[Cailean]

Hey, I think I just learned something and thought I'd share....

I just took 2 photos from the same spot... the first is at 35mm and the second at 420mm. Both pics were shot with an F number of 4.0 but with different shutter speeds (1/250 & 1/150, respectively). Everything I've read has said that depth of field is determined by aperture... the smaller the aperture, the shallower the DOF. It is obvious in these two photos, however, that #2 has a much shallower DOF than #1 even though they were shot through the same-sized aperture. Am I making sense at all? I wouldn't think the different shutter speeds would account for the DOF, or would they?

#1


#2 - it's a garbage photo but it illustrates the point, methinks.

[Rufus]

I, like you, always believed that aperture was the controlling factor, but no. Focal length also is.....

[Cailean]

Ok, so does changing the aperture change the focal length? or is focal length strictly a function of the lens position?

[Rufus]

No, the aperture cannot influence focal length. Focal length is defined as the distance from the centre of the lens to the focal plane or CCD.

[Cailean]

OK, that's what I thought.

[shuttertalk]

Hmm... interesting results! Like you and Rufus, I thought it was just aperture which mainly controlled the DOF.

[peter]

thats is some extremely shallow depth of field though ..
i guess the distance from the object would count as well ..
cause the thing is .. to get the same depth of field from different focal lengths ...
with the same appeture and shutter speed ..
i think youd have to be framing the same image in both shots. .
that might be a factor ..

[Rufus]

That's right. Distance is a factor. This is closely related to the "flattening" and the "deepening" effect of long and wide lenses respectively.

[shuttertalk]

You learn something new every day..

[adam]

That's why you use telephoto in your subjects; to get nice background blur

You can also see this on many bird shots, the background is blurred a lot

[shuttertalk]

Hey I was just reading... apparently depth of field with digital is also greater than film, due to the smaller sensor size... which makes it even harder to get the blurry background effect.

http://www.wrotniak.net/photo/dof/

[Rufus]

True, but it depends very much on the sensor size.