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**UncleVanya** · 02-04-2016, 09:04 AM

Originally posted by ffking

I'm not an engineer or electronics whizz by any stretch of any imagination, but would it be that difficult for the focusing mechanism to be linked to the daipragm, and to stop ultr-fast menses down to f2.8 during focusing? - just interested - have no knowledge of practicalities.

In Live View this does seem to happen, however this leads to errors in focus when you are shooting wider open than the lens stops down to during live view focusing since depth of field is increased.

**clackers** · 02-04-2016, 12:41 PM

Originally posted by pathdoc

It would seem odd that Pentax's system can't cope with f/1.4, seeing as both legacy (F or FA) and digital AF glass both exist with this focal length. Whether f/1.2 is just a bridge too far, I can't say, as there is no KNOWN f/1.2 Pentax AF lens currently in existence.

Are any of the brands' cross AF points rated beyond f2.8?

I use a Katzeye with Optibrite on my K-30 with fast manual glass. Spot metering seems unaffected - I don't know why.

**Digitalis** · 02-04-2016, 05:21 PM

Originally posted by clackers

Are any of the brands' cross AF points rated beyond f2.8?

I don't think so, even Canon and Nikon AF modules that can focus at f/8 aren't quoted in the literature as having increased accuracy with lenses faster than f/2.8.

Considering my experience with the Canon 50mm f/1.0L* on the 1DX I can't say there is any difference in precision between earlier models.

*AF lenses this fast are a real test of any canon DSLR's AF system.

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02-04-2016, 09:04 AM	#16
UncleVanya Loyal Site Supporter Join Date: Jul 2014 Photos: Gallery \| Albums Posts: 28,472	Originally posted by ffking I'm not an engineer or electronics whizz by any stretch of any imagination, but would it be that difficult for the focusing mechanism to be linked to the daipragm, and to stop ultr-fast menses down to f2.8 during focusing? - just interested - have no knowledge of practicalities. In Live View this does seem to happen, however this leads to errors in focus when you are shooting wider open than the lens stops down to during live view focusing since depth of field is increased.

02-04-2016, 12:41 PM	#17
clackers Otis Memorial Pentaxian Join Date: Jul 2013 Location: Melbourne Photos: Albums Posts: 16,397	Originally posted by pathdoc It would seem odd that Pentax's system can't cope with f/1.4, seeing as both legacy (F or FA) and digital AF glass both exist with this focal length. Whether f/1.2 is just a bridge too far, I can't say, as there is no KNOWN f/1.2 Pentax AF lens currently in existence. Are any of the brands' cross AF points rated beyond f2.8? I use a Katzeye with Optibrite on my K-30 with fast manual glass. Spot metering seems unaffected - I don't know why.

02-04-2016, 05:21 PM	#18
Digitalis Site Supporter Join Date: Mar 2009 Location: Melbourne, Victoria Photos: Gallery Posts: 11,695	Originally posted by clackers Are any of the brands' cross AF points rated beyond f2.8? I don't think so, even Canon and Nikon AF modules that can focus at f/8 aren't quoted in the literature as having increased accuracy with lenses faster than f/2.8. Considering my experience with the Canon 50mm f/1.0L* on the 1DX I can't say there is any difference in precision between earlier models. *AF lenses this fast are a real test of any canon DSLR's AF system.

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