Quick question re. depth of field

Derben

Derben

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A bit of an emarrising question really - I should know this - but don't seem to be able to get it right.

Am I right in saying...

The higher the F number "equals" smaller aperature "equals" sharper picture

The smaller the F number "equals" larger aperature "equals" blurred background.

I'm trying to do portrates using the AV setting on my 400D - setting it to a small F number, but the pictures are not getting a blurred background when I focus on the face. Everything is in focus - am I doing something fundamentally wrong ?
 
Derben said:
A bit of an emarrising question really - I should know this - but don't seem to be able to get it right.

Am I right in saying...

The higher the F number "equals" smaller aperature "equals" sharper picture

The smaller the F number "equals" larger aperature "equals" blurred background.

I'm trying to do portrates using the AV setting on my 400D - setting it to a small F number, but the pictures are not getting a blurred background when I focus on the face. Everything is in focus - am I doing something fundamentally wrong ?
Click to expand...

DOF is a function of aperture and magnification. In general terms, longer FL lenses magnify more so have smaller DOF for a given aperture...I say general because the minimum focus distance will also determine the potential mag.
There is an online DOF calculator here www.dofmaster.com and you can see what DOF to expect with your chosen FL and F-stop.
If you have problems, come back with the lens and distance to subject info and we'll have a quick calculate.

Bob
 
Distance between the subject and background is also important, especially with shorter focal lengths. Increase the distance between them is a simple way to increase the blur.
 
Here's something I did to demonstrate to my girlfriend how aperature effects DOF and how you can adjust it to focus only on the subject:

f/29:
2151678006_60e5bee5ef.jpg


Both of these are f/4:

2150887909_60b18fac96.jpg


2151678254_5c2d449db9.jpg
 
Hit the nail on the head there redeyeshev most lenses tend to be best about 2 or 3 stops down from maximum unless you can afford really expensive glass though for the majority of shots it won't be a problem if they are slightly less sharp. If I were doing some huge gallery enlargements though i'd be going for better lenses.
 
So which is the best lens to use. I'm currently playing with the 18-55 which is standard with the camera. Would I get good results from this lens for portrait/blurring?
 
You've got good answers above, but don't assume that just because a smaller aperture (higher f/number) produces more DOF that smaller apertures that shots taken at very small apertures produce sharper images - they don't.

The problem is an optical effect called diffraction limitation, ususually abbreviated to diffraction, which causes an overall loss of sharpness at small apertures.
With a 35mm or full frame digital camera, diffraction starts to kick in at apertures smaller than about f/16, with an APS-C digital (most DSLR cameras) it's f/11, with a point and shoot with a tiny sensor it can be as low as f/2.8.

Diffraction limitation is progressive - it gets worse as the f/number becomes higher and it's a product of f/number not actual aperture - the reason it becomes apparent at larger apertures with smaller films/sensors is that the images need to be magnified more, showing the faults more.

To answer your next question, no I don't know why some lenses can stop down to f/32 or so when doing so produces lousy image quality but that's manufacturers for you:'(
 
Is it me or are people missing the point of the question posed by the OP?

Derben, with the 18-55 to get separation between the subject and background you'll need to have a considerable amount of distance between them even shooting at f/5.6 @ 55m. Ideally you want to be as close to the subject as possible but with a background several hundreds meters away. The tighter you get to the subject the better the effect, but you'll find you're shooting close-ups of the face rather than a traditional head & shoulders portrait.

In reality a different lens, the 85mm f/1.8 for example, would be better for the effect you're after.
 
Derben said:
A bit of an emarrising question really - I should know this - but don't seem to be able to get it right.

Am I right in saying...

The higher the F number "equals" smaller aperature "equals" sharper picture

The smaller the F number "equals" larger aperature "equals" blurred background.

I'm trying to do portrates using the AV setting on my 400D - setting it to a small F number, but the pictures are not getting a blurred background when I focus on the face. Everything is in focus - am I doing something fundamentally wrong ?
Click to expand...

Hi

I have really struggled to get to grips with this. The books tell you high f number big DOF, low f number small DOF. They don't tell you about needed the background to be away from the subject and you need to be close to the subject, they don't tell you the effect increases with telephoto lenses, and they don't tell you how it alters depending on where you focus.
Which is why i love this place cos they remember stuff like this.

I would suggest you put you camera on a tripod, get a could of objects and place them away from each other on a table or what ever, then take different photos at different f stops to see the difference you get with your lens. if you have more than one lens you can keep the camera in place and switch the lens to see what difference you get with different lenses. This will give you the best idea on what your lenses can do.

hope that helps
 
Toothie said:
I would suggest you put you camera on a tripod, get a could of objects and place them away from each other on a table or what ever, then take different photos at different f stops to see the difference you get with your lens.
Click to expand...

Pretty much what I did above. Thats with the standard 18-55 lens too
 
pxl8 said:
with a background several hundreds meters away.
Click to expand...

That's a touch excessive, it wouldn't need to be anywhere near 'hundreds' of meters. Hell, you probably won't even need more than 10 in most cases.
 
divine` said:
That's a touch excessive, it wouldn't need to be anywhere near 'hundreds' of meters. Hell, you probably won't even need more than 10 in most cases.
Click to expand...

Depends on the effect you're after. For something like this:

gypsy.jpg


with a fairly short focal length then even at f/5.6 a background at 10m will still be showing some signs of detail. To get the classic "so out of focus it's just smooth gradients" effect you'll need a much greater distance but as I said, in reality it really need a different lens. The above shot was a 135mm @ f/2.8 with the background 50m or so away.
 
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