Understanding Histograms

If you've been using digital cameras for some time now, chances are you have already seen lots of histograms. They are shown by most cameras and imaging software and they are a very powerful tool to review your exposure. Today we are going to take a look at them and we're going to answer the question:

How do I interpret a Histogram?

A histogram is a graphical representation of a series of data, it's not only used in digital photography but also in many statistics representations. Every column in a histogram represents a case and its height represents how often this case is presented. Translated into digital photography, every column represents a tonal value, while its height represents how often this value is present in our picture.

The light values in a histogram go from 0, at the left meaning completely dark to 255, at the right meaning completely white.

This means, if a picture is dark the columns in the left will be higher, because a bigger part of the image will be contained in them.

It might sound complicated but let me show you some examples to make it easy.

This is the histogram of a normally exposed image. The tones are well distributed along the histogram from the darker to the lighter areas, there is a very high peak at the brightest end of the histogram showing the overexposed area caused by the light at the lower right corner of the image. These peaks are something to avoid, since parts of the image with so bright tones will end up just white without any appreciable detail. In this image though this is tolerable since the burned area is quite small. Also, we can see a wider peak in the bright area of the histogram caused, that shows the bright area on the wall that's being lit by the light.

What this histogram is telling us about the image is that it has a burned area, it has well distributed tones and an important area of the scene is brighter than normal.

Let's take a look at a completely different Histogram.

This image is was intentionally underexposed to give it a different mood, which is clearly visible in the histogram. If you look at its darker end, you will see how it contains a big part of the image. The brighter end is very low, filled only by some of the light rays and the clouds on the sky. Another important detail we can recognise in this histogram is that there are parts of the image in the very dark end of the histogram, as opposite to the image before, this shows that a part of the image will be shown black without any appreciable detail, once again this is something acceptable in this photograph because it creates the silhouette at the bottom of the picture. However you should try to avoid having values at the lower end of the histogram if you don't want any part of the image to be completely black.

So summing up, we have seen that a Histogram allows us to see how bright or how dark our image is, without having to rely on its brightness on the LCD screen, which can be affected by other factors like ambient light. It will also show us if we have burned or dodged any part of the image, so that we can notice if we have lost details in our picture, which is very difficult to see on the LCD screen only.

I hope this was a clear article, if you have any doubts or suggestions, don't forget you can let me know in a comment. Thanks!

Shooting silhouettes

In our last post, I mentioned that we could create silhouettes in a very easy way, just by using the central spot metering mode . Now I'm gonna show you how to create this effect that can give such a distintive touch to a photograph.

What is a silhouette? A silhouette is created in the limit between an underexposed and a normally exposed area. This means that to create a silhouette we will need a source of light behind our subject that makes it darker than the rest of the scene, like in the example above.

But having that source of light won't create this effect by itself. If we use matrix metering mode (default mode) the camera will try to expose the whole image properly, spoiling the silhouette effect. What we want is the camera exposing properly for the lighter areas only, leaving the rest underexposed to create the silhouettes. To do that we will use central spot metering mode, you can read infomation in more depth in the last article, but basically you want to center the source of light in the frame and then use the AutoExposure-Lock button to tell the camera to expose properly for that lighter area. The darker zones will automatically be underexposed, creating the silhouette effect.

This technique is not very easy to use and will probably require some practice from you, but once you are used to finding light sources behind your subjects, you will be able to create very nice silhouettes.

TIP: As with many effects in digital photography, this one can we created in Photoshop too, so if you can't get to shoot silhouettes or you just don't get them perfectly, you can tweak them later on the pc. Here's how it's done in Photoshop

• Open your image

• Go to Image->Adjustments->Levels
• Take the lever at the lower end of the brightness range and shift it to the brigther areas, like in this screenshot

• Once the areas you want to silhouette are dark enough, click ok.
• That's it, this is what it will look like now.

Note: This only works if the shot was taken with a light source behind and has already some silhouette effect. If the shot has none of that effect you'll need to select the area you want to silhouette (I recommend using quick mask mode, pressing Q) and then repeat the same procedure.

And that's it for now, I recommend you not to take the easy way and try to take the silhouettes straight in the camera, but if not you can always cheat your way to perfection :D . Experiment and have fun

Understanding Metering modes

In our series of articles about "Mastering the basics" we learned about the different settings to use to get the right exposure, but we never talked about the way we metered available light.

Now, we're going to take a look at the camera's metering modes.

Every modern camera has a sensor that measures how much light is getting through the lens. In most cameras we can chose how we want the camera to do it. There are 3 different metering modes:

Matrix metering: The most used of all modes, the camera measures light from all points in the scene, and attemps to set the exposure in a way that the whole scene is properly lit.
This is a very easy to use and effective metering mode because the largest possible part of the image will be properly exposed. Similarily to the auto mode, though this metering mode has its limitations when we want to have some advanced control over the resulting image.

Center weighted spot metering: This mode comes in very handy when shooting differently lit scenes. Imagine for example, that you're shooting at a concert. The whole scene is dark, except for the performer in who is lit by spotlights. Using the matrix mode, the camera will try to expose the biggest part of the scene, as a result the preeminent darker zones will be properly exposed, while the performer will be overexposed. This will be in many cases far from the ideal result. In these situations we'll use the center weighted metering mode, now the camera will prioritize the area in the middle and expose properly for a portion of approximately 25% of the image around its center.

Photo by sint

Central spot metering: This is the most selective mode, the camera will expose properly for a point situated in the exact center of the image. The rest of the scene will not be taken into consideration. I do not recommend this mode in most of the cases because light can change a lot from one point to another and the readings won't be very useful if we use one point only. It can be a good mode to use if we want to select an exact point for the camera to expose properly though which can be useful for example for creating silhouettes or for complicated light situations.

Now, when you read about the last two metering modes you were probably asking yourself, what about the framing? What if I want the performer in the concert to be properly exposed, without framing him in the middle?

Well, camera designers were aware of this problem and that's what the Auto Exposure Lock button is for. Most DSLRs have this button that allows you to lock the exposure settings once your camera has taken the light readings and adjusted its settings accordingly. This will allow you to take the light readings from the performer by placing him in the center and then, with the AE-Lock button pushed frame the shot as you want to.

Summing up: Center weightered and center spot metering modes, allow you to chose the part of the image you want to be correctly exposed, while matrix metering mode will try to have the whole image exposed properly.

Shooting star trails

In our last article we learned how to shoot all kinds of light trails, now we're going to take a deeper look at how to shoot star trails.

First of all. What are star trails?

When you look at the stars at night, they seem to be stuck, but if you stare for a couple of minutes you'll start to realize that they're moving, only very slowly. This effect is created by the earth's rotation and not by the movement of the stars themselves. Every star in the sky makes a whole turn around the pole star. This particular one is right in the axis of the earth's rotation and this causes it not to move in the sky and always head north, that is why this star has been used by travellers thousands of years ago to guide themselves in the night, yes I like science :D but we're moving away from our main topic.

What is the effect of this motion when we shoot stars at night?



If we shoot stars with a shutter speed long enought to capture this movement, the stars will create trails in the form of circles around the polar star, decorating our shot with a very interesting sky.

How do we set up our camera to achieve these results?

This will be a little more complicated than with standard light trails. First of all you will need a remote control, or any other device that allows you to use the bulb mode of your camera. Setting up the maximum 30 sec exposure configured in your camera won't be enough to capture the star's movement.
To set up bulb mode we'll need to go into manual. Se the ISO to minimum, aperture preferably to a high value and shutter speed to bulb mode. Set the focus to manual and focus on infinity.
I also recommend using a long focal length because stars will appear to move faster when the lens is zoomed in.

And this is it, you can start to experiment with different compositions and shutter speeds, keep in mind however that very long shutter speeds will create too much noise in the image, remember turning the noise reduction function of your camera to minimize this effect. If you want to have longer trails, you can create a series of relatively short exposures and then blend them together in photoshop.



It is important to be able to identify the pole star, since all the stars will turn around it, making it a strong subject of your composition. To find it look north and find a very bright star.

And last but not least, keep in mind that this kind of photography works best at clear skies without luminic contamination, so run away from big cities.

Photographs by Corica, Warner Strauss and me.

Shooting light trails

If you've read the "Mastering the basics" section you already know a lot about how to use the exposure settings creatively. In this article we're going to take a look at using long shutter speeds to get light trail pictures.

What you're going to need:

A tripod. When shooting at speeds longer than a second the use of a tripod is a must, trying to take these images handheld will result in very blurry images. You can however try to lay the camera on some stable surface but this limits your creative possibilities a lot.

A DSLR or Bridge Camera. Most point and shoot cameras don't allow for exposures longer than 1 or 2 seconds so we'll need to use a DSLR or bridge camera that lets us choose the shutter speed.

How is a ligh trail created?

The creating of light trails is very simple. When we let the shutter open for a long time any bright light that goes through the camera's field of view, will create a trail of light through the image. Used properly these trails can create very nice effects on roads and can be an interesting addition to a cityscape shot.

The first thing you have to do for getting a light trail shot is finding a place with moving lights, a typical location to start at is a driveway. Set your shutter speed to a value between 5 and 30 seconds, I recommend using the Shutter speed priority mode . Now mount your tripod and frame the shot. Once a car approaches, click the shutter button. The lights of the car will create a light trail on the image as long as the shutter is open. Review your results and try using different shutter speeds.

Using flash. Although flash is not often used in light trail shots it can create a nice effect showing the object that creates the trail and the trail itself. I recommend using rear curtain flash, in this mode the flash is shot just before closing the shutter and it makes the subjects lit by the flash appear at the end of the trail. Something like this:

Going back to the techincal aspects, i recommend using a low ISO and enabling noise reduction in your camera's menu to reduce digital noise from the long exposure.

An interesting addition to our gear if we want to shoot light trails often is a remote control. Triggering your camera from a remote has 2 advantages:

1. We won't need to touch any button on the camera, eliminating every chance of creating blur in the image.
2. It will allow us to use bulb mode, in this mode you'll be able to open the shutter with one click of the remote and closing it with a second click. This way you can't exactly control how and where the trail is created.

A very special kind of light trail are star trails, but we'll talk about that in the next article so stay tuned!

Camera modes

Now that we know the effects that every setting has on our image, we're going to learn how to tweak those settings in our camera's menu. It is possible that your camera has a different menu system but usually all DSLRs have the options I'm going to talk about.

Auto: Not much to say about this mode. It is fast and useful for the times where you just haven't got the time to think about the settings. The camera makes all the decisions about the exposure and even the metering modes, but now that we've learned how to use every of those settings, we want to have control over them, right?

P mode: This one is similar to auto mode, you set the ISO and the camera will automatically set the shutter speed and aperture for the shot. Also P mode allows you to control the metering modes, something we will talk about in future articles

Shutter Speed priority: This is were we start to take control of the settings, this mode will allow us to set the shutter speed and ISO we want and the camera will adjust the aperture to match the available light. Very useful for taking action shots or for playing with motion like we saw in the article about Shutter speed.

Aperture priority: This mode allows us to control aperture, and ISO while the camera will take care of adjusting the shutter speed. This mode is very useful for taking control of the depth of field like we saw in the article about aperture.

Manual: This is the mode, where you control everything. ISO, shutter speed, aperture, metering modes, white balance... .This might sound like it's the ultimate mode to use once you understand every setting, but in my opinion this mode makes it too difficult to get a correctly exposed shot, since it will be you that will have to tweak every setting until you get the correct amount of light. There are moments though where this mode will be the only option, like when you want to use the bulb mode of your camera. This mode allows you to have the shutter open for as long as you want and can be helpful for making very long exposures, like in star trail shots.

So, you will be asking yourself, which mode should I use?

The answer to this question depends on your skills and the situation you're in, as i said before you can use auto mode if you want to take a photograph quick without losing time on adjusting the settings, but the rest of the modes will give you more control over the shot.
I personally find myself shooting usually with P mode that allows me to control the ISO so that the camera doesn't accidentally bump it too high ruining the shot because of the noise. When i want to use some of the exposure settings creatively I just go into aperture or shutter speed priority mode, since they're much easier to use and will give you identical results to the manual mode.
Sometimes you'll just need to use the manual because the camera won't be able to get the shot exposed like you want. Don't be afraid of using it now that you know what every setting does, do keep in mind though that it will probably take some time to get the shot properly exposed. But hey you're doing this just for fun right? Just take your time and enjoy experimenting!

Mastering the Basics: Understanding Aperture

After having learned about shutter speed and ISO settings, now we're going to take a look at the aperture.

The aperture setting is the size of the opening in the lens.

Every lens has a couple of blades, called iris, that are put together to regulate the light that gets through, depending on the aperture setting used.

The aperture is indicated in the camera as the f number. A high f number will represent a small aperture while a small f number will represent a big one. This might sound counterintuitive but is easier to understand if we imagine the f number in the formula 1/f. The higher the f number the smaller the end result.

But enough with the technical details, what is aperture good for?

Tweaking the aperture settings will not only control the light intake, but will also control the size of the depth of field (In short, the depth of field is the part of the image that is in focus). This will allow us to use the depth of field that we find appropiate for our shot. Imagine for example that we are taking a portrait picture. We want everything in the background to be out of focus, so that it doesn't distract from the main subject. In order to achieve that we will use a small f number, that will result in a big aperture, creating a shallow depth of field.
If we were shooting a landscape for example we would do the exact opposite so that all the parts of the image stayed in focus.

The maximum aperture setting will depend from each lens and is an important factor to have in mind when buying new gear, prime lenses, those without zooming possibilities, are the ones which allow higher apertures. It is easy to find cheap prime lenses allowing for f1.8 apertures and more expensive ones even allow for f1.4 or f1.2. Zooming lenses instead have smaller apertures and their maximum aperture varies depending on the focal length used. The 18-55mm kit lens from the D40 for example will allow for a f3.5 aperture at 18mm but only f5.6 at 55mm. Zooming lenses with bigger apertures are only found in the professional level and are used by sport photographers for being able to capture fast shots even in low light.

Summing up, rising the aperture setting will let in more light while making the depth of field smaller and the maximum aperture depends on the type and the quality of the lens.

Mastering the Basics: Understanding ISO

In the last article about the exposure triangle we took an in-depth look at the shutter speed and we mentioned how sometimes it was better to use a slow shutter speed than a high ISO. Now we're going to analyze why this happens.

First of all what is the ISO setting?

The ISO setting represents the sensors sensitivity to light.

Getting into more technical details, the ISO setting in digital photography is the gain of the sensor's signal. Although we speak about digital photography the camera's sensor itself is an analog device, just like a microphone for example, and as such its signal can be amplified to get more light into the picture.

This can be useful because it will allow us to use a faster shutter speed or smaller aperture when we need it (see article about shutter speed), but amplifying the signal will create noise, which will translate into small color dots in our image like in this example photograph. Depending on the camera and the ISO setting this noise can go from slightly visible to spoiling our image. Most modern DSLRs will be able to handle noise from ISO 200 to 800 at acceptable noise rates, but it's always a good idea to keep ISO at its minimum and only rise it when the we're forced to.
As an example a typical situation where you're forced to rise the ISO is at concert photography, where there is dim light and you can't use slow shutter speed because the subject would be blurred, in that case you can only rise the ISO, although you know you will have an image with some noticeable noise.

Summing up all you need to know about ISO is that you have to keep it as low as you can by using, when possible a slow shutter speed or a big aperture.

Mastering the Basics: The Exposure Triangle

Before we go into more advanced aspects we have to master the basic aspects of photography. The first concept to understand is the exposure triangle.

The exposure triangle is a representation of the 3 settings that will determine how much light reaches the camera sensor.

It consists of three elements.

ISO: The sensor's sensitivity to the light
Shutter Speed: The amount of time the sensor is exposed to light.
Aperture: The size of the opening in the lens that lets light in.

An easy way to understand this is to imagine a hosepipe: The aperture is the width of the pipe, the shutter speed is the time the pipe is open and the ISO setting is the pressure.

As you can imagine from the example above, there are several possible combinations that will let in the same amount of light, like for example increasing the shutter speed and then compensating that by decreasing the ISO setting, something common for night shots.

When we use the auto mode of our camera, it will automatically chose a combination to let the correct amount of light in, but while the amount of light will probably be correct, the combination used may not be the best one to get the results we expected from the shot. This happens because each of the combinations has its advantages and disadvantages that will depend on how and what we're shooting. If we want to be able to chose the combination of settings ourselves we will have to take a look in greater depth to each of the three settings: ISO, shutter speed and aperture, which we will do in the three upcoming articles.