How is it possible to have shallow depth of field with a small aperture? (The tele-macro paradox.)

Question

Some longer tele lenses have a macro mode that allows for really shallow depth of field that have a lot of background melting when using relatively narrow apertures; like f/4.5-6. I have tried and cannot recreate this shallow depth of field with my fast prime lenses at similar apertures. I was under the impression that aperture accounted for depth of field, not focal length.

Internet searches often lead to very technical reports and descriptions of how the optics in the lens are arranged to achieve this. I would like to know in lay terms how narrow depths of field are achieved with such a narrow aperture?

Answer 1

Here's my attempt at a lay explanation for what's going on...

The reason that depth of field exists, i.e. the reason every photo doesn't have every single object tack sharp is that light coming in at different angles will be bent different amounts by the lens and come into focus at different distances behind the lens.

There are many factors which influence depth of field, but ultimately it's about the relative angles of incoming light. You can determine where the light comes into focus for a particular angle by focussing the lens, but you can't do anything about the difference in angle between the front and back of an object.

Imagine a triangle a 100 meters long and a few meters high. The slope will be very shallow. Now imagine another triangle 101 meters long and the same height. The slope will be almost identical. The slope represents the angle of incoming light from an object about 1 meter long at a distance of 100 meters.

Now imagine a triangle 2 meters long and 1 meter high, and a triangle 1 meter long and 1 meter high. The slopes are now very different, being 45 degrees and 27 degrees. Because the two angles of incidence in the first case were almost the same, with a 1 meter long object at 100 meters you can set the focus on the lens to get the front and back in focus.

But when the same object is much closer to the camera, you can either have the front or the back in focus, but not both, because the angles are different.

The aperture controls how much blur exists for light at different angles, but no aperture setting can eliminate blur if the angles are very different. Thus focus distance is the most important property when determining depth of field, so with close up (macro) photography you can get shallow depth of field with almost any aperture because of how quickly the incoming light angles change.

Answer 2

Aperture is no the only things that effects depth of field - there are three factors: subject distance, focal length and aperture.

Subject distance - the closer the subject to the sensor the narrower the depth of field - in macro photography the subject is very close.

Focal length - longer focal length cause narrower depth of field, in a lot of lenses in macro mode the lens's elements move in a way that increases the real optical focal length (see those technical reports you mentioned)

Aperture - from the 3 factors aperture is the weakest and you can't fully compensate for the other two by using a narrower aperture.

Answer 3

The main reason for your tamron lens's ablity to achieve such shallow DOFis because of the minimum focusing distance and the focal length combination.

In your tamaron lens , you can use the trigger "marco" mode only if you have a focal length of 200mm. and your minimum focusing distance is 3meters for that lens. thus what am trying to say is u can achieve high DOF when focusing a subject closer and increasing the focal length.

DOF is directly proportional to (1/focal length+focusing distance)

so the closer you place your subject, the lesser DOF

and respectively the larger the focal length, the lesser the DOF.

this cannot be achieved using normal lenses and prime lenses unless and untill they are macro lenses :)

i hope i have explained properly .. :P happy clicking :)

Answer 4

There's also a special technique that allows for a shallow DOF in situation where this usually doesn't seem possible:

If you use a tele lens, set to fixed focus, and make a "gigapixel" photo of a scene, i.e. you take lots of pictures of the scene, in a mosaic layout, and then stitch them together on the computer, you'll get this amazing effect.

While this may not answer your question in particular, it may still be of interest to other photographers when they look up this question, so please don't vote this down for being a bit off-topic :)