Focusing via Live View giving inaccurate result

[Alex]

Hi,

In trying to get to the bottom of why some the edges of my images are out of focus, I thought I'd do some depth of focus tests with my main lenses. With the aid of a well illuminated Spyder LensCal focusing target I used the fine focusing controls in Phocus at widest aperture (120mm at F4), to obtain the true point of focus (@ 5m) from which the depth of focus in front and behind could then be measured.

However after a couple trials the front of the measure rule (+6 cm), always seemed sharper than the focused point (0), and try as I might the sharpness measure graph in Phocus always gave the same result when focusing on the 0 point. So in trying find where the point of focus really was, I edged the the target closer until I found it: 30cm in front of the Phocus's confirmed focus point.

If this was an autofocus issue I'd obviously send it to service for them to recalibrate the autofocus on the camera, but as this has been done via the Live View which is essentially reading off of the sensor and confirming different levels of edge contrast, I'm not sure how this can be fixed. Does anyone have any ideas - am I using the wrong technique/methodology ?

Thanks,

Alex

[NickT]

Hi Alex
I must admit i'm a bit mystified... The Live view is indeed reading directly off the chip so what you see as being in focus should be exactly what you get when you shoot.

The only thing I can think of is that something is changing between you focussing with Live view and shooting.

I don't think it's mirror slap for vibration as that would make the whole image soft.

I wonder is the camera set to auto or Manual focus?

[jerome_m]

Do you stay at f/4 when taking the picture? The focus point changes slightly with the aperture of the lens.

[Alex]

Thanks for the suggestions guys, I've gone back and redone the tests with your points in mind

First though I tried Live View focusing the same lens on another camera body and didn't encounter the same problem - so it must be the camera.

- I went back to the 1st camera - made sure True Focus was off and both the LCD and Phocus was reading MF under focusing.
- After focusing and then subsequently at F4 & F4.8 I achieved the same results as before - focus was out by 30cm:
- I then tried using the camera's autofocus and the Focus target was completely in focus - Crazy!

Problem with this 'solution' is that sometimes Autofocusing is not just possible (central area focal point forced to look at a blank uniform area), and I don't feel comfortable relying on the Autofocus without any way of confirming its result as being spot on.

Do you think I should send it in to Service for checking? - Not sure how they'd approach the issue.

Alex

[NickT]

Alex have you adjusted the diopter on the prism? Adjust it so the LCDs in the VF are sharpest.

I find AF is *always* better than my focussing abilities just FYI

[Alex]

Hi Nick,

Alex have you adjusted the diopter on the prism?

Does changing the diopter on the prism have an effect on the focusing within Live View? I had always previously imagined that its help was for when visually focusing manually with the lens barrel - something I'm too shortsighted to accurately use & only touch when stacking :-)

Thanks,

Alex

[Alex]

No joy with the Diopter adjustment (But wow, it's really effective and well made!), and so have sent some images to service to see what they think.

One small discovery I found is that on my versions (Windows build), of Phocus, the Green focusing doughnut (LV @ 100%) intermittently changes size when in fact it's not supposed to...maybe this may be linked - I need to set the camera up on a Mac version of Phocus - Do you think a 2011 i5 Mac Mini would stand up to Live View for the purposes of this test?

Alex

[Alex]

One small discovery I found is that on my versions (Windows build), of Phocus, the Green focusing doughnut (LV @ 100%) intermittently changes size when in fact it's not supposed to

Upon further testing I've found (using Phocus 2.8.4), that the Green/Orange focusing Doughnut defaults to its correct size (as per the Mac build), if I initially start the Live View just once using the iPhone/iPad App; after that it keeps to the same size - until I change the lens or unplug the camera; then it goes back to being intermittent.
Having the Focusing doughnut ring working at its correct size is seemingly helping find the focusing points more accurately (and quickly).

[NickT]

Thanks for the update Alex.