[Guest]

With the winter months upon me, and a new to me 50D with MFA, and some vacation days to spend, I have tested my 50mm f1.8 with three illuminant sources.



First, all testing has been at constant aperature of f2.8.



The three illuminants are:



1. Natural daylight (overcast, snow).

2. Samsung LED monitor calibrated to D65.

3. Tungsten recessed lighting. (dark outside, no natural light)



I have carefully derived the MA at f2.8 with each illuminant.



1. Natural daylight: MFA = -2

2. Samsung LED monitor: MFA = -2

3. Tungsten recessed: MFA = +9





In the case of tungsten lighting, my 50mm f1.8 set at f2.8 suddenly front focusses and must be compensated.



In daylight the lens can get by with MFA 0, but, careful measurements indicate the lens very slightly back focusses in natural lighting.



I am interested in precisely the mechanics of how such a front focussing effect can occur with Phase Detect AF and this lens. If anyone can reproduce this result, let me know.



If anyone knows why the long wavelength of tungsten, relative to D65 outdoor lighting, results in focus shift with phase detect AF, please let me know.



Thank you.