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09-05-2011, 06:50 AM   #1
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Sigma 18-50 F2.8 worth it over the Kit lens ?

I have the pentax K-x kit lens 18-55 F3.5-5.6. Is the Sigma lens worth paying $325+ over it ?

I read the reviews, seems to me that it has same or more barrel distortion at 18mm and is not that sharp at 18mm.

09-05-2011, 06:59 AM   #2
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QuoteOriginally posted by StDevious Quote
I have the pentax K-x kit lens 18-55 F3.5-5.6. Is the Sigma lens worth paying $325+ over it ?
IMO not worth.

QuoteQuote:
I read the reviews, seems to me that it has same or more barrel distortion at 18mm and is not that sharp at 18mm.
Distortion at 18mm is relative small, but consider that it is possible to correct Pentax image geometry by the camera itself. Not sure if this is possible with a Sigma lens. You gain a f-stop but you loose 5mm over 50. Depends very much on your shooting style if you are upgrading or downgrading yourself with this.
09-05-2011, 07:01 AM   #3
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Yes, it's worth it. Having f/2.8 available at all focal lengths allows you to get a more shallow depth of field, and to use faster shutter speeds and/or lower ISO in low light conditions.
09-05-2011, 07:07 AM   #4
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Hi Devious,

I know it is worth spending $$'s on a BETTER lens, also you can have a look at the even better lenses.
In the end, my (expensive) experience is that the quality of the lens matter much more than the quality of the camera.
Spend your dollars / euro's or whatever on glass more than on bodies.

Also, make sure to fine calibrate your camera to your lenses.
I have the DA*16-50mm and looking at the results on a big 23" screen, there is *so much difference* with my std 18-55mm or Tamron 18-250mm!

I'd advise you to even take it a step further and look at some more expensive lenses.

Why don't you have a look at the review of some of them to make a better judgement for yourself?

Have look here at lenstip (some Polish guys that know what they are doing):

Your choice:
LensTip.com - lens review, lenses reviews, lens specification - Lenstip.com

Or an even better option (my preference) the new HSM OS 17-50mm f2.8:
LensTip.com - lens review, lenses reviews, lens specification - Lenstip.com

Or the Tamron option, the 17-50mm f2.8 screwdrive lens:
LensTip.com - lens review, lenses reviews, lens specification - Lenstip.com
This Tamron is the cheapest, very good option!

There are other lens review & test sites to check, like slrgear.com:
The Sigma's
Sigma Lens: Zooms - Sigma 18-50mm f/2.8 EX DC Macro (Tested) - SLRgear.com!

Sigma Lens: Zooms - Sigma 17-50mm f/2.8 EX DC OS HSM - SLRgear.com!

The Tamron
Tamron Lens: Zooms - Tamron 17-50mm f/2.8 XR Di II VC LD Aspherical IF SP AF (Tested) - SLRgear.com!

Although the Tamron is cheap, I prefer the quiteness of SDm or HSM lenses.
The K-5 plus a SDM/HSM lens is a stealth combination,

Cheers, Bert


Last edited by bymy141; 09-05-2011 at 07:13 AM.
09-05-2011, 08:05 AM - 1 Like   #5
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Here's a real world example of why f/2.8 is important. Let's say you're wanting to take a portrait indoors. You'll probably be at the long end of your zoom lens, since that is where portraits are most flattering, and where you don't have to be right in your subject's face. With the kit lens, that will put you at f/5.6. But with the Sigma 18-50mm f/2.8, you can still be at f/2.8 at the long end.

To demonstrate the advantage of f/2.8, I just snapped the following picture of my daughter at the kitchen table. The lens was the Tamron 28-75mm f/2.8 mounted on my Pentax K-x. Light was coming in through the back door. Settings were as follows:

50mm
f/2.8
1/40 sec
ISO 200

You can click on the image if you'd like to open the full-size image.



If I were using the kit lens at 50mm, my aperture would be f/5.6...two stops less than f/2.8. So in order to get the proper exposure with the smaller aperture, I would have to compensate by lowering the shutter speed, or raising the ISO, or some combination thereof. In this case, that would mean lowering the shutter speed to 1/10 sec, or raising the ISO to 800. In order to demonstrate the effects of these compensations, I took pictures at f/5.6, and made the required compensations.

Here is a 100% crop from the f/2.8, 1/40 sec, ISO 200 image:



And here is a crop from the f/5.6, 1/10 sec, ISO 200 image. As you can see, the slower shutter speed has resulted in a soft image due to subject and/or camera movement.



And here is a crop from the f/5.6, 1/40 sec, ISO 800 image. In this case, there is no problem with motion blur, but the boost in ISO has resulted in introducing grain and noise into the image.



So this is why I say that it is worth it to spend the extra money on the f/2.8 lens. Having a large aperture will allow you to capture clean/sharp images in a much wider array of lighting conditions. This is especially important if you ever find yourself shooting indoors, even during the day. Plus, as I mentioned above, a large aperture allows you more control over depth of field, allowing you to better separate your subject from the blurred background.

Last edited by Edgar_in_Indy; 09-05-2011 at 08:26 AM.
09-05-2011, 08:11 AM   #6
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QuoteOriginally posted by Edgar_in_Indy Quote
Here's a real world example of why f/2.8 is important. Let's say you're wanting to take a portrait indoors. You'll probably be at the long end of your zoom lens, since that is where portraits are most flattering, and where you don't have to be right in your subject's face. With the kit lens, that will put you at f/5.6. But with the Sigma 18-50mm f/2.8, you can still be at f/2.8 at the long end.

To demonstrate the advantage of f/2.8, I just snapped the following picture of my daughter at the kitchen table. The lens was the Tamron 28-75mm f/2.8 mounted on my Pentax K-x. Light was coming in through the back door. Settings were as follows:

50mm
f/2.8
1/40 sec
ISO 200

You can click on the image if you'd like to open the full-size image.



If I were using the kit lens at 50mm, my aperture would be f/5.6...two stops less than f/2.8. So in order to get the proper exposure with the smaller aperture, I would have to compensate by lowering the shutter speed, or raising the ISO, or some combination thereof. In this case, that would mean lowering the shutter speed to 1/10 sec, or raising the ISO to 800. In order to demonstrate the effects of these compensations, I took pictures at f/5.6, and made the required compensations.

Here is a 100% crop from the f/2.8, 1/40 sec, ISO 200 image:



And here is a crop from the f/5.6, 1/10 sec, ISO 200 image. As you can see, the slower shutter speed has resulted in a soft image due to subject and/or camera movement.



And here is a crop from the f/5.6, 1/40 sec, ISO 800 image. In this case, there is no problem with motion blur, but the boost in ISO has resulted in introducing grain and noise into the image.



So this is why I say that it is worth it to spend the extra money on the f/2.8 lens. Having a large aperture will allow you to capture clean images in a much wider array of lighting conditions. This is especially important if you ever find yourself shooting indoors, even during the day. Plus, as I mentioned above, a large aperture allows you more control over depth of field, allowing you to better separate your subject from the blurred background.
Holy cow! what a comparison! Thanks a lot.

She's beautiful and a great model.
09-05-2011, 08:45 AM   #7
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This is at F3.2 instead of 2.8 (DA 21mm) but shows the advantage of large aperture shooting available light in a dark theater.


K20D ISO 3200 F3.2 1/45 second.

BTW, I'm using the Tamron 17-50 as my main lens, which beats the daylights out of the kit lens for sharpness, contrast and saturation. It has noticeable barrel distortion at the wide end but that's fixable in post.
09-05-2011, 08:48 AM   #8
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QuoteOriginally posted by StDevious Quote
I have the pentax K-x kit lens 18-55 F3.5-5.6. Is the Sigma lens worth paying $325+ over it ?
Yes it is. I had that Sigma and liked it at all focal lengths.

09-05-2011, 08:49 AM   #9
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QuoteOriginally posted by Edgar_in_Indy Quote
Here's a real world example of why f/2.8 is important. Let's say you're wanting to take a portrait indoors. You'll probably be at the long end of your zoom lens, since that is where portraits are most flattering, and where you don't have to be right in your subject's face. With the kit lens, that will put you at f/5.6. But with the Sigma 18-50mm f/2.8, you can still be at f/2.8 at the long end.

To demonstrate the advantage of f/2.8, I just snapped the following picture of my daughter at the kitchen table. The lens was the Tamron 28-75mm f/2.8 mounted on my Pentax K-x. Light was coming in through the back door. Settings were as follows:

50mm
f/2.8
1/40 sec
ISO 200

You can click on the image if you'd like to open the full-size image.



If I were using the kit lens at 50mm, my aperture would be f/5.6...two stops less than f/2.8. So in order to get the proper exposure with the smaller aperture, I would have to compensate by lowering the shutter speed, or raising the ISO, or some combination thereof. In this case, that would mean lowering the shutter speed to 1/10 sec, or raising the ISO to 800. In order to demonstrate the effects of these compensations, I took pictures at f/5.6, and made the required compensations.

Here is a 100% crop from the f/2.8, 1/40 sec, ISO 200 image:



And here is a crop from the f/5.6, 1/10 sec, ISO 200 image. As you can see, the slower shutter speed has resulted in a soft image due to subject and/or camera movement.



And here is a crop from the f/5.6, 1/40 sec, ISO 800 image. In this case, there is no problem with motion blur, but the boost in ISO has resulted in introducing grain and noise into the image.



So this is why I say that it is worth it to spend the extra money on the f/2.8 lens. Having a large aperture will allow you to capture clean/sharp images in a much wider array of lighting conditions. This is especially important if you ever find yourself shooting indoors, even during the day. Plus, as I mentioned above, a large aperture allows you more control over depth of field, allowing you to better separate your subject from the blurred background.
A very good example of why a fast lens should be in everybody's bag. The 18-55 kit lens does a good job for a lot of stuff. Mine is going nowhere but for portrait shots, nothing beats a fast lens. Another thing to consider is a fast prime which is the route I have taken. If budgets are an issue, there is a lot of high quality MF glass available for very little. I think if I was going to go for a fast zoom, I would choose the Tamron 28-75 simply because I have seen so many great shots around the forum from that lens.
09-05-2011, 09:06 AM   #10
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QuoteOriginally posted by reeftool Quote
I think if I was going to go for a fast zoom, I would choose the Tamron 28-75 simply because I have seen so many great shots around the forum from that lens.
The extra reach of the 75mm facilitates blurring the background and inducing bokeh, letting you get more separation between your subject and the background. It also doesn't require you to get as close to the subject, which may make for more comfortable/less intrusive shooting. The 28-75mm also doesn't have to deal with the extreme barrel distortion of the wider zooms, and as a full frame lens does not have noticeable vignetting on APS-C.
09-05-2011, 09:15 AM   #11
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QuoteQuote:
The 28-75mm also doesn't have to deal with the extreme barrel distortion of the wider zooms, and as a full frame lens does not have noticeable vignetting on APS-C.
The 28-75 has a lot of barrel distortion at the wide end :-( It is a good choice if you need a bit more reach, with the trade-off of not going very wide.
09-05-2011, 09:28 AM   #12
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QuoteOriginally posted by SpecialK Quote
The 28-75 has a lot of barrel distortion at the wide end.
Relative to the 18-50mm and 17-50mm zooms, the 28-75mm has much less barrel distortions. Here are the figures for distortion at the wide end according to photozone.de:

Tamron 28-75mm f/2.8
1.5%

Tamron 17-50mm f/2.8
2.7%

Sigma 18-50mm f/2.8
2.93%

Sigma 17-50mm f/2.8
2.91%

Obviously, it's not fair to compare a 28mm to 17/18mm for barrel distortion characteristics, but it still speaks to the fact that with the 28-75mm, barrel distortion is much less of a concern than with a wider zoom. I almost never feel the need to correct for barrel distortion with my 28-75mm. Then again, I probably spend a lot more time near 75mm than I do near 28mm.
09-05-2011, 09:36 AM   #13
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QuoteOriginally posted by Edgar_in_Indy Quote
The extra reach of the 75mm facilitates blurring the background and inducing bokeh, letting you get more separation between your subject and the background. It also doesn't require you to get as close to the subject, which may make for more comfortable/less intrusive shooting. The 28-75mm also doesn't have to deal with the extreme barrel distortion of the wider zooms, and as a full frame lens does not have noticeable vignetting on APS-C.
QuoteOriginally posted by SpecialK Quote
The 28-75 has a lot of barrel distortion at the wide end :-( It is a good choice if you need a bit more reach, with the trade-off of not going very wide.
The 28-75 looks good with longer reach but my objective is to replace the kit lens with a better walk around lens
09-05-2011, 10:10 AM   #14
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Either the Sigma 18-50 or Tammy 17-50 will do the job nicely if you want a single lens solution.
09-05-2011, 11:37 AM   #15
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Let me share a bit of my experiences since I have both the :
-Sigma 18-50mm f2.8 EX DC MACRO
-Tamron 17-50mm f2.8 XR Di II

Pretty much all of my photography is urban night scenes @ f2.8, used mostly at the wide end.

Purchased the Sigma at same time as my K-x thinking it was 'higher quality' than a Tamron,
always curious so I tested a Tamron a few months later; it back-focused too much, sharpness
was on-par with my Sigma, not better (reviews always quote the Tamron as the sharpest one).
Handling felt 'cheaper' than the Sigma. Soon returned it, not impressed.

For some reason my Sigma gradually got soft over time, not as sharp as when new,
it's currently sent back for service/warranty repair, so purchased another Tamron as a spare replacement while it's gone.

Tested a few copies of the Tamron in store and picked the one that worked best with my camera.
Once home fine tuned it with the 'DEBUG_MODE' trick in the K-x menu, which I didn't know of when did the first test.

WOW! really really sharp lens and focus is spot-on,
now the Sigma will become the 'spare' lens.

The Sigma is good, but in my opinion the Tamron is the better choice.
(plus it's 1mm wider @17mm, and yes it's noticeable)

Michel

update: 25th December 2011,
My Tamron is out of service, defective zoom mechanism.
So it's back to using the Sigma for a while.

Last edited by mlatour; 12-25-2011 at 03:53 PM.
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