Calculating Mag Increase with Diopters

[athinkle]

I'm trying to decide on whether or not to get myself a close-up lens

to add to my macro arsenal. My question is: How does one calculate

the change in the final max magnification ratio for the lens+diopter

combo? I'm wondering if there's a way to measure it based on the

focus thread to focal length ratio that one uses to calculate

magnification with tubes. I need to know if one of the diopters I'm

looking at will bring my maximum mag on my tele lens to at least

1:1, so I just need a way to make the calculation.

[greg_erker]

When the camera lens is focued at infinity it is pretty easy

to calculate. From my achromatic close-up web page (www.angelfire.com/ca/erker/closeups.html):

 

Another approach is to calculate magnification. Convert the diopter strength to millimeters focal length (CU FL = 1000/Diopters). Then divide the camera lens FL by the CU lens FL to get the magnification when the lens is focused at infinity. Thus a +2 CU is 500mm FL. If you use it on a 50mm lens you will get 1:10 life size. The same +2 on a 300mm lens will give you 0.6 of lifesize (1:1.66).

 

I'm not sure how the magnification is calculated when both effects

are combinded.

 

Greg

[alex_lofquist]

When you wish to find the "magnification" when the lens is focussed closer than infinity, then there is no simple technique except to try it out. Some lenses (internal focussing) actually change the focal length of the lens. This makes a simple calculation impractical. I use both 1.5 and 2.9 diopters on my lenses and did actual measurements for near and far focussing distances, and image magnifications. I put this data on index cards so that I can decide immediately what combinations to use in a given situation. It is the pragmatic approach!

[eosdoc]

If you need to make a calculation, I suggest a calculator:<BR>

<a href=

http://ca.geocities.com/lokejul/jlcalc.htm

>http://ca.geocities.com/lokejul/jlcalc.htm</a>