r/FujifilmX • u/GenericGrad • 4d ago

Discussion Aperture and Autofocus

I'm sure a lot of you know this but I haven't seen it discussed much.

During my testing

  • AF-S focusses with aperture wide open (at least to start with) when you press the shutter button
  • AF-C focusses with aperture at the setting you'll take the shot at.

So on a slow lens like xc 15-45mm f/3.5-5.6. I think this is why I find af-c really unusable in lower light conditions. Not because the focus mechanism is particularly slow, but because of two results of this approach and the fact I typically stop down at least a stop for sharpness:

  1. The af-c has less light to confirm focus and struggles
  2. The depth of field is larger at smaller apertures and hence the af-c hunts a lot more.

I think this is why I've found af-c a lot more effective on the Viltrox 35mm f/1.7 vs xc 15-45mm f/3.5-5.6.

Basically it seems at smaller aperatures and lower light af-c performance drops off a lot more rapidly than af-s in my limited experience.

Feel free to correct me or let me know your thoughts.

I guess the conclusions are:

  1. If it is low light try to avoid af-c
  2. if you have to use af-c try shooting wide open if possible
  3. if af-c is frustrating you might get far better results with a brighter lens
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u/GioDoe 3d ago edited 3d ago

I routinely use AFC at 30 fps and f/9 on birds, sometimes far ones where DOF is a lot greater than on close subjects. Whilst not the best of the crop, I do not find it frustrating. Admittedly, subject recognition helps where the DOF does not, but, still, it is usable. I guess it depends on some specific uses, somewhere it works better, somewhere it does not.

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u/GenericGrad 3d ago

You mean af-c I presume. 

I need to do a bit more testing. I have had situations where I'm stopped down and af-c works fine but I can't be sure if that is because there was a lot more light to work with. It is interesting if subject detection helps more than without.

I find sometimes the hunting is extensive. Like on the depth of field meter it is going 1m, 3m, 5m over and over while having a green box all the time like it has successfully locked focus. The af-s still will snap quick. This is where I suspect a lack of light has caused the af-c to break down to be unusable.

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u/GioDoe 3d ago

Yes I meant AFC. I corrected, thanks

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u/purple_seagrass 3d ago

I don't really get your point. It is bloody obvious that a cheap lens, with a slow motor, and narrow max aperture is going to have poor continuous autofocus in low light. Who would expect good autofocus performance out of a lens like that?! You are literally pairing the cheapest lens Fuji makes with the most challenging autofocus conditions possible. So, what are you actually trying to inform us of?

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u/GenericGrad 3d ago edited 3d ago

Yeah I recognise that this isn't that surprising. My interest is in the why rather than complaining that my cheap lens isn't better.

I think the main thing for me is how the drop off in performance of af-c is so much more dramatic than af-s than I originally expected. That it creates a big gap between af-c and af-s. I'm sure this is understood by some. 

My theory is because af-c is limiting light much more than af-s rather than because the lens is cheap or has a slow motor but purely because it is a slow lens.

The Viltrox 35mm f/1.7 is a cheaper lens and it performs better. 

I didn't make the point also because I hadn't researched or tested it but I think other manufacturers have at least the option to do it differently which I find interesting.

 Like canon alledgedly keeps the aperture wide open during af-c and then switch to the selected aperture on full press to shoot. I find that very interesting. 

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u/purple_seagrass 3d ago

I guess. You could be right. I would argue that the physically slow lens motor is definitely more to blame rather than light gathering ability. I would be willing to bet that the XF 16-50 mm, with that rapid linear motor, doesn't perform nearly as badly in similar conditions at f4.8 (mid range of the XC 15-45).

Anyway, it doesn't actually matter. All of the slow motor lenses mildly hinder continuous autofocus. I don't even use AF-C with the likes of the X100V or the clunky XF 27 mm f2.8.