Sigma's response to my soft 2.8 lense problem

[sean_mclennan]

I asked a question in another thread about the soft results I was

getting with my Sigma 2.8 24-70 EX. This was my first 2.8 lense and

I was disappointed with how soft it shot at 2.8. It works great at

F8, but anything lower was really soft...but only at either end of

the lense. (24 or 70, at 40mm the lense was tach sharp)

 

I emailed Sigma asking for clarification if this was a technical

issue...or the results I should expect from this lense. This is the

response I received:

 

Sigma Tech: "Are these results happening at close distance?"

 

Me: Well, yes, most of my test shots were within 20 inches...but I

have shot a couple test shots from at least 3-4 feet with the same

results.

 

Sigma Tech: "Especially with a high speed zoom lens, there might be

focusing offset at close distance because of spherical aberration.

The slight focusing offset with Digital cameras exists not only on

the Sigma but also on any other Camera/ Lens manufacturer. This can

be corrected by re-aligning the focus on the camera (adjusting the

distance from the lens mount to the sensor), but if you want to send

the lens in for testing, please do so."

 

 

*****************

How the hell do I adjust the distance from the lense mount to the

sensor? Secondly, I have heard of some people having back focus

issues with the Canon 70-200 F4...could this be the same thing?

 

what are everyone's thoughts on Sigma's response?

 

sean

[maury_cohen]

At least they're offering to test the lens.

I'd take them up on this.

[casey mcallister]

My 2 cents:

 

Well I think what they are trying to say is that the distince between the rear element and the exposure plane are different for DSLR's than Film SLR, and that the lense is optimized for film, ergo OOF's w/ DSLR.

 

I would also surmise from this repsonse that Sigma can adjust this distince to be more suited toward DSLR.

 

Sounds dicey to mess with the lense...get it back and it's worse?

 

I have the Tamron 28-75 Xr Di f 2.8 and must say that his lense produces wonderfully focussed images on both film and DSLR in close ups.

[carina]

It is basically the response you would get from most manufacturers of optics. Maybe the tech didnt do a thorough job explaining it all, but he is describing what is probably the cause of your problem, assuming you are workng at very short distances. Also, he is telling you this "problem" can/does occur on other lenses (others made and not made by Sigma) due to the nature of the design of such a zoom. He also points out a difference between film and digital uses, and that there is a way to adjust for this on digital cameras. He never actually recommends that you attempt to change the sensor-plane distance, nor that he would do it. He then tell syou that if you suspect the lens is in need of calibration, that you could/should send it in and they will test it. Maybe not the answer you wanted, but not half bad compared to many companies. This particular lens offered a wider view than the 28-70, but is not as sharp, especially wide open. This has nothing to do with the problem, nor the possible/probably sub-par or defective lens, but out of curiousity what do you photograph so close (20 inches?), wide open, and with a fast zoom (most of which are definitely not designed for such work)?

[bobatkins]

Lenses tend to show more spherical aberration when focused close.

 

In the presence of spherical aberration there isn't an exact focal plane. There are three different "focus" planes (actually an infinite number, but we'll just look at three!). There's the marginal ray focal plane for rays coming in at the edge of the lens, there's the paraxial focal plane for rays coming through the center of the lens and there's the plane containing the circle of least confusion (i.e. the best "compromise" focus).

 

I've no idea which focal plane autofocus will pick. It probably should go for the plane of least confusion. However in the presence of spherical aberration the image will always be soft, even at "best focus".

 

My guess is just that the lens is soft wide open and that focus is probably as good as it gets. You can tell by using manual focus and seeing if it gets any sharper. It's not a very expensive lens (1/3 the cost of the nearest Canon equivalent) and you tend to get what you pay for.

[sean_mclennan]

I don't really photograph anything at 20 inches regularly. I shot some stuff wide open at 70mm and found it soft. So I took some pics of some fairly close objects...as it is far easier to manually focus on a close object of high detail than one 30 feet away.

 

i have shot object from a distance. I shot a detailed lamp from at least 4 feet....and even in the LCD preview you could clearly see a clarity difference between wide open and stopped down to F8. This prompted me to do these tests with newprint...

 

Someone in another forum was saying this was really a dark secret the lense companies didn't want people to know....that there was a reasonably high chance for slight errors of this nature on DSLRs and their zoom lense compatability with these camera's was/is dodgy at best...

 

don't know if I beleive that...but I have heard of focus issues on the Canon 70-200 f4 on DSLRs as well.

 

In any case, I only have 7 days left to get a full refun, so I don't think I'll send it away to be tested. If I love the lense I would, but so far, it's been a good lense, but nothing special...and it is heavy. I'm going to get the Tamron from B&H....

 

thanks all

[tom.s]

I've been trying for years to reach the Plane of least confusion.The more I look the more confused I get.(-:Grasshopper.

[mayorson_kirk]

try your best to use canon body with mirror lock up and use heavy tripot for support. Remove any filters and use the suppied hood and shoot at the mount at f/2.8 or f/4 and see the results. Also exam the lens elements carefully under spot light and see whether any bubbles spread on the lens element. In that case, the element may go wrong and need to send back.

[carina]

Sorry, I misunderstood. English is my second language and I thought you bought the lens for the purpose of closer photography, as opposed to the way you were simply testing it. Though you might now be soured on Sigma altogether, if you are willing to lose the 24mm wide end and go for the Tamron 28-75mm, one other option is the original Sigma straight f2.8 28-70mm (the film version, not the newest digital version). I have seen this lens $100+ less than the Tamron, but it might fail you to some degree, in the same way - though it is better reviewed wide open and on the wide end of the zoom. I have used the new Tamron bt would definitely not call it tack sharp wide open, though it is very nice lens. I do not know that my co-workers new Tamron is any sharper than my old EOS 24-85mm USM, though it is faster to be sure. The original Sigma, the EOS 24-85, and the new Tamron due have one small bonus in that they use smaller filters (67mm, all - I think) than many of the other newer, faster lenses - which are often 77mm or even 82mm.

[sean_mclennan]

1. I am not soured on Sigma at all. I appreciate all 3rd part lens makers and have a great deal of respect for some of your great lenses. Some of them outperform all others in their class. not an easy feat.

 

2. I am still confused over the problems I am hearing about DSLRs using film lenses. There seems to be a very tight tolerance to which the lenses must conform or you have focus issues. This is not simply a Sigma issue, as I've heard about it with various lens manufacturers...including even Canon L lenses.

 

Therefore, I am forced to ask why you would recommend the Sigma 28-70 (film) over the 28-70 (digital)? Would the digital not be better suited for my digital SLR?

 

finally, I DO NOT expect a soom to perform like a prime. However, I am sorry to say that the Sigma lens I had did not perform as well as others who have the same lens and have sent me samples of their prints. I realize that Sigma could have tested/calibrated/fixed the lens, but it all honesty, I can't be bothered. It is too time consuming to be without my most used lens and first impressions matter. This isn't my first impression with Sigma...but it is with a 2.8 quality Sigma.

 

In anycase...I have purchased the Tamron and returned the Sigma. If the Tamron is not up to par, perhaps I will try the Simga 28-70 as you suggest. Or maybe I just have to drop the $$$ for a Canon L to achieve the results I expect?!

 

sean

[carina]

..."Therefore, I am forced to ask why you would recommend the Sigma 28-70 (film) over the 28-70 (digital)? Would the digital not be better suited for my digital SLR?"...

 

Sorry for any confusion. I was indicating -or trying to (second language, again!)- that Sigmas original fast zoom, 28-70mm, was rated higher than their later fast zoom, 24-70mm. When they opened the lens up another 4mm, they "lost some". One difficulty is that Sigma makes and has made so many different lenses of similar or same focal length. I only recommended the old Sigma 28-70mm film lens based the fact that you were opting out of the wide 24mm end of the zoom when you mentioned the Tamron, which starts at 28mm, not 24mm. If it is true that all the third-party fast 2.8 zooms are a little soft shot wide open (and this may well be true, and not just of 3rd-party lenses), and if you are willing to accept a 28-70 instead of a 24-70, then maybe the now much cheaper old Sigma would satisfy you - if the only other alternative was the costly L lens. Though my EOS USM 24-85 is hardly fast, I notice little difference to the Tamron or the Sigmas -- seriously. This was partly the reason for buying the EOS 100mm f2.0 and the 24mm f2.8 as an insurance policy for low light when I knew I would be enlarging. The zooms were fine for most work, but didnt come through when shot wide open, and for me if I am going to pay the $$$ for a straight zoom, it should be sharp wide open and nothing I looked at was anywhere near the old 28-70L, so I lived without the zoom rather than live without the LPM quality.

[cjtower]

I would second much of what she says about the zooms, with one caveat, which she might have meant or presupposed, I am unsure. The old 28-70mm Sigma is sharper than their new 24-70mm Sigma, but still suffers from some softness wide open, as -I think- does the Tamron. So, in the end I think you either get the "L", or live with a little softness (how much, depends on the third party lens you have). The only reason to try the old Sigma would be, as she hinted at earlier, if you are going to accept a little softness, then you might as well do so at a reduced price - which the Sig 28-70 is at these days. I recently bought the Sigma 28-70 f2.8 used for only $185. She is right about Sigma pumping out the lenses and lens designs. I believe they made three versions of the 28-70mm f2.8, then a 24-70mm f2.8, and now a 24-60mm digital f2.8 (rumored to be an answer to the soft 24-70mm f2.8!).

[miles stoddard]

I think the last lens you mentioned is definitely the best of the wider straight f2.8 zooms from Sigma. The closest thing to wide-open sharpness of the Canon L's (and even those are not spectacular, as seen in all the reviews) in the wide to mild-tele range is the new Sigma. The 24-60mm Sigma lens is very nice, and sharp wide open. 60mm is too short for portraits for film users, but since this is a "digital-design" it is much more on a digital body (1.6 for Canon). This lens, when I bought mine, was maybe $65 less than the 24-70mm f2.8 Sigma, and only $65 or so more than the 24mm f1.8 Sigma prime. No regrets on my part.

[sean_mclennan]

ok, thank you for the clarification. It does makes sense. I guess the hardess thing for me to wrap my head around is the quality difference. When I shot film, the fastest zoom lense I shot was an f4...and it was tack sharp. I was soooo happy to be able to afford a faster lense, I was truly dissapointed with the sharpness of the Sigma at wide open. Of course, I had nothing to compare it against as it was my first 2.8 lense. I do have a Tamron on the way from B&H...and I might very well find the same results. In which case, I can return it and ask for the 28-70 if they still have any. We'll see.

 

thanks again for all the replies

 

sean

[danny_lee2]

but I thought the sigma 24-60 is has poor sharpness at 2.8 ?