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04-12-2009, 04:02 AM   #1
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70mm Ltd... not again!

So I got a 40mm ltd and it had serious backfocus issues. The shop i got it from replaced it and the new one blows me away.

So i got my myself a 70mm ltd. Of course i tested it for backfocus with a few quick shots and it seemed fine. I got out and shot with it over the last 3 days and my happiness slowly faded. I kept finding myself pushing it to f/5.6. And at 5.6 i am happy. But every time I go any more open than that it is a bit of a lucky dip.... it is so hit and miss and wide open it is very miss.

So i set up a proper test tonight and...DAMN! this one front focuses. Why why why?!?!

Both shots below are from different distances. I shot at least 20 pics from each distance and the results are the same every time. For petes sake! That is two ltd i have bought and 2 of them have issues.

Needless to say i am very unimpressed with pentax quality control.

When these lenses get it right they get it VERY right but so far 100% of the limited that i have bought have started off as lemons.

The shop will think i am a freak, 2 lenses, 2 complaints.

I am cursed.










cheers
mike

04-12-2009, 04:25 AM   #2
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You are testing in daylight, are you not?

Your pictures do show a possibility you are testing under indoor lighting. You will get incorrect results that way
04-12-2009, 04:53 AM   #3
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Yes these tests are indoors. I will retest outdoors in the morning. I was intending on running a few more tests before contacting the shop on tuesday (monday is a holiday).

However, I am quite sure that the issue will still exist. The only reason i started testing was because when shooting at any aperture larger than f/5.6 (ie 2.4 - 4.5) the results are very inconsistent with far too many missed focus shots. The lens misses more than it hits.

This became apparent when flicking through 70ltd images and (good copy) 40 ltd images. The 40 ltd images where almost all in focus at the open end of the scale. The 70 ltd images are soft at least 50% of the time.

BTW, why does indoor light make a difference? Does that mean that these lenses are unrealiable indoors? Isn't that one of the big reasons to get a fast lens? to shoot indoors?

mike
04-12-2009, 05:10 AM   #4
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I don't know the scientific reason why you incandescent light affects autofocus but it does appear to do so. There is nothing wrong with lenses when shhoting indoors, it is the autofocus system that has a problem.

As well as the focus chart test , stick the camera on a tripod outdoors and manually focus until you know focus is correct (you will need to take the card out and look at the pictures on your computer to comfirm). Then note where the distance scale is on the lens, and try repeated AF tests to see if they agree.

I got such inconsistent results with the latter with many lenses I have come to one inescapable conclusion.........autofocus on cameras is nowhere near as accurate as any manufacturer would have you believe. That is one reason they dont sell cameras with (or with options) for a split prism focus screen anymore.

04-12-2009, 05:21 AM   #5
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QuoteOriginally posted by pschlute Quote
As well as the focus chart test , stick the camera on a tripod outdoors and manually focus until you know focus is correct (you will need to take the card out and look at the pictures on your computer to comfirm). Then note where the distance scale is on the lens, and try repeated AF tests to see if they agree.

.
I always use a tripod for the tests. Usually a cable release too but in the above tests i just dropped the ISO to UP the shutter speed to avoid any shake.

I see what you are saying above but different lenses have different behaviour with AF. eg, the first 40mm ltd i had backfocused badly, now this 70mm ltd appears to be front focusing.

If the AF system itself was flawed then i would expect both lenses to either backfoocus or both to front focus.

When i was testing the 40 i tested in multiple bodies and it happened with all of them. I will go and borrow another body tomorrow too. If it misbehaves on two bodies then that is good enough for me.

Rant at pentax: what good is it to buy a fast lens if one can not use it above F/5.6

cheers
mike
04-12-2009, 06:17 AM   #6
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Get a Katzeye or Chinese knock-off split focusing screen...

Problem solved!

Cheers...
04-12-2009, 08:23 AM   #7
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Michael where did you get that screen from

QuoteOriginally posted by Michaelina2 Quote
Get a Katzeye or Chinese knock-off split focusing screen...

Problem solved!

Cheers...
And what options did you order it with?

This is the first time I am hearing that the focus screens solve the FF/BF issues with the lenses. That would be great... where did you get your Screen from and what style did you get?

Unless you are saying that he should get a screen to manually focus the DA 70... if that is the case the DA pancakes have such a short focus adjust, I would not think that would be a good option for the OP.
Plus 450 bucks is a lot to pay for a AF lens that only works well as a MF.
04-12-2009, 08:34 AM   #8
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The focus screen will do nothing for AF , except help you know where perfect focus is by manually focussing. Assuming that it is made correctly and fitted correctly that is.

04-12-2009, 08:37 AM   #9
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yeah he is joking...no focus screen will solve an af issue.... but certainly get a focus screen if you dont have one, they are awesome
04-12-2009, 09:10 AM   #10
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Oh I am

confused!

I fell victim to the "cool face"
I thought he was saying "Cool... I have found a solution to the problem"

I am feeling a little embarrassed



When I am trying to be clever I tend to go for the , If I am trying a lame attempt at humor I may go with the or the or even the ... seldom using the LOL face since I know I am not that funny...
04-12-2009, 11:39 AM   #11
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QuoteOriginally posted by Igilligan Quote
And what options did you order it with?

This is the first time I am hearing that the focus screens solve the FF/BF issues with the lenses. That would be great... where did you get your Screen from and what style did you get?

Unless you are saying that he should get a screen to manually focus the DA 70... if that is the case the DA pancakes have such a short focus adjust, I would not think that would be a good option for the OP.
Plus 450 bucks is a lot to pay for a AF lens that only works well as a MF.
AF gets you close, then fine tune with manual focus to 'spot on.'

Also, with the split screen in place, one can adjust the focus (K20D) using a high contrast vertical item (telephone pole at say, 50 feet). It's very easy to do with almost no fussing around. Mount camera on your tripod, open aperture wide, then (+/-) adjust until the pole appears to be a continuous line.

I use a Chinese knock-off on my K20D and K100D (both are the same screen size). They cost about U$30 each, delivered (took about 10 days to US).

Works great...
04-12-2009, 12:27 PM   #12
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When I first got my FA 31mm Ltd I was concerned that it was a dud as focus was sometimes off, ever so slightly. I never use AF with it now....always manual with the split-prism screen.

It feels like I have re-taken control over my photography.
04-12-2009, 12:31 PM   #13
Damn Brit
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QuoteOriginally posted by Igilligan Quote
confused!

I fell victim to the "cool face"
I thought he was saying "Cool... I have found a solution to the problem"

I am feeling a little embarrassed



When I am trying to be clever I tend to go for the , If I am trying a lame attempt at humor I may go with the or the or even the ... seldom using the LOL face since I know I am not that funny...
Don't be confused Gus, send the K20 back to Pentax.
04-12-2009, 02:12 PM   #14
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I don't have a k20d but i know it comes with AF adjustment that can be set on a per lens basis. Is this pretty acknowledging that their lenses are not calibrated that well?

Maybe i should just buy a k20d..... let me just go out side and pick a few more notes from the money tree.

mike
04-12-2009, 02:55 PM   #15
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Hmmm..... tested again under natural lights. Same results. Shows two things. That AF not liking indoor lighting is probably a myth and that this lens is no good.

Both shot at different distances again. Shot at least 20 at each distance and reset focus between each shot.

At the greater distance the focus point is not even with the DOF. Even at the short distance it may sneak in by a bees dick. Pentax! Delivers junk again. I need to go and have a soothing herbal tea before I turn into rice high.






cheers
mike
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