INTRODUCTION:

There have been a lot of questions floating around concerning DOF or Depth Of Field. That’s to be expected because it can be a bit tricky to understand at first. Why? Because your DOF is not adjusted by one setting on your camera. DOF is governed by THREE contributing factors. Now once you understand those, it really is not too difficult to comprehend.

So I thought I would develop a short informative tutorial on the subject to explain how all three elements affect your DOF.

It will of course take practice, but grasping it will come quickly once it is explained in detail.



I also thought I would explain in it’s most basic form, and with no technical jargon, so that the most unlearned person can get it. So for all you experienced people, cut me some slack, please? I am just using the most basic form of explanation to describe this.

OK so what are the three contributing factors?


1. Aperture setting
2. Distance To Subject
3. Focal Length


So now on to the next section below to see examples and an explanation of the first one. Your Aperture setting….

PART I - Aperture Value



Now your aperture sits in your lens. Not your camera. It is like an iris that can close down to restrict the amount of light that gets in to the sensor of your camera for recording an image. It also can be opened up wider to allow more light in

We measure the size of that opening in what we call “F Stops”. Why we use that term is not really important in this explanation. We are only talking about depth of field, remember?

Now your aperture has effects on your image in a couple of ways, but since we only want to understand depth of field, we will only look at this way.

Now before we move on, you need to know that when we close that iris down, the higher the F Stop will go in number. The more we open up that aperture, the lower that F Stop number will go. It sounds backwards, I know, because we want to think that the wider something is, the larger the number for measuring it becomes. But it’s backwards from that.


The wider the opening – The lower the value.
The more narrow the opening – The higher the value.


And how that effects DOF is shown on this chart.

So again, the larger the F Stop value – The smaller the opening – The more area that is in focus.

The smaller the F Stop value – The larger the opening – the more narrow the area that is in focus becomes.

Here is a real world example of how that looks.

This image was shot with a 100mm lens with an F stop value of “f/2.8” and you can see how much background is out of focus. A nice pleasing blur that really makes the dog jump off the screen is created.

Because we used such a wide open aperture setting – or small F Stop value – our depth of field is very narrow and the background goes out of focus quickly.

With this one you can see a huge difference in how the background looks.

The only thing that changed, was changing our F Stop value to “f/11”.

All we did was close the aperture down to create a smaller hole for the light to get through, but doing so meant that a larger area was in focus, and therefore our background blur is reduced as well.

Starting to make sense?



So let's pretend for a second....

So let’s pretend you are at the zoo taking pictures of a few penguins. You are using a zoom lens because they are so far away. Using a wide open aperture of say f/2.8, will allow you to isolate one happy penguin and have just that one be in focus. All his friends will be out of focus.

But changing your f/stop value to say f/11, will allow more happy penguins to be in focus as well.

Now there are some conditions to that. You will need to consider the other two contributing factors to DOF, but we will cover that shortly.

PLUS, your aperture value effects other aspects of your image as well. But remember, we are only talking about how it effects DOF.



Now move on to the next section to understand the second contributing factor – Distance to subject. Move on to Part II

PART II - Distance To Subject




The second contributing factor to DOF is “Distance To Subject”. It is a pretty simple concept to understand. Promise. It goes like this….

The closer you physically get to your subject, the more narrow your DOF - or in focus area – becomes.

The further way you physically get from your subject, the wider your DOF – or in focus area – becomes.



Now let’s see what that actually looks like. For these next three examples, the only thing that changed was I actually stepped closer to my subject as I took the pictures. Aperture remained the same, and I continued using the same 100mmm lens. Again, I only walked closer to the subject. That’s it.

So for this first image, you see that both figurines are in focus. It is kind of hard to tell, but the figurine on the right was about 6 inches behind the one on the left. We are looking at a 2 dimensional photograph, so it is kind of hard to tell.

But you will see how moving closer will make that more apparent. For this shot I was 10 feet away from my subject. Again, the figurine on the right was 6 inches behind the figurine on the left.

Oh and just for information - I was focusing on the figurine on the left.

Now in this next shot, all I did was take a few steps closer. Now I am 6 feet away.

You can now see, that by moving closer, our DOF – or in focus area – decreased, and now the figurine on the right is a bit out of focus.

Again, the only thing that changed was me walking closer to the subject. That’s it.

In this last shot, I once again stepped closer. Now I am only 3 feet away.

You can clearly see that the figurine on the right is even more out of focus than before. Because I got closer, my DOF – or in focus area – got even smaller.

Now the figurine on the right is very much out of focus. And if you look at the figurine on the left, even parts of that one are starting to go out of focus as well.

I told you this part would be easy.

Now on to the last contributing factor to DOF – Focal Length. Move on to Part III.

PART III - Focal Length




Focal Length.

This has everything to do with the lens you are using. Is it a wide angle? Is it a telephoto? What is it? Because that will make a huge difference on your DOF as well. Understanding it is very simple as well. In fact, it is very much like distance to subject. You can even look at it in the very same way.

Why?

Because a wide angle lens has the effect of looking like you got farther away from your subject, and a telephoto lens looks like you got closer.

So let’s look at some examples. Now for these next shots I used a zoom lens that went from a wide angle setting (24mm) to a telephoto setting (105mm).

BUT… The distance I stood from the subject never changed. The aperture value never changed. It stayed at f/4 for all the shots.

So for all three of these shots, I was 7 feet away and my aperture value was f/4. That never changed.

So this first image was taken at 24mm. See how much is in focus? Most of the blue bars in front of, and behind our subject, were in focus. Even the trees in the background, while out of focus, retain detail. Only the closest blue bars are a bit soft and out of focus. Only the elements farthest away are a bit soft and out of focus.

Now we don’t move from where we are. We just zoom in with the lens, to 50mm.

When we do, the effect to our DOF is exactly the same as when we just walked closer to our subject. The DOF - or that in focus area – gets smaller.

Now the handrail closest to the camera is out of focus. The blue bars behind our subject is out of focus. The tree in the background has lost detail as well.




Again, we stayed where we were and our aperture remained the same. But because or focal length grew, our DOF narrowed.


See where this is going?

For this last image, again, all we did was zoom in further with our lens.

We changed the focal length to 105mm.

The impact to the DOF is very apparent. We have good out of focus blur behind our subject. Our DOF was narrowed even further. In fact, now even the dog’s nose is a bit soft and out of focus.

I think you get the idea.

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CONCLUSION:




As we demonstrated with the above three sections, DOF is affected by 3 specific things.

Aperture Value
Distance to subject
Focal Length of the Lens.

So for you to get the desired DOF you are looking for, you have to juggle all three things in your head to get the shot you are wanting.

If you are wanting good background blur for a portrait of your kid, you have to consider all three things first. What lens are you using and what is the focal length of that lens? Based on that answer, how close must you get to the subject for that blur to really be noticeable? And how wide can you open your aperture up on that lens?

If you are taking a big group photo, You have to ask the same set of questions. If you want to get everyone in focus, you have to run through the same list, so you can make that happen.

I really hope that this was easy to understand. I just wanted to help some of the new people out.

And I really hope the seasoned and more experienced people, don’t nick pick it to death, and understand what the intent was for.


Here is a link to a DOF calculator for helping you determine the DOF for a shot. Will not help, of course, if you are in the field. But if you are practicing at home....

DOF Calculator

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