Strange infinity on Pentax 18-250mm lens


WaypointCharlie

Link Posted 13/04/2012 - 09:49
I've managed to get hold of a used Pentax 18-250mm lens to go on my K-5. It appears to be in good condition and I've checked that the auto focus is correct in the middle and ends of the focal length range.

Using auto and visual focus I notice that the infinity focus point reduces towards the wider end. At 250mm infinity focus is pretty much at the infinity mark on the focus ring. At 18mm the focus reaches infinity by about the 7m marking on the focus ring. Winding the focus up to the infinity marker doesn't de-focus or change the focal length, meaning it focusses to infinity from the 7m mark all the way to the infinity mark.

I don't believe this is claimed to be a parfocal lens, so some degree of variation with focal length is to be expected. However, is this sort of amount normal? Is this what Tamron mean by, "The infinity position on the A18 is made with certain allowances to insure proper focus under a variety of conditions"?

Is there some internal adjustment which is out of calibration? Is this down to lens sample variation? I've read some people say they focus at 250mm, then zoom out to 18mm to take the shot at that focus. If infinity starts at 7m at the wide end then this obviously cannot work. Perhaps they are the same people who say you need to stop down the lens a lot to get any decent IQ!

I'm not too bothered by this as otherwise the lens does what I'd expect of it, but it would be nice for manual focussing if it could be adjusted so that focus at 250mm was good for 18mm.

johnriley

Link Posted 13/04/2012 - 09:58
You can't use the technique of focusing at the longest focal length and then zooming out with many zoom lenses. They are not really true zooms, but varifocal lenses. The focus point will shift as you change focal length. We call them all zooms for simplicity, although the real meaning is probably getting a bit lost.

Zoom first, then focus and you should be fine. Any problems with your lens will soon become apparent.

It's a great lens, really versatile and should be great fun to use.
Best regards, John

DNA

Link Posted 13/04/2012 - 10:27
I would like to have a zoom like this, but you can't order this lens any more I did notice. Is that correct?

WaypointCharlie

Link Posted 13/04/2012 - 10:45
Thanks John. So reaching infinity at about 7m on the focus ring (about half the focus ring range) would be considered normal for this lens at the 18mm wide end?

Yes DNA, these lenses are only available used now. I believe they are basically the Tamron design with minor tweaks to badge as Pentax. I held out for a copy of the Pentax version, in spite of them generally fetching a higher price. I understand the in-body lens correction only works with the Pentax lens, not the Tamron and I'm not sure if the camera can pick up the MTF information with the Tamron (MTF is my preferred P-line). Also some say the lens coatings are different.

I see from the Pentax roadmap that a new 18-200mm (ish) lens may be launched this year. I also see there may be an DA* 18-85mm (ish) lens next year or later. That could be very nice...and of course very expensive.

johnriley

Link Posted 13/04/2012 - 10:58
I missed the 7m reference, and that does seem a bit excessive, but when i check out Sue's lens I'm surprised to find that hers is the same. It depends how far away the infinity subject is of course but at quite long distances and 18mm the focus point does approach the 7m mark.

Her lens is very sharp so I assume it's a characteristic of the lens. Well spotted.
Best regards, John
Last Edited by johnriley on 13/04/2012 - 11:01

milamber

Link Posted 13/04/2012 - 11:00
I've just checked mine and it's the same. Infinity focus at 18mm is roughly at 7m on the focus ring and doesn't change up to infinity.

To be honest it's not something I've ever considered. I assumed it was normal to refocus when the focal length is changed.

I love my 18-250mm. It's a brilliant travel lens and don't let the odd negative comment about IQ put you off. They are largely from people who have never used one and don't believe a zoom with this range can be this good.

WaypointCharlie

Link Posted 13/04/2012 - 11:49
Thanks guys, it's good to hear that my copy isn't anything abnormal.

When I got the lens a few days ago I played around checking the focus and working out how best to use it. I tied focussing at 250mm then zoomed out to 18mm, keeping the same focus. Needless to say this resulted in it being way out and severely back focussed. It obviously clicked when I found infinity started at 7m at the wide end!

Judging by feedback on the Amazon sites most users seem to really like this lens. A few people are critical of the IQ but, given the varifocal nature of this lens, I do wonder if some of them are trying to focus at 250mm and zoom out. I can see how they'd only be happy severely stopped down!

Here are a couple of 250mm test snaps (bear in mind these may be more like 220mm, given that you only hit 250mm at infinity with this lens). The lamb isn't that sharp, perhaps because of the slow shutter speed, although the original doesn't seem to show signs of shake. Perhaps I should have stopped down to get a wider depth of field (coming from a compact camera it's something I haven't had to worrying about before).

http://www.flickr.com/photos/waypointcharlie/6924084150/
http://www.flickr.com/photos/waypointcharlie/7070162659/
Last Edited by WaypointCharlie on 13/04/2012 - 12:07

johnriley

Link Posted 13/04/2012 - 12:05
Glad it's all OK and sorted and we've all learned something new.

The pictures look good, it will be nice to see some more in due course.
Best regards, John

JAK

Link Posted 13/04/2012 - 12:43
I took this photo of the QM2 passing Scarborough in October 2009 with my GX20 and 18-250 (set to 250mm.)




It was still several miles out to sea so it's cropped from the original image. I sent it in to the BBC for their weather bulletin photos and it was on screen later that night!

John
John K
Last Edited by JAK on 13/04/2012 - 12:45

DNA

Link Posted 01/05/2012 - 20:56
WaypointCharlie wrote:
Thanks John. So reaching infinity at about 7m on the focus ring (about half the focus ring range) would be considered normal for this lens at the 18mm wide end?

Yes DNA, these lenses are only available used now. I believe they are basically the Tamron design with minor tweaks to badge as Pentax. I held out for a copy of the Pentax version, in spite of them generally fetching a higher price. I understand the in-body lens correction only works with the Pentax lens, not the Tamron and I'm not sure if the camera can pick up the MTF information with the Tamron (MTF is my preferred P-line). Also some say the lens coatings are different.

I see from the Pentax roadmap that a new 18-200mm (ish) lens may be launched this year. I also see there may be an DA* 18-85mm (ish) lens next year or later. That could be very nice...and of course very expensive.

What about that DA 18-85mm? What will be the price then?

JAK

Link Posted 01/05/2012 - 21:54
WaypointCharlie wrote:
I understand the in-body lens correction only works with the Pentax lens, not the Tamron and I'm not sure if the camera can pick up the MTF information with the Tamron (MTF is my preferred P-line). Also some say the lens coatings are different.

For info, the Tamron lens does pass the lens info for in-body lens correction. (I've got both a Pentax and a Tamron version.) If there is any difference, the Tamron version seems a little warmer in its colour rendition for some reason, but that's about all.


DNA wrote:
What about that DA 18-85mm? What will be the price then?

You tell us! I know it's hinted at on the Pentax Road Map but I've not seen details of it yet, have you?

John
John K
Last Edited by JAK on 01/05/2012 - 21:54

WaypointCharlie

Link Posted 02/05/2012 - 09:51
Quote:
JAK wrote:
For info, the Tamron lens does pass the lens info for in-body lens correction. (I've got both a Pentax and a Tamron version.) If there is any difference, the Tamron version seems a little warmer in its colour rendition for some reason, but that's about all.

Quote:
audiobomber wrote:
There was a lot of confusion about this, because the lens correction option is selectable when you mount a Tamron 18-250, and the processing speed slows as if corrections are being made. But in the end the corrections are not applied, the photo is unaltered. So if lens corrections are important to you, get the DA lens.

From PentaxForums.com: http://www.pentaxforums.com/forums/pentax-slr-lens-discussion/138601-pentax-18-2...

In-body lens correction isn't something I plan on using but as there's confusion as to whether or not it can be done with the Tamron I suspect it may impact slightly on the resale value. Someone said they had the Tamron then bought the Pentax lens, simply for the lens correction. Were they misled?

I'd be slightly bothered if the camera couldn't pick up the Tamron MTF info, although I'm not sure how well the camera utilises it and am growing out of using MTF P-Line. Not easy to test that I guess.

JAK

Link Posted 02/05/2012 - 11:23
WaypointCharlie wrote:
In-body lens correction isn't something I plan on using but as there's confusion as to whether or not it can be done with the Tamron I suspect it may impact slightly on the resale value. Someone said they had the Tamron then bought the Pentax lens, simply for the lens correction. Were they misled?

Sorry, have to eat humble pie - the Tamron doesn't have it. When I got the lens and tried it I thought it did, but it doesn't. Since trying the Tamron it has been kept in its box so my initial quick impression was wrong. Given the lens was made by Tamron as a joint venture with Pentax, one would have thought Tamron would have included the Pentax data in the lens though. And yes, the Pentax one does.

WaypointCharlie wrote:
I'd be slightly bothered if the camera couldn't pick up the Tamron MTF info, although I'm not sure how well the camera utilises it and am growing out of using MTF P-Line. Not easy to test that I guess.

I have no idea how they're programmed, I wonder if the camera uses a default if that detail isn't in the lens chip.

John
John K
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