Autofocus sensitivity/accuracy question

**clackers** · 08-14-2019, 03:55 PM - **1 Like**

Originally posted by kypfer

so whilst waiting to win the lottery I'll be carrying my 70-200mm f/2.8 + matching 2x, which may become my "default" lens

Well, good luck with that. People like WPresto have shown that the Pentax HD 1.4 TC is outstanding and only degrades the image quality by a tiny amount.

A 2x converter is a mixed blessing. Yes, you have now got in theory a 400mm f5.6, so that's okay by the autofocus module, but that extra bit of unexceptional glass getting in the way of the sensor may mean that it's a tossup whether it was better for you to leave it off and just crop in.

kypfer · 08-15-2019, 12:29 AM

Originally posted by clackers

Well, good luck with that. People like WPresto have shown that the Pentax HD 1.4 TC is outstanding and only degrades the image quality by a tiny amount.

A 2x converter is a mixed blessing. Yes, you have now got in theory a 400mm f5.6, so that's okay by the autofocus module, but that extra bit of unexceptional glass getting in the way of the sensor may mean that it's a tossup whether it was better for you to leave it off and just crop in.

No contest there ... all of my image viewing these days is on a computer screen or TV and a lot of my pictures are 100% cropped for final viewing anyway. I'll agree that some of the older converters, that are manual focus and even manual exposure, can be more of a liability than an enhancement, but my experiences with the matching Sigma EX converter on my Sigma 70-200mm f/2.8, purchased new when I bought my original *ist film camera, has always been very satisfactory!

A Sandpiper or similar at 100 yards is not a big subject, so the more pixels I can get covered by subject matter the better

I'm not shooting for National Geographic or even a local contest, I just want to be able to demonstrate a recognisable image and maybe show the differences between two similar species that were present on a particular day or take a record of something unusual or rare.

Recognising the limitations and requirements of my equipment will help me achieve this a little more regularly

**clackers** · 08-15-2019, 01:12 AM

Originally posted by kypfer

I'm not shooting for National Geographic or even a local contest, I just want to be able to demonstrate a recognisable image and maybe show the differences between two similar species that were present on a particular day or take a record of something unusual or rare.

Just ID pictures or recording the experience? Sure.

I have an older Tamron 2x TC I don't bother to put on my Tamron 70-200 f2.8. The Pentax 1.4 works really well on it, though.

beholder3 · 08-15-2019, 01:27 AM

Originally posted by stevebrot

The analogy is useful because of the broad familiarity in photography circles with the spit-image and also because the blackout behavior is similar as the limits are tested. Also demonstrable is the how/why of AF fine adjustment works...too much stuff for discussion here.

I do agree that modern PDAF in essence is very much like the way the human user uses the prism in a split image matte screen. But I do assume few readers here will easily understand why that is.

If one understands the principle similarity it is also easy to understand that the angle at which the two half parts of the split image prisms are made in essence makes them a F5.6 or F2.8 or whatever split image thingy, because it makes them look that far to the edges of the lens.

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08-14-2019, 03:55 PM - 1 Like	#16
clackers Otis Memorial Pentaxian Join Date: Jul 2013 Location: Melbourne Photos: Albums Posts: 16,397	Originally posted by kypfer so whilst waiting to win the lottery I'll be carrying my 70-200mm f/2.8 + matching 2x, which may become my "default" lens Well, good luck with that. People like WPresto have shown that the Pentax HD 1.4 TC is outstanding and only degrades the image quality by a tiny amount. A 2x converter is a mixed blessing. Yes, you have now got in theory a 400mm f5.6, so that's okay by the autofocus module, but that extra bit of unexceptional glass getting in the way of the sensor may mean that it's a tossup whether it was better for you to leave it off and just crop in.

08-15-2019, 12:29 AM	#17
kypfer Pentaxian Join Date: Jan 2018 Location: Jersey C.I. Posts: 3,594 Original Poster	Originally posted by clackers Well, good luck with that. People like WPresto have shown that the Pentax HD 1.4 TC is outstanding and only degrades the image quality by a tiny amount. A 2x converter is a mixed blessing. Yes, you have now got in theory a 400mm f5.6, so that's okay by the autofocus module, but that extra bit of unexceptional glass getting in the way of the sensor may mean that it's a tossup whether it was better for you to leave it off and just crop in. No contest there ... all of my image viewing these days is on a computer screen or TV and a lot of my pictures are 100% cropped for final viewing anyway. I'll agree that some of the older converters, that are manual focus and even manual exposure, can be more of a liability than an enhancement, but my experiences with the matching Sigma EX converter on my Sigma 70-200mm f/2.8, purchased new when I bought my original *ist film camera, has always been very satisfactory! A Sandpiper or similar at 100 yards is not a big subject, so the more pixels I can get covered by subject matter the better I'm not shooting for National Geographic or even a local contest, I just want to be able to demonstrate a recognisable image and maybe show the differences between two similar species that were present on a particular day or take a record of something unusual or rare. Recognising the limitations and requirements of my equipment will help me achieve this a little more regularly

08-15-2019, 01:12 AM	#18
clackers Otis Memorial Pentaxian Join Date: Jul 2013 Location: Melbourne Photos: Albums Posts: 16,397	Originally posted by kypfer I'm not shooting for National Geographic or even a local contest, I just want to be able to demonstrate a recognisable image and maybe show the differences between two similar species that were present on a particular day or take a record of something unusual or rare. Just ID pictures or recording the experience? Sure. I have an older Tamron 2x TC I don't bother to put on my Tamron 70-200 f2.8. The Pentax 1.4 works really well on it, though.

08-15-2019, 01:27 AM	#19
beholder3 Pentaxian Join Date: Dec 2011 Posts: 3,112	Originally posted by stevebrot The analogy is useful because of the broad familiarity in photography circles with the spit-image and also because the blackout behavior is similar as the limits are tested. Also demonstrable is the how/why of AF fine adjustment works...too much stuff for discussion here. I do agree that modern PDAF in essence is very much like the way the human user uses the prism in a split image matte screen. But I do assume few readers here will easily understand why that is. If one understands the principle similarity it is also easy to understand that the angle at which the two half parts of the split image prisms are made in essence makes them a F5.6 or F2.8 or whatever split image thingy, because it makes them look that far to the edges of the lens.