Focal length, focus distance, and depth of field

[robertwsimpson]

There seems to be a lot of confusion about how focal length (the mm part of what your lens is called) and focus distance (how far away from the front lens element you are focusing) affect a photo's depth of field. People have been arguing that a photo taken with a longer photo length (ie the long side of a zoom lens) will have a smaller DoF, given the same f-stop setting. As I learn best by doing, I have some exhibits that should help show what is actually happening. Hope this helps. Keep in mind that the basis of this is "similarly framed photographs."

Exhibit 1:

This photo was taken with my 18-55mm f/3.5-5.6 kit lens with it set at 18mm and f/3.5. The first thing that everyone should take notice of is that a blurred background is very achievable with one of the worst and cheapest lenses that canon makes.

Exhibit 2:

This photo was taken with my 18-55mm f/3.5-5.6 kit lens with it set at 55mm and f/5.7. This is the widest aperture setting available at this zoom.

Exhibit 3:

This photo was taken with my 18-55mm f/3.5-5.6 kit lens with it set at 18mm and f/5.7. This is the same aperture value as the maximum available at 55mm.

First, lets dispel the myth that on a variable aperture lens, the DoF becomes smaller at the long end of the lens. This is clearly not the case. The easiest way to tell this is by looking at the coffee cup behind the stuffed animal. At 55mm and 5/5.7, the black line at the top of the cup is distinct. At 18mm and f/3.5, it is barely distinguishable at all. Another way to tell is by looking at the dark spot above the hind leg of the stuffed animal. In the 18mm shot, the spot is blurry. In the 55mm shot, some of the fibers are distinguishable. In this instance, logic prevails. A shot at 18mm and f/3.5 does in fact have a smaller DoF than a similarly framed shot at 55mm and f/5.7.

Next, lets compare the two shots with the same f-stop values. Comparing the coffee cups reveals similar levels of detail. The black ring is distinguishable, although blurred. The spot on the stuffed animal's leg is blurred, but some of the fibers are visible. I would confidently assert that these two photos have very similar depths of field.

Thus ends the data dissection of this experiment. I do have some other observations. First, Depth of Field has far more to do with focus distance than focal length. Let me explain. Everyone is familiar with the focus graph. If not, it looks something like this:

So, the closer the object is to the minimum focus distance of the lens, the smaller the DoF becomes. We see this with macro lenses. That is why giant f-stops (tiny apertures) are used in macro photography. The farther away an object is from a lens' minimum focus distance, the larger the DoF becomes. That is why the maximum focus distance on a lens is infinity, rather than some arbitrary distance. The farther away one focuses, the DoF becomes so gigantic that for intents and purposes, everything far away is in focus.

One reason that I think people assume longer focal lengths give a shallower depth of field is that when compared to shorter focal lengths, longer ones act to compress the apparent distance between an object and its background. This can be seen in the size of the coffee cup in the 18mm shots vs the 55mm shot above. I did not move the cup, but it appears closer to the camera in the 55mm shot. Since it appears closer to the camera, and it is still blurred by the same amount as the 18mm shot, it appears that the 55mm shot has a shallower DoF than the 18mm shot.

I hope that this helps everyone. Also, I hope that when people make assertions on the internet, no one just believes them without reading up or doing his own experiments. Not only is it silly to just accept what other people say, but you can learn a lot by checking things out for yourself.

 

[robertwsimpson]

By the way, comments and questions are very welcome. We're all here to learn. If you think I've gotten something wrong here, please let me know. Just be respectful.

 

[robertwsimpson]

also, please excuse my tax documents and our business plan, which are sitting there for the world to see.

 

[Dao]

Longer focal length do have a shadower DoF

i.e.
Camera with crop factor of 1.6

Focal Length = 50mm
Aperture = F/5.6
Subject Distance = 10ft
DoF = 2.62 ft

Focal Length = 200mm
Aperture = F/5.6
Subject Distance = 10ft
DoF = 0.15 ft


Now try another experiment.
Use a 50mm Lens and take a portrait shot with background far enough to create a blur. Set the Aperture at F/5.6

Then use a lens with 200mm focal length and take a portrait shot with the same background. But this time, the camera need to move back further away from subject so that the framing is about the same as 50mm.

You will see the result.

 

[robertwsimpson]

Longer focal length do have a shadower DoF

i.e.
Camera with crop factor of 1.6

Focal Length = 50mm
Aperture = F/5.6
Subject Distance = 10ft
DoF = 2.62 ft

Focal Length = 200mm
Aperture = F/5.6
Subject Distance = 10ft
DoF = 0.15 ft


Now try another experience.
Use a 50mm Lens and take a portrait shot with background far enough to create a blur. Set the Aperture at F/5.6

Then use a lens with 200mm focal length and take a portrait shot with the same background. But this time, the camera need to move back further away from subject so that the framing is about the same as 50mm.

You will see the result.

I just did that experiment. The results are posted above?

I'm not arguing that a 50mm lens and a 200mm lens both have the same DoF at a focus distance of 10 feet.

 

[Rosshole]

Play around with different setting on this page, then duplicate the settings with your camera.

http://www.dofmaster.com/dofjs.html

 

[robertwsimpson]

How are you guys not getting that I just did all of this in real actual life and posted the photos above so that you can see them? No need for theoretical calculations. Just look at the pictures.

 

[Rosshole]

Ok, I understand your testing, but you didn't post how far you wer from the focus point, which is an important part of the equation.

Your 18 mm shot seems VERY close to the subject. The shorter the distance to the subject, the shallower the depth of field. These principals are easier to see when comparing a 70 mm shot to a 200 mm shot.

EDIT: because there are three variables in this equation, a 3d graph would be in order.... the 2d graph doesn't express all of the proper values. Also, you are changing two of the variables in each photo (distance to focus point, focal length), try again and change only one at a time. The results should be apparent then

 

[robertwsimpson]

Well, clearly, to frame a photo the same, one must be farther away from it at 55mm than at 18mm. I even talked about this in my original post. Did you read it all?

 

[robertwsimpson]

Ok, I understand your testing, but you didn't post how far you wer from the focus point, which is an important part of the equation.

Your 18 mm shot seems VERY close to the subject. The shorter the distance to the subject, the shallower the depth of field. These principals are easier to see when comparing a 70 mm shot to a 200 mm shot.

EDIT: because there are three variables in this equation, a 3d graph would be in order.... the 2d graph doesn't express all of the proper values.

3d graph is not needed, as the aperture value remained constant for the main part of the test.

 

[Rosshole]

Your DoF will remain essentially the same if you increase the length to focal point while increasing the focal length.

 

[robertwsimpson]

Your DoF will remain essentially the same if you increase the length to focal point while increasing the focal length.

Exactly. That is why I said that the DoF is essentially the same for similarly framed photos with equal aperture values at different focal lengths. It's all about focus distance.

 

[Rosshole]

I guess that I missed your whole point of starting this thread then...

 

[robertwsimpson]

I guess that I missed your whole point of starting this thread then...

it was to dispel a myth that is rampant on this forum.

 

[Stosh]

There are 2 different kinds of blurriness and many people don't recognize that. There is blurriness from depth of field and there is another called background blur.

Depth of field rules apply to objects "near" the subject (or wherever the lens is focused). As long as you use the same field of view (or framing as you say), depth of field is determined only by f-ratio, and your pictures clearly show that. In your pics, depth of field can be found by looking mostly at the stuffed animal. When both are shot at f/5.7 with different focal lengths, they have the same depth of field.

Background blur rules apply to objects "distant" beyond the subject. Distant can be loosely defined as anything beyond twice subject distance. You don't have many details in your examples to show how background blur changes among different lenses, but the amount of background blur is determined by physical aperture. Larger aperture makes more background blur. Lots of people think that aperture means f-ratio, but it doesn't. Physical aperture is calculated by focal length/f ratio. So in your examples 18mm/3.5=5mm. 55mm/5.7=10mm. 18mm/5.7=3mm.

Part of the reason bokeh and blurriness is such a mystery is because it's usually a combination of both types of blur. Most people aren't even aware of the rules of background blur, only depth of field. When people say"that's nice bokeh", they're usually seeing background blur. And since background blur is attained by large apertures and distant backgrounds, longer focal length lenses usually have larger physical apertures, hence all the talk about using longer focal lengths.

Do some more experimenting and put more distance between your subject and background. Then you'll start seeing how larger apertures will blur the background more. Very nice pics illustrating depth of field.