Autofocus Mirror/Cat Lens with Adapter: Worth Trying?

Started 3 months ago | Discussions thread
ProfHankD Veteran Member • Posts: 7,860

xaprb wrote:

I’m curious if anyone has stacked a manual-focus mirror/catadioptric/reflex lens onto an autofocus adapter like one of the Leica M-mount to Sony E-mount adapters, and how it works.

Yup. Both Samyang 500mm f/6.3 and Spiratone 300mm f/5.6.

They work great, but you're just auto-tweaking manual focus. 4mm of movement is nothing for long mirror lenses.

Theoretically, I think a lot of the cat lenses need to focus by moving elements relative to each other, but in practice I wonder how well it’d work to move the whole lens.

Works fine, and would no matter what for such long focal length lenses because 4mm movement is so small.

The other thing I’m curious about is focusing near infinity. That’s where it seems to get really difficult with the cat lenses, because the adjustments need to be so delicate. Do these autofocus adapters have the precision required for that?

Yup. At least the TechArt Pro LM-EA7 does on my A7RII.

Speaking of that, I think the close-focus range is often quite a range of movement; maybe the adapters don’t have the travel required for that. Can they be set to limit to focus from, say, 50m to infinity? Then you could set the lens to inifinity and let the adapter twitch back and forth a tiny bit.

Again, just 4mm AF movement range. That's just a tiny tweak on manual focus.

I don’t have one of these adapters, so this is all theoretical for me, but something I’m interested in. If it seems promising, or if someone has experience or could test this, I’d love to know how it works. I could also get one of those adapters and try it myself if that doesn’t seem like a fool’s errand.

As described, they work well. However, the only truly autofocus mirror lens is the Minolta/Sony 500mm f/8 AF Reflex, and it really does autofocus on an LA-EA4 on my A7RII. You really only get the center AF point in the SLT adapter with the AF reflex lens, but it's pretty quick and accurate too. Major disadvantage is that the AF reflex lens doesn't focus very close, while the Samyang gets near macro range. I'm still not sure if the IQ of the Minolta or Samyang is better; the Minolta seems better when manually focusing the Samyang, but using the LM-EA7 to tweak focus, it's a pretty close race.

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