Is there a link between focal length and focus speed on telephoto?

[LancsMak]

Curious as to whether what I'm observing is my technique or a fact of lenses. Lens in question is an EF 70-200 f2.8 L Mk2 on an R6 Mk2. When doing pan action shots I consistently have a better hit rate on targets 5m+ away than targets 3m or so away. I'm wondering whether this is down to needing to pan more slowly due to the greater distance/larger arc or whether focus speed performance varies across the zoom range on a telephoto.

If anyone is able to satisfy my idle curiosity I'd be very interested!

Thanks.

 

[Sangoma]

The closer the subject, the further the lens has to travel, ie to change focus from 1m - 3m would take more movement of the lens than changing from 10m to 12m

 

[sk66]

As Steve said... it's called "focus throw;" notice the travel distance between 1.5m to 2m vs 10m to infinity on this focus scale.
You can also just think of it as DoFocus/DoField... it becomes greater the farther away the subject/focus is (i.e. more is already "in focus").

 

[LancsMak]

Thanks both. At least the answer isn't me being s***!

 

[Musicman]

This may or may not apply to you, but the with larger subjects, you have to take into account relative motion of different parts of the subject during the exposure.

If the subject is further away, then it's relatively easy to keep everything in focus during the pan as everything is more likely to moving in the same plane


London General WHV26 by Rob Telford, on Flickr

If it's closer to you, then any relative movement of different (such as the outer corners turning in this example) is going be more visible in the pan, even if the lens is in focus (on the nearest corner here) perhaps even with sufficient depth of field to cover the whole subject

London General SE279 by Rob Telford, on Flickr

The further away the subject is, and the longer the focal length you are using, the less this will impact the image.

 

[sk66]

If the subject is further away, then it's relatively easy to keep everything in focus during the pan

This has more to do with magnification... if the subject is farther away then the magnification is less, relative speed is less and DOF is greater. But if you compensate by using a longer lens, then magnification increases, relative speeds increase, and DOF decreases, to the same degree.

Similarly, cropping in post or using a crop sensor is also more magnification (for same size output); and has the same effects.

 

[LongLensPhotography]

Would this be at 200mm and perhaps at f4 or 5.6? They have wild focus shift on long end when stopping down. Better off just using wide open or all the way up at f8. It is at least reasonable performer on r6 for the most part, but don't bother trying on higher Res bodies!

 

[LancsMak]

F3.2 typically

 

[Phil V]

Thanks both. At least the answer isn't me being s***!

The same phenomena happens with very fast shorter lenses, moving bigger bits of glass is more difficult than moving smaller bits - ought to be obvious.

 

[LancsMak]

The same phenomena happens with very fast shorter lenses, moving bigger bits of glass is more difficult than moving smaller bits - ought to be obvious.

Thanks but since I have no understanding of the torque of the USM it wasn't...

 

[Phil V]

Thanks but since I have no understanding of the torque of the USM it wasn't...

Nowt personal at all - but it's not about precise specifications.

You don't need to be an F1 engineer to understand that a double decker bus wont accelerate like a hot hatch Even though there's actually more power

 

[sk66]

Thanks but since I have no understanding of the torque of the USM it wasn't...

There's also a wide variety of focus motors in use; even "USM" types (ring, micro, nano, etc)... without doing a bit of digging it can be hard to know exactly what you are comparing.