Originally posted by raider Hi

I am currently investigating what is the closeup lens suitable to bring a normal lens close to 1:1 and after searching the forum, I came across this post:

I think I recognize this as my scribblings so I will answer this. Note that I am currently working on a spread sheet to perform all these calculations, which I keep in my PDA,

Quote:

Okay. In my case, my lens is a 75mm with 1:3.9 magnification. So assuming I am using a +5 dipoter, the

FL_close_up = 1/5 = 200mm.

New focal length = (FL_old x FL_close_up) / (FL_old + FL_close_up)

= ( 75mm x 200mm ) / (75mm + 200mm)

= 54.54mm

ok, so far so good, you have established the new effective focal lenght.

Quote:

What does this 54.54mm relating to? The "1:3.9" never come into place in the above equation.

you've got it, the 1:3.9 is the maximum magnification ratio of the origonal lens at closest focus

Quote: So what would a +5 diopter bring the magnification of 1:3.9 to?

Thanks for any insights.

go back to the concept that magnification ratio is the ratio of subject to lens distance over lens to film(or sensor) distance

you have made a new lens with a 54mm focal length, but which is 75mm from the sensor or film at infinity, and which at infinity focus can focus out to 200mm, therefore your new lens can go from 1:200/75 or 1:2.66 to something higher.

That something higher you need to calculate.

This needs more math, and some estimations about your lens.

I have not yet got all the formulas set up for this, so I have worked backwards with my current version of the spreadsheet, which i also use for calculating the benefits of extension tubes.

You lens, from the magnification ratio, can close focus to about 370mm from the front element, and to do so, effectively moves the front element out away from the film/sensor 19mm

When you consider the new focal length, with the 5 diopter close up, it is a 54mm lens, extended out 21mm at infinity. using this, and the lens extension to minim,um focus of 19mm for a total extension of 40 mm , I estimate your maximum magnification will be 1:1.5

You can get the formulas on

Photography - Wikipedia, the free encyclopedia
As I said, this is a work in process so I don't have all of them yet in a simple form to post