Depth of field question

[ernie]

Hey,

Got my new DSLR yesterday: brand new D80 with 17-55mm f/2.8 lens. First thing I tried out was experimenting with DOF, as this is something my old point & shoot couldn't handle. I understand how DOF works - aperture, distance to the subject etc.

So I set my camera to manual, focal length at 55mm and fiddled with f numbers and shutter time a bit. Distance to subject was variable (0.5 meters to 5 meters). Then I noticed I could just adjust the DOF (I was going for shallow) with the zoom ring in front of my lens, and any adjustments to the aperture and shutter time just had a brighter/darker photograph as a result. Which makes perfect sense of course, since those control the amount of light let in.

Anyway, what I'm trying to say is that I'm confused. I thought ap. and shutter time determined the DOF. So what exactly does this zoom do to DOF? I would have figured I'd get a different DOF adjusting shutter and ap. whilst leaving the zoom alone. But all it does is make my picture brighter/darker. Can someone enlighten me please?

 

[Tolyk]

Are you sure that's not the focusing ring? Zoom will affect DOF because you're changing the perspective, getting closer or further from the object. Distance is a big factor when trying to achieve a blurred out background.

 

[ernie]

Eeh, focusing ring yes. Sorry. I had it zoomed in at 55mm all the time.

 

[altyfc]

Good luck with the new camera, ernie.

Like you were, I'm currently using more of a point-and-shoot camera and not getting the flexibility I would like. I plan to get a DSLR next year some time, I hope... I just haven't really had the time lately to justify the purchase.

I'll be interested to hear how you get on, and whether you find you're able to get far better shots than previously.

 

[Big Mike]

Sounds like you were manually focusing the lens, rather than changing the DOF.

You don't need to put the camera into manual mode to play with your DOF. The shutter speed, on it's own, has no affect on DOF...but of course the shutter speed affects the exposure, as does the aperture, which does affect DOF.

Put the camera into A (Av) mode (aperture priority). This way, you can choose the aperture and the camera will give you the appropriate shutter speed. Choose an aperture based on the DOF that you want.

Also, the focal length (zoom) will affect your DOF.

 

[ernie]

Right, thanks for the advice.

What I'm still wondering is: why would I need DOF control if I can just blur out what I want with focusing? I'm probably missing something here ...

 

[Patrice]

Generally you would focus on the subject you want sharp and then use DOF to control how far in front of the subject and how far in back you want other objects to be sharp as well.

 

[djf]

DOF is determined by focal length of the lens, the distance to the subject, and the F Stop. Basically, the DOF will get narrower if the focal length of the lense is longer, if the F stop is widened, or if the distance to the subject is decreased (same effect as increasing the focal length).

Here is a page with a DOF calculater. Plug in your numbers and you will begin to understand:

http://www.cambridgeincolour.com/tutorials/depth-of-field.htm

What I'm still wondering is: why would I need DOF control if I can just blur out what I want with focusing? I'm probably missing something here ...

Remember that the camera will have the F stop wide open while you are focusing. It will only stop down the F stop when you hit the shutter release. So the DOF will be at it's minimum when you are focusing. This is why some SLR cameras have the DOF preview button.

If you are really confused, then set the camera to A priority and step through all the different F stop settings. That will eliminate the shutter speed confusion.

A good piece of advice a friend gave me was "Focus far and the near will follow." I didn't get that until I was trying to focus on a close up object and also have a clear background.

 

[altyfc]

Sorry, this is a bit of a naiive question, but I don't have a DSLR... when you look at the preview screen at the back of camera, and adjust the aperture/depth of focus, does the preview screen show what is and isn't in focus, as well as the final shot? I appreciate it's much smaller, but would you still be able to see this?

 

[sabbath999]

Sorry, this is a bit of a naiive question, but I don't have a DSLR... when you look at the preview screen at the back of camera, and adjust the aperture/depth of focus, does the preview screen show what is and isn't in focus, as well as the final shot? I appreciate it's much smaller, but would you still be able to see this?

No, it shows what the scene looks like with the lens wide open at its maximum aperture. There is a button on many (most) DSLR's and most film SLR's called the "depth of field preview" button. You push the button, and it closes the lens aperture so you can see what the depth of field looks like. Of course, this also reduces the amount of light in your viewfinder so it sometimes is difficult to see.

The good thing about the DSLR is you don't have to guess. You can use the preview, shoot the shoot then use the view screen to exactly see your depth of field.