Tuesday, January 26, 2010

Wide Angle Distortion

I explained last time how focal length changes the perspective of our images. Here are a couple of examples of how a really wide angle lens can expand the scene. Candy Land seems smaller than I remember it from childhood, but it was never this big…

In the second image, notice how much longer the right arm seems compared to the left because of the wide angle stretching effect. If that were a nose, it would look huge. That's why you have to be careful if you ever use wide angle lenses for portraits.

Wednesday, January 20, 2010

Composition: Zoom and Perspective

Often when we zoom in or out all we are thinking about is how much of a scene we are capturing - or how "close" our shot is going to be. But zooming has another, very important, effect on our images. Zooming can greatly alter the perspective of an image by compressing or expanding a scene. As we zoom in, the distance between objects in our picture seems less and less. Conversely, as we zoom out, the distance between objects in our picture seems greater and greater. Consider the following two images:

Notice the tree in the foreground (red arrow) is basically the same size. However, the tree marked with the blue arrow seems much farther away in the second image than it does in the first. You can see that all the elements in the two pictures are affected in a similar manner. The fist image was taken at 180mm and the second at 17mm. I ended up with two substantially different images because of the focal length.

Sunday, January 17, 2010

Picture of the Week

  • Shutter Speed: 1/40
  • Aperture: f1.8
  • ISO: 800
  • White Balance: Sunlight
  • Focal Length: 35mm
  • Exposure Comp: 0

Friday, January 15, 2010

What's Wrong With This Picture? (3)

I took this picture a couple of weeks ago (while we still had our Christmas tree up) and it has a pretty glaring problem: it is overexposed. If you can't tell right away from looking at it, that's ok - you will learn with experience. The most obvious clue is the neon yellow in the cheek and nose. If that doesn't seem obvious, consider the histogram:

All of you faithful readers will immediately notice the spike on the right side of the red-channel graph. So the problem is that the colors in the skin look unnatural and uneven. One way to correct this problem is to adjust the exposure compensation, which I did, to -.7 (in this case.) Here are the results:

So I ended up with a better looking histogram. The image is better too, but now seems dark. That's ok though. It's more important to get a good exposure that doesn't fall out of the range of what your sensor can record. Because I captured all the information, a quick adjustment in photoshop produced this:

This was a quick fix, because the second exposure produced an image file with all the information intact. It would have been impossible to "fix" the first image because once you blow a channel(s) out, you can't get that information back.

Thursday, January 14, 2010

Would You Be Interested in a Forum?

Photomoms, I'm thinking about starting a forum or group so that all of you could upload pictures, ask questions, give feedback, and (dare I say) show off some of your own images. I think this would be a great way to learn more about photography and help everybody get better no matter where you are now. Let me know in the poll in the sidebar if that sounds like something you would participate in.

Monday, January 11, 2010

Pictures of the Week - Mr. Mom

This past weekend I was Mr. Mom while mommy was away. So that means extra time in PJs.

And when it is time to get dressed, my go-to outfit is overalls, or "oree-yalls" according to our eldest. And also, the girls get to pick any bow they want, whether it matches or not.

Tuesday, January 5, 2010

Histograms Part 6

Now that you are all experts on how to use histograms to evaluate exposure, here are instructions on how to access them on a Nikon D40 (I would guess that the D40x, D60, D3000 and maybe the D5000 are probably about the same.)

After you do this one time, the camera will remember your choices making it a lot easier every other time, so don't get overwhelmed. 
  1. Start with a picture displayed on the LCD and press "ok" in the center of the thumb pad. 
  2. Toggle down to "filter effects" and press "ok" again.
  3. Toggle to "color balance" and press ok
Now you are looking at the three histograms you need to evaluate your exposure. They are being displayed as part of a gimmick-tool used to change the color balance in your picture - which you don't want to use - so be careful that you don't make any changes to the color balance by accidentally moving things around. (If you make a mistake, just press the play button to cancel as it indicates.)

After you've done this once, you won't have to do any toggling on the menus because the camera will remember your previous choices. So all you have to do is press "ok" three times in a row and you're there.

If you have another Nikon camera, there is an easier way (typically) to access the histograms. When you are looking at a picture on your LCD, just press the directional thumb pad on whatever axis you don't use to switch to a different picture. In other words, if you navigate through your images using left and right, you will use up and down to scroll through a few information pages associated with the currently displayed picture. And if you navigate using up and down, you will use left and right to see the information. Just go through those information pages until you see the three colored graphs.

If you have another brand of camera, I'm sorry to say I can't tell you how to access the histograms. But most cameras have them in there somewhere, so dust of your manual to find out how to get to them. 

Using histograms to evaluate exposure is a lot more useful on your camera when you can make an adjustment and take another shot than it would be later on your computer. 

Sunday, January 3, 2010

Histograms Part 5

So now we get to it. It may seem obvious at this point what you can do with histograms (if so, great,) but just in case, I'm going to try to explain exactly how to use them.

When you look at a histogram (hopefully a three-color histogram,) you are looking for mountains of data spreading across the graph that don't bump up against the edges of the graph. If you do have a peak or a spike against an edge of the graph, you will be losing information in your file and consequently, texture and contrast in your image. Additionally, you don't want a large space between your data-graph and an edge (the right edge especially.) So here are some examples…

In one of my very first posts on exposure, I gave same ideas to help judge exposure based on what you can see on your camera's LCD. Those judgements are be based on visual cues which lead to an evaluation that is ultimately subjective. I'm going to use those same images, but this time evaluate them objectively using histograms. Here are the images with their corresponding histograms following each:

The above image is underexposed. Even though the graphs are not bumping up against the left edge very much, the fact that there is no information in the right quarter of any of the graphs tells us that this image is underexposed. Very badly underexposed images may have graphs that are more extreme than this, but those will be more obvious without a graph. This type of underexposure is where histograms are really valuable. If you see a graph like this, use exposure compensation to adjust your exposure.

This image is overexposed. You can see in the red channel, that the graph is running into the right edge - this is often the case in overexposed images of people since most skin (black, white or in between) will show up in the red channel. The overexposure of the red channel causes the neon look in the skin (notice the nose in particular.) If you see a graph like this, use exposure compensation to adjust your exposure.

This is properly exposed image. The graphs come right up to the right edge with out running into it. These are the types of graphs you should look for. Here are some other good looking histograms following their corresponding images:

This last one is a curveball - and an important lesson. The histograms are running into the right edge indicating that there is some overexposure. However, this is a properly exposed image of my daughter. What is overexposed is the snow. That is why there are patches of pure white in the snow where you can't see any texture. The only way to save the detail in the snow would be to underexpose my daughter. I didn't do that since, obviously, she is the most important part of the picture.

In many images, there will be decisions like this you will have to make. So don't just depend on histograms without thinking. If you can identify why your graphs are hitting the edges, and your ok with losing that information (like the snow,) then don't worry about it.

Read Part 6 next.