Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
03-22-2010, 04:16 PM   #1
Moderator
Loyal Site Supporter
BigMackCam's Avatar

Join Date: Mar 2010
Location: North East of England
Posts: 6,655
K-7 back-focusing, and lens issues

Hi all

I just made a big switch from Nikon to Pentax after losing faith in customer service and technical support from the former. I won't bore you with the detail... what matters is, after extremely careful thought and research, I upgraded from my Nikon D40X and low-end Sigma lenses to a K-7 with 18-55mm and 50-200mm WR kit lenses. Time will tell whether my choice was a wise one, but for now, I'm very happy

Alas, the first K-7 had to go back to the supplier, as it was consistently back-focusing by a good inch or so on both lenses . The supplier, Cameraworld in London, was fantastic about this... I had the first camera and both lenses for over a week and carried out more than 200 test shots before returning the whole kit. They instantly swapped both the camera and lenses for completely new kit along with a reassuring "come straight back to us if you have any further problems". How refreshing!

Anyway...

The replacement K-7 is working just fine, and I absolutely love the camera. The build quality and overall design is just superb, and from the short time I've played with the camera, I feel like a Pentax convert already!

JPEGs at higher ISO sensitivities do look a fair bit noisier than with my D40X, but that's OK - I understand why, and expected it when I chose the camera. Actually, I kind of like the balance between detail and "grain" on the K-7 - I'm a film guy at heart, and I don't mind some graininess at higher sensitivities.

What I'm less impressed with is the performance of the lenses - in particular, the 18-55mm. Wide-open - and, particularly, in high-contrast / flash situations - there is a considerable amount of "blooming" around brighter areas (for example, the letters on a keyboard have a "glow" around them). If I stop the lens down a little, the problem reduces or goes away completely, but in my book this means I have a slower lens than advertised!

In contrast, the cheap-and-cheerful equivalent I use on my D40X - the Sigma 18-50mm F3.5-5.6 DC HSM - seems way better at similar focal length and aperture settings. In fact, I can't get that cheapo Sigma lens to show more than the faintest blooming in any circumstances whatsoever.

And so to the point of my first message here (other than to say "Hi" and humbly ask for acceptance into this esteemed group!)...

I'm delighted with the K-7, and - for now - happy to compromise for the flexibility these two kit lenses offer me. I don't think my specific 18-55mm lens is at fault, as the first one (coupled to the slightly faulty / mal-adjusted camera body) also showed the very same blooming problem. So, am I right in thinking that the lenses I've bought - particularly the 18-55mm - have serious limitations compared to the low-end Sigmas I'm used to?? If so, I'm kind of surprised that Pentax would package such a wonderful body with comparably poor glass.

I don't mean to offend anyone here - I'll say it again, I already feel like a Pentax convert, and I am very happy with the kit I bought overall. I know when I've figured out the limitations of the lenses, the remaining limitation will be me and NOT the K-7! But I'd love to know what others think about these "kit" lenses.

Cheers

Mike

03-22-2010, 05:05 PM   #2
Forum Member




Join Date: Oct 2009
Location: Maryland
Posts: 80
Hi Mike and welcome to the club. I can't address your specific issue because I don't do much flash photography and have never noticed the problem you describe. But with respect to the kit lenses, they get very high marks from most people on this and the dpreview forums. I started with the original 18-55mm and 50-200mm kit lens 3-1/2 years ago on my K10D, used them for well over a year and was very pleased with the results - many of my favorites were taken on your beautiful island. The new kits are improved and I have had great success with the latest 18-55mm kit on my wife's K-x.

I'm making preparations for another visit to the UK in May and will have my trusty Pentax 18-250mm lens as well as my new Pentax 12-24mm lens to use on my new K-7. I had similar backfocus problems with the K-7 and 12-24mm lens and chose to send them to Pentax for calibration instead of getting a replacement.

Bob
03-22-2010, 05:41 PM   #3
Moderator
Loyal Site Supporter
BigMackCam's Avatar

Join Date: Mar 2010
Location: North East of England
Posts: 6,655
Original Poster
Hi Bob - thanks so much for the response.

You know, the more I play with the camera and lenses, the less I'm inclined to comment until I know what the heck I'm talking about.

I'll say this - at the same (or almost the same) focal lengths and apertures, white lettering against a black background, shot at macro distance (around 1 ft or less) with or without flash *does* have more of a bloom around it with my Pentax 18-55mm / K-7 combo than with the Sigma 18-50mm on my D40X. That said...

When I focus a little further away, there's nothing in it to my eyes. So, perhaps I'm expecting a bit too much of the close focusing for the Pentax lens?

I just played around taking some flash-based shots of my wife's "Mother's Day" flowers, fromt about 2 ft away, and the Pentax combo did just as well as my Nikon. So I guess perhaps I simply need to get used to this new rig and understand the differences?

Interesting to hear that you chose to have your kit calibrated by the manufacturer, Bob. Now that I'm basically happy with my setup, I think I should play with it for a few weeks and - if I'm unsure of the performance - I may choose to do the same as you. Somehow, though, I suspect I just need to settle in with my Pentax gear! One thing's for sure - I really do like the K-7, so I reckon it's worth taking some time to (a) figure out the rig in detail and learn the idiosyncracies, and (b) if necessary, accept that micro-adjustments may be required and that these don't detract from the fundamental qualities of the equipment.

Thanks for the nice comments about our "beautiful island", Bob Maybe the grass grows greener on the other side... I have a burning desire to re-visit New England since becoming an amateur photographer, so I can properly record all the wonderful sights that CT, RI, MA & MN have to offer!

Take care & thanks again

Mike
03-22-2010, 05:46 PM   #4
Damn Brit
Guest




Moved to lens forum.

Welcome.

A picture (and associated settings) is worth a thousand words. Was SR on?
The kit lens even against your old Sigma is still a cheap kit lens (best kit lens mind you).
It will always be at it's weakest wide open and at the 18 and 55mm ends. Stop it down to f/4 - f/8 and that's where you'll see it perform the best.
As for the 'blooming' (again, a picture and settings help), that could have something to do with saturation, contrast or sharpness etc. If you haven't gone into the menu to change those from the defaults, then you aren't getting the best out of the camera (don't ask what the best settings for those are, that's subjective, you just need to play around to see what you like best).


Last edited by Damn Brit; 03-22-2010 at 06:01 PM.
03-22-2010, 05:50 PM   #5
Veteran Member
tokyoso's Avatar

Join Date: Jan 2010
Location: Tokyo
Photos: Albums
Posts: 723
Congrats on the fine purchase!

Are you using your nikon flash with k-7?
03-22-2010, 06:01 PM   #6
Veteran Member




Join Date: Nov 2008
Location: Melbourne
Photos: Gallery
Posts: 3,549
Mike, the K-7 should have a function in the 'set-up' menu that allows you to adjust the AF no?
03-22-2010, 07:11 PM   #7
Veteran Member
Pentaxor's Avatar

Join Date: May 2009
Location: Vancouver, B.C.
Photos: Gallery
Posts: 6,513
what metering mode was used, btw?
03-22-2010, 11:04 PM   #8
Pentaxian
Marc Sabatella's Avatar

Join Date: Sep 2006
Location: Denver, CO
Photos: Gallery
Posts: 10,686
Most people who have compared the two find the Pentax far better than the Sigma overall. In fact, that's been an issue because in some countries, the Sigma lens used to be packaged with the camera instead of the Pentax for some reason, and many people were not happy about that. But with the DA18-55, I'd say the sort of softness you describe in high contrast close focus situations is typical. I mean, without samples I can't say if you're seeing something beyond what I'd expect, but what you're descirbing is definitely a relative weak spot for the lens. I assume, BTW, you aren't deliberately compromising IQ with a filter?

03-22-2010, 11:08 PM   #9
Veteran Member




Join Date: Nov 2009
Location: Pittsburgh, PA
Posts: 479
I think the "glow" and "blooming" you might be referring to are chromatic aberrations. Please post some examples with the EXIF data intact!
03-23-2010, 11:17 AM   #10
Senior Member
summonbaka's Avatar

Join Date: Dec 2009
Location: Kagoshima, Japan
Photos: Gallery | Albums
Posts: 237
BTW, the k-7 has per-lens micro adjustment for up to 20? back/front focusing lenses. Read the manual for more details.
03-23-2010, 03:56 PM   #11
Moderator
Loyal Site Supporter
BigMackCam's Avatar

Join Date: Mar 2010
Location: North East of England
Posts: 6,655
Original Poster
Thanks for all the helpful responses, folks.

Yes, I'm aware of the AF "fine tuning" feature and have tried that... however, it still seems to me that the wide open performance of the 18-55mm kit lens isn't all that great. I would love to be proven wrong, but I've now seen two examples of this.

Please see attached the various comparative images - hopefully these will demonstrate what I mean.

The first image, K7iso200-55mf56.jpg, is the centre-section of a 100% crop photo of my keyboard (ie. this is where I centre-focused), taken on the K7 with 18-55mm WR kit lens, about 1ft away with flash, shutter speed around 1/100, f5.6 (ie. wide open).

Second image, K7iso200-55mF20.jpg is exactly the same settings but with the lens stopped all the way down to f20.

Third image is from my 10mp Nikon D40X with Sigma 18-50mm lens - again, centre-section of 100% crop image, taken about 1ft away with flash, shutter speed 1/60 and f5.6 (wide open).

To me, the Nikon / Sigma combo is sharper at f5.6.

I can guess what some will say - the K7 image at f5.6 is over-exposed. Well, I did try exposure compensation and dialled down the EV to -2 (not -0.2, but -2), and the problem was certainly reduced, *but* was still way more noticable than on the D40X + Sigma.

Cheers

Mike
Attached Images
     
03-23-2010, 04:01 PM   #12
Moderator
Loyal Site Supporter
BigMackCam's Avatar

Join Date: Mar 2010
Location: North East of England
Posts: 6,655
Original Poster
I should add, with the lens a couple of stops down, the performance is excellent and I've no reason to complain... so, per my earlier post, please don't take this as a "downer" on this camera / lens combo. I love the camera... I'm just unsure about the 18-55mm kit lens wide-open.

Cheers

Mike
03-23-2010, 05:06 PM   #13
Pentaxian
Marc Sabatella's Avatar

Join Date: Sep 2006
Location: Denver, CO
Photos: Gallery
Posts: 10,686
Looks like focus is not on the "G" in the picture above and that plus the fact that it's exposed are contributing. Also, since the K-7 has quite a bit more resolution than the D40x, you're blowing up the 18-55 image a lot larger. That will magnify any differences.

But like I said, that's one respect in which the Sigma may indeed be better, since it's a known weak area on the Pentax. In most other respects, though, virtually everyone who has ever compared finds the Pentax superior overall.
03-24-2010, 05:26 PM   #14
Moderator
Loyal Site Supporter
BigMackCam's Avatar

Join Date: Mar 2010
Location: North East of England
Posts: 6,655
Original Poster
Hi Marc

Thanks for that.

I've conducted a few more experiments this evening and have drawn the following conclusions (non-scientifically, I might add, but methodically):

1. Focus appears to be fine - I've used test subjects of various colour lettering on various colour backgrounds, and most seem pretty good. Black-on-white news print focuses beautifully, and at wide open aperture the area I focus on is crisp, while the areas just in front and behind of the focus point start to blur as expected.

2. Wide-open, the lens does seem to struggle a bit with bright, light colours (white, yellow etc.) against a black or very dark background, producing a "glow" or "halo" around the edges of the brightest areas. If this is due to a focusing problem, then I can only suspect an idiosyncracy in the firmware, as black and other coloured lettering on light backgrounds focus perfectly.

3. Stopped down to f4 at 18mm and f8 at 55mm (as recommended by Damn Brit), the problem ceases to exist - in fact, the lens produces *very* nice results indeed at these settings!

What I've learnt, then, is that my replacement K-7 camera does appear to focus properly (where my first one didn't), as does the 18-55mm kit lens, but the latter doesn't like certain very high contrast subject matter when close-focused at widest aperture. In these situations, stopping down the aperture a little improves matters no end...

... all of which makes absolute sense.

The only slight disappointment I have (if that's not too strong a word) is that the cheap Sigma 18-50mm lens on my D40X doesn't appear to share that same limitation - but now that I understand this "quirk", I can work around it.

All in all, I'm happy with the outcome of my tests, and thank everyone here for their feedback. My camera and lens(es) are working just fine, and it's up to me now to learn how to use them to their best effect!

Thanks

Mike
Reply

Bookmarks
  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
18-55mm, camera, d40x, k-7, k-mount, kit, lens, lenses, pentax, pentax lens, sigma, slr lens
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
how do i check a lens for front/back focusing and general sharpness? ohyouloveme Pentax SLR Lens Discussion 3 04-16-2010 09:23 AM
Back focusing AF lens an issue on MF film bodies? RichyX Pentax Film SLR Discussion 6 11-11-2009 01:38 PM
Focusing issues with AF Alex2007 Photographic Technique 11 04-16-2009 05:06 AM
screwmount lens focusing issues heatherslightbox Pentax SLR Lens Discussion 17 04-07-2009 01:08 PM
DA* 50-135 Back focusing...is it the lens, or me? azcavalier Pentax SLR Lens Discussion 12 06-09-2008 08:49 PM



All times are GMT -7. The time now is 06:29 AM. | See also: NikonForums.com, CanonForums.com part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
register
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]
Top