Understanding An Autofocus Mode In Modern DSLR Camera

Autofocus is one of the great features available in modern DSLR camera. Some cameras are equipped with an Autofocus motor inside the camera and some are not. Similarly, there are lenses which come with the built-in Autofocus motor inside itself. Benefits of having an Autofocus motor inside the camera is that it can Autofocus the lens which doesn't have a built-in Autofocus motor. Nikon D40, D40X, D60, D3000 and D5000 models lack an Autofocus motor. But Nikon D90 does have an Autofocus motor built-in inside so that it can fully Autofocus the lenses which lack an Autofocus motor. There are two ways for the DSLR camera to focus the subject; manually and automatically. When your lens is setup to focus automatically (by using the switch available on the lens), camera offers few different ways to achieve that autofocus.

Different camera brands provide different kind of Autofocus mode and sometimes only the naming is different but the core functionalities are same. Most of the Nikon DSLRs including D90 come with the three different Autofocus modes and one Manual mode. To change between different Autofocus mode in D90, you have to press the AF button on the top of the camera near the monochrome LCD screen and rotate the main command dial. While changing the modes, you can see currently selected Autofocus mode on your top LCD screen as well. Most of the high end semi-pro or pro DSLRs come with the dedicated lever to switch/lever between different autofocus modes. Now, let's discuss about these different autofocus modes and when should we use them.

D700 Autofocus switch

1. AF-S (Autofocus Single)

In this mode, focus is set once when you press the shutter release button halfway down and the camera keeps the focus locked on the subject until the shutter button is fully pressed to take a picture. If you release the shutter button without taking a picture, you will have to re-focus again to take the picture. This mode will be your best choice if you are shooting subjects that do not move such as landscapes, cityscapes or portraits etc. In this mode, by default, you might not be able to take a picture until the focus is locked on the subject.

2. AF-C (Autofocus Continuous)

This mode is good if you are shooting moving subjects like in wildlife and sports photography. When the camera is set to AF-C mode, once the shutter release button is pressed halfway down, the camera sets the focus on the subject but continues to monitor the subject in order to re-focus if it's moved from the original position. While shooting using AF-C mode, focus and exposure aren't really locked until you press shutter release button all the way down to take a picture. In this mode, by default, you can take a picture even if camera has not fully focused the subject but you end up getting blurry images.

3. AF-A (Autofocus Automatic)

This mode is actually a combination of the two modes described above. When your camera is set to AF-A mode, camera focuses subject using AF-S mode if the subject is not moving but it changes automatically to AF-C mode if subject starts moving. Isn't that pretty cool? This mode is pretty good when you are shooting mixture of the actions. This mode is probably the default mode in your camera. However, like in AF-S mode, subject needs to be sharply focused to release the shutter release button all the way down and then only camera takes the picture.

4. Manual Focus

You can use the manual focus mode when your lens is set to the manual focusing mode (using the switch on the lens itself) or if your lens is not equipped with an Autofocus motor. In this mode, you can manually focus your subject using the focusing ring on the lens barrel. Manual focus is popular among the landscape photographers specially when they want to achieve sharp focus from the foreground to the background (some prefer to focus into infinity and some prefers to focus into one third of the frame to achieve everything in focus).

Some photographers like to shoot in a manual focusing mode all the time but I prefer to use Autofocus mode most of the time except in some tricky landscape shot where I also use manual focus.

Understanding An Aperture And The Depth Of Field

In my previous blog post, we discussed about the shutter speed and it's effect on the exposure. Now, lets discuss about an Aperture, one of the three pillars in photography along with the shutter speed and an ISO, and it's relation with the shutter speed in exposure setting. To set an aperture value, your camera should be either in Aperture Priority mode (A) or in Manual Mode (M). If you are using an Aperture priority mode, you change an aperture value and the camera adjusts the shutter speed automatically to balance the exposure. And if you are using Manual mode, you can set the shutter speed and an aperture value independently. I already discussed about ISO and shutter speed in my previous blogs and today I am going to explain about an Aperture and how it affects our photographs.

What is an Aperture?

When the shutter is opened to allow lights into the censor, light must pass through a window called an Aperture. Sometime it's compared with the pupil in a human eye. Wider the aperture, more lights come into the censor and vice versa. An Aperture is also called a F-stop and it's value is called F-stop value. F-stop value is indicated by f and followed by the number value.

F-STOP Values

For example f2.8 (also written as f/2.8) means wider aperture and allows more lights. Similarly f22 means narrow aperture and allows less lights into the censor. Difference from one aperture value to immediate next value is called one full stop value. This term is frequently used when we deal with the exposure setting. For example, if your lens F-stop value is in f2.8, f4, f5.6, f8, f11, f16 and f22 series, moving an aperture from f2.8 to f4 is called one full stop and decreases the brightness of your picture by one full stop (it will allow only half amount of lights than in f2.8). So once again, changing an aperture value by one full stop up (from 2.8 to 4) will allow half the light and one stop down (from 4 to 2.8) will double the amount of light hitting to the censor of the camera. To calculate the aperture values in a full stop value (1 EV), you can use the formula below.

Nowadays, in modern digital cameras, aperture value, shutter speed and ISO values can be set in a fractional value rather than one full stop value. Almost in all modern digital SLRs, these exposure values can be adjusted and decreased or increased in the faction of one-third, two-thirds or one full stop value. The most common fractional value is one-third stop (​1⁄3 EV) but some cameras use half-stop value as well. If you look at the diagram above, we jumped the aperture value from f/2.8 to f/4 which is one full stop change. But, if you want the aperture one-third stop smaller than f/2.8, you would set the aperture value to f/3.2 and if you want two-thirds smaller, it would be f/3.5, and finally one full stop would be f/4. The next few f-stops in this sequence would be f/4.5, f/5, f/5.6, f/6.3, f/7.1, f/8, etc. which can be calculated by using the formula below.

And if you want to calculate the aperture values in half-stops, you can use the formula below.

What is Depth of Field?

Depth of field is an effect of using different aperture settings. When you use a small aperture value (that means wide opening of the lens and allows more lights), you will get the sharper image of a small area or the foreground area and the blurry image of the background area. Less aperture value means less area of the subject will be in focus and results into shallow depth of field. But when you use big aperture value (that means narrow opening of the lens and allows less lights), more wider area is in focus and everything that you see through the view finder will be in equal focusing mode resulting into deep depth of field. Generally, we use shallow depth of field while shooting macro objects like flowers, insects etc and deep depth of field is required while shooting landscape to be everything from foreground to background in focus.

Shallow depth of field

Shallow Depth of Field

Focal Length - 32mm     Shutter speed - 1/60    Aperture - f5     ISO - 200

Deep depth of field

Deep Depth of Field

Focal Length - 85 mm    Shutter Speed - 1/2    Aperture - f/16     ISO - 200

Take and Give relation

Exposure can be set by changing an aperture value, shutter speed or different combination of shutter and aperture value. When you take one F-stop from an aperture, you can give one F-stop to the shutter speed to match the same exposure. Moving from one shutter speed to another one is also one full F-stop jump like in an aperture value.

Aperture Full Stops:

1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 45, 64

Shutter Full Stops:

1/1000s, 1/500s, 1/250s, 1/125s, 1/60s, 1/30s, 1/15s, 1/8s, 1/4s, 1/2s, 1s

Fundamentals Of The Shutter Speed

In last few weeks, I have got several requests from the readers to write about Shutter Speed, Aperture and ISO; three pillars of photography also known as exposure triangle. If you are reading about photography or want to learn the basics of photography, you would see these terms almost everywhere. These are the foundation and very fundamental settings for your camera. When you shoot in Auto mode, camera calculates these parameters for you but if you want to have more control of the lighting or exposure, you have to use either semi-manual or Manual mode and select these values manually. To change the shutter speed, your camera should be either in Shutter Priority mode (S) or in Manual Mode (M). If you are into Shutter priority mode, you change the shutter speed and the camera adjusts an Aperture value automatically to balance the exposure. And if you are using Manual mode, you can set the shutter speed, an aperture value and ISO independently. In this blog, we will discuss more about the shutter speed.

What is the Shutter Speed?

Shutter speed indicates how fast the shutter of your camera opens and closes while taking picture. When you press the shutter release button, it opens the shutter of the camera, allows lights from the subject to the censor of the camera and closes it. The time difference of the opening and the closing of the shutter depends upon the shutter speed. Higher the shutter speed faster the shutter opens and closes. Similarly, lower the shutter speed, shutter will open for a long time. Shutter speed is directly related to the exposure settings of your camera. Higher shutter speed means it will allow less lights into the censor but captures fast moving subject (freezes the motion of the moving subject) by closing the shutter quickly. Lower shutter speed is used when you want to allow more lights into the censor and capture the activity. While using the slower shutter speed, you have to be careful of the camera movement. If you are shooting with the slower shutter speed without using tripod, you might get blurry picture because of the camera shaking.

Tip: Relationship between the Focal Length and the Shutter Speed

The general rule of thumb is: If your lens is not a VR (Vibration Reduction) lens and you are not using a tripod, your shutter speed should be at least 1/(focal length) of the lens to avoid blurriness on the picture due to camera shake. For example, if you are shooting handheld with 105mm non-VR lens, you should use 1/105th seconds of the shutter speed to get sharper image. If you are using a VR lens, you can decrease the shutter speed up-to 2 to 4 stops depending on the lens because you would gain that much light by turning the VR feature on.

Now, let’s look at how the shutter speed is indicated on camera and what does it mean to you as a photographer.

Shutter speed is indicated by the whole number or the fraction or special notation, BULB. The whole number is generally followed by a double quote to indicate that it is in a second format. The fraction value is represented as a number without a quote. For example, if you see 2" in your viewfinder, it means the shutter will open for 2 seconds. But if you see 60 in your viewfinder, that means your shutter will open for 1/60th of a second. Generally in a viewfinder, you won't see it in a 1/60 format but if you see a whole number without a quote, you have to understand that it's a fraction of a second. When you see a BULB, it means that your shutter will be opened as long as you press the shutter release button and until you release it.

Faster shutter speed to freeze motion

I took this picture above with the shutter speed of 1/1000th of a second and you can see how the birds’ movements are frozen.

Slower shutter speed giving silky water effect

This picture was taken at the shutter speed of 10 seconds using a tripod. I wanted to get the silky water effect with a long exposure setting and I was shooting in a very low light condition (at night) as well.

You want to use a faster shutter speed to freeze fast moving subjects like running players in a sports events or playing kids or racing cars etc. But you should use a slower shutter speed to capture a motion of your subject or if you are shooting in a low light condition or setting up for night time photography or to get silky effects of water fall using ND filter etc.

1" - Slower shutter speed and allows more light come into the censor.

1/4000 - Faster shutter speed and allows less light come into the censor.

I will discuss about shooting fireworks and it's relation with the shutter speed for the exposure settings in my next blog post.