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12-05-2006, 06:11 PM   #1
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If a lens is "soft" wide open, then why spend the extra money for this?

Hi everyone

I was reading about the SIGMA 18-50MM F2.8 DC EX, PENTAX and the reviews all say that it is very soft at 2.8 through the range.
I was wondering if 2.8 is soft throughout the range, is it worth spending the extra for a 2.8? wouldn't a 3.5-4.5 that is sharp wide open (and could be less money) make more sense?

just learning here...... pardon the lens "ignorance"

any opinons welcome!

cheers


Last edited by slip; 12-05-2006 at 07:02 PM. Reason: "learing" was changed to "learning"
12-05-2006, 06:48 PM   #2
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Here's my ignorant answer:
1) soft might not hurt some images (portraits, perhaps)
2) a bit better low light capability
3) a bit shallower DOF capability
4) possibly sharper at f4 than the wide open 3.5/4.5 lens
5) hoping you get lucky and get a good copy. (Sigma has a reputation for high sample variability. I don't know if this is deserved or not.)

Others?

-Mark

QuoteOriginally posted by slipchuck Quote
Hi everyone

I was reading about the SIGMA 18-50MM F2.8 DC EX, PENTAX and the reviews all say that it is very soft at 2.8 through the range.
I was wondering if 2.8 is soft throughout the range, is it worth spending the extra for a 2.8? wouldn't a 3.5-4.5 that is sharp wide open (and could be less money) make more sense?

just learing here...... pardon the lens "ignorance"

any opinons welcome!

cheers
12-05-2006, 07:00 PM   #3
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Sigma 18-50mm DC EX f2.8 is sharp wide open except at extreme ends. Softer at 18mm than 50mm. But It did not mean it is too soft to use. The sample variation is more to do with background blurr and colour rendition (rich or washed out). It is generally a good lens.

As Mark as suggested, all are very true. Usually one stop down would make the images invinsible in terms of MTF. Wide open is still usable in a lot of low light situation and it is usually not that possible to get sharp images in low lit situations anyway.

Most of the time, it is the playing of DOF being fun and creative when darker lenses are unable to produce similar effects
12-05-2006, 08:31 PM   #4
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All sharps are not created equal

Because sharp to you might mean digicam-snapshot to me?

Or, more importantly, because you might catch something at f/2.8 that would be completely lost at f/5.6 and the tiny amount of 'unsharp'-ness is insignificant in the meaning of the whole picture.

BTW, I posted a full-size image taken with my SMC Pentax-A 50/1.2 wide open at f/1.2 ("The Unsharpest of the Unsharp") with the K10 and posted it here:
(WARNING - BIG FILE!)
50/1.2 wide open photo

As long as you're getting adequate resolution (this lens clearly is) then you'll get adequately sharp images, IMO.

--Sean

12-05-2006, 10:11 PM   #5
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On aperture and sharpness

QuoteOriginally posted by slipchuck Quote
I was wondering if 2.8 is soft throughout the range, is it worth spending the extra for a 2.8? wouldn't a 3.5-4.5 that is sharp wide open (and could be less money) make more sense?
Others have already correctly answered good part of your question.

I would like to add one more thing: every lens is sharper at mid-range apertures. Thus, an f/4 or f/5.6 lens will still be sharper at smaller apertures, say f/8 or f/11.

Important thing to remember is that no lens in the world can have equal sharpness at all apertures. While measuring lens resolution reviewers often provide relative comparison (the same lens at different apertures) pointing out that a lens is "soft" wide open and sharper stopped-down. No surprise here, that is always true for any lens and should not be tested at all.

Here is the principle behind this: at wide apertures lens aberrations are more pronounced (chromatic, spherical, astigmatism, field curvature...) and this effect becomes less visible with smaller apertures. But with apertures becoming too small aperture blades diffract light and image again looses sharpness. Obviously, there is a mid point where decreasing aberrations cross increasing diffraction influence, and that point is the sharpest aperture.

My tests with some 5 or 6 different lenses confirmed that ALL of them are sharpest at f/8 and f/11. Larger the maximal aperture and this interval extends down. Examples: my A50/1.4 is sharpest at f/4 to f/11; A28/2.8 is sharpest at f/5.6 to f/16; FA35/2AL is sharpest between f/4 and f/11 (and only slightly softer and f/2.8 - an excellent lens BTW!).

But there is one final and subtle detail: at large apertures depth of field is so small that there is more risk of having softness due to a focusing error than to any lens "softness" at wide apertures.

Some interesting resources:

1) Interesting articles on optics and camera lenses:

www.pinnipedia.org/optics.html

2) Famous Yoshihiko Takinami lens resolution test page -- you can clearly see how lens resolution increases and then decreases with various apertures for all lenses tested:

www.takinami.com/yoshihiko/photo/lens_test/index.html

3) WARNING: a totally geeky resource!!! Laboratory optical tests of many Pentax lenses with light paths and lens optical designs. Note that XY diagrams are not to scale -- to compare various lenses make sure to read numbers at X/Y axes (do not compare visually only). Check it out here:

pentaxstudy.bufsiz.jp
12-05-2006, 10:19 PM   #6
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"I was wondering if 2.8 is soft throughout the range, is it worth spending the extra for a 2.8? wouldn't a 3.5-4.5 that is sharp wide open (and could be less money) make more sense?"

it would be less money.. its the 2.8 thruout the range that costs money and is harder to achieve relatively speaking..

as for the dodgy sigma thing.. my son just bought a used f2.8 28 to 70 EX sigma off ebay..

he popped it on my k100d did a few compare shots with my sigma f2.8 50mm prime.. it compared well..

he then popped it on his own k100d and wide open all he got was a fuzzy blob.. it seems the variation isnt just with sigma..

it also seem the wider the opening the more likely the pentax is not to get it right..

cheap is best (f3.5 to 5.6) unless u really need the fast glass.. it can cause more problems than u think.. a nice old pentax 1.7 50mm manual pime (ebay) will do for the odd occasion u might really need a fast lens.. the auto focus on the camera can handle the f3.5 to 5.6 a lot easier than it can the 2.8 or faster openings..

trog

ps.. i overlapped the previous post..

"But there is one final and subtle detail: at large apertures depth of field is so small that there is more risk of having softness due to a focusing error than to any lens "softness" at wide apertures."

100% yes.. u wont hear of anybody complaning about a cheap 3.5 and less lens front or back focusing..

Last edited by trog100; 12-05-2006 at 10:25 PM.
12-05-2006, 11:10 PM   #7
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Lenses are generally softer wide open than when stopped down and some are better than other's when wide open. The FA31 f1.8 LE and FA77 f1.8 LE are actually very good wide open and very useable(except DOF is VERY shallow at portrait distances), whereas the FA50 f1.4 is very soft wide open.

There are a few things in favour of a fast lens over a slow lens:
1) A fast lens of say f2.8 may start to be quite sharp at f4 which means that you can shoot portraiture at this aperture and get pleasing OOF effects due to the shallow DOF.

If on the otherhand you had a lens which had a maximum aperture of f4, then it isn't until probably f5.6 or f8 that it becomes sharp and you will not be able to get the nice OOF effects or shallow DOF.

2) A fast lens usually costs more due to the larger glass elements required and sturdier build to hold these elements. Also, the fast glass is usually for more"pro" oriented aspects of photography and are built to a higher standard.

Due to the fact that these fast lenses are going to be more costly and they can charge more, manufacturers use better, more exotic glass elements to correct for many of the aberrations associated with lenses(especially zooms). These faster lenses may also be made to tighter tolerances due to their "pro" spec design. Due to all the aspects outlined above, many of these lenses can be used nearer their wide open aperture than their less expensive consumer grade cousins.

As I said in point 1), the FA31 and FA77 can be used wide open with good results due to the better glass, more exotic elements to correct aberrations and tighter tolerances in manufacture.

I can use all of my f2.8 lenses at f4 and they will be very sharp. I have some old zooms that have a maximum aperture of f4.5 and they aren't even sharp at f8!
12-06-2006, 07:28 AM   #8
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Another factor not sharpness related: with a faster lens the focusing should be quicker especially in low light situations, as more light is available for the mechanism. In certain conditions this could mean the difference between getting AF lock or not... Plus the view finder image will be brighter as compared to a slower lens.

12-06-2006, 03:31 PM   #9
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QuoteOriginally posted by Nesster Quote
Another factor not sharpness related: with a faster lens the focusing should be quicker especially in low light situations, as more light is available for the mechanism. In certain conditions this could mean the difference between getting AF lock or not... Plus the view finder image will be brighter as compared to a slower lens.
Yes, these are good points that I had forgotton to mention.
12-06-2006, 03:45 PM   #10
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QuoteOriginally posted by Nesster Quote
Another factor not sharpness related: with a faster lens the focusing should be quicker especially in low light situations, as more light is available for the mechanism. In certain conditions this could mean the difference between getting AF lock or not... Plus the view finder image will be brighter as compared to a slower lens.
The viewfinder issue is most certainly important. In my experience though, the AF benefits are not as great as one might think. Often with a faster lens, what is weakest wide open is not so much sharpness as contrast. My FA50/1.4 is a pretty good example. It is actually quite sharp wide open, but it lacks contrast---images sharpen up nicely with a moderate S-curve---. Since AF systems rely on contrast, a lens which is fast but lacking contrast doesn't always focus that well. I think this is a big reason why my FA35/2 is faster to focus than my FA50/1.4. Of course, all things being equal, the faster lens will have the advantage.
12-08-2006, 10:16 PM   #11
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Low Light

QuoteOriginally posted by slipchuck Quote
Hi everyone

I was reading about the SIGMA 18-50MM F2.8 DC EX, PENTAX and the reviews all say that it is very soft at 2.8 through the range.
I was wondering if 2.8 is soft throughout the range, is it worth spending the extra for a 2.8? wouldn't a 3.5-4.5 that is sharp wide open (and could be less money) make more sense?

just learning here...... pardon the lens "ignorance"

any opinons welcome!

cheers
Sometimes a speedy lens (1.4-2.0) will allow you to bring home a picture that you couldn't pull off with a slower one. It may not be perfect, but may be better than none.

Jer
12-09-2006, 06:43 AM   #12
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having fast glass for when u really need it is no bad thing but.. its expensive and in more normal conditions is less likley to produce any advantages over cheaper slower glass.. i know two k100d cameras that are hit and miss with A/F at F2.8.. lens flare is also more likely..

trog
12-09-2006, 10:49 AM   #13
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this doesnt really answer the question, but hell, i love the softness produced by wide apertures for artistic/experimental reasons. they are great for producing dreamlike images.
12-09-2006, 03:04 PM   #14
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Yeah, I second that

QuoteOriginally posted by potatostar Quote
this doesnt really answer the question, but hell, i love the softness produced by wide apertures for artistic/experimental reasons. they are great for producing dreamlike images.
Being soft is not a sin. Just do not post them. People will tell you: "your images are soft" or "soft at corners" etc.

Dream like images are harder to produce these days ...
12-09-2006, 10:32 PM   #15
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QuoteOriginally posted by roentarre Quote
Being soft is not a sin. Just do not post them. People will tell you: "your images are soft" or "soft at corners" etc.

Dream like images are harder to produce these days ...
harder - how so? or just more rare?
i suppose it depends on where you post the images. in an art community one isnt nearly as likely to get those comments as it would in a more general photography community.
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