[MDRatliffe]

I've seen it said that autofocus doesn't work for a 1.4x past a aperture of f/4 and so on, but is that a hard limit, or can good strong lighting still allow for it? I don't expect fully sharp images, or fast autofocus, I just want to get an idea as to whether autofocus could work on a f/5.6 lens, under conditions like good sunlight or if I should think about a cheap manual one to mess with.

[Adam]

I've seen it said that autofocus doesn't work for a 1.4x past a aperture of f/4 and so on, but is that a hard limit, or can good strong lighting still allow for it? I don't expect fully sharp images, or fast autofocus, I just want to get an idea as to whether autofocus could work on a f/5.6 lens, under conditions like good sunlight or if I should think about a cheap manual one to mess with.

In good lighting it should be possible only at the expense of speed, especially if you use a modern body that can otherwise focus in -3 or -4 EV (K-3 III, K-3, K-1, K-5 II, etc.).

[bertwert]

Some people use a TC on the 55-300 which is a third stop slower at f/6.3 with great results, so it's possible. I believe @normhead; is one.

[beholder3]

I've seen it said that autofocus doesn't work for a 1.4x past a aperture of f/4 and so on, but is that a hard limit, or can good strong lighting still allow for it? I don't expect fully sharp images, or fast autofocus, I just want to get an idea as to whether autofocus could work on a f/5.6 lens, under conditions like good sunlight or if I should think about a cheap manual one to mess with.

My experience is that Pentax DSLR cameras with their F5.6 sensors can function down to "wide open" F9 lens/converter combos.
But that needs bright and contrasty subjects (due to the effective area of the remaining AF sensor now being so small; I guess the size of the AF cross then is less than 1/3 of the full thing and within that tiny area there needs to be a good contrast edge).

[MDRatliffe]

In good lighting it should be possible only at the expense of speed, especially if you use a modern body that can otherwise focus in -3 or -4 EV (K-3 III, K-3, K-1, K-5 II, etc.).

Alright thank you, I'm not to concerned about autofocus speed, more that it can work for simplicities sake. I'm using the K-S2, so assuming it would be fine since it's newer than the K-3 and K-5 II

[Wasp]

I have the SMC Pentax 1.7x AF converter. It works fine with my SMC Pentax A 400mm f/5.6, as long as the light is good.

[Adam]

Alright thank you, I'm not to concerned about autofocus speed, more that it can work for simplicities sake. I'm using the K-S2, so assuming it would be fine since it's newer than the K-3 and K-5 II

The K-S2 has the K-5 II's AF system, so yes you should be in good shape.

Adam
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[vector]

It can also help if you have a quick shift lens to manually get the focus into the ballpark before pressing the AF button. I have done that with stacked 1.4TCs which generally doesn't work but will if you help it out by getting close first.

[Paul the Sunman]

What is the experience with the DFA 150-450 and the 1.4 DA TC? I guess it would work with the K3iii in decent light.

[5ks]

What is the experience with the DFA 150-450 and the 1.4 DA TC? I guess it would work with the K3iii in decent light.

This is what interests me a lot, too (not quite yet being able to try myself...). Depending mostly on the amount of light, I guess?

[kypfer]

I use my Sigma 150-500mm f/5-6.3 with a Kenko 1.5x tele-converter, in AFS and central spot focussing, with good light on a contrasty static subject, with little problem, provided I can wait for the focus confirmation to "lock", which can take a couple of tries sometimes.
My K-3iii is better in this regard than my KP, the K-70 often struggles (all with the latest firmware).
Hoping to get a bird in flight or similar with the 1.5x fitted is relying on a very great lump of luck, though it can happen sometimes
As mentioned, quick-shift focus and/or manually pre-focussing can be a great help … the ergonomics of the lens being a factor here. My "bigma" isn't optimally designed to be hand-held and manually focussed!
To be honest, the resultant image can be very similar to a heavily cropped image without the tele-converter on a 24 megapixel camera, in some circumstances, though this may be due to heat haze or shimmer (over water, especially).
For the price of the "genuine" Pentax item I really would like to "try-before-I-buy", though I would expect it to perform better than my "budget" option, especially on a Pentax lens.
The main criteria for reliable results seems to be good contrast, rather than necessarily bright light, though, obviously, in lower light conditions, high ISO noise may become a factor if the shutter speed needs to be kept up.
Good luck

[Bob 256]

It all boils down to the camera and lighting conditions. I've used my K-1 with a x2 TC and it auto focuses satisfactorily when there's enough light available. As the light decreases, AF can become sluggish or quit altogether. Some cameras need more light than others. In fact, the K-1 has issues if the contrast AF is used and a slower maximum aperture lens is attached, but the phase (viewfinder) AF is much better. I doubt however that you will have any issues with an f5.6 lens and a x1.4 TC in bright daylight. Note that you'll need a coupled TC which will open the lens all the way while obtaining a focus. If the lens stops down for any reason during AF (e.g., manual aperture and set at f11), then it may not work.

[alfa75ts]

What is the experience with the DFA 150-450 and the 1.4 DA TC? I guess it would work with the K3iii in decent light.

This is what interests me a lot, too (not quite yet being able to try myself...). Depending mostly on the amount of light, I guess?

I just tried it, it's currently a solid 'neutral grey' overcast day in Melbourne and it worked fine on the K3 (maybe a little slow) even in darkish shadows. The K3-iii was even better, as expected.

[Rider0974]

I've just had a quick test with my camera.
K1 ii, FA28-70MM F4, Kenko DG P/AF Teleplus MC7 converter.
Indoors at 28mm F10 ISO400 AF set to spot auto auto focus works but I can't
hand hold for 5 seconds without moving a little, so the image is more than a little blurry.

[35mmfilmfan]

If my memory serves, the intended use of a TC is on a telephoto lens - short range zooms lenses are not considered ideal.