[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.

[popo]

Just thinking generally... the camera will focus wide open regardless of the actual aperture used. f/2.8 will increase the depth of field and possibly mask small focus errors. Looking at the 50mm f/1.8 II review, there is some longitudinal chromatic aberration, caused by the focal planes of different wavelengths of light not coinciding exactly in the same plane. I guess, if the subject or lighting was skewed towards some colours more than others, the AF sensors would only see that and might pick a different focus point. Even if that were the case, I don't know how strong an effect it would be.



I could test it some time, but it probably wont be soon.

[Guest]

[quote name='popo' timestamp='1293128822' post='5159']

Just thinking generally... the camera will focus wide open regardless of the actual aperture used. f/2.8 will increase the depth of field and possibly mask small focus errors. Looking at the 50mm f/1.8 II review, there is some longitudinal chromatic aberration, caused by the focal planes of different wavelengths of light not coinciding exactly in the same plane. I guess, if the subject or lighting was skewed towards some colours more than others, the AF sensors would only see that and might pick a different focus point. Even if that were the case, I don't know how strong an effect it would be.



I could test it some time, but it probably wont be soon.

[/quote]

[bryan conner]

Is there an echo in here?

[wim]

Actually, the 50D is much better in this regard than the 40D was, because the AF sensors of the 50D use two wavelength bands, compared to the one with the 40D. Same difference between 5D classic and 5D II, BTW.



For fun, you should also check AF with fluorescent lighting <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='' />.



Kind regards, Wim

[Guest]

[quote name='wim' timestamp='1293228357' post='5167']

Actually, the 50D is much better in this regard than the 40D was, because the AF sensors of the 50D use two wavelength bands, compared to the one with the 40D. Kind regards, Wim

[/quote]



Wim,



Thank you for your information and reply. I was not aware that the 50D autofocus sensor had changed at all relative to 40D.



Also, sorry for the echo, I had quickly tried to reply thank you to the post.

[genotypewriter]

[quote name='wim' timestamp='1293228357' post='5167']

Actually, the 50D is much better in this regard than the 40D was, because the AF sensors of the 50D use two wavelength bands, compared to the one with the 40D. Same difference between 5D classic and 5D II, BTW.

[/quote]

Thanks, very cool info... I too noticed these AF improvements but I thought they were mainly related to the algorithms than the hardware itself. Mind sharing where you found this info so I can quote it elsewhere?



GTW



PS: Wish you a happy new year <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/smile.gif' class='bbc_emoticon' alt='' />

[wim]

[quote name='genotypewriter' timestamp='1293923314' post='5309']

Thanks, very cool info... I too noticed these AF improvements but I thought they were mainly related to the algorithms than the hardware itself. Mind sharing where you found this info so I can quote it elsewhere?



GTW



PS: Wish you a happy new year <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/smile.gif' class='bbc_emoticon' alt='' />

[/quote]

Hi Geno,



This info was in one of the Canon papers they produced when the cameras were introduced.



Of course I had to try this out, and I found that in tungsten, halogen and fluorescent light AF was remarkably better than with the 40D and 5D classic indeed, let alone artificial mixed light. Other than that the AF algorithms have been improved as well as far as I can tell. The 50L is a good example of that. My 50L focuses well within Canon specs at all distances and apertures, but on the 40D and 5D it occasionale missed focus completely and utterly. About 1 in 20 shots or so with the 40D, and a bit less with the 5D, but on 50D that certainly was better (couldn't get it to do it, but only borrowed, never owned one), and I have never had a problem with the 5D II in this regard.



A very happy New Year to you too!



Kind regards, Wim