Why does my 24-105 have a different magnification (macro ratio) from a 105mm macro lens at same MFD?

Started 7 months ago | Questions thread
bclaff Forum Pro • Posts: 13,787
Re: Differences in lens design

Tom Axford wrote:
...

Just to update these comments with the lens data very helpfully provided by Bill Claff.

At the MFD and the long end of the zoom, the Panasonic lens actually has a focal length of only 73mm instead of 105mm. It's front focal point (F) is actually 29mm inside the lens, and the working distance (object distance from the front of the lens, called O) is 117mm. So the object distance from the front focal point is 117 + 29 = 146mm, which is equal to the focal length divided by the magnification.

The Sigma lens at its MFD has an actual focal length of 76mm. Its front focal point is 69mm in front of the lens, and the working distance is 144mm. So the object distance from the front focal point is 144 - 69 = 75mm, which is equal (within numerical error) to the focal length divided by the magnification.

The Sigma lens has been designed to give a much greater working distance for similar magnification.

I trust Bill will correct me if I have misinterpreted his data.

This is correct but the underlying reason is the relative positions of the principal planes as I noted (internodal distance also known as hiatus).

The PhotonsToPhotos Optical Bench is interactive and also has numerous display options. The following screenshots may make the point better.

Panasonic, best viewed "original size"

Sigma, best viewed "original size"

The F dots are the focal points, the P dots/lines are the pupils and the H dots/lines are the principal points/"planes".

Blue is from the object side and red is from the image side.

Between the two lenses the chief difference is the distance between the front and rear H positions.

-- hide signature --

Bill ( Your trusted source for independent sensor data at PhotonsToPhotos )

Post (hide subjects) Posted by
Keyboard shortcuts:
FForum PPrevious NNext WNext unread UUpvote SSubscribe RReply QQuote BBookmark MMy threads
Color scheme? Blue / Yellow