How do extension tubes affect focusing distance?

Question

1. How do extension tubes affect focusing distance? More specifically, what I mean to ask is : why does the maximum focusing distance become shortened when an extension tube is attached? - Why does the maximum focusing distance become reduced (from infinity to XYZ)? I don't understand why moving the image plane further back would have any effect :(

2. On a related topic, am I correct in stating that the reason the minimum focusing distance decreases (when a lens is used with extension tubes) is because the lens's focal length stays the same, but the light has to travel further to reach the sensor/film? Does that make any sense, it does (kinda) in my head - but I am a silly person aha ;)

I should make it apparent that I have tried to look for an answer, but I have yet to find an explanation. I have read from multiple sources e.g. ( http://www.cambridgeincolour.com/tutorials/macro-extension-tubes-closeup.htm , http://www.divephotoguide.com/underwater-photography-techniques/article/super-macro-underwater-photography--definitive-guide-part-2b/ )

If you could provide a basic answer as well as a more technically savvy answer that would be greatly appreciated :) Also, I know it is a lot to ask, but you guys please provide a picture or a video in your answer? <3 :)

I would like to thank you all in advance for your answers and support! :)

Answer 1

With a certain distance to the subject, it is in focus at a certain distance to the image sensor. As you move closer to the lens, the place it is in focus moves back. this is because the lens property to bend light is in principle fixed and as you move closer, you change the incoming angles. Naturally, something has to compensate for this, and the distance form lens to sensor plane is one way. And the beauty of this solution is that it is easy for you to do.

When you focus your lens, it will also move the lenses. Old and simple designs move all the lenses away from the sensor as you focus from max to minimum distance. See [left] far focus [right] close focus:

You can see the entire lens holding group moved further into the lens house.

They have to make a choice to stop the mechanical movement somewhere. But when you add a spacer ring to move the entire lens further away, you can focus closer, at the expense of infinity - because now the lens cannot move close enough.

New internal focus moves exploit that the "objective" is made form many "lenses", and it displace them relationally to keep the focus on the sensor distance.

You can say that the oldschool method acknowledge and makes use of the fact that closer subjects are on focus farther away from the lens, and the internal focus change the angle bending properties to compensate.

You might then ask, why they often stop moving the lenses at around 50-85mm minimum focus distance. Are they sneaky and want to sell you expensive macro lenses? Obviously, size is a competing factor. Also, when moving to macro work there are some optical challenges that needs to be fixed other than the min. focus distance such as flatfield focus. It this range the DOF is extremely narrow and for macro you want corner sharpness. You also want a longer manual focus span for finetuning of where to place that narrow plane.

Answer 2

Take a look at the thin lens formula (image taken from Wikipedia):

S₁ here is the distance between the subject and the lens, and S₂ is the distance from the lens to the spot where the image of the subject is formed. f is the focal length of the lens.

You can see that as distance S₁ gets smaller, S₂ has to get larger to compensate. That is, as you move the subject closer and closer to the front of the lens, the place where the lens focuses the image moves farther away from the (other end of) the lens.

The optics of a camera lens are obviously more complex than a simple thin lens, but the same idea is at work. A camera lens has a mechanism that lets you move the optics to adjust the focal point, but that mechanism can only move so far. Adding a spacer between the lens and the mount effectively changes the range over which the lens can focus, letting the lens focus on objects nearer to the lens than it could otherwise, but preventing it from focusing all the way to infinity.

Answer 3

As a beginner I have also wondered about this and have come up with the following:

Simplified assumptions: 1. The thin lens equation (1/object_dist + 1/sensor_dist = 1/f) holds for the lens as an approximation. where, object_dist = distance of object from the lens. sensor_dist = distance of sensor from the lens.

2. Focusing is done by moving the "lens" away or toward the sensor (i.e. by changing sensor_dist) but the lens can only move a finite distance.

   2.1 When the lens is focused at infinity, the lens is nearest from the sensor and sensor_dist = f, let's call this the infinity focus location. 1/object_dist + 1/sensor_dist = 1/f at infinity, 1/object_dist = 1/infinity = 0. therefore, 0 + 1/sensor_dist = 1/f --> sensor_dist = f.

   2.2 When the lens is focused at its nearest focusing distance it is farthest from the sensor. It can only move away so far because of physical design limitations.

   2.3 In summary, Focus at infinity: object_dist = infinite, sensor_dist = f, nearest from sensor. Focus at nearest: object_dist = nearest, sensor_dist = farthest from sensor (by design limitations).

Now the answers.

1. Because the lens can only move a certain distance, the addition of an extension prevents the lens from moving back to its previous infinity focus location (f, nearest from sensor) and thus prevents the lens from focusing at infinity.

2. You are correct in stating that the reason why the minimum focusing distance decreases (when a lens is used with extension tubes) is because the lens's focal length stays the same, but the light has to travel further to reach the sensor/film.

   The thin lens equation: 1/object_dist + 1/sensor_dist = 1/f

   If f remains constant but sensor_dist (distance from sensor or distance light has to travel to reach sensor) increases, object_dist has to decrease to compensate.

Answer 4

The answer is the a Len has two parts a focal length (zoom) and a focal distance (focus). Sticking to a fixed focal length lens. They are sold with the ability to focus from infinity to a close distance.

What this is the ability to focus to a point lines the hit the front of the lens parallel (infinity) and bend those to hit the focal plane (film or sensor) and light beams form. A close point (min focus distance) also gets bent to the focal plane.

So the lens has a range of bending it can achieve to a plane behind it. When you add an extension tube it is moving the plane further away. So when the plane is further away the exit angle has to be larger so the therefore the input angle has to be less. Less than 90 input (infinity) means you lose distance focus.

So to the second point of minimum distance. It decrease because output angle of the lens in larger so the minimum bent the lens can handle now allows for a small angle input.

On a zoom lens you are already change the effective focus point and the focus angle. But the lens still only has a limited flexibility.