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10-24-2017, 05:16 PM   #1
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Pentax 16-50 f2.8 woes

Phil_Rose wrote:

Hi all, I just bought a used lens (16-50) on ebay and I'm finding I have some focusing issues.It seems to frequently (though not always) focus just a little behind where it looks like it s focusing. This is only in AF mode. It seems worse on people and leaves and better on highly textures objects. I have had an f4 version of the same lens for 10 years or more and I never saw this issue. Is this something others have seen? Is it just that at 50mm and 2.8 it's hard for a lens to be accurate? Surely it's not too much to ask!

Should I return this as having problems?

Update I did return this and have had it replaced and finding the same problem. I did a location portraiture shoot this afternoon and focusing on the guys eyes at f2.8 to get a nice shallow depth of field and 50 mm and about 3 feet distance is focusing not on his eyes but slightly behind his eyes by the look of it. I have tried playing with the fine focusing adjustment on the camera to no avail.

Can anyone help?

Phil

10-24-2017, 05:24 PM - 1 Like   #2
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QuoteOriginally posted by Phil_Rose Quote
I have tried playing with the fine focusing adjustment on the camera to no avail.
Did you test the lens methodically? Is there a specific focal length where it's performing worse than at others? Is the focusing accurate in live view mode?

Here are the steps needed to test the lens:
Fixing Front and Back Focus - Introduction - In-Depth Articles

It could be the lens, or it could be the camera simply focusing somewhere else. You may need to tweak where you position the AF point. Which body are you using?

If in-camera adjustments don't help, you should send in the lens along with the camera for calibration. May be more economical to buy the lens with with warranty so that you don't have to pay extra for that service.

Also, bear in mind that the lens will be softer at F2.8 than what you may be used to, but with accurate focus the file should be perfectly usable.

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10-24-2017, 07:51 PM - 1 Like   #3
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If I'm at 3' or so at less than f4, with a small target, I always use live view to focus. Only reliable method I know. The other choice is to change drive mode to high and fire off 10-15 shots. One will be in focus.
10-25-2017, 07:24 AM   #4
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I too find that this lens back-focuses, but only until it is adjusted for in the camera menu (see the article Adam linked to).

Actually I owned the lens for quite a while before running into the calibration issue, because I had only used it on a K-01, which focuses in contrast-detect mode and is immune to these differences.

10-25-2017, 07:45 AM   #5
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Thanks all, I'll try that article and get back to you.
10-25-2017, 08:59 AM - 1 Like   #6
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Hi there. I did the test where I change the Micro Focus whilst shooting a flat test piece and I uploaded the images here

Micro Focus Tests - philrosephoto

I have also uploaded one image from yesterday shoot which seems soft to me. Today I volunteer as I do every Wednesday at the local Humane Society where I Photograph cats. I will take both the 16 to 50 and the 16-45 and do some with one and some with the other. This will be a baptism of fire since cats in boxes are notoriously difficult subjects! I will let you know when I have added the results to the same folder and you can tell me if I am being a pixel peeper. It seems like perhaps the best bit of advice I've had is that I just need to take lots of photographs and hope one of them comes out in focus which is clearly not ideal. This doesn't seem to happen and anything over about 3 feet, by the way.

Oh, the photograph taken 16-45 had no micro-adjustment. I have never found I needed to use that feature. Somebody asked whether maybe my focus point was slightly off but since no other lens has this problem I'm assuming that that can't be the case? It can't be that the focus point is off for one lens and not for others, could it? The fact that it seems to do a very good job focusing on flat subjects does kind of suggest that that could be the case. What do others think? Would it be possible for the focus point be off for one lens and not for others? I usually shoot spot or customisable spot (is that the correct name?) And don't find Pentax is very good on multipoint focusing so pretty much never use that.

Thanks again to everyone to sticking with me on this.

Phil

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10-25-2017, 06:33 PM   #7
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QuoteOriginally posted by Phil_Rose Quote
Oh, the photograph taken 16-45 had no micro-adjustment. I have never found I needed to use that feature. Somebody asked whether maybe my focus point was slightly off but since no other lens has this problem I'm assuming that that can't be the case? It can't be that the focus point is off for one lens and not for others, could it? The fact that it seems to do a very good job focusing on flat subjects does kind of suggest that that could be the case. What do others think? Would it be possible for the focus point be off for one lens and not for others? I usually shoot spot or customisable spot (is that the correct name?) And don't find Pentax is very good on multipoint focusing so pretty much never use that.
It's possible (and not considered a defect) for one lens to requite calibration and another not to. But the position of the point would be the same.

Are you still shooting with the K10D and K20D? Is the focusing performance between the two consistent?

Lastly, you'd get more reliable focusing performance with the K-5 II or newer, as those bodies are considerably faster and more sensitive in low light, and can leverage the faster aperture. It's also easier to apply corrections via the menu.

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10-25-2017, 06:46 PM   #8
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I checked out your photos, and shooting a flat resolution chart is going to be difficult to dial in the fine focus of the lens. You can do it, but it is much easier to shoot at something at a 45 degree angle, so you can see how far in front/behind the point you are focusing on is really in focus. Going back to your charts, I can't tell if you were really square to them or not. Being really accurately aligned is critical for any evaluation - the right side is sharper than the left. Is it because you were skewed a degree or the lens is out of alignment? Can't tell. I'd put a level on the chart, and make sure it is really square on the wall. Also use a level on your tripod to make sure it is true. Then with the camera on the tripod, position the whole assembly so the chart is really square in the frame with even spacing all around. Physically move the tripod up/down/sideways/front back/ whatever to get the chart square, but don't rotate/swing it o do so - you need to be really square. If you can get it so you have even space all the way around, you're probably at a point you can start to make a better judgement about the lens and it's optical performance and alignment.

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