because of the movement. As the settings change, the spinning hook looks substantially different. This is a result of the shutter speed (the changing number not next to the "f".) Here is the hook close up:

So what can we learn here? If there is something moving in your picture, you can change the way it looks by controlling your shutter speed. If you want to "freeze" the action, use a larger number, if you want a motion blur, use a smaller number. But that's not all that happens. Tomorrow, we'll consider another way shutter speed affects out pictures.

Wednesday, December 9, 2009

First Week with a DSLR: Part 5

Experiment Results (Part 1)
Read about the experiment here.

I took these pictures of the rails in a fence with our  Nikon D40 and  Nikon 35mm f/1.8 DX Lens.  I followed the process of the experiment, but then reordered the pictures from one extreme to the other.  The first picture (top left) was set at 1/5sec and f22, while the last was 1/500sec and f1.8.

Even if you don't understand what aperture is or what is happening as you perform this experiment, you can see that something is changing.  All these pictures have the same exposure (for the most part,) but they do not look the same.  Consider the first and last image a little larger:

So what can we learn here? As the number in your view finder next to the "f" changes, it changes how much of your image appears in focus. If you want blurry backgrounds, make that number smaller; if you want everything to look sharper, make that number bigger.

P.S. Both images are in focus. Don't make the mistake of thinking the focus of the lens is causing this - it's not. Both images are focused on the first full rail in the picture.

Sunday, December 6, 2009

First Week with a DSLR: Part 4

An Experiment
Your DSLR can do lots of great things so it's hard to know where to start when everything is new. Here is an assignment that can teach you a lot about your camera if you pay attention to what's happening.

The Setting:
For this experiment you still want to be outside in the daylight. And you'll want to go through this process multiple times with different subjects. Be sure to choose a moving subject a few times and be sure that there are lots of things in your image that are different distances from your camera. In other words, don't take a picture of a flat brick wall. Don't do any zooming or repositioning when you are going through one set of pictures.

The Settings:
You'll want your camera in Program Mode. This means the winding dial on the top should be on "P." Everything else should be normal - assuming you haven't fiddled with other settings.

What you need to know:
When you take a picture, your camera collects a proper amount of light to give you a good exposure. Your camera uses two variables to control how much light is collected. The key to this experiment is understanding that different combinations of these two variables can produce the same exposure.  It's the same as being able to multiply different combinations of numbers to get to 12.
  • 1 x 12 = 12
  • 2 x 6 = 12
  • 3 x4 = 12
You don't have to understand everything yet, just know that different settings can give you the same amount of light.

The Process:
Aim your camera at something and take a picture.  Now, while you are looking through the viewfinder, wind the command dial (pictured below) one click (either direction.)

In the viewfinder, you should see some numbers at the bottom change. In addition you should see "P" with an *.  (If all the numbers disappear, just push the shutter release down half-way.) What you have just done by winding the command dial is changed your two multiples without changing the product. So, for example, your camera may have chosen 2 and 6 to make 12 and turning the dial changed it to 3 and 4. So with these new settings take another picture. Wind and repeat until winding doesn't make a change to the numbers anymore.  Then go back to where you started and wind the other way.  You'll end up with a series of pictures with the same exposure using different settings. Do this all again aiming at different subject.  Then do it a few more times. If you ever feel like you've totally lost track of where you are, just turn your camera off.  When you turn it back on, it will start over.

Once you have done this four or five times (or more,) you can look at the result on your computer. Try to figure out what's happening. Check out some results here: Part 1, Part 2, Part 3.

Saturday, December 5, 2009

Picture of the Week

  • Shutter Speed: 1/400
  • Aperture: f1.6
  • ISO: 200
  • White Balance: Sunny
  • Focal Length: 50mm
  • Exposure Comp: -.3

Thursday, December 3, 2009

First Week with a DSLR: Part 3

Learning Lessons
If you follow my pointers from last time, you should end up with some encouraging results right off the bat.  After you've taken 500-1000 pictures - which you can easily do in afternoon - here are some things you should be learning:

  • Your DSLR can take pictures right when you push the button.  This is a huge advantage that DSLRs have over point and shoot cameras.
  • You can take pictures in rapid succession without having to wait.  This is another huge advantage that DSLRs have over point and shoot cameras.  Take advantage of this by taking lots of pictures at a time.
  • It matters a lot more what you focus on with a DSLR when compared to a point and shoot camera.  This has to do with multiple factors which aren't important during week one, but what is important is knowing how to focus your camera.
  • Your camera gives you a lot of information when you look through the viewfinder.  Once you learn what it all means, you'll be able to control your camera more effectively and efficiently.  So start reading my DSLR Guide.
  • Composition may be harder than you thought.  If not, great - you're a natural.  If so, here are some composition tips.

Tuesday, December 1, 2009

First Week with a DSLR: Part 2

Dispelling the Myth

So you've just opened the box of your brand new DSLR.  What next?  Well first you'll probably want to attach the camera strap, charge and insert the battery, attach your lens, and put in a memory card (preferably not the small-capacity freebie that came with your camera.)  Once you've done that, you should have a working camera.  When you turn it on, you'll probably be asked to set the time and date, but that is most likely your only hurtle to overcome before snapping away.

So let's say you've done all this, you've put the kids to bed and you are ready to experiment.  Prepare to be disappointed.  Why?  Because most people who haven't used an SLR or DSLR think that big heavy cameras take great pictures.  Wrong.  Photographers take great pictures.  One of my photography-pet-peeves is hearing people say "Sure his pictures are good, look at his camera."  I especially hate this when people say it about me.  For two reasons.
  1. I've got a nice camera, but I want some credit for my pictures.  (This is called pride)
  2. Plenty of people with nice cameras take bad pictures.  (This is reality)
So don't think that your new camera is going to make you the next Ansel Adams or Annie Leibovitz without any effort.  If you can get past this, then the first few weeks with your new camera will be a lot more pleasant.

Where to Start

To avoid disappointment, here are a few pointers:

  • Start your experimentation outside in the daylight.  Then you will get to see how great your camera can be.  You'll have to work to get good pictures in darker situations, but don't start with the most difficult stuff.
  • Start with your camera in AUTO or Program mode.  Let your camera do the work at first so you can get used to holding it, zooming and taking pictures.
  • Don't change settings if you don't know what they do.  You'll probably do more harm than good - and then you'll forget what you did.
  • Use your camera's display to review your pictures immediately.  Take advantage of the fact that you have a digital camera that can give you immediate feedback.  Learn and make adjustments right away.
  • Don't delete anything at first.  Wait until you can see the pictures on your computer and find out what you are doing well or poorly.  Looking at your mistakes can help you to learn a lot quickly.

A Message of Hope

Don't let my sobering message mislead or discourage you.  Your new DSLR will absolutely help you take better pictures than your old point and shoot camera.  Your new DSLR can get good pictures in situations that your P&S cannot.  Your purchase was worth it.  It just might take some time for you to learn how to use your DSLR in a way that maximizes its potential.