Shift in focal plane by adding a glass plate - tutorial article

Occasionally we are interested in how the addition of a flat "glass" plate downstream from an image-forming lens shifts the location of the plane of focus. This comes up, for example, in the matter of the addition of a fresnel field lens in front of the ground glass in a vew camera.

I have just relased to my technical information site, The Pumpkin, a new tutorial article entitled, "The Effect of Inserting a Flat Glass Plate into the Optical Path Downstream from a Lens". It is available here:

http://doug.kerr.home.att.net/pumpkin/index.htm#Glass_Plate

Thanks Doug for the article. Seems well written but the formulation I have not checked but it seems correct.

From what you are saying, even a polaroid or other filter will alter the focus. That's important to remember!

Asher

Asher Kelman said:

Thanks Doug for the article. Seems well written but the formulation I have not checked . . .

Click to expand...

Aw, shucks!

From what you are saying, even a polaroid or other filter will alter the focus. That's important to remember!

Click to expand...

Well, for a filter in front of the lens, the shift occurs in object space (affects the distance to the object plane that will be in focus). So for a filter 2.0 mm thick, with an index of refraction of 1.5 (just to use a tidy number), assuming that the camera had been first focused at a certain distance and then the filter mounted, the distance to the object plane of focus would shift out by 0.67 mm .

But of course if focus were done by ground glass (either directly or SLR-style), or by any type of through-the-lens autofocus system, even that tiny shift would automatically be corrected for, and no error in focus would occur.

In most cases, this matter is only of impact to us if the "glass plate" is inserted into the path between the lens and either the ground glass or the film, but not both.

Best regards,

Doug