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Understanding Depth of Field

Depth of Field

One of the most important things to learn in photography is the concept of Depth of Field. Depth of Field refers to how much of the picture is in focus. When you focus on a particular subject, other objects in the photo are going to be out of focus if they are at a different distance from the focused subject. A shallow depth of field means that only the subject is in focus while everything else is out of focus. A deep depth of field means everything is in focus.

Controlling Depth of Field

The Aperture is the size of the opening in the lens through which light enters the camera and exposes the film. The aperture also controls how much of the photo is in focus. If the aperture is small, everything will be in focus, while a large aperture will make objects, even slightly far from the subject, blurred. The aperture is usually expressed as an F-number and is a ratio of the focal length to the diameter of the aperture. When the camera is set to F/2, it means that the diameter of the aperture is half the focal length. At F/20, the diameter is 1/20th the focal length. Hence F/20 (with a smaller aperture) will given a very deep focus, while F/2 (with a large aperture) will give a shallow depth of field.

With a Large Aperture, objects get out of focus quickly

 

With a small Aperture, all objects will be in focus

Here’s another example. The focus of the lens is on the letter ‘J’. In the shot with a large aperture (small F-number), the rest of the keys quickly fall out of focus and are blurred. In the shot with a small aperture (large F-number), almost all the keys are in focus –

Using Depth of Field

So, now we know how to control the depth of field, but the question is “why is it important to control depth of field?”. Why don’t we want all our pictures to have a very high depth of field and everything being in focus? Well, it turns out that often times, the background to a photo can be very distracting. To reduce the distraction in the photos, a photographer will make the depth of field shallow. This is often used in Portraits and Macro shots. Experiment with different depth of field settings to see how it affects your photos.

Advanced Topic – Depth of Field and Relative Distance

The rate at which the objects become blur depends on the relative distance from the camera and the object focused on and the other objects. If the subject which is in focus is very close to the camera, then the sharpness will fall very rapidly with distance. For example, if the focused subject is 1 foot away from the camera, a object 1 feet behind it will be much more blurred than the same object when it is one foot behind the subject at 10 feet distance. This is what makes Macro photography so challenging. The following diagrams will illustrate this concept –


Shooting (for) the Moon

The moon shot with a 300mm lens on a clear night. This picture was cropped from the original picture

Shooting the moon can be tricky but rewarding. Here are some tips

Plan your shoot

If you already have a composition in mind, you may want to check out the phase of the moon and it’s location in the sky so that you can plan the time to go take the picture. Some websites like – http://www.moongiant.com/ are pretty informative. Also check out apps like – https://play.google.com/store/apps/details?id=com.lunafaqt&hl=en for planning out your shoot.

The moon is small

You need a camera with a good telephoto lens (good zoom) to capture the moon. Here are 3 different shots taken at 18mm, 55mm and 300mm on a 2/3rd frame DSLR. With a 18mm lens, the moon with look like a little dot in the frame. With a 300mm lens, you can capture enough details to make out some of the big craters.

 

The Moon gives out a lot of light

Have you ever shot a landscape photo with the moon and saw just a bright ball with no details? That happens because the moon is actually pretty bright and the rest of the landscape is dark. This means that the camera’s meter ends up suggesting an exposure for capturing the dark foreground. To capture details of the moon, you actually need to pick an exposure similar to shooting during the day.

Photomontage

You may have noticed a lot of landscape photos with a large moon in them. These are tough to take because of the two reasons explain above. Firstly, the landscape has to be shot with a 300mm lens to get a decently large moon. Secondly, the exposure has to be such that both the moon and landscape are properly exposed. This is often possible only in the late evening around sunset. In most cases, these photos are created by digitally cutting the moon from an underexposed picture of the moon and pasting it on a picture with the rest of the landscape. Here’s an example of such a photomontage –

 


Macro photography – Background is important

Macro photography is a tricky topic and we’ll deal with it in another post as well. The subject of this post is the background. Take this photo by Holger Magnussen, for example, the flowers have a good contrast to the background and the background doesn’t distract from the subject. Remember, the background in any photo is as important as the subject itself. A busy background can lead the eye away from the subject and is a big distraction.

The great contrast between the pink flower and the black background makes this photo POP.

 

Here’s an example of of a photo with a busy background

Example of a Photo with a Busy Background. The 2 colors in the background (blue and green) distract the viewer and draw attention away from the subject.

 

 

A similar photo with a much cleaner background

A single colored background distracts less from the flower and bees.

 

Here are some tips on how to make the background cleaner

 

Hang a cloth behind the subject

While photographing flowers, I often place my camera on a tripod and hold a black cloth or even my camera bag behind the flower. This helps me get a nice black background and makes the subject pop. You could also rig up a home made stand to hold up an old T-shirt or any cloth of your choice.

Wild Flowers shot with a black camera bag held up behind them. The black background brings the contrast to the white flowers making the photo pop

 

Choose your background

You do have options in choosing your background in a macro shot. Don’t get stuck as one spot and one angle. Try moving around a little to find the background of the right color and at the right distance. If you want a green background, move the camera to an angle where there’s a leaf behind your subject. You can also increase the amount of blur that the background has by moving the camera to an angle from where the background is farther away from the subject.

 

Use a lower depth of field

Open up the aperture to reduce the depth of field. A lower depth of field means less busy backgrounds, but it also means less sharp subjects, so whenever possible try and follow options 1 and 2 instead.

 

Crop the photo to remove the background

Even if you can’t get a clean background in the original shot, you can always do some post processing. Here’s an example of a crop to remove the distracting background. Notice how it brings attention to the flower and bee. It was further enhanced using some airbrushing in Gimp/Photoshop


Photographing Waterfalls and Streams

cascada

Waterfalls can be interesting subjects to capture. This photo, taken by Paul Bica, uses a long exposure to capture the water in this smooth silky flow. The exposure for such shots needs to be at least a couple of seconds. Setting the exposure to more than a second during the day time can be tricky. To achieve this, you should first set the camera’s ISO setting to the lowest ISO possible (most likely 100). I usually take such shots in AV mode (aperture priority) with the aperture set to the smallest possible size. This works well since you usually wouldn’t mind getting an extremely high depth of field. The small aperture allows the minimum possible amount of light and ensures that the right exposure happens with a long/slow shutter speed.

In most day light conditions, you’ll probably still find it difficult to get a exposure long enough to give the smooth effect. If you are only a couple of f-stops away, you could try adding a polarizer which you may have or underexposing the photo a couple of stops. It’s very likely that this wouldn’t help much either. In most cases you’re probably going to have to get your self a neutral density filter. The neutral density filters block all visible light equally hence the name ‘neutral’. Since some of the light is blocked by the filter, you can now reduce the shutter speed further, getting the blurry white water you want. As with all long exposure shots, you will need a good tripod to keep the camera steady with the shutter is open.

Another important tip for such photos is that using a remote shutter release helps avoid camera shake due to the pressure of the finger depressing the shutter button.

Another long exposure shot