What focal length should I set for IBIS when using a reverse-mounted lens for macro?
Asked 11/9/2018
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2 answers
0
My Sony A7R II lets me manually enter a focal length for SteadyShot. I’m using old Olympus lenses reverse-mounted for macro work, sometimes with extension tubes.
Examples:
- 50mm lens reversed
- 28mm lens reversed
- sometimes with 16mm + 10mm extension tubes
Because reversing a lens and adding extension changes the magnification, I’m unsure what focal length value to enter for stabilization. Is there a practical way to estimate the correct IBIS setting when a lens is reverse-mounted, with or without extension tubes?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
7y ago
2 Answers
1
The calculator on http://extreme-macro.co.uk/extreme-macro-reversed-lenses/#calculator can be used to determine the focal length
Entering the values from sample shot 2 (28mm lens reverse mounted, displays 50mm on 36mm gives 0.72:1)
Adding the extension tubes (26mm) shows the measured maginification (1.64:1) and the focal length of 119mm. So the best setting would be 115mm without extension tubes and 120mm with extension tubes.
The formulars used by the calculator:
Given values:
t = extension in mm
l = lens focal length
m = original magnification
f = intended f/stop
Calculated values:
The new magnification (mag:1)
mag = m + t/l
The new focal length in mm
focal = l*(1+mag)*(1+mag) / mag;
The new fstop
fstop = f*(1+mag)
Originally by user76026. Source · Licensed CC BY-SA 4.0
user76026
7y ago
0
Generated from our catalog & community — verify before relying on it.
A practical approach is to enter the lens’s effective focal length at the working magnification, not simply the marked focal length.
From the community answers, one useful macro formula is:
- new magnification: mag = m + t / l
- effective focal length: focal = l × (1 + mag)^2 / mag
where:
- l = lens focal length
- t = extension tube length
- m = original magnification
Using the examples given:
- reversed setup without extension came out to about 115mm
- with 26mm of extension, about 120mm
So for those specific setups, setting SteadyShot to roughly 115–120mm is reasonable.
Another answer notes that if you treat stacked optics by diopter power, a 50mm lens combined with a reversed 28mm lens gives an equivalent focal length around 18mm. That can help describe the optical combination, but for macro IBIS setup the effective focal length based on magnification is the more directly useful method from the answers provided.
In practice, exact IBIS performance at high macro magnification is limited anyway, so use the nearest sensible value and expect only modest stabilization gains.
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UniqueBot
AI7y ago
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