Where's my depth of field gone?

willery · Jan 2, 2007

Hi Folks, I have a Canon Powershot S2. It's on automatic most of the time as I use it mainly for my business. It produces good images and I have no complaints considering all the things it can do in one package. One thing that has me asking for your professional opinion is the manual setting option. I wanted to take a few snaps of the birds on my bird table, through the window glass, so I set it to manual, zoomed in and manually focussed. The manual focus is not very precise and the images were disappointing so I tried closing the aperture down as much as possible using manual overide hoping to get a good depth of field. I thought this would overcome the limited manual focussing adjustemnt on the camera. I was surprised that the depth of field hardly altered regardless of the aperture? Is this something to do with the compact digital system. I think I remember my film SLR cameras that, when stopped down, almost did away with the need to focus! Perhaps I need to get a digital SLR. Any advice would be most welcome.

ksmattfish · Jan 2, 2007

Increasing focal length (zooming in) decreases DOF.

How far away is your bird table? The closer you are focused to the camera, the less DOF also.

Alex_B · Jan 2, 2007

willery said:
I need to get a digital SLR. Any advice would be most welcome.

SLRs tend to have a narrower depth of field ...

so you went to higher f/stops? then your DOF should increase for a given focal length.

why not do two testshots at the same focal length, one with a wide aperture, say 2.8 and one with a rather small aperture, say 8 or beyond (depending on what your camera allows) .. and then compare. you could use a ruler with inches or cm marks on it ... at an angle of 45 degrees. there you can easily see your DOF.

willery · Jan 2, 2007

Thank you for your kind replies to my query. The distance from the house to my bird table is approx. 5Mtrs. I have found that the minimum aperture is 7.1, otherwise camera shake is too noticeable. I have done some further tests as you recommend and now I see that there is a difference in DOF from 7.1 to 3.5. It would appear to be 200 - 300mm in total. I am using maximum zoom, (which I believe is the equivalent of 430mm on this camera), and I am then enlarging on the moniotr to crop out the image. Perhaps I am expecting too much from the optics on a basic all in one compact like the S2.

markc · Jan 2, 2007

If no part of the image is sharp, are you sure it isn't something as simple as camera shake? What shutter speed are you using?

willery · Jan 2, 2007

markc said:
If no part of the image is sharp, are you sure it isn't something as simple as camera shake? What shutter speed are you using?

Thank you for your reply. I'm sure it's not camera shake, the image is sharp in the DOF, (as sharp as it gets with a compact camera), it's just the DOF is small and it's difficult to focus accurately enough to get focus spot on the target. The little blighters fly around so quickly I get no time to focus anyway, hence I was hoping to get a DOF of about 1000mm to encompass the table top and bag them quickly! I tried focus bracketing but all I get is one shot of the bird and two of the desserted table! I think the speed range is around 1/20 to 1/40 at f/7.1, depending on the light of course. Any higher than f/7.1 drops the shutter speed and the camera shake effect you mention is noticeable.

ksmattfish · Jan 2, 2007

Seach online for a "DOF calculator", and input your specs. You may find that you are increasing the DOF, but it's just not very noticable. For instance you may be going from 8" of DOF to 16" of DOF, which may look about the same depending on the scene layout.