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Depth of Field (grrr ....!)


Lulu01 5 156
19 Mar 2009 1:08PM
I understand DOF technically and know what it is in photographs but cannot, for the life of me, achieve it in my images. I'm use a Canon 40D with a 17-85mm f/4-5.6 EFS lens.

As I understand it, DOF its made up of 3 elements in simple terms:
1. Distance from camera to subject
2. Focal length of lens
3. Aperture

I went out yesterday and kept taking the same shot over and over whilst changing the aperture or moving further away/nearer to the object. I only managed to get one shot that's even close and that feels more like a complete fluke than a considered shot.

Does anyone have any tips, techniques, advice, anything that will help me get over this block I seem to have with it.

Also, does anyone use the same camera/lens who could give me a link to their pictures showing me what the combination is capable of, and how they achieved them. I was thinking I could go out and try to recreate the shots until such time as I can do it on my own. I have tried a couple of books, but they forget to tell you how far away the photographer/camera was whilst taking the shot.

Any and all advice WILL be much appreciated!

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Coleslaw e2
9 13.4k 28 Wales
19 Mar 2009 1:09PM
Upload the shot, tell us the technical details, tell us what you want to achieve.
User_Removed 7 2.2k 3 United Kingdom
19 Mar 2009 1:15PM
If you're trying to achieve a good bokeh, which is the blurred background around an otherwise in-focus subject, then it helps to shoot with a large (eg. f4 rather than f22) aperture using the longer end of the telephoto as this (generally) makes the plane of focus narrower. It also helps to get the background as far away as is practically possible behind your subject. This helps the bokeh work better.
spaceman 10 5.2k 3 Wales
19 Mar 2009 1:24PM

Quote:I went out yesterday and kept taking the same shot over and over whilst changing the aperture or moving further away/nearer to the object.


Perhaps you were changing too many things and going round in circles. When doing test shots it's usually better if you concentrate on a single aspect until you fully understand it and then move on to another. What effect were you after - did you want shallow DoF for a blurry background or extensive DoF to keep everything sharp?
digicammad 11 22.0k 37 United Kingdom
19 Mar 2009 1:28PM
As a rule of thumb, your focus point is 1/3 of the way into your depth of field. In other words twice as much will be in focus beyond as in front of the focus point.

To start with, try the two extremes of aperture and the mid-point, that should give you big enough steps to be able to see the difference.

Have fun.

Ian
ljesmith 10 1.1k United Kingdom
19 Mar 2009 1:47PM
Actually there is the fourth element of format to take into account as well. Wink
Lulu01 5 156
19 Mar 2009 1:56PM
Thanks for the responses. COLTRANEKOH - photo uploaded (DOF question in General box), details:
Aperture priority, TV 1/40, aperture f/14, ISO 100, Focal length 50mm, WB shade.
I was only a few inches from the flowers. The cross was a about 8 feet away, above me, on a mound.
I know its a rubbish shot, but its the only one that's even remotely got some DOF blur.
Lulu01 5 156
19 Mar 2009 1:59PM

Quote:Actually there is the fourth element of format to take into account as well.


I'm confused enough, but tell me ... what is the fourth element?
FatHandedChap 8 1.3k England
19 Mar 2009 2:10PM
As Luke hinted, the format that you're using will affect DOF.

If distance, focal length and aperture remain constant then you will see different DOF on a crop camera compared to a full frame camera, to medium format, to large format.
ljesmith 10 1.1k United Kingdom
19 Mar 2009 2:15PM
Basically, if you have a small sensor camera (which i think your's is) and use a wide angle lens you're going to naturally get a lot of DoF. Opening up to f4 may not be enough to get the effect you're after. You could try moving away and using a longer focal length to reduce the lens factor, but obviously that then changes the shot completely.
Lulu01 5 156
19 Mar 2009 2:17PM
I
Quote:Quote:IWhat effect were you after - did you want shallow DoF for a blurry background or extensive DoF to keep everything sharp?


I was trying for a blurred background with one thing in focus. I seem to have got the hang of extensive DOF, ie everything sharp.
Lulu01 5 156
19 Mar 2009 2:23PM

Quote:Basically, if you have a small sensor camera (which i think your's is) and use a wide angle lens you're going to naturally get a lot of DoF. Opening up to f4 may not be enough to get the effect you're after. You could try moving away and using a longer focal length to reduce the lens factor, but obviously that then changes the shot completely.

I tried f4 through to f22. If I move further away, I lose the focus of the shot. I was trying to get, for example, one headstone in a graveyard as the main focus, with the blurred outline of other headstones/trees/the church etc as the backdrop.
If my lens doesn't really allow for it, what lens should I consider?
StuartAt e2
10 1.0k 6 England
19 Mar 2009 2:23PM
Looks like the problem is the aperture. To achieve shallow DOF, you need a large aperture (i.e. small number). Your F14 is doing the reverse.
Lulu01 5 156
19 Mar 2009 2:24PM

Quote:Looks like the problem is the aperture. To achieve shallow DOF, you need a large aperture (i.e. small number). Your F14 is doing the reverse.

This shot was a complete fluke. Having read the various posts, I understand what you are saying re f/14, but I tried all f stops and this was the only shot out of dozens that had any DOF.
spaceman 10 5.2k 3 Wales
19 Mar 2009 2:25PM
Use the lens at 85mm and use the largest aperture (5.6). That's as much as you can do with the lens. If the background still isn't blurry enough then your only option is to use a lens with a larger aperture - a so-called "fast" lens.

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