how to get bokeh

[fwellers]

Hi,
I just got my first dslr yesterday, a Nikon D90. After much research, and a month or two of pouring over this and other forums, I know enough to fit into a thimble.
Been reading the manual all night and half the morning, and finally started trying some pics. Haven't even stored any on my pc yet. I just installed the Viewnx and capture nx2, but haven't used them yet.

I just say that as a preview so you know you're dealing with a rank beginner here.

Anyway, for my first of many questions. I've tried to take a pic of objects and get a bokeh ( or any blurring of the backround ), by opening up the aperture all the way, which is 3.5 on the 18-105mm kit lens that came with it. I used Aperture priority mode. But no matter how I focus on the subject, I still get the backround in focus also.
This is in my house, with about 10 feet or so of a backround.

is this because I'm not doing something I should be doing or is it because I just need a faster lens. Is it just impossible to get the bokeh effect at 3.5 aperture ?

Thank you,
Floyd

 

[Ron Evers]

Try f 3.5 @ full zoom & see what you get.

 

[Dao]

If you have a fast lens, it is easier. But I think you can still do it with your current lens.

In order to create a shadow depth of field, aperture is one thing. But the distance between the camera and the object as well as distance between the object and the background are also important.

If you set your lens focal length as 100mm and aperture as F/5.6
The distance between the object and your camera is about 5 feet.

The depth of field is going to be 0.16 feet. In other words, anything within that 0.16 feet is going to be in focus. Anything outside this range is out of focus. And if the background is far away from this range, it is going to be blurry.

Example, this photo was taken at St. Louis Zoo with aperture set as F/5.6 and focal length of 210mm.

 

[FlyingFly]

You can search for "depth of field" to find some calculator for it. Aperture, focus length, foreground distance and background distance decide the degree of out-of-focus.

Use F5.6 at 105mm and focus on an apple at 1 foot and leave background 100 feet away. You will find bokeh taking place.

 

[fwellers]

Ok thank you all. I'll hunt up some basic forumlas for that to get a better idea.
I wasn't thinking. My backround obviously was too close to get the effect.
apple at 1 foot with f/5.6 needs about 100 foot backround with focal length of ~100mm,
but with all else the same, if I am 5 feet away from my subject, I need much much less backround, if I'm reading Dao's post correctly.

At least I know what to play with now. )

 

[FlyingFly]

Sorry, the exact distance I've mentioned about is not accurate. I just mean it shall create enough effect of out-of-focus for background.

As Dao said, the 0.16 feet depth of field means, anything between 4.95 feet and 5.11 feet are in focus, otherwise out of focus. Note that fore depth of field is half of rear depth of field.

However it doesn't mean something at 5.12 feet immediately being very blurry. Actually,
The farther the background is, the more blurry it becomes.
The nearer the foreground is, the more blurry the background becomes.
The larger the aperture (the smaller the F number) is, the more blurry the background becomes.
The longer the focal length is, the more blurry the background becomes.

 

[fwellers]

Thanks Flyingfly. I wasn't taking your numbers for gospel. But your extra explanation does help.
I just tried it. My rooms are smallish. I have a lamp on a table in the middle of a room against a wall. The only way I can get any effect was to use the max (105mm) focal range. I was about 7 ft from the lamp, and the nearest other thing was the far wall, about 8 feet away from it.
Since the max aperture I can get was f/5.6, I was able to see slight blurring on the power outlet of the far wall.
Then I moved closer to the lamp, and the blurring on the far wall increased.

Your last 2 statements:
The larger the aperture (the smaller the F number) is, the more blurry the background becomes.
The longer the focal length is, the more blurry the background becomes.

I guess they are not directly proportional though, because as I reduce the focal length, and my aperture opens up, I am not able to produce the effect with the same backround and foreground distances.

So it seems that the focal length has more of an effect on the blurring than the aperture does, at least for the lens I'm using.

Thanks again for your help.

 

[fwellers]

Try f 3.5 @ full zoom & see what you get.

Ha ! I only wish.
I can only get f/5.6 at full.

 

[FlyingFly]

...
Your last 2 statements:
The larger the aperture (the smaller the F number) is, the more blurry the background becomes.
The longer the focal length is, the more blurry the background becomes.

I guess they are not directly proportional though, because as I reduce the focal length, and my aperture opens up, I am not able to produce the effect with the same backround and foreground distances.

So it seems that the focal length has more of an effect on the blurring than the aperture does, at least for the lens I'm using.
...

Of course they're not directly proportional. Actually they have different unit (focal length has the unit of millimeter and F number has no unit) and thus you can't compare them directly.

You can only accurately calculate the depth of field to know what is the final effect if you change multiple factors in reverse direction.

By the way, if you shorten the focal length, you would have to approach the foreground to make it in same size on picture (as using longer focal length).

 

[Ron Evers]

Ha ! I only wish.
I can only get f/5.6 at full.

Quite right, I was forgetting that the aperture would decrease as you zoomed but you now get the idea.

 

[tirediron]

Okay first of all, "bokeh" refers to the manner in which the background is out of focus, not the degree. Ken Rockwell has an excellent explanation here. Depth of field and the degree to which the background is out of focus is based upon several factors. First and foremost is the aperture of the lens; the larger the aperture (smaller 'f number') the small the distance that will be in focus. Second is focal length, the longer the lens, the less depth of field at a given aperture. IE, an 8mm lens at f4 will many times the DoF of a 400mm lens at f4. Lastly is placement; how far you are from the subject and how far the subject is from the background.

The best backgrounds (as I understand you to want them) will be achieved with a large aperture and a long focal length, with the subject well removed from the background. This excellent DoF calculator will let you see how all this works by plugging in various numbers.

 

[fwellers]

Great Stuff Tirediron. Thanks !!