Canon 24-105 IS is parfocal on 5D3, but not on EOS-M?

[vstrglv]

I have tested Canon 24-105 on 5D3 and EOS-M. The result is very strange.

EOS-M


5D3

• Left picture - Focus length -105mm, manually focused at zoom10 and Varavon viewfinder 3x
• Centre picture - focus length was changed  to 24mm without focus adjustment.
• Right picture- after adjustment

What is the reason?

[dmilligan]

Probably the distance from the end of the lens to the camera sensor is slightly different since the eos-m is mirrorless, and that is throwing off the parfocalness, IDK.

(In the future please use the [img] tag for pictures in your posts, most folks don't like to follow links)

[vstrglv]

As I remember there is no error on the focus length scale on EOS-M, if the 24-105 scale set to infinity focus is at infinity. But I have to check

[dmilligan]

See if the cameras are parfocal to each other. Manual focus on one, take a pic, switch the lens to the other camera and take another

[maxotics]

One is a full-frame sensor, the other an APS-C. 

[dmilligan]

One is a full-frame sensor, the other an APS-C. 

Why would that effect the lens parfocalness?

[maxotics]

Sorry, just guessing.  Most DSLR lenses aren't parfocal (which is a film/video priority).  I know that lens is supposed to be.  But I've read, when wide open, even DSLR parfocal lenses aren't completely parfocal through the whole range.  That lens is made for full-frames I believe.  The adapter to the EOS-M does seem to put the lens further out, like the same distance on a Canon DSLR.  But again, that lens is optimized for full-frame sensors.

Anyway, I'm curious too!

[vstrglv]

See if the cameras are parfocal to each other. Manual focus on one, take a pic, switch the lens to the other camera and take another

I do not own ROS-M, so can check 3 or 4 days later. BTW I used the same 24-105 lens for both cameras. Then I checked another 24-105 lens for both cameras - the same result.

[vstrglv]

I have checked 24-105 on EOSM and 5d3 again today.
1. Focused at 105mm eosm on the moon. Scale position at infinity.
2. Installed this lens at 5d3 without changing focus and focus length. Moon was in focus too.
3. Same like 1. , but focus  distance was 5m.
4. Same result - in focus at 5d3 too.
5.If i change focus length to 24mm i see image in focus at 5d3, but out of focus at eosm
6.In order to have image in focus on eosm, i have to shift focus scale to about 2.5m.
I guess that there is a glass plate or filter in front of matrix and its thickness is too high in EOS-M

[dmilligan]

It's probably because the mount is different. The lens doesn't sit at exactly the same distance from the sensor for each camera. That makes the parfocalness not work since the optical design to make that work expects the sensor to be a very specific distance.

[vstrglv]

If distance is different it is impossible to focus to the same point (infinity for example) and have the same position at the lens distance scale. This takes place for Focus length 105mm.

[dmilligan]

No, not neccessarily, DOF is much larger at infinity than it is close for a given aperture, so it is possible to be within a certain tolerance, still achieve infinity focus, but have the distance ever so slightly different, enough different to notice at very close focus distances where DOF is much smaller and the effect is much more amplified.

Technically speaking, the DOF at infinity is infinite, everything from the hyperfocal distance to infinty is in focus, so checking focus for something that is farther away than the hyperfocal distance doesn't really mean anything.

To really test this you need to use very small distances where the effect of focus is greater and the DOF is shallower. Testing at infinity doesn't mean much, there's a lot more room for error.

[vstrglv]

I have tested ar10m and 3m. The result is the same