It's Depth of Field...NO, it's not.

I heard some people talking about using Telephoto lenses and how they offer a shallower Depth of Field than a normal lens to make their portrait subject pop out of the image. So I thought I would talk about what really happens.

Now we know that Depth of field (DOF) is controlled by: Aperture, Distance to Subject and Focal Length. So from what we know,  we can say that; If we increase the focal length, that will decrease our DOF. But is THAT really what happens in real life.

Let's look at some examples to see what really occurs.

Look at the four Images below and tell me which one has the Shallowest Depth of Field
All were shot at the same Aperture, varying Focal Lengths & Distance to Subjects.

So, from just looking at them, we would say that the image shot with the 200mm Lens has the shallowest DOF.

Well, we would be wrong. Every image has the exact same DOF!     .06 feet.

But why then do they all look different and why would it not be so since Focal Length changes DOF.

Because, Focal length's effect on DOF would assume that we did not move. But what happens in real life is that we move forward or backward to keep our subjects in the same framing in the image. So we move a exact proportionate amount closer to the subject as our focal length decreases. If we go from 200mm to 100mm, We move, in this case, from 6 feet from our subject to 3 feet.

Okay then, what makes the images look different?  Perspective Compression The objects in the background are no more out of focus in any of the images, but by using a telephoto lens it brings those far away objects closer to our eyes (compresses) so that we can now see more clearly how out of focus they in fact are. We also are decreasing (narrowing) the field of view which make us focus (no pun intended) more on the background

I've taken a slice of the image shot at 200mm. I've taken a slice of the same part of the image from the image shot with the 50mm but magnified the back ground by 3X, escentially moving it closer to our eye. As you can see, they are... the same

So from this we can know that by using a telephoto lens we can increase the apparent DOF without the need for wide open apertures. Which can come in handy sometimes, especially when doing fill flash outdoors which when using a more wide open aperture may make our shutter speed exceed the sync speed of our Camera/Flash

I would also like to address one thing,  I Hear photographers talking about an image having a shallow depth of field and calling it Bokeh. That is NOT Bokeh, that is just a shallow depth of field. Bokeh is a descriptor used to describe the QUALITY of the out of Focus areas of an image. And is used to describe the quality of  a lens. "That lens has a very smooth and creamy Bokeh" But you cannot use it as: "I decreased my aperture to give the image more Bokeh"