[bigbaz]

Pentax KP with 55-300 PLM with 1.4x Aw rear convertor or Sigma 120-400mm f4.5-5.6 APO.

Your thoughts please, I know there’s obviously a great reduction in both weight and size sticking with Pentax but is that way optically better and what are the advantages/ disadvantages of either option.

I already own the 55-300plm and the Sigma and I’m now seriously considering the Pentax rear convertor option.

[BigMackCam]

The problem with using the 1.4x rear converter with the 55-300 is that you'll lose a full stop of light. Given that the 55-300 PLM is already pretty slow at the long end (f/6.3 without the converter), you'll need very good lighting conditions to use auto-focus reliably. In anything but the best light, you'll also start pushing the ISO up, especially if you want to use reasonably fast shutter speeds. The KP will handle this better than most cameras, but it still means you'll lose some image quality due to noise...

[pepperberry farm]

don't do the rear converter, save your money for a serious 300mm+ prime.... if you aren't happy with the IQ of either zoom at that distance, adding a teleconverter is not going to help....

[kypfer]

I'll agree with BigMacCam ... a teleconverter on an f/6.3 is pushing it!
OK in good contrasty light with a static subject. With a moving subject, especially in flatter light, virtually no chance using autofocus.
I use a Kenko 1.5x with my Sigma 150-500mm f/5-6.3 and it does work well under ideal conditions, but not something I'd want to rely on ... just can't afford anything better
Given the OP's options, I'd stick with the Sigma, especially given the price of the Pentax teleconverter!

[TER-OR]

Watch for the DA300 - it's a fantastic lens. Occasionally Pentax runs a discount via the stores like B&H. Alternately you can look for a used one.

[UncleVanya]

If zoom is required, look for a used copy of the Sigma 100-300 f4 or the Pentax Da* 60-250 f4, both work reasonably well with a teleconverter.

If prime could work, then look for any Pentax 300 (F*, FA*, DA*)

[ChristianRock]

If you're really serious about your telephoto needs, consider saving up for the DFA 150-450mm Pentax lens... yes it's expensive, even used, but probably similar to buying something like a DA*300mm plus the Pentax teleconverter... and it will give far more flexibility.

[swanlefitte]

There are other posts where I make my case that the 1.4x isn't worth it for the 55-300 by itself compared to other options. Auto focus is not a reason to me.
I don't know how many of you have used the 1.4x, 55-300 combo but it auto focuses great until it is pretty dark. I don't have problems until I need f/9.5 1/45 iso 800 under low contrast. this duck picture focused fine on the mothers eye.mother reunited with babies after 2 days - PentaxForums.com It was shot at 9.5 1/125 ISO 800 and pushed +2.40 in post. The photo is as bright as it looked in real life. The autofocus just worked.

It failed in this picture of a catbird. I was even darker and there is a tangle of branches and a fidgety bird.We are talking f10, 1/250 Iso 800 pushed +2.65 in post. I was in a forest with low light and the bird was in brush deeper in the low light with me handheld trying to keep focus on a moving eye between branches. I could not identify the bird at the time because it was a shadow. The top of the head color couldn't be seen.

[mikesbike]

Pentax KP with 55-300 PLM with 1.4x Aw rear convertor or Sigma 120-400mm f4.5-5.6 APO.

Your thoughts please, I know there’s obviously a great reduction in both weight and size sticking with Pentax but is that way optically better and what are the advantages/ disadvantages of either option.

I already own the 55-300plm and the Sigma and I’m now seriously considering the Pentax rear convertor option.

You are better off getting a suitable 1.4x TC matched for the Sigma, which should work well enough. However, if you like the compact lightweight aspect of the DA 55-300mm PLM along with the compact KP design, as well as its very good AF performance, it delivers very fine results. But with this lens you'd be better off to just crop your images rather than trying with a TC. The IQ of the KP is such you can get this degree of cropping without significant IQ loss, less so than with a TC.

[swanlefitte]

The IQ of the KP is such you can get this degree of cropping without significant IQ loss, less so than with a TC.

I have tested this twice now. Today in fact. The 55-300 PLM +1.4x tc aw testing extreme detail - PentaxForums.com
With my ks-2 under static conditions the tc is slightly better. In real world with slow speed and moving elements that goes away. The combo is better than not in many ways. It is not better enough to cost double either alone. The plm is worth the price, the combo is worth about 1.2x that price not 2x.

[BigMackCam]

I've used the 1.4x converter on my HD DA55-300 screw-drive model with some success - but it begins to struggle in lower light where my DA*60-250 f/4 continues to work perfectly, especially when the AF target doesn't have a lot of contrast. That's the thing - I'm not saying autofocus won't or doesn't work with the proposed combo, but that it's not an especially good idea to couple the converter with a slow variable-aperture zoom lens. The amount of light required for speedy and successful AF operation is greater with the 1.4x converter than without, regardless of the lens used... and, as the light level drops, AF will (obviously) struggle earlier with a slower lens than with a faster one.

NB. The OP lives in the same region of the UK as me where, for a large part of the year, we don't often benefit from clear, sunny skies. It's a beautiful, bright and largely-cloud-free morning as I write this, but such conditions aren't common, even in summer. Often, we have overcast skies with thick cloud that steals a lot of light, and at winter's peak there's just seven hours between sunrise and sunset...

[swanlefitte]

I've used the 1.4x converter on my HD DA55-300 screw-drive model with some success - but it begins to struggle in lower light where my DA*60-250 f/4 continues to work perfectly, especially when the AF target doesn't have a lot of contrast.

Well you got me wondering so I decided to test it. My results are here. Interesting results that leave plenty of questions. The 55-300 PLM +1.4x tc aw testing extreme detail - PentaxForums.com

[bigbaz]

I've used the 1.4x converter on my HD DA55-300 screw-drive model with some success - but it begins to struggle in lower light where my DA*60-250 f/4 continues to work perfectly, especially when the AF target doesn't have a lot of contrast. That's the thing - I'm not saying autofocus won't or doesn't work with the proposed combo, but that it's not an especially good idea to couple the converter with a slow variable-aperture zoom lens. The amount of light required for speedy and successful AF operation is greater with the 1.4x converter than without, regardless of the lens used... and, as the light level drops, AF will (obviously) struggle earlier with a slower lens than with a faster one.

NB. The OP lives in the same region of the UK as me where, for a large part of the year, we don't often benefit from clear, sunny skies. It's a beautiful, bright and largely-cloud-free morning as I write this, but such conditions aren't common, even in summer. Often, we have overcast skies with thick cloud that steals a lot of light, and at winter's peak there's just seven hours between sunrise and sunset...

Thank you everyone for your input, going on what you’ve said I’m going to stick with my Sigma for the time being. Ideally I would like the DFA 150-450 but that’s going to take a lottery win.

BigMacCam is correct about the weather here in the North East of the UK, and even my KP a with the 55-300plm can struggle at times.

Once again thank you all

Barry

[Des]

The OP lives in the same region of the UK as me where, for a large part of the year, we don't often benefit from clear, sunny skies.

Masterful understatement Mike!

I have the Kenko 1.5x TC with pz contacts. AF works using this with the PLM in ideal conditions, but even in this part of the world conditions for photographing wildlife aren't always ideal. When I have used it I have wondered whether I gained anything compared to just the PLM and cropping.

The comparison by @Swanlefitte is interesting. See also comparisons posted by @Normhead between TC and cropping. My takeaway from these would be that a TC can capture more detail with a fast sharp lens in certain situations, but the returns diminish when the lens is less fast or less sharp. Looking at Chris's test I would say that there is some gain in detail from the TC + PLM at 300mm v 300mm and cropping, but it is pretty marginal. There was more gain when he backed off to 210mm, because the PLM has more resolution there. But I would expect that a good 300mm prime would have done better with or without the TC.

Given the cost of the DA converter, personally I would put the money towards a good 300mm prime instead. I have the FA*300 f4.5 and it's a very sharp lens - around $A800 here, compared with around $A600 for the TC. Or consider selling the Sigma and getting the DFA 150-450.

[jgnfld]

Testing on the fine print on some storage tanks a mile and a half away, I can detect zero additional detail with the teleconvertor over simple cropping. This has been true with my former K-50 and K-70 and remains the same on the KP.