DOF across focal lengths and shooting distances.

q quick question regarding depth of field across focal lengths and at respective shooting distances.

I.E:

shooting some street photography with my 30mm at 1.4 at say a distance of 3 meters of "X" subject i will get "Y" DOF

now say i change to my 50mm at 1.4 still and step back from the subject further to get the same field of view as with the 30mm will i get the same DOF as the other lens?

another version.

say i have an 85 1.8 and i step even further back to get the same field of view and shoot it at 1.8 will i get the same DOF seeing as im now further away from the subject that i want to be in focus?

sorry if thats too confusing/doesnt make sense. im just trying to figure it all out/

-isaac-

--
http://flickr.com/photos/isaac_owen_photography/

For a given camera and distance to subject significantly greater than the focal length, DOF is only a function of f stop and subject magnification (or field of view). 30 1.4 at 3m would give you the same DOF as 50 1.4 at 5m, or 85 1.4 at 8.5m.

On the other hand, background blur (assuming a distant background) is a function of subject magnification and aperture (focal length divided by f stop). Blur increases with magnification and with aperture. Because of this, the 85 1.8 at 8.5m will give you the most background blur and subject isolation with a little more DOF than any of the other two options. This is because this lens has a greater maximum aperture than the others (85/1.8 = 47mm, 50/1.4 = 36mm, 30/1.4 =21mm)

isaacarus said:

q quick question regarding depth of field across focal lengths and at
respective shooting distances.

I.E:
shooting some street photography with my 30mm at 1.4 at say a
distance of 3 meters of "X" subject i will get "Y" DOF

now say i change to my 50mm at 1.4 still and step back from the
subject further to get the same field of view as with the 30mm will i
get the same DOF as the other lens?

another version.
say i have an 85 1.8 and i step even further back to get the same
field of view and shoot it at 1.8 will i get the same DOF seeing as
im now further away from the subject that i want to be in focus?

sorry if thats too confusing/doesnt make sense. im just trying to
figure it all out/

-isaac-

--
http://flickr.com/photos/isaac_owen_photography/

Click to expand...

sssanti said:

For a given camera and distance to subject significantly greater than
the focal length, DOF is only a function of f stop and subject magnification
(or field of view). 30 1.4 at 3m would give you the same DOF as
50 1.4 at 5m, or 85 1.4 at 8.5m.

Click to expand...

There should be one more restriction on this: it only applies at distances significantly less than the hyperfocal distance. (That does include all shallow-DOF situations.)

At longer distances, distant objects become sharp or nearly so, there is more DOF behind the subject than in front of it, and the DOF depends on focal length in addition to f-stop and magnification. (An example is 30mm f/16 @ 3m vs. 50mm f/16 @ 5m.)

sssanti said:

On the other hand, background blur (assuming a distant background)
is a function of subject magnification and aperture (focal length
divided by f stop).

Click to expand...

Good point... background blur is often lumped in with DOF, but it does not follow the same rules.

The reason for the difference, and for the restriction on the rule for DOF, is perspective. At the same framing and f-stop, the amount of blur as measured on the blurred object is independent of focal length. But a longer focal length requires a longer subject distance, and that enlarges the background relative to the subject - blur included.

Paul van Walree's depth of field article has a good illustration of this, along with lots of other information:
http://www.vanwalree.com/optics/dof.html#backgroundblur

--
Alan Martin

The article you mentioned is a very good reference about the topic.

Alan Martin said:

Paul van Walree's depth of field article has a good illustration of
this, along with lots of other information:
http://www.vanwalree.com/optics/dof.html#backgroundblur

--
Alan Martin

Click to expand...

isaacarus said:

q quick question regarding depth of field across focal lengths and at
respective shooting distances.

I.E:
shooting some street photography with my 30mm at 1.4 at say a
distance of 3 meters of "X" subject i will get "Y" DOF

now say i change to my 50mm at 1.4 still and step back from the
subject further to get the same field of view as with the 30mm will i
get the same DOF as the other lens?

Click to expand...

Yes

isaacarus said:

another version.
say i have an 85 1.8 and i step even further back to get the same
field of view and shoot it at 1.8 will i get the same DOF seeing as
im now further away from the subject that i want to be in focus?

Click to expand...

Hey, now you're not at 1.4 anymore, so no, DOF will change.

Also note that available DOF will start to move to the front now, at 30mm you had about 35% in front of the focal plane, at 85mm you're up to about 45% with the rest obviously behind the focal plane

isaacarus said:

sorry if thats too confusing/doesnt make sense. im just trying to
figure it all out/

Click to expand...

Won't we all?

In a perfect world, this is what we do. We look at the subject.(this we usually can do) We move away from the subject until we get the perspective we want (at this stage we have usually been run over by cars or the subject is being obscured behing buildings). We than mount the lens that will give us the framing we want. (except the 400mm 2.8 was too expensive or too heavy to bring), we select the aperture that will give us the wanted depth of field (except the light is too bad so we either need to stick to f/2.8, or the light is so strong that f/22 is the only option) and pick our shutter speed (that we can do) and select the iso needed for correct exposure (usually ISO12 or ISO185000).

Isn't photography fun, huh?

Cheers

--
Anders

'It is nice to be important but it is more important to be nice'