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03-21-2009, 05:09 PM   #1
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Backfocusing 40mm Ltd

Hi,

I picked up a brand new 40mm Ltd a few days ago. Obviously i started shooting everything with it. Unfortunately as soon as i saw the results i was quite disappointed. Wide open it is very soft. Once stopped down a bit it very sharp and the color is great, nice and contrasty, smooth bokeh etc. But so soft wide open. To see example of how soft it is click the below link (there are more on page 3 too). I am starting this thread to stop cluttering up the one where people have good copies.

https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/54584-40-ltd-b...e-using-2.html

Thanks to the folks on here i figured out that some testing for backfocus was needed. So i did the below test : Focus Testing - photo.net

BTW, i recommend everyone to do this. It's a great way to calibrate your body / lens.

I tested my 40mm ltd on two k100d bodies. I also threw the A 50mm f1.7 into the test to give my self as much data as possible.

Sure enough the 40mm ltd back focused on both bodies but was worse on my body. The 50mm back focused on my body but not on the loaner.

Essentially i found that by k100d backfocuses a little and the 40mm ltd backfocuses on both bodies.

I adjusted my k100d to a tiny bit of front focus with the 50mm f1.7 but with this adjustment there is still too much backfocus with the 40mm.

At this adjustment range it is still too soft wide open. If anyone wants to see specific images from the test, please let me know and i will upload them.

My main question is this: am I within my rights to get a direct exchange lens?

My prob is that i am a bit of a pushover and hate yelling at people so before i do that i want to be positive that i am not asking for too much. I am 100% sure the lens is wrong (the 50mm at f1.7 is sharper than the 40mm at f8). I guess that i am expecting 'pushback' from the store.

Maybe they will surprise me and be good about it.

thanks for reading!


mike

03-21-2009, 05:22 PM   #2
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Trying to get another lens may be hit and miss, the next one may be the same or perhaps fix your problem. I think sending your camera in for calibration would be the best (and most painful) solution.

For what its worth, my K10D also back focuses. I added some adjustment and it seems fine with all lenses including manual focus with a Katz-eye.

- Andrew
03-21-2009, 05:30 PM   #3
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Once i calibrated the camera to a little bit of front focus (with other lenses) the 40mm still backfocused.

It also backfocused on the 2nd body.

It is the lens.

mike
03-22-2009, 01:25 PM   #4
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If it's the lens, then I don't think you have other chance then go back to seller and ask for refund/exchange or ask them to send it back to Pentax. In all cases, it's faulty item and the retailer should deal with it...
good luck and sorry to hear that it the lens...

BR

03-23-2009, 12:59 AM   #5
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I got in touch with the store today and asked for and exchange lens. No way. "bring it in for testing" or even better, "take it to the importer for testing as that is where we would send it". Can I have a loaner lens for the time they have it? NO we don't have one.

So i called the importer. They will test it. I have to post it to them or go in myself. What a pain in the ass. I get a dodgy product and then I have to be without while they dick around with it. Needless to say I am very annoyed.

Why should I do their quality assurance for them. They (Pentax) but the 'LTD' label on it.... they at least should have tested it.

So..... essentially i am lending them money (coz i won't have a lens for weeks) while they attempt to 'adjust' it. It just makes me lose confidence in their entire system, from the product to the after sales support.

I understand that there will be some lemons but WHY do i have to PAY to get one. THe product IS NOT FIT FOR THE PURPOSE FOR WHICH IT WAS SOLD.

Anyway, that is my rant.

mike
03-23-2009, 01:08 AM   #6
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sorry to hear that schmik...
which country are you in?
frankly here in UK I'd tear the head of salesman off if he would refuse to take the product back. You have your rights!!! but it may vary in your country

BR
03-23-2009, 01:20 AM   #7
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I'm in Aus. I may have another go tomorrow. I can only deal with them over the phone..... can't get anytime till the end of the week to actually go and do something about it.

mike
03-23-2009, 11:46 AM   #8
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QuoteOriginally posted by schmik Quote
I tested my 40mm ltd on two k100d bodies. I also threw the A 50mm f1.7 into the test to give my self as much data as possible.

Sure enough the 40mm ltd back focused on both bodies but was worse on my body. The 50mm back focused on my body but not on the loaner.
The 50 is manual focus, right? So it doesn't really provide any sort of meaningful basis for comparison.

testing for FF/BF is a much more difficult process than people give it credit for. I spent months being sure I had all sorts of problems with a variety of lenses and bodies before finally getting my testing methodology down to the point where I was actually testing the equipment and not my own ability to conduct good tests. Once I got to that point, I discovered all my problems were actually the result of faulty tests - when performed properly, all the lenses and all the bodies in question actually turned out to work perfectly.

That experience, plus seeing posts from people who are also clearly (from the images they post) not performing the tests properly, is what makes me immediately skeptical when someone reports an FF/BF problem based on their first attempt at ever performing such a test.

I say all that so hopefully you won't take it personally if I suggest you post the images from your test and your description of your methods, to make sure you really do have a problem, before you go to all the trouble it looks like you'd have to go to to exchange the lens.

03-23-2009, 01:13 PM   #9
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Marc, Thanks for the reply. It is much appreciated. I do not take it personally. Especially seeing as last night I did a heap more testing and let's just say that I did not get the exact same results.

I definately get back focus on the 40mm and MY k100d. It seems that the loaner k100d and this lens combo is OK. Also, the images from the loaner are sharp wide open (just as sharp as they are at f/8.

I will test it more tonight to be sure.
I use the test that I have linked to in the first post. I also line up a series of AAA batteries at 45degrees and shoot them. All tests are from a tripod and using a cable release.

BTW, using the A 50mm, I look at the AF indicator and turn on the 'beep'. Does this not test the AF function? (obviously not the motor but the inteligence).

Care to share any tips on testing for BF/FF?

Also, using the debug mode, it seems i can adjust the BF/FF issue a little but the images are still soft wide open. I wonder if this is the best way to calibrate the camera? Is this all that pentax would adjust if they got their hands on it.

Also, apologies to all for my rant. Seems it may not be the lens.... it may be the body (another piece of pentax gear). I will post some test pics later......... after tonights testing.

cheers
mike
03-25-2009, 03:57 PM   #10
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I had time again to test it last night. I only tested it using the better k100d. i.e the one the lens performs much better on. Here a random sequence of 10 shots. There are two or three that are acceptable.....out of 10 and the whole thing is much worse on the other k100d

Doesn't matter now, the retailer has stated that they will replace it when another 40mm LTD comes in. Thank god they stepped up and are willing to provide good service.

Have a look and the pics and tell me what you think.

mike
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03-25-2009, 03:59 PM   #11
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Her are the next 5.
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03-25-2009, 05:27 PM   #12
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QuoteOriginally posted by schmik Quote
I do not take it personally. Especially seeing as last night I did a heap more testing and let's just say that I did not get the exact same results.
Frustrating, isn't it?

Independent of whether it turns out there is a problem with the lens, one lesson to take from this is that AF is a highly complex operation that depends on a number of variables, some of which are easy enough to control but others are much harder to control. That's what makes focus tests so difficult to perform reliably. The trick is discovering what some of those variables are and then eliminating them.

Assumign you eventaully get to the point of reasonably repeating results, it will mean you have figured out how to control focus better. So the good news is, the effort you spend doing this *can* actually make you a better photographer - at least in terms of your ability to nail focus. As you discover the things that you do in the focus tests that cause it to miss, you can stop doing those things in real life. My own focusing abilities - both using AF and MF - improved *tremendously* as a result of all the time I spend obsessing over focus charts. I'm much better at anticipating the things that will cause the focus to not where end up where I want it.

So again, regardless of the outcome regarding your lens in particular, you will better off for having gone to the trouble to try to deal with these issues and figure out for yourself how to get repeatable results. For whatever that is worth.

QuoteQuote:
BTW, using the A 50mm, I look at the AF indicator and turn on the 'beep'. Does this not test the AF function? (obviously not the motor but the inteligence).
I think what it will test is whether the AF sensors are physically out of alignment, as that would make the camera think something is in focus when it isn't. It won't test for for any problems that come after that - ones involving the camera's ability to communicate to the lens how to fix the focus. And no one really seems to know for sure how much of any focus problem is due to misaligned sensors versus anything else.

QuoteQuote:
Also, using the debug mode, it seems i can adjust the BF/FF issue a little but the images are still soft wide open. I wonder if this is the best way to calibrate the camera? Is this all that pentax would adjust if they got their hands on it.
My understanding is that if the AF sensors are physically out of alignment, that can be fixed by turning a few screws underneath the camera, and the test systems they have in place are supposed to do this if appropriate. I think someone once even posted directions on doing this yourself (on dpreview).
03-25-2009, 05:44 PM   #13
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QuoteOriginally posted by schmik Quote
Her are the next 5.
Are these crops? As it is, it is difficult to tell what the camera is supposed to be focusing on - there is a confusing jumble of stuff at the center of the frame and the camera could have legitimately focused pretty much anywhere it wanted. The point of a controlled focus test is to give the camera something to focus on twhere it cannot possibly choose anything else. so in the pictures here, the vertical line that appears at the bottom of the frame should have been centered, not placed at the bottom as it appears here. If these are crops, then, could you post the whole image they were taken from? Just one would do, so I can get a sense of what the scene actually looked like.
03-25-2009, 06:39 PM   #14
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Yes, they are just crops. Here is the full image. Camera height relative to the test chart is not ideal in this test (it was in previous tests) but i don't think this matters too much.

Focus is always on the centre line. Confirmed with the red square through the view finder.

And I thoroughly agree about how many lessons are learned through this process.
Messing with formware is one thing.... and it is easy to record the seetings and restore them to factory. Focusing srews on the other hand are a bit harder. I'll tackle it one day.... when the k100d is the backup body.

Thanks for your time Marc!!!!

mike
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03-25-2009, 07:53 PM   #15
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I used the focus charts from pentaxdslrs.blogspot.com.

I had a grim couple of hours when I was convinced I had front-focusing problems, based on tests with a book, but with this chart, focus was very accurate.

Part of the directions that come with these charts show how to ensure that the sensor is detecting only the target, which is harder than I thought it would be. In particular, the red square is considerably smaller than the sensed area.

Good luck!

Reid
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