Thursday, November 19, 2009

Light Metering

Metering Zones
It's fall, and it's time for the back to basics style. What I'll be posting next are a few sections on the basics of your SLR camera, and how to use them. My thoughts on this are that I'm not the only one that would rather play with a camera than read the manual. In most cases, knowing the intricacies of your camera's settings will allow you to consistently capture the best shot in a situation.

Functionally speaking, one of the most basic aspects of photography is correctly metering the light in a scene. Although modern cameras do really well at this, knowing your metering modes is still important and will keep pictures looking great.

First, lets look at how metering works on a fundamental level, and then we can dive into each mode to see how they can be used in specific lighting situations you will encounter.

Metering Zones

Before we can discuss the fundamentals of metering, what is a metering zone? To answer this simply, the modern cameras will break the viewfinder into a number of metering zones as seen in the sample picture above. Generally more metering zones will allow the camera to give more accurate results.

Fundamentals of Metering

One of the easiest ways to think of this is to look at the sample picture as the camera would meter it; it looks at the image in the viewfinder as all gray scale. Each of the zones gets all of their values averaged together, and in the end has one value per zone.

Here is an example of how the camera might "see" a picture when metering with 49 zones.
Mouseover to see the average values for each zone!

In order to properly expose the picture, the camera adjusts the exposure so that metered zones are equal to what is called "middle gray". This value is effectively what the camera's metering mode attempts to achieve, a middle gray in the area that is being metered.

So if you were to take a picture of a completely white sheet of paper, your camera would expose it as "middle gray". Similarly, a picture of a completely black sheet of paper would also appear as "middle gray".

The reason for this is that the universally accepted method for properly exposing using shutter speed, f-stop, and ISO is designed to render subjects as "middle gray". Anything that is "middle gray" will reflect around 18% of the light that hits them, and cameras then assume that in any given scene, the average should reflect 18% of the light.

Taking a picture of something that is all white (like a picture of snow) will then come out underexposed because the picture is going to reflect more than 18% of the light that hits your subject (the snow).

Given this, when we describe a metering mode that only meters 9.4% of the picture in the center, this means the camera will average the zones in that area together, and then attempt to make the grayscale version of that specific portion of the image "middle gray" after exposing.

Middle Gray

Surely by now you're wondering "What is middle gray?! And why does this guy keep putting it in quotes?!" Well, I was putting it in quotes because the actual value of it will depend on the white balance you have chosen. Perfect middle gray is 100G-100R-100B. However, if you have set your white balance to a slightly warm value, you'll see more red and less blue in the middle gray.

Finally, Metering Modes

Now that you are a certified genius in relation to metering, lets discuss what each of the modes on your camera does.

Evaluative Metering

Spot Metering

Partial Metering

Center-Weighted Metering

Visual examples of the different metering modes in a Canon EOS camera with the green focal box in focus.

  1. Evaluative Metering (aka Matrix Metering)

    This is the Auto Mode of metering - where the camera uses some sort of advanced algorithm to determine the lighting of the scene and adjust appropriately. Different manufacturers will implement this different ways, but it can be as complex as actually having a library of typical "scenes" that it will compare the image lighting pattern to in order to appropriately expose your scene.

    Canon's specific implementation of this will link the metering directly to the active Autofocus point and compare the light values from that point with surrounding metering zones to attempt and provide accurate exposure. This works really well when taking a picture of a subject, but may not work well for large objects that are majority black or white (because they take a large area of the viewfinder and do not reflect 18% of the light).

  2. Spot metering

    This mode uses just the center of the image in the viewfinder, and does the averaging only in that zone.

  3. Partial Metering

    Similar to Spot metering, this mode will cover an area that is slightly larger, 9.4% of the total viewfinder area.

  4. Center Weighted Metering

    Using a weighted average, this mode will assign a heavier weight to the zones in the center of the image, but will take into account all zones in the viewfinder..

And Then?

You should have a slight idea of when each of these would be useful, but lets go over a few situations and see which metering mode would be the best suited.

1. A person standing in the snow or another very bright background.
As long as the person is the active focus point, evaluative metering will do a decent job. However, I recommend you choose spot metering for this; it will ensure that the person is properly exposed. If the background is blown out, then try using a flash for fill lighting on the person, and switch to partial metering so that the entire image isn't over exposed.

2. A black car that takes up most of the viewfinder.
Since the subject is not reflecting the expected amount of light, any of your modes that only meter the black subject will not look great. Try using Center Weighted Metering, and then you may still need to adjust the image to under-expose an f-stop or two.

3. Inside of a house, taking a picture of the view through a small window.
So here, we get into the problem of dynamic range of the image. Either you'll have the window's details properly exposed, or inside of the house properly exposed (unless you do some dynamic range photography).

Choose partial or spot metering, and using your camera's exposure lock, you can get the proper exposure of the window or the inside of the house, and then adjust the framing of the shot.

If you try to use exposure lock with evaluative metering, it may still over expose if the zones being evaluated have too much wall in them, or under-expose if they have too much window in them.

4. General Purpose
For general purpose, you should just use the Evaluative Metering, and then if you're using Canon, be aware of the focus point - that's what is going to be properly exposed so long as there's not a large area around it that's a deep dark or light color.

Your mission, should you choose to accept it.

Take a picture of a backlit subject, and change the metering modes to see what the differences are. Don't use a flash for fill light on this exercise.

Can you produce an image where the subject is properly exposed? What about one where the backlight is properly exposed, and the subject is just a silhouette?

Monday, October 12, 2009

Photography Glossary

See F-stop.
Aperture Priority
A shooting mode on modern cameras where the aperture is set to a desired value and the camera automatically determines the shutter speed to properly expose the picture.
Refers to the size of the optical sensor in digital cameras, or the size of the film’s frame in film cameras. APS-C is approximately 40% smaller than a full frame sensor or 35mm film. (Full size sensors are the same size as 35mm film frames.) What this means, is that with a standard lens on your Digital SLR with an APS-C size sensor you will get 1.6x the focal length of the lens.
Refers to the aesthetic quality of the portions of the image that are out of focus.
A method of taking multiple pictures with different exposures or settings either for High Definition Photography, or when it is difficult to obtain the proper settings with a single shot . Typically your camera will allow you to set three or more values at which the pictures or taken; for example you may bracket based on f-stop, or even shutter speed with some cameras where holding down the shutter button will take a picture at -1 f-stop, 0 f-stops (properly exposed), and +1 f-stop.
CMOS Sensor
The image sensor that has replaced film in modern cameras. The CMOS stands for Complimentary Metal Oxide Semiconductor, and is just one type of sensor modern cameras are using.
Depth of Field
A term used to describe the amount of the depth of an area that is in focus in a picture. In other words, a higher depth of field will give more in focus than a lower one given the same subject and picture. The f-stop and lens focal length are just two things that will impact your depth of field.
See Depth of Field
Digital Single Lens Reflex Camera-See SLR.
Stands for (Electro-Optical System) and is specific to the Canon line of cameras, the EOS camera system was introduced in 1987, and is still in use today.
Exposure is a general term to indicate the brightness of an image. Something with improper exposure generally does not have a good light distribution, and will lose detail in the darker parts of the image. Similarly, over exposing an image may cause you to lose detail in the brighter parts of the image.
Film Speed
Focal Length
When referencing lenses, this is the distance it takes for light to converge beyond the lens. Subjects inside of the focal length of a lens will have a more shallow depth of field than those outside of it. With a 35mm camera, if you have a lens with a focal length of 35mm, you will capture images at a ratio 1:1 (no zoom, no wide angle). Focal lengths less than the size of your sensor are considered Wide Angle, and greater are considered zoom lenses.
Focal Ratio
See F-Stop.
Directly correlates to the diameter of the entrance pupil in a lens. The smaller the f-stop, the larger the pupil and more light allowed into the camera as it is expressed in terms of focal length (f/1.4 means focal length divided by 1.4 where 1.4 Is the f-stop). Each increment of an f-stop will reduce the amount of light let in by half! Standard values are: 1.4, 2, 2.8, 4, 5.6, and 8.
Full-Size Frame
Typically accepted as a 35mm sensor or 35mm film.
Previously referred to as Film Speed, the ISO used to determine the sensitivity of film to light. The higher the ISO, the less time needed to properly develop the film – even if it was at the expense of a grainy or less defined picture. In modern digital cameras where there is no film, changing to a higher ISO value will instruct the camera to properly expose the image in less time, typically resulting in increased image noise.
A type of photography where the focal distance is extremely small. This typically produces images of subjects at multiple times their original size.
Relative Aperture
See F-Stop.
Shutter Priority
A shooting mode on modern cameras where the shutter speed is set to a desired value and the camera automatically determines the f-stop to properly expose the picture.
Shutter Speed
The speed that the shutter will open and close- giving the exact time that light is allowed to hit the sensor or film. Shutter speeds are typically defined in seconds, so a shutter speed of 1/40 is 1/40th of a second. A faster shutter speed means less light is allowed into the camera.
Stands for Single Lens Reflex. Historically refers to a type of camera where there is a movable mirror between the lens and sensor or film. This mirror allows the viewfinder to see the image as it appears through the lens. When the picture is taken, the mirror flips up before the shutter opens, and falls back into place once it closes if in standard operating mode.
The visual distortion of the areas around an image historically caused by poor lens optics. Recently, it is done digitally or with specially crafted lenses for artistic effect.

Monday, October 5, 2009

Simple Color Enhancement in Photoshop with Lab Color Curves

Lab Color CurvesHave you ever taken a photo of a particularly stunning sunset or vibrant autumn leaves only to get home and find that the colors are just not quite as bright as you remember them? Well, there's a good reason for that.

Without being too technical, the primary colors of light are red, green, and blue (as opposed to the primary colors of pigment which are cyan, magenta, and yellow). Thus, your color TV, your laptop's LCD, and the HD Jumbotron at Auburn University's Jordan-Hare stadium (war eagle) all use red, green, and blue lights to display any color they want.

So, it stands to reason that when your camera's CCD gathers raw data in red, green, and blue channels that your camera would store the data in RGB format. The problem is, though, that your brain is a little more complicated than this. Your brain adds expectations based on light, shadows, and past experience. So, when your pictures don't look like you remember, that's because the camera doesn't know what your brain said it should look like.

There's an App for That

The good news is that this problem was solved way back in 1931 when the International Commission on Illumination came up with the CIE 1931 color space. This color space has been expounded upon through decades of experimentation and is now generally called Lab (the L stands for lightness; a and b stand for two color-opponent dimensions (long story). This color space was designed to mimic the color distributions of human perception.

What does this mean for you? Well, simply put, you can easily adjust your images using a curves adjustment layer in Photoshop. Before I show you how, let me show you a few samples.


These images have been color enhanced leveraging the Lab color space. To see the original picture, just hover over the image with your mouse cursor.

Boats on New River with Lab Color CurvesBoats on New River without Lab Color Curves

New River with Lab Color CurvesNew River without Lab Color Curves

Oakland Cemetery with Lab Color CurvesOakland Cemetery without Lab Color Curves

Oakland Cemetery Statue with Lab Color CurvesOakland Cemetery Statue without Lab Color Curves

Using Lab Color Curve Adjustment

You can make Lab adjustments with gimp and there's a Paint.Net plugin too. I may publish instructions for these applications if more people start using them, but here's how you do it in Photoshop.

Here's how you do it:
  1. Open your image
  2. Click Image >> Mode >> Lab Color
  3. Click Layer >> New Adjustment Layer >> Curves
  4. Name your layer and click OK
  5. Adjust the curve for channel a
    1. Drag the top right corner in one quarter of the way
    2. Drag the bottom left corner in one quarter of the way
    3. If you did it right, the window will look like this:
      Photoshop Curves Dialog for Lab Color
  6. Repeat the process for channel b
  7. Press Ok
  8. Click Image >> Mode >> RGB to convert it back to RGB color so you can save it
That's it! Now, disable the adjustment layer and then re-enable it a few times to see the difference it makes. When you're ready to save your image, merge the visible layers and save a copy of your image.


Once you've got the hang of it, you'll be able to get much more real life color out of your images. As usual, we'd love to see what you have so your homework is to apply this effect to some of your own images. If you see any dramatic results, post them on Picasa or Flickr. Leave us a comment and link to your images. If you see any pictures you think are better without the color enhancement, we'd love to see those too.

Friday, October 2, 2009

Adding a Vignette with Photoshop

Vignette DemonstrationVignetting is any technique that reduces clarity around the corners or edges of an image. Generally, you'll see the vignette as darkened corners and edges around the center of the image. This technique has been used to draw the eye to the center of the picture where the subject will be bright and in sharp focus.

There are several ways to produce vignettes in your own images. For example, using a lens hood that is too long or too narrow for your lens will cause the hood to block some of the light in the periphery. Alternatively, using a lens that is too small for your sensor can produce the same result. There is, however, a vastly easier (and reversible) way to enhance your images with a vignette if you have the right tools.


But, before we get into that, why don't we take a look at a few demos? Both of these images have been enhanced with a vignette. To see the original picture, just hover over the image with your mouse cursor.

New River with VignetteNew River without Vignette

Rapids with VignetteRapids without Vignette

How to Add Your Own Vignette

I've tried this with a number of tools including two free image editing packages called gimp and Paint.Net (to be sure, there will be future blog posts about both of these). But, for now . . . the popular one is the very expensive Adobe Photoshop.

Here's how you do it:
  1. Open your image
  2. Create a new layer (Ctrl + Alt + Shift + N)
  3. Fill with white (Shift + Backspace)
  4. Set blend mode to multiply (Alt + Shift + M)
  5. Select Filter >> Distort >> Lens Correction
  6. Look at the vignette section and play with the settings in there
  7. Press Ok

That's it!


So, that's all there is to it. It's a quick way to add some spice to your pictures. So, here's your homework. Add some vignettes to your own photos. If you see any dramatic results, post them on Picasa or Flickr. Leave us a comment and link to your images. If you see any pictures you think are better without vignette, we'd love to see those too. For bonus points, see if you can create a vignette effect optically and let us know how you did it.

Saturday, September 19, 2009

Time Priority (Shutter Priority) Creative Mode

Time Priority DemonstrationHaving completed the introduction to the basic shooting modes in Getting to Know Your Shooting Modes, we're ready to take a look at our first creative mode called Time Priority or Shutter Priority. The indicator for this shooting mode differs from camera to camera and your best bet is to read your manual for your device; however, generally the time priority setting is identified as Tv on Canon cameras or S on Nikon and Sony.

As you probably know, the camera has two ways to control the amount of light that contacts the sensor (whether it is film or digital): shutter speed and aperture. Aperture refers to the size of the opening in the lens and we'll talk a lot more about aperture later when we talk about Depth of Field and portrait photography, but for now just know that your exposure is set primarily by shutter speed and aperture setting.

The time priority shooting mode allows you to chose exposure time and the camera will set your aperture for you. This way you can concentrate on how you want motion to appear in your photograph without having to worry about getting the right exposure. In the demo photo above, you can see two different effects you can make with shutter speed.

What's it for?

One thing you might want to do in your photo is capture the feeling of motion in your photo. In the waterfall photo, we wanted the viewer to be able to feel the intensity of the water as it rushes over the smooth rocks and crashes into the pool below. We decided to keep the shutter open longer to let the motion blur of the water appear giving the image the look and feel of being in motion.

Another thing you might want to do is freeze time stopping all motion to record an exact moment in time. You might want to catch the exact moment a balloon pops or a water droplet hits the ground. In the demo, we set the shutter speed to about 1/2000th of a second to freeze the rapidly fluttering wings of a hovering hummingbird.

By the way, this is how two of the basic modes work. Sports mode increases your shutter speed (decreases exposure time) to about 1/500th of a second in bright light to stop motion and give you a clear crisp shot of all the action. Night mode decreases shutter speed (increases exposure time) to gather as much light as possible from the background of the night shot. The night shot will also enable the built in flash (if your camera has one). There's a good reason for this; however, we'll be discussing that in a future lesson all about using the flash.

Finally, a long exposure isn't always just for showing motion. Sometimes, a long exposure can bring out a lot of subtle details. For example, a long exposure can help you bring out the vibrant colors of a sunset or sunrise. Just keep in mind that longer exposures will require a tripod to keep them clear. A good rule of thumb is 1 / focal length so if you're shooting at 100mm (effective) then you'll need a tripod for any exposure longer than about 1/100th of a second.


With that, you have another homework assignment. Set your cameras on Time/Shutter priority and go get some pictures. Put your camera on a tripod (or on anything stable really) and take the same picture twice: once with a fast shutter speed and once with a slow shutter speed. What do you notice about the motion of the objects in the picture? Do you notice anything about the feeling that's conveyed in each? Is there any difference in the colors and clarity of each of the pictures? What happens to the aperture setting (f-stop) as you change your exposure time?

Try taking a bunch of handheld shots at different exposure speeds. What's the slowest shutter speed you can use and still get a crisp image? What if you use a tripod?

Take some long exposure night shots. What could you do if you wanted to freeze the action at night? If you want some advanced homework, go out with a friend and try to get some night portraits. What do you notice about objects in the foreground? Try using the flash and a short exposure time. Now, try using the flash and a long exposure time. What happens? Why?

Be sure to put your answers in the comments and share your favorite photos on Picasa or Flickr. Please let us know what you think of our blog and what you'd like us to write about in the future.

The best way to learn is to teach. If you have an idea for a class, write it up and send it to us. If we like it, we'll post it. Be sure to include your name and a link to your organization and/or portfolio.

The Photography Professors

Thursday, September 17, 2009

Getting to Know Your Shooting Modes

Shooting ModesThis probably looks familiar to many of you; the shooting mode dial varies from camera to camera but most of them have the same general settings. I'm sure that some of you undoubtedly know what these settings are, but others just dial in the green rectangle for "Automatic Mode" and call it a day.

The first step to taking better pictures is to get out of the so called "Basic Mode" settings and start using the "Creative Modes." The basic modes include: portrait, landscape, close-up, sports, night, and no-flash. In this blog, we'll usually focus more on the creative modes, but the basic modes deserve an introduction.

There's nothing you can do in the basic modes that you can't do in "Manual Mode." Problem is, manual mode can be pretty daunting. Over the next several lessons, we'll explain what each basic mode is, what it's used for, and why it creates a particular configuration. We'll teach you how to reproduce the same effects in the creative modes.

That's when you can really begin tweaking your photos to get exactly the effect you're looking for. But, not so fast. First, a homework assignment for you.

Take a few pictures in each of the basic shooting modes on your camera. Do some experimenting while you do it.
  • What happens if you use night mode during the day? What about landscape mode when taking a portrait?
  • When you're shooting in portrait mode, is there anything you notice about objects in the background?
  • When shooting in landscape mode, is anything ever out of focus?
  • When you shoot in portrait mode does the flash go off? Even during the day?
  • What happens when you take a picture of a moving object in sport mode? In night mode?

Answer those questions, make some observations of your own, post some pictures on Picasa or Flickr, and come back and leave us a comment. Tell us what you learned. Share some theories with us. Link to your pictures. We're looking forward to hearing from you!

The Photography Professors

Welcome to the Photography Professors!!

Antique CameraHello there and welcome to the Photography Professors! We're glad you found us and we hope you enjoy our blog. Our goal is to help amateur and professional photographers alike broaden their horizons and learn new tricks and techniques to put in their photography tool kits.

We'll be discussing all kinds of topics like basic photographic techniques, fancy tricks like bokeh effects, product reviews, and even Photoshop quick tips to help add that extra spice to your favorite photos.

We know there are a lot of photography blogs out there, but we (like all of them) think we're different. Here's a little insight into our format:

Each blog post will begin with a sample photo that demonstrates the technique we'll be learning. Then, we'll describe what the effect is and how you can reproduce it. Next, we'll describe why it happens. Finally, we'll give you a homework assignment!

I know what you're thinking, "Homework? Yuck!" Don't fret; it'll be fun. We know that everybody learns a little differently but almost everybody learns best from experience. Besides, we like feedback, so we're excited to see what you learn from each post.

So, follow us on Blogspot, subscribe to our RSS feed, and leave us lots of comments! We hope you get as much out of participating in this blog as we do.

The Photography Professors