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Old Dec 26, 2009, 3:07 PM   #11
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My advise. Don't worry about. ;-)

What you need to know is that you may need to use a shorter focal length to get a wider angle of view (where you can fit more into the frame from a given shooting distance). Or, you may need to use a longer focal length for more apparent magnification. 18mm is a typical starting focal length on a "walk around" lens used for a camera with an APS-C size sensor, giving you roughly the same angle of view you'd have using a 28mm lens on a 35mm camera.

Short answer (repeated from above):

If you use a smaller sensor or film size, the angle of view will be narrower (more apparent magnification) for any given focal length.

If you use a larger sensor or film size, the angle of view will be wider (less apparent magnification) for any given focal length.

The only reason to even have a so called crop factor or focal length multiplier is so that users familiar with using lenses on 35mm cameras have a better understanding of how angle of view compares using a DSLR with a sensor smaller than 35mm film.

If 35mm cameras were not so popular, there would be no need to use them at all.
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Old Dec 29, 2009, 3:22 AM   #12
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Right, well I understand more of it than before at any rate
Crop factor is applied to every lens on an aps-c body because the aps-c sensor draws a smaller rectangle in the image circle of the lens; lenses made for aps-c have a smaller image circle, und so weiter.

Sony's 16-105mm seems like a very good lens, but unfortunately it would be at least 130USD or so more than Sigma's 17-70mm.
The full frame Sony / Minolta 24-105mm can be had for around 130USD less than the 17-70mm but its optical characteristics may be noticably worse than the two aboves.

There is also Sigma's 24-70, and while it fetches a price somewhere between the 17-70 and Sony 16-105m it has a constant max f/2.8 and 'carries some weight', ahaha. Decisions decisions. 24-70 is the minimum range though. For better range, the Tamron 24-135mm has gotten alright reviews, but it appears rather impossible to find for sale, even on ebay. (For a current lens, that's a bit of a mystery)

Right, well, these are the main contenders and their price ratios:
(Cost : 100%): Minolta 24-105mm
150% : Sigma 17-70mm
200%: Sony 16-105mm

With those relative costs in mind, what do you say?

Last edited by Lindinblade; Dec 29, 2009 at 6:05 AM.
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Old Dec 29, 2009, 6:55 AM   #13
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I think the Sigma 17-70 would better suppliment the lenses you've already got.
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Old Jan 9, 2010, 8:13 AM   #14
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Well, I got the last lens from the last reseller to have it in stock ins Schweden, it seems. Now, if AF functions OK and nothing else is terribly amiss, I feel certain I shall feel pleased. (Elsewise it's a Sigma issue, this reseller is good at everything besides returns, it is said..but, when ebay sellers won't even reply, what choice does one have?)

I do feel like comparing the three lenses I have that can do 70mm in a vaguely systematic fashion too. Won't be possible until next weekend however.
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Old Jan 22, 2010, 12:31 PM   #15
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Did a quick, potentially critically flawed test today, a comparison betwen the Kit lens (Sony DT 18-70, f/3.5 - 5.6) and my new Sigma 17-70 f/2.8 - 4.5. Handheld, iso200, AS on, focus mode 'wide'.

[email protected] 18mm, f5.6, 1/80, center crop(100% - unless photobucket or this forum does something :


Same settings, Sigma:





[email protected] 18mm, f8, 1/50, center crop (100%):


Same settings, Sigma:

I'd suspect the Sigma had focused somewhere else, but looking at the whole images I don't think so.

Anyways, let's look at these four pictures again, but in the right-hand bottom part:

[email protected] 18mm, f5.6, 1/80, bottom-right 100% crop:

Green and reddish CA evident.

Sigma, same settings:

Green and purple CA can be seen.

Stopping down might eliminate the CAs?

Kit @18mm, f/8, 1/50, bottom right 100% crop:

Not really, no. Though I'm hardly experienced with 'pixel peeping' and I didn't really know what to expect.

Sigma, same settings:

CA still evident.


The flaws of this is of course the limited scope (one focal range, two apertures), handheld - 1/50 - could be some motion blur, but with AS maybe not.
If any conclusions at all can be drawn from this, it's that the Sigma isn't very good at 18mm (supposedly 17mm as well)- sharpness and CA-wise?

Will make further tests with tripod, what suggestions do you have for that?

Last edited by Lindinblade; Jan 22, 2010 at 12:34 PM.
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Old Jan 22, 2010, 12:43 PM   #16
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You're going to have a bit of CA with most lenses like that on their wider end. See the bottom of this page for how one tests in Nikon mount:

http://www.photozone.de/nikon--nikko...report?start=1

P.S.

It does look like your focus point was different between those photos. I'd make sure to select the center focus point and make sure you're focused on the same subject when comparing lenses. IOW, from what I can tell from the bottom crops, the Sigma was probably locking on a closer focus point somewhere compared to the kit lens.
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Old Jan 22, 2010, 1:32 PM   #17
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Quote:
Originally Posted by JimC View Post
You're going to have a bit of CA with most lenses like that on their wider end. See the bottom of this page for how one tests in Nikon mount:

http://www.photozone.de/nikon--nikko...report?start=1

P.S.

It does look like your focus point was different between those photos. I'd make sure to select the center focus point and make sure you're focused on the same subject when comparing lenses. IOW, from what I can tell from the bottom crops, the Sigma was probably locking on a closer focus point somewhere compared to the kit lens.
Hello, thanks for the quick reply

The review does mention CA at 17mm, but says that 'stopping down to f/8 reduces CA to a very good level' and in my two shots I can't honestly tell the difference in CA between f/5.6 and f/8. Maybe corner CA at f/2.8 would have been worse, though?

As for focus, I did focus using the middle point and then aimed for the 'head' of the snow-covered log, every time. At 18mm from where I was standing though it was a bit diffuse and far away so I'll try something better defined next time And use a tripod, my fingers can't stand the sun and they can't stand the cold

Last edited by Lindinblade; Jan 22, 2010 at 1:35 PM.
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Old Jan 22, 2010, 2:24 PM   #18
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It does like a tad soft. But, it's hard to tell what the focus was on from the crops, and it looks like the focus point was different from what I can see of the bottom right crops. So, I'd try to find a good target and make sure you're focusing on the same thing with both lenses for comparison purposes. Also, CA is going to be worse in the out of focus areas (and if focus was off, that would have contributed to it).
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Old Jan 22, 2010, 4:03 PM   #19
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Congratulations on being so analytical. Yes, the Sigma is a much better lens than the older kit lens, and you just proved it for yourself.
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Old Jan 23, 2010, 12:34 PM   #20
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Quote:
Originally Posted by TCav View Post
Congratulations on being so analytical. Yes, the
Sigma is a much better lens than the older kit lens, and you just
proved it for yourself.
Can't form a suitable reply: unable to determine sarcasm levels.
However: If you do think that and is merely being cryptic, would you
care to enlighten me? I clearly didn't prove anything of the sort to
myself.

I did do a new test today though (handheld still- couldn't get to the tripod, but shorter exposures as well as whole pictures) and all comments are
welcome. Mostly interested in focus now, hence only center crops.

All are spot focus: somewhere around the centers of the center crops.

[email protected] 1/320 f/5.6 (probably fully open, probably not fair):


100% center crop:


[email protected] same exposure:


100% center crop.


As expected, the Sigma looks better. If it's defect, it isn't entirely

defect anyways

Same scene>

[email protected] 1/320 f/8 100% center crop:


[email protected] 1/320 f/8 100% center crop:

Not much of a difference now that I can tell.

Continued in next post due to 12-imager-per-post-limit - >

Last edited by Lindinblade; Jan 23, 2010 at 2:10 PM.
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