Thank you for your excellent article on Understanding Depth of Field. This web site is awesome.
I'm a little confused a by the statement that with constant magnification, depth of field is virtually constant regardless of focal length. My confusion stems from the fact that in order to keep the same magnification you have to change your distance from the subject. You are changing two variables simultaneously. It seems you could just as easily argue that with constant magnification, depth of field is virtually constant regardless of distance to subject. (Maybe that's obvious but I've never heard it stated that way before.)
I found another couple of excellent articles (http://toothwalker.org/optics/dof.html and http://www.scottbideauphotography.co...s-compression/) that point out that moving the camera position changes the perspective and so comparing a photo that was taken at a distance of 1 m with a focal length of 20 mm to another photo that was taken at a distance of 5 m with a focal length of 100 mm is really comparing two different photos.
This got me thinking. It seems the whole concept of depth of field is based on:
- Limitations in our ability to discern minute differences in focus.
- The size of the out of focus object (circle of confusion.) If the object is larger we can more easily discern its "sharpness" or "bluriness."
If that's the case then the illusion of depth of field is impacted by two or three things: aperture, magnification, and resolving power. Magnification happens in a lot of ways:
- Increasing the focal length
- Moving the camera closer to what is being photographed
- Enlarging an image taken by a cropped sensor to the same size as an image taken by a full frame sensor
- Post-production cropping and enlarging
- Moving your eyes closer to a print
Each of these ways to magnify an object may have a different level of impact, but a generalization might be that any time you enlarge/magnify something you can see more detail. Increasing focal length (magnifying) while simultaneously moving the camera further away (de-magnifying) sort of cancels each other out, doesn't it? And it's no wonder that taking a small portion of the background from one image and enlarging it to compare with another "un-enlarged" background will result in almost identical perceived depth of field. In the end, smaller things always appear sharper.
Further, anything that impacts our ability to resolve minute differences in focus should have an impact. The resolution of the lens, the resolution of the sensor, the resolution of a monitor or printer, the resolution of our eyes -- these all should impact the illusion of depth of field, right?
I'm sure I'm glossing over a ton of detail but I'm trying to understand the basic concepts that I can leverage to impact my photography. Among these should be:
- Only one plane in the photo is truly in focus
- Aperture impacts perceived depth of field
- Magnification or enlargement impacts perceived depth of field.
- Viewing distance impacts perceived depth of field.
- Depth of field is almost never spaced equally in front of and behind the focal plane
Am I understanding this correctly?
Thanks again for your great article and this wonderful web site.