Pentax/Sigma 50 f1.4 Comparison

Hello all.

As a newbie and recent recruit to the Pentax fold, I recently bought my first Prime lens, an old Pentax-M 50mm f1.7 from Ebay, in excellent condition. I am now really enjoying the experience of being able to shoot indoors (at this dull time of year) whilst also creating images containing lots of bokeh.

I'm not too upset either at having to manually focus. However, when I roll the rear wheel and zoom in to check for sharpness on the camera's screen, I do notice that I don't always achieve the subject sharpness I'd like. I gather this is not unusual as even tiny head movements (after focusing) can affect this sharpness. I do get lucky sometimes though!

It's therefore possibly time for me to consider buying one of the modern-day equivalents: either the Pentax 50mm f1.4, or the Sigma.

I have to admit that whilst the Sigma is considerably more expensive, it does appear to be a 'better' lens. Perhaps that statement is a tad naïve,
but my question is; does anyone in the forum have experience of using this lens, or indeed, just an informed opinion...unlike myself!

Regards.
Alan Lowe-Jones.

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Camera: K20D.
Lenses: Pentax DA* 55mm f1.4 SDM, Pentax DA* 16-50 SDM, Pentax 18-250 SMC DA, Pentax-M 50mm f1.7, Sigma 70-300 F4-5.6 DG macro.
Flash: Pentax AF360.

"Tiny head movements" affecting "sharpness" is to do with depth of field (DOF) - the amount of depth that is in focus. DOF can be very shallow indeed at large apertures such as f1.7 or f2. You will have the same issues with any lens with a similar maximum aperture.

A sharper lens won't give you an increased DOF at the same aperture. It just means that the part of the image that is in focus will be sharper.

You need to either be careful and skilful with what you put in focus with a shallow DOF, or use a small enough aperture (large enough f number) that all of your subject is easily in focus.

Anyway, Pentax 1.4 lenses are no sharper than what you have already at around f1.8 or f2. I don't know about the Sigma. I hear the new Pentax 55mm f1.4 is sharp at f1.4, but it's a very expensive lens.

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~Pete

Hi, Pete.
Thanks for replying so quickly.

What I wrote may be misleading, as I'm comfortable with how DOF works, and how it becomes even more shallow (and critical) with increased aperture.

My thread is more to do with the benefits of using autofocus on a lens such as the 50mm f1.4 : is autofocus better than manual at achieving sharpness? We have to assume of course that neither the photographer or the subject moves after focusing.

Also, I guess the autofocus is quicker than manual.

My real question to the forum though is just about the Sigma lens itself, to ask if anyone has experience of using it.

Many thanks.
Alan.

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Camera: K20D.
Lenses: Pentax DA* 55mm f1.4 SDM, Pentax DA* 16-50 SDM, Pentax 18-250 SMC DA, Pentax-M 50mm f1.7, Sigma 70-300 F4-5.6 DG macro.
Flash: Pentax AF360.

Sometimes you'll find it easier to achieve sharp focus with AF, other times with MF. I can't be more precise than that because it very much depends on the subjects and your skills.

I find MF sometimes more accurate than AF when the subject is well lit and shows up clearly through the viewfinder - even more so with a Katz Eye focusing screen, but also with the standard screen.

(I have an MF 50mm f1.7 and a AF 50mm f1.4, Pentaxes).

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~Pete

Thanks, Pete.

As this appears to have been a 'slow' thread, I gather there aren't too many Sigma 50mm f1.4 owners within the Pentax Forum. Interesting!

Anyway, I very much appreciate your input about your own experience of the auto & manual lenses. It's enough for me to probably delay a while and just keep going with the old manual lens, which btw, I think is excellent.

Have to see what else I can spend my money on!

Cheers.
Alan.

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Camera: K20D.
Lenses: Pentax DA* 55mm f1.4 SDM, Pentax DA* 16-50 SDM, Pentax 18-250 SMC DA, Pentax-M 50mm f1.7, Sigma 70-300 F4-5.6 DG macro.
Flash: Pentax AF360.

Pentax 50's are regarded as some of (and by some people *the*) best lenses in the business. Given a new FA50/1.4 is cheaper than the Sigma it's probably not as attractive to the average Pentaxian as it it to owners of some other brands.

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Joining the Q

There is an AP test comparing the Sigma to the Nikkor and although they conclude that the Sigma is "better" if you look at the detail it is more prone to flare and has lower contrast.

I suspect I would prefer the Nikkor to the Sigma, and that probably I would also prefer the Pentax for the same reasons.

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Best regards, John

Good test in the BJP link Major flare problems when the sun is just out of shot.... can't be seen through the viewfinder.

shim

To be honest, if you're not getting the results you'd like with the Pentax-M 50mm f1.7 then it's not the lens that's at fault...

If you want to see if AF would help, then try using trap focus. Set your camera to AF-C and it'll only let you take a picture when it thinks the lens is in focus (this only works for manual lenses).

Thanks to all for picking up this thread.

Stevo, I will try 'Trap Focus'; that looks like it could be very useful.
Presently, I've just left the camera on AF-S, although it does beep and give me the red square when, I presume, it thinks it's in focus. I'll try the experiment to see what the difference is.

Shim. Thanks for that link to the review of the Sigma lens. Excellent reading, and I certainly wouldn't be too unhappy now about spending the extra on the Sigma lens.

Changing subjects completely, I followed-up the thread about Jessops selling off stock yesterday, and bought my first flash-gun, the Pentax AF360. I'd been reading the excellent material recently on this forum about flash photography, and had just about made my mind up that I'd go for the AF360 anyway...so it was nice to get it for £121 inc. p&p.

Flash photography, now there's a can-o-worms!

Many thanks to all.

Alan.

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Camera: K20D.
Lenses: Pentax DA* 55mm f1.4 SDM, Pentax DA* 16-50 SDM, Pentax 18-250 SMC DA, Pentax-M 50mm f1.7, Sigma 70-300 F4-5.6 DG macro.
Flash: Pentax AF360.

This is a common misunderstanding. The red square only shows where the camera is focusing, not that it's in focus. Focus confirmation is indicated by the green hexagon at the bottom of the viewfinder. If the green hexagon is steady then focus has been found.

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Best regards, John

The red square shows where and when the camera is focusing. It shows again if and once the camera has focused, with a beep. The camera does not beep if focused is not achieved. It is an indication of focus along with the green hexagon.

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~Pete

The red square shows the position being focused on and it does flash, but once focused it disappears. The green hexagon and indeed the beep confirmation confirm focus is achieved. If the green hexagon continues flashing, then the systemm has not focused properly.

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Best regards, John

Thanks, Pete/John.

I should have known that already! Indeed, I have read that in the camera's manual. Doh!

I feel a bit silly now, but in my defence, I'm trying to take in a lot of info at the moment! The good thing is, I'll not forget this little lesson now.

I shall have another play when I get home tonight.

BTW, John, it's Paul Nemeth I work with, not Paul Jones. I think my subconscious had seen Paul Jones on your camera club's website.

Regards.
Alan.

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Camera: K20D.
Lenses: Pentax DA* 55mm f1.4 SDM, Pentax DA* 16-50 SDM, Pentax 18-250 SMC DA, Pentax-M 50mm f1.7, Sigma 70-300 F4-5.6 DG macro.
Flash: Pentax AF360.

Reporting back to the forum ...

I did some tests last night with my K20D and Pentax-m 50mm f1.7 lens.

No problems I'm pleased to say!

Stevo/John, I found that the 'in-focus' beep/red-square/green hexagon behaved the same regardless of whether the camera was set to AP-S or AP-C. But it was nice to be fully aware of how these 3 indicators worked.

I found the focusing (on the centre of a flower) to be very sharp (zooming using the camera's screen and roller-wheel) although not always! I'm pretty steady at hand-held phtotgraphy, but I guess at f1.7 even the slightest movement is enough to affect the sharpness.

I did all of this using the camera's M mode of course, which I find is a pleasure to use, especially the top display's indication of light-metering/exposure. Also, using M mode, it's very easy to adjust the shutter-speed and the ISO to suit.

Which brings me on to another point: I have found that the ISO can be easily manually set using the OK button and front wheel together. This is a really handy usable feature. Is this common knowledge, as I've NOT found this feature in the manual...or am I just being dim again?!

Regards to all.

Alan Lowe-Jones.

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Camera: K20D.
Lenses: Pentax DA* 55mm f1.4 SDM, Pentax DA* 16-50 SDM, Pentax 18-250 SMC DA, Pentax-M 50mm f1.7, Sigma 70-300 F4-5.6 DG macro.
Flash: Pentax AF360.