[JohnBee]

This has been on my mind for awhile now.

If the Pentax SMC-AF 1.7x TC was limited to f/4 lenses on average due to AF limitations, does this mean that the new K-5 AF will raise this threshold?

You thoughts...

[Entropy]

This has been on my mind for awhile now.

If the Pentax SMC-AF 1.7x TC was limited to f/4 lenses on average due to AF limitations, does this mean that the new K-5 AF will raise this threshold?

You thoughts...

Probably not, if anything it may get worse. It's a function of lens/AF system physics - the more tolerant an AF system is of small apertures, the less precise it is. (Which is why Nikon has those special f/2.8-only AF sites.)

So if they've improved AF speed and precision, they've likely tightened down the tolerances on minimum aperture size (maximum aperture number).

[Lowell Goudge]

forget about improved AF and the 1.7x.

I think if you look at the specifications, they have not changed the EV requirement for functioning AF and therefore the minimum functioning aperture of the lens will not change either.

It is also very important to note that while everyone here, self included talk about the AF TC working with F4 lenses, the datasheet for the TC claims support for F2.8 and faster only.

Now I don't have the first generation pentax AF, but the PZ-1 works with this TC to F4, which is why I bought it in 1991, I was able to test it and confirm function before I bought it.

The question is, what about the SF-1


I guess the answer is that there might have been a change in EV sensitivity and AF early on, but my perception is not much has changed in 20 years.

[JohnBee]

Okay, I guess there is lots to learn with respect to AF functions.
What about live view AF, you think that would be a way to overcome TC focus limitations?

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Last edited by JohnBee; 09-28-2010 at 08:49 AM.

[Eruditass]

I thought the new AF system was supposed to work at lower EV? Just one of those old rumors?

[JohnBee]

It is also very important to note that while everyone here, self included talk about the AF TC working with F4 lenses, the datasheet for the TC claims support for F2.8 and faster only.

Interesting, I never knew that.

The thing of it is, I've been shooting with an f/3.8 lens lately and i've found the TC to be more or less a hit and miss affair. However when it does work(in direct sunlight) it's pretty sensitive in terms of needing a hard contrast region in order to achieve focus lock.

So I was thinking... if the new K-5 has an all new and improved contrast detect AF, then shouldn't that improve the AF performance with the AF TC as well?

[Lowell Goudge]

Okay, I guess there is lots to learn with respect to AF functions.
What about live view AF, you think that would be a way to overcome TC focus limitations?

Interesting point, this might or moght not work, I have not really looked hard at LV with my K7 to have any idea whether there is an improvement, but I doubt it, this is essentually video AF which is pretty slow in comparison

I thought the new AF system was supposed to work at lower EV? Just one of those old rumors?

the spec for the K5 is -1EV to +18EV, same as my PZ-1

It is also very important to note that while everyone here, self included talk about the AF TC working with F4 lenses, the datasheet for the TC claims support for F2.8 and faster only.

The question is, what about the SF-1


I guess the answer is that there might have been a change in EV sensitivity and AF early on, but my perception is not much has changed in 20 years.

Interesting, I never knew that.

The thing of it is, I've been shooting with an f/3.8 lens lately and i've found the TC to be more or less a hit and miss affair. However when it does work(in direct sunlight) it's pretty sensitive in terms of needing a hard contrast region in order to achieve focus lock.

So I was thinking... if the new K-5 has an all new and improved contrast detect AF, then shouldn't that improve the AF performance with the AF TC as well?

Yes it is SAFOX IX+ compared to SAFOX II on the PZ-1, note it was upgraded from SAFOX VIII+ on the K7 and SAFOX VIII on the *istD

I just looked up the SF1 specifications that were EV2 to 18, so the AF on the PZ1 and later, as I suspected, had a significant upgrade in terms of AF sensitivity, but there has not been much since that time.

Note the SF1 manual also states F2.8 or faster with the SMC-F-1.7x AF Adaptor. If we took F2.8 plus 3 stops that should get us to F* or better native aperture before the AF TC will stop working, buyt experience tells me that the newer systems really don't perform that low, and F4.5 is really the limit, so only 1.5 stops in real functional gain

[JohnBee]

Yes it is SAFOX IX+ compared to SAFOX II on the PZ-1, note it was upgraded from SAFOX VIII+ on the K7 and SAFOX VIII on the *istD

I just had a moment of youth and realized that I never bothered to mention that my question was coming from a "K20D" AF/TC scenario.

Even writing it out, I get the feeling that this will most likely change the nature of many of the answers posted here(or not).

I do apologize for leaving that out of my initial post

[Lowell Goudge]

I just had a moment of youth and realized that I never bothered to mention that my question was coming from a "K20D" AF/TC scenario.

Even writing it out, I get the feeling that this will most likely change the nature of many of the answers posted here(or not).

I do apologize for leaving that out of my initial post

hard to tell but I suspect the changes are probably minimal, as you can see it appears the K7 was an upgrade only marginally on the origonal *istD, which was 17 camera releases earlier.

BTW the K20D also used the SAFOX XIII system. Maybe there have been 2 upgrades since the K20, i.e. one in the K-X and the other in the K7, but the K5 is the same as K7.

[JohnBee]

BTW the K20D also used the SAFOX XIII system. Maybe there have been 2 upgrades since the K20, i.e. one in the K-X and the other in the K7, but the K5 is the same as K7.

I didn't know the K20D used the SAFOX XIII,
However doesn't the K-5 use the + version of this?
If memory serves me right, I seem to remember something about the new AF having drastically improved contrast detection.

[Lowell Goudge]

I didn't know the K20D used the SAFOX XIII,
However doesn't the K-5 use the + version of this?
If memory serves me right, I seem to remember something about the new AF having drastically improved contrast detection.

The K7 uses the SAFOX XIII+ the K5 uses the SAFOX IV+, I cannot find any camera that uses an intermediate between the K7 and K5.

so the K7 is better than the K20, and all other pentax cameras that use the SAFOXXIII, but the K5 is still a further revision

[JohnBee]

The K7 uses the SAFOX XIII+ the K5 uses the SAFOX IV+, I cannot find any camera that uses an intermediate between the K7 and K5.

so the K7 is better than the K20, and all other pentax cameras that use the SAFOXXIII, but the K5 is still a further revision

Okay, sounds great!
I shall hold my breath until then

[Entropy]

I think if you look at the specifications, they have not changed the EV requirement for functioning AF and therefore the minimum functioning aperture of the lens will not change either.

Incorrect assumption here that those two are guaranteed to correlate, although they likely will.

Minimum aperture requirement is determined not by amount of light allowed in, but on physical aspects of the lens that are dependent on aperture regardless of the brightness of the scene. This is why an f/2.8 lens in very dim light (such as the AF illuminator only) will blow away an f/8 lens in bright sunlight as far as AF performance, despite far more than 3 EV difference in ambient light. Think of how split prism focusing screens tend to "go dark" regardless of ambient light past a certain aperture, it's the same principle for phase detect AF.

The "special" f/2.8 focus areas in some Nikons can be thought of like a split prism that diverges far faster than normal - it allows you to see more clearly how far out of focus you are, but will "go dark" faster.

However, what is interesting will be if the K-5's contrast detect AF is decent - this isn't subject to the same "brick wall" limitations as phase detect AF, so might work decently with small aperture lenses (but requiring bright light).

[Lowell Goudge]

Incorrect assumption here that those two are guaranteed to correlate, although they likely will.

You will note I commented in the origonal review of the AF specifications for EV that although the SF1 is specified from EV2 to EV18 and from PZ-1 on from EV-1, the three stop change in EV for the AF system to work only resulted in a 1.5 stop change in the ability of the AF system to use slower lenses, based upon performance of F2.8 lenses with the AF adaptor, but known performance to F4.5 on all the cameras from PZ-1 on.

Now if only someone here has an SF1 to try slower lenses???

conclusion is some relationship but not 1:1

[telfish]

You will note I commented in the origonal review of the AF specifications for EV that although the SF1 is specified from EV2 to EV18 and from PZ-1 on from EV-1, the three stop change in EV for the AF system to work only resulted in a 1.5 stop change in the ability of the AF system to use slower lenses, based upon performance of F2.8 lenses with the AF adaptor, but known performance to F4.5 on all the cameras from PZ-1 on.

Now if only someone here has an SF1 to try slower lenses???

conclusion is some relationship but not 1:1

I have a sf10 and the 1.7 af converter will that do?