[JinDesu]

I'm looking to get a Sigma 50 F1.4 because of the reviews stating it has fast silent autofocus. I currently have a Sigma 17-70, and on my k-x, it isn't the fastest focusing lens in my collection. My old kit lens (18-55) would beat out the 17-70 in focusing speed. I mention this because in the many reviews of the Sigma 17-70 on this forum, everyone says it has "fast AF".

So - does anyone know how quick the Sigma 50 F1.4 AF is in comparison to the Sigma 17-70? What about in comparison to the DA 35 F2.4 (which is fast as lightning, and knows kung fu fighting)? Or any video showing the Sigma 50 F1.4 focusing on a Pentax camera? I tried to find some youtube videos, but the mass majority were not Pentax cameras.

Thanks in advance

[snake]

I got to demo the 50 1.4 from Sigma, but ended up getting a nice, used F50 1.7 and if there's any difference, it's tiny. To be honest, the F50, despite being old (mine has signs of being nearly unused), is superfast. Maybe the screwdrive adds to the feeling of speed, but it's incredible, really. The Sigma is a nice lens, but it's very large compared to the F/FA 50 lenses and perhaps the only thing it gives over the FA lenses is quiet operation.

I think optical quality is quite good, however and comparable across all. I think the F/FA50 1.7 is sharper across the frame. I know it's not a specific answer, but if you like the speed of the DA35, the F/FA 50 lenses should do right by you, so long as you don't mind the noise.

[JinDesu]

I got to demo the 50 1.4 from Sigma, but ended up getting a nice, used F50 1.7 and if there's any difference, it's tiny. To be honest, the F50, despite being old (mine has signs of being nearly unused), is superfast. Maybe the screwdrive adds to the feeling of speed, but it's incredible, really. The Sigma is a nice lens, but it's very large compared to the F/FA 50 lenses and perhaps the only thing it gives over the FA lenses is quiet operation.

I think optical quality is quite good, however and comparable across all. I think the F/FA50 1.7 is sharper across the frame. I know it's not a specific answer, but if you like the speed of the DA35, the F/FA 50 lenses should do right by you, so long as you don't mind the noise.

Thank you for the info! I have considered the F/FA 50 1.7, but I do want the silent AF (there was a cheap F50 1.7 on sale recently, but I missed out). Optical quality, I've heard that it is as good or better than the FA 50, and as good or just below the DA 55 1.4. Unfortunately, as I want both silent and fast, it looks like the Sigma fits the bill for me.

Just to be clear, when you demo'd the Sigma - the AF speed difference from the FA 50 was tiny?

[snake]

Yeah, it was pretty small and oddly, the F50 didn't hunt as much to lock. I can't give more specifics than that, however, since it was a shorter-duration demo and I didn't push them as much as I wished.

However, I don't think you'd have an issue with the optics of the Sigma. Just maybe the size and stopped down to 1 to 1.5 stops, it's pretty damn good. I know, it's tough to buy a 1.4 lens to stop down, but it depends on what you're using it for and if the sharpness and increased DoF is important.

[JinDesu]

Thanks for all your help. I'll give it a go and see - my biggest worry was because the Sigma 17-70 that I have had a pretty well rated AF system, but it hasn't performed amazingly in my opinion. That's why I wanted to see if anyone had comparisons for the Sigma 50 1.4 against a screw-drive lens.

[snake]

I think screw-drive gets maligned because it's an older system, but far from "clunky". It doesn't have that futuristic sound to it, like "supersonic" or "ultrasonic" or "spaceage", which makes people think the lenses are automatically fast.

In reality, screw drive is phenomenally fast, sometimes faster than equivalents with these supposedly fast motors in them. Just have to deal with the noise.

[JinDesu]

I think screw-drive gets maligned because it's an older system, but far from "clunky". It doesn't have that futuristic sound to it, like "supersonic" or "ultrasonic" or "spaceage", which makes people think the lenses are automatically fast.

In reality, screw drive is phenomenally fast, sometimes faster than equivalents with these supposedly fast motors in them. Just have to deal with the noise.

Yep - I've have good experiences with screw-drive lenses, and their speed is what I will consider my standard, especially with prime lenses. I do use my camera in more quiet environments occasionally (my camera has the beep off too) - so I want to have a few silent motor lenses.

[snake]

You'll probably do ok with the Sigma. To be realistic, when we're comparing AF speed, in many cases, we're splitting fractions of a second, which look huge on spec sheets. It's much more important for AF lock to take place and be correct when you're at this speed range.

If you're interested, check my signature link, particularly the "Christopher Street Day Parade". I did many of those shots with the f50, stopped down often to F2.2, often bumping against the upper limit of the k-5's shutter speed. I honestly think the Sigma would have performed just as well in such a varying, chaotic situation. Basically, one would not have missed any shots with it. It's splitting hairs and in good light, the variance severely narrows. Wide open apertures also tend to create hunting due to narrow DoF, but can work when you have compliant subjects.

[JinDesu]

Thanks again for all your advice!

[Bestzoom]

My Sigma 50f1.4 focuses faster than FA77Ltd. It is accurate in AF even at f1.4..
The Sigma 50 is sharp at f1.4. I don't know how it compares to FA50f1.7. I may get a copy of Pentax 50 f1.7 to compare side by side with the Sigma one day.

The big factor to take into consideration is its bulk. Perhaps bigger than most 50f1.4 from other manufactures. It is also prompt to flare. Otherwise it is very sharp when stopped down to 2.2 or 3.2...

[JinDesu]

I got the Sigma 50 F1.4 today after this discussion. Yes - it's as fast as my screwdrive lenses. Pretty accurate - gonna test for any signs of FF and BF, but so far so good. A bit soft at F1.4, but it was sharp at F2.

Thanks for the advice guys!