Monday, April 18, 2011

Using extension rings

My previous post, “Lens focal lengths – an illustration” showed how longer focal length lenses deliver a smaller field of view. You might also have noticed that the last image, acquired with a 50mm lens at a working distance of 350mm, was slightly out of focus. That happened, not because I was too lazy to adjust the focus ring, but because the 50mm lens just won’t focus at that distance.

So what do you do when you can’t back the lens away any further from the target? (Or you don’t want to increase the field of view?)

Well this is where extension rings come in. As you can see in the picture above, placing a 2mm ring between the lens and camera flange allows a focused image to be formed on the sensor.
You’ll also notice that the field of view is slightly smaller (and I tried hard not to move the camera between the two images.) What you can’t see, well I can’t anyway, is that the extension ring also slightly reduces the quantity of light reaching the sensor. In other words, the image is a little darker.
That’s not an issue with just a few millimeters of extension, but it can become so as you start to use longer extension tubes.
And why would you need to use longer extension tubes? Check back another time for that.

1 comment:

Spencer Luster said...

Hi Brian,

Another useful post! My one very minor quibble is with your comment: "...the extension ring also slightly reduces the quantity of light reaching the sensor."

If the desired image just fills or overfills the sensor, then what you say is true. As the distance from the lens to sensor increases, the (presumably) expanding cones of light from the lens spread out more and thus less of it hits the sensor. But, if the initial image underfills the sensor, you can add spacers, have the light cone expand, but still have all the light from the object land on the sensor. Of course I know that what you meant is that the light flux density decreases -- each pixel sees less light even though the total leaving the lens is the same.

There is, however, a notable quasi-exception to the above. The case of double or bi-telecentric lenses in which the image rays are all parallel cones of light. If you increase the lens to sensor distance, the same light flux density strikes the sensor -- or so close to the same that it makes no practical difference. It's another advantage of the double telecentric design -- such as those from Light Works! :-)

One can also imagine a lens in which all the exiting cones of light converge. In such a case you could increase the lens to sensor distance and actually see an increase of light flux density at the sensor!