How to calc magnification with close-up lenses: Part 2
Now onto the mysteries of calculating magnification if you focus the lens with the close-up filter closer than infinity. Unfortunately we now have to distinguish between lenses with IF-design and lenses with a "normal" non-IF design. Now what’s the catch about IF-design? IF (= internal focusing) designs tend to shorten their focal length when focusing close, while normal non-IF designs simply move the whole lens further away from the sensor without shifting any internal lens-groups. Those normal lenses keep their focal length when focusing.
Now that you have understood from the first part that the focal length is the important factor to calculate the magnification with a close-up filter it is clear that IF-design with an effectively shrinking focal length tends to
reduce the effect of the close-up filter. So to calculate the magnification at closest focus distance we have to calculate the "
shrink-factor" of an IF-lens first. But we'll get to this later.
Let’s have a look at
non-IF design lenses first.
What you need to know for this calculation is the maximum magnification that the lens alone is capable of. This is normally part of the technical data or you can test this for yourself easily by photographing a ruler and dividing the mm on the ruler by the width of your sensor.
In our prototypical case our 70-200mm zoom has a max magnification of 1:8 (throughout the zoom range) and the 400mm lens a magnification of 1:3. So the zoom has quite mediocre close-up capabilities whereas the fixed focal is pretty good for a “non-macro” lens. Now let’s follow the necessary calculations with the 400mm lens:
1. We calculate how far the lens has to be extended to get a magnification of 1:3. That is 400mm/3 = 133mm. I.e. the front element of the lens is now 133mm+400mm = 533mm away from the sensor
2. We calculate what the focal length of the 400mm combined with the 500mm close-up filter is. That is 1/(1/400mm+1/500mm)=222mm
3. For this combined focal length of 222mm and the distance of the front element 533mm we calculate the magnification as 533mm/222mm-1 = 1.4 or
1:0.71 if you like.
4. If you feel so inclined you can also calculate the distance from the front-element to the subject as 533mm/1.4 and get 381mm
Phew, that's it! This is our first calculation to this subject and you can now easily calculate the resp. Numbers for the 70-200mm zoom.
I'll be back in a minute and give you the results. And then we go on to the dreaded shrink-factor
Have fun!
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In answer to your question Citruspers, unfortunately I'm not into lens reversing. So I cannot rely on personal experience, but perhaps the math is easy enough for me.
If that's the case I might dip into this water after the current thread comes to a close...