Photons to Photos Optical Bench and Focus Breathing

[RobertOToole]

For some focus-by-wire brands, there may be a way to get even higher repeatability and precision than the best mechanical lenses. Some brands of camera have tethering interfaces that allow single-microstep changes in focus distance. Thus you can get essentially perfect repeatability by focusing all the way to the lens' closest focusing distance, and then stepping the focus backwards (while counting the steps) until you get perfect 1:1. This will be repeatable by using the same number of steps and the same starting point every time (unless the temperature is very different). The Panasonic Image App works this way, and it is wireless, so you can do it from your phone without touching the camera. This makes it very precise, and the camera doesn't shake as you focus, making it even more precise (particularly with long lenses).

Thanks for that Lou, interesting!

You gave me an idea, with Sigma S or Sports lenses you can add the Sigma lens dock and on the Sigma app you there is a AF adjustment with a scale from speed on one side to precision on the other (I forgot the actual labels), but it slows the AF speed down for more precision. I wonder that would improve the MF function? I should email the Sigma service manager.

Best,

robert

[RobertOToole]

For some focus-by-wire brands, there may be a way to get even higher repeatability and precision than the best mechanical lenses. Some brands of camera have tethering interfaces........

That was easy :

[physicsmajor]

Really cool tool.

Some very interesting optical designs are in here, for example you can see how the Zoom Micro-Nikkor 70-180mm manages to focus from infinity to minimum distance with constant effective aperture at a given zoom (and no focus breathing in actual use). I always wanted to see how they managed this.

https://www.photonstophotos.net//Genera ... sO,OffAxis

[Lou Jost]

Robert, that's a great observation! I have one of those docks and I will try to use it. Unfortunately my Sigma lenses are a mix of Nikon, Canon, and Leica L mounts (one of the joys of mirrorless cameras is the ability to use almost any lens), but my dock is just for nikon.....

[rjlittlefield]

Really cool tool.

Some very interesting optical designs are in here, for example you can see how the Zoom Micro-Nikkor 70-180mm manages to focus from infinity to minimum distance with constant effective aperture at a given zoom (and no focus breathing in actual use). I always wanted to see how they managed this.

The part about constant effective aperture has a simple explanation: the physical aperture and all refracting elements behind it are fixed in position with respect to the sensor. Any lens having that property will have exactly constant effective aperture, because of no change to the exit pupil.

The part about no focus breathing is more challenging. To achieve that requires that the focal length changes in exactly the right way as focus moves. The Micro-Nikkor does not quite achieve that, as you can see in the simulator by watching the Angle of View as focus is shifted from min to max. It is very close, though.

--Rik

[RobertOToole]

Robert, that's a great observation! I have one of those docks and I will try to use it. Unfortunately my Sigma lenses are a mix of Nikon, Canon, and Leica L mounts (one of the joys of mirrorless cameras is the ability to use almost any lens), but my dock is just for nikon.....

Got an email a few minutes ago, the Sigma Mgr says only L-mount has that feature as far as he knows. No USB dock for E-mount also so thats a dead end. But it sounds like that full time MF setting would effect how the focus ring responds, it might help with the delay issue.

Best,

Robert

[Lou Jost]

Actually the L cameras have a menu item that can help with this behavior. You can choose a setting that makes the focus depend only on the position of the focusing ring, not on its rate of rotation.

[RobertOToole]

Actually the L cameras have a menu item that can help with this behavior. You can choose a setting that makes the focus depend only on the position of the focusing ring, not on its rate of rotation.

Nice feature!

The Sigma optimization software only allows changes to sensitivity of the ring input to stop AF and go into MF control. Doesn't sound as good as the L-mount solution.

Best,

Robert

[JKT]

I heartily agree about the Sigma Art macro lenses. I have the 70 and the behaviour is similar. If you don't care about the exact magnification, you can focus by fine tuning the distance, but when that is not an option things get difficult. Additionally Sigma claims non-combatibility with Canon TC:s. Actually manual focus seems to work with at least EF 1.4x II and EF 1.4x III. However, with RF 1.4x even the manual focus won't work. That was an unpleasant surprise, which speeded the acquisition of EF 1.4x III.

Thanks for the TC info JKT.

As usual things are not quite that simple with third party options. The exact combination that DID NOT work was Canon EOS RP, RF 1.4x/2x, Commlite basic RF to EF adapter and Sigma 70 mm Art Macro. It turns out that the problem was the basic Commlite. With the control ring version at least manual focus does work. It would likely work with the Canon adapters too, but you'll need an extension tube between the TC and the adapter.