General Lens Question

[colinbm]

I am trying out some of my Sigma SA lenses on the Sigma fp.

One lens the 18-200 was not happy on the Sigma SD15, but it is shining on the Sigma fp, even thought it has some vignette it is suitable for square or cropped panos.

The other lens the 105 macro was beaut on the SD15, but misses focus on the fp ? Why would that be ? The Sigma fp is contrast focus ?

Is adjusting shims in the lens mount going to help ?

[Andrea B.]

Don't some Sigma lenses have associated firmware? Maybe either the lens or camera needs a firmware update? Just a thought.

[colinbm]

Don't some Sigma lenses have associated firmware? Maybe either the lens or camera needs a firmware update? Just a thought.

 

Thanks Andrea, these are just before that can in....

[dabateman]

That's not too surprising. A lens designed for a PDAF system was kind of like a once and done focusing system. The weight and size of the focusing element matters but not nearly as much as it matters for a CDAF system. Also the focusing motors are designed differently.

 

For CDAF you pulse the element rapidly back and forth to find optimal contrast. You don't know direction, so you need to move in both directions rapidly to find it.

An old PDAF lens, motor might stop as fail safe to avoid burn out or too much power drain. Many Tamron lenses have this problem or will suck up too much power trying to find best focus. There is a warning on the Metabones site about them from early adaption.

 

One way you can help that macro, which no doubt has a slow autofocus motor, is to half press the shutter at least 3 times. Kind of like AF OCD. Get near, release, get better, release, ete. This is something I still do from habit, as my older four thirds lenses are similar and weren't designed for CDAF.

[colinbm]

That's not too surprising. A lens designed for a PDAF system was kind of like a once and done focusing system. The weight and size of the focusing element matters but not nearly as much as it matters for a CDAF system. Also the focusing motors are designed differently.

 

For CDAF you pulse the element rapidly back and forth to find optimal contrast. You don't know direction, so you need to move in both directions rapidly to find it.

An old PDAF lens, motor might stop as fail safe to avoid burn out or too much power drain. Many Tamron lenses have this problem or will suck up too much power trying to find best focus. There is a warning on the Metabones site about them from early adaption.

 

One way you can help that macro, which no doubt has a slow autofocus motor, is to half press the shutter at least 3 times. Kind of like AF OCD. Get near, release, get better, release, ete. This is something I still do from habit, as my older four thirds lenses are similar and weren't designed for CDAF.

 

Thanks Dave, I understand & will try..