This is the second post in the “You Asked For It” series and today I will discuss depth of field.  What is depth of field and why should I care ?

Depth of field is the ability to isolate your subject from the background – shallow depth of field

Depth of field is also the ability to have everything from close to infinity in sharp focus – greater depth of field.

Whaaat ?  How could the definition of depth of field mean two different things ?

Before I explain here are a few things to keep in mind:

1.   The smaller the sensor on your camera the more difficult it will be to achieve shallow depth of field.  This simply means if you have a compact point and shoot camera it will be very hard to take photographs with a shallow depth of field.  The lenses on these cameras are simply not fast enough (the aperture does not open wide enough).  There are of course some exceptions one being the Fujifilm X10 and X20 cameras which have very fast zoom lenses.

2.    The wider the field of view (or the shorter the focal length) of the lens the more difficult it will be to achieve shallow depth of field (wide angle).  The narrower the field of view (or the longer the focal length) the easier it will be to achieve shallow depth of field (telephoto).

3.    Most kit lenses on DSLR cameras are not fast enough to achieve shallow depth of field.   Now before everyone reading gets mad at me let me explain.  Most of the time when you go out and buy a camera kit you are paying 80% of the price for the camera body and 20% for the lens.  What does this mean ?  The manufacturer is basically throwing in the lens to sweeten the deal and keep the price of the camera reasonable.  Most of these kit cameras sold on the market today are also equipped with APS-C size sensors which are smaller than full frame sensors on professional cameras.  There is nothing wrong with APS-C size sensors and they could be used for professional looking results but remember what I mentioned in the first comment (the smaller the sensor the more difficult it will be to achieve shallow depth of field).

Anyway lets get back to the lenses.  Most kit lenses supplied with cameras will be 18-55mm f/3.5-f/5.6 which simply means at 18mm the widest aperture you can use will be f/3.5 and it will slowly decrease as you zoom out to 55mm where the widest aperture available will be f/5.6.   These are called sliding aperture lenses and are usually used on consumer level cameras.   Professional level lenses will usually be fixed aperture lenses such as a 24-70mm f/2.8 lens.   On this type of lens the widest aperture available will be f/2.8 throughout the entire zoom range which is a big difference from a kit lens.  These lenses also carry a much higher price because they are more expensive to manufacture.  There are some aftermarket fixed aperture zoom lenses available at reasonable prices such as the Tamron 17-50mm f/2.8 which also happens to be an excellent lens.

4.    Full frame sensor cameras will be the easiest cameras to achieve shallow depth of field with.  What is a full frame sensor ?  The original format for a SLR camera (single lens reflex) was 35mm and that described the size of film it used which was approximately 36 x 24mm in size.   When we say full frame DSLR (digital single lens reflex) we are referring to the size of the digital sensor which is approximately the size of a frame of 35mm film.

As you can see in the diagram above your lens opening or aperture varies depending on what is required for correct exposure.  The lens opening works in conjunction with the shutter speed on the camera.   When your camera is in Program mode the camera decides the correct exposure by selecting a combination of shutter speed and aperture.  In Program mode your camera decides everything so you have no control.  The first thing you should do to be able to control depth of field is put your camera in Aperture Priority mode.  By setting your camera to Aperture Priority mode you will control the lens opening or aperture and the camera will select the correct shutter speed to obtain correct exposure.  Notice we are taking baby steps here and still working with a safety net.  Your camera is still making decisions just not all of them.   Now you will be able to control depth of field by selecting the lens opening of your choice.  The first test I want you to do is to take a photo of a picket fence with a fully open aperture, then take the same photo with the aperture fully closed.  Take the two photos from the position of standing right next to the fence and the pickets are moving away from you.

Go ahead I’ll wait I’m retired 🙂   Now I want you to upload these images to your computer and compare them and see if you can tell the difference in depth of field.  Which one was taken with the wide open aperture and which was taken with the fully closed aperture.  Are you starting to understand the concept of depth of field ?

Shallow Depth Of Field

So why do I care about shallow depth of field I want all my photos to be totally sharp ?

OK if thats what you want you should close this article and move onto the next photo in the WordPress reader.   But if I took the time to read this far into this post I would stick it out until the end.  Sometimes we want to separate our subject from the background such as in a portrait.  If you are taking a photo of someone standing in front of a Rhododendron that was blooming you would not want the flowers to be tack sharp because that would detract from your subject.  You want your subject to be tack sharp and the flowers to be less sharp or blurred.  That would be an example of shallow depth of field.

How do we achieve this ?

By using a wide lens opening or a lower number (f-stop or aperture).  A typical outdoor portrait with a pleasing background (soft focus) should be taken at an aperture of less than f/3.5 or a lower number.  This is where the problem of the kit lens comes in because most of them do not go lower that f/3.5.

Everyone who is serious about photography should purchase a 50mm f/1.8 lens or if your budget is a little more generous an f/1.4.  Most manufacturers offer a 50mm f/1.8 lens for under $200 and the optics on a lenses such as these will probably be loads better than a kit lens.

Yes but if I buy a 50mm I will lose the zoom function ?

Well my answer to that would be use your feet to zoom, move back from the subject if you want a wider view or closer if you want a tighter view.  Most people that buy 50mm lenses tend to use them a lot more than their kit zooms because of the increase in picture quality.

The photo below illustrates shallow depth of field where the fingertips of the gloves are in sharp focus but the background quicky falls off into a blur (a large lens opening or lower aperture number).  This photo was taken with a Fuji X100s and the lens opening was f/2.0 the widest available on that camera.

Greater Depth Of Field

So far all I have been talking about is shallow depth of field.  So why would I want greater depth of field ?  If you are a person who likes to photograph landscapes you would want everything from near to far to be in focus.  How would you do this ?  By using a smaller lens opening or a higher number (f-stop or aperture).  You can also achieve this effect by using a wider angle lens which inherently have greater depth of field.

The photo below illustrates greater depth of field where everything from the rocks in the foreground to the bridge all the way in the distance is in sharp focus (a small lens opening or higher aperture number).  This photo was taken with a Nikon D7000 and Tokina 12-24mm lens at f/8.0 which is about the middle of the aperture range for that lens.  Be careful when using  f-numbers or apertures smaller than f/8 or f/11 because an effect called diffraction will degrade your image quality (a whole different subject).

Now for all of you with kit lenses out there whom I just alienated I apologize.  There is still hope, just remember a wider lens opening will give you a shallow depth of field (a lower f-number), and a smaller lens opening (higher f-number) will give you a greater depth of field.    Go out and experiment with your camera and kit lens.

The reason I told you to put your camera in  aperture priority mode earlier in this post is because I didn’t fully explain aperture and how it relates to shutter speed.  I would rather you not get confused with formulas when you are still trying to grasp the concept of depth of field.

Don’t worry by the end of these  lessons we will take your safety net away and turn you loose with your cameras in fully manual mode 🙂

The quicker you take your camera out of program mode or fully automatic mode the quicker you will take your images to the next level.

Next Friday we will explain – Shutter Speed