Lens ? Stupid question -- But I really need HELP.

zendianah · Oct 31, 2007

Hi there,
I purchased a Sigma - 24-70mm 2.8 lens.... I hear that the Sigmas have good batches and bad batches... blah blah blah. My images seem to NEVER be sharp. I tryed all the advice that I get here.

(kids -- use F11 with a faster shutter speed)

Heres the stupid question part.... How can I test the lens for sharpness?-- Besides stating the obvious... take a picture and see if its sharp..obvious... I'm really embarrased about this question.. BUT I dont know who else to ask?

Mods - If this idiotic question needs to be moved to beginners feel free.. I had no idea where to post.

Big Mike · Oct 31, 2007

You can check to see if it's front or back focusing. An easy way to check is to put the camera on a tripod and shoot a ruler at a 45 degree angle. Set the focus on a particular point and shoot wide open. Then open the image and examine it...you are looking to see if the point that you focused on, is actually the sharpest part of the image. You could also use a book, magazine or newspaper...and just focus on a particular line of text.

There are other ways to test a lens, try a Google search for 'lens test'.

EOS_JD · Oct 31, 2007

My Sigma 24-70 f2.8EX was soft wide open. Sharpened up well as it was stopped down. this was why I bought the Tamron 28-75 f2.8 and it was much better however I now use the 24-105 and the Tamron rarely gets out the bag (my brother has it at the moment)!

zendianah · Oct 31, 2007

Thanks Mike!! I'll try it out !

JerryPH · Oct 31, 2007

My favorite was the focus test. You can download for free a PDF file, print ONE page out, and follow the instructions from the others... this is an easy test to do and will tell you right away if your lens front or back focuses.

http://focustestchart.com/chart.html

Enjoy!

zendianah · Oct 31, 2007

JerryPH said:
My favorite was the focus test. You can download for free a PDF file, print ONE page out, and follow the instructions from the others... this is an easy test to do and will tell you right away if your lens front or back focuses.

http://focustestchart.com/chart.html

Enjoy!

Thank you !! That is very useful!!! I just printed it out!

itoncool · Oct 31, 2007

Well, it's not a stupid question and I'm grateful you started this thread, zendianah. And thanks to you all.

PS : I'm in the middle of the preparation, just printed out the chart too

zendianah · Oct 31, 2007

I used the tutorial.. and my 2 main lens are sharp so its user error.

Now I need to test my very used 80-200 lens -- I bought that one used so now I would have to work with the softness.

JerryPH · Oct 31, 2007

zendianah said:
I used the tutorial.. and my 2 main lens are sharp so its user error.

Now I need to test my very used 80-200 lens -- I bought that one used so now I would have to work with the softness.

All my lenses (excluding the 30mm F/1.4 that I just ordered yesterday) are focusing properly too. I have seen a verson of the Sigma 18-50 with the Canon mount that front focuses about 10-12mm using that same test page... he is planning on sending it in with the test sheet to be hopefully re-adjusted.

I'm glad to see that all your lenses are focusing properly!

JerryPH · Oct 31, 2007

EOS_JD said:
My Sigma 24-70 f2.8EX was soft wide open. Sharpened up well as it was stopped down.

The Sigma 18-50 is very sharp at F/2.8, but it is a completely other lens.

kundalini · Oct 31, 2007

Front focusing - Back focusing. What should the lens be? If it's the other, how do you do the ring around - i.e. work with what you've got. Or should it be returned?

Garbz · Oct 31, 2007

Sorry to nudge the topic a bit but the thread is all over the place anyway. How could back / front focusing possible be the fault of the lens? I don't see how this could be anything other than a camera fault and only something to do with the focusing screen being a different distance away from the mirror as the film plane. Yet I have seen constant discussions regarding lenses which are backfocusing.

RacePhoto · Oct 31, 2007

JerryPH said:
My favorite was the focus test. You can download for free a PDF file, print ONE page out, and follow the instructions from the others... this is an easy test to do and will tell you right away if your lens front or back focuses.

http://focustestchart.com/chart.html

Enjoy!

Thanks. I forgot about that one, and I have a friend who's photos are all "soft focus" At least I can check for short or long focus and probably see if the edge sharpness is any good at the same time.

JerryPH · Nov 1, 2007

Garbz said:
How could back / front focusing possible be the fault of the lens? I don't see how this could be anything other than a camera fault and only something to do with the focusing screen being a different distance away from the mirror as the film plane.

The question one should be asking is... why is Sigma accepting these returns, reporting to do something, returning the lens to the owners and the focus issue being resolved after testing again?

Yes, I agree that there is a posibility that the camera is at fault, but if it is not at fault for 5 out of 6 of the camera owner's lenses, but one specific lens consistantly displays this behavior, and it comes back from "repair" with the issue significantly reduced or eliminated, one has to agree that a "faulty camera" is not the only possibility, yes?

JerryPH · Nov 1, 2007

kundalini said:
Front focusing - Back focusing. What should the lens be? If it's the other, how do you do the ring around - i.e. work with what you've got. Or should it be returned?

It should be neither. It should focus on what your aim it at. If it is recently purchased, it should be returned. If its been 4 months (arbitrary number just randoly picked by me), and you just did the test and see this, get it repaired under warranty (it helps if you send a copy of this picture to them explaining how the test was done and why you feel there is an issue). Now as to much much you can live with, thats totally up to you. For me, I think that up to 2-3mm is liveable, but for someone else, 3mm is intolerable. Your lens, you decide how much leeway you can live with.

On the left, an example of a front focusing Sigma 18-50 (I *believe* this is an earlier non HSM model... pic used with owner's permission) and on the right an example of a proper focusing Sigma 18-50:

I sincerely apologize for bringing this thread off-topic, but I hope that the information is beneficial to the users here in this thread and on this forum.