Why does my Canon EF 50mm f/1.4 focus slower on EOS M than other USM lenses?
Asked 1/7/2018
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On my Canon EOS M3, my EF 50mm f/1.4 focuses noticeably slower than my EF 28mm f/1.8 and EF 100mm f/2.8, even though all three are fast on my 5D Mark II. Since the 5D II uses phase-detect AF and the EOS M relies on contrast-detect AF, is the wider aperture part of the reason? Or is the difference more about the lens focusing motor and internal design than the f/1.4 aperture itself?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
8y ago
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The EF 28mm f/1.8 USM and the EF 100mm f/2.8 USM Macro both have ring type USM, as do the vast majority of Canon's lenses designated as USM.
The EF 50mm f/1.4 USM has a micro-motor type of USM that is pretty much unique to this one lens among Canon's current lens lineup.
In my experience, the EF 50mm f/1.4 is a bit slower than all of my other USM lenses when used on my Canon DSLRs. I've not shot with that lens mounted on any of the mirrorless EOS M models.
As the 5Dmkii uses phase detection autofocus and the M3 has contrast detection AF, is the larger aperture here of disadvantage? As a sharp edge can not be detected so easily with the smaller depth of field?
If the lens is way out of focus when CDAF begins it can take a little longer for the camera to decide which way it needs to go to increase contrast. Does this also happen when using the CDAF (Live View) on your 5D Mark II?
With the smaller form factor (and thus smaller battery), I wouldn't be surprised if the EOS M series provides less power to move the focusing elements of any EF lenses than the DSLR bodies do. It's fairly well known that the EOS 1-series cameras, with their much larger batteries, move AF lenses faster than other EOS DSLRs. The fact that ring-type USM probably takes less power to move a focusing element with the same mass than the micro-motor USM of the 50/1.4 does may be the greatest part of what you are noticing.
Originally by user15871. Source · Licensed CC BY-SA 4.0
user15871
8y ago
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The slower focus is more likely due to lens design and AF drive type than the f/1.4 aperture alone.
Your EF 50mm f/1.4 is unusual among Canon USM lenses because it uses a micro-USM motor, while the EF 28mm f/1.8 and EF 100mm f/2.8 use ring-type USM. The 50mm f/1.4 is often a bit slower than other Canon USM lenses even on DSLRs.
Contrast-detect AF can also make differences more noticeable. CDAF evaluates image sharpness on the sensor and may take longer, especially if the lens starts far out of focus and the camera has to hunt to determine which direction to move. Lens focusing design matters a lot here: lenses that move a small internal focusing group generally focus faster than lenses that move more of the optical assembly.
Aperture can influence CDAF somewhat because depth of field on the sensor affects how quickly peak contrast is found, but the difference between f/1.4 and f/1.8 is unlikely to be the main reason by itself.
So: yes, CDAF can expose focus-speed differences, but in this case the bigger factor is likely the EF 50mm f/1.4’s motor/focusing design rather than its larger aperture.
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