smc-m out at infinity?

Quick question: is there any particular reason why my 28 might be out at infinity? up to around 10m it's stupidly sharp, but after that it drops right off. Front focus, maybe? It's difficult to adjust for a manual lens on the K5, no?

Bret

There is so much DOF at infinity that there must be something seriously wrong with the lens. Perhaps someone has dimantled it at some point and made a mess of it, who knows, but I would suggest it needs attention.

hmm. Not good. Samples, all uncropped:

this was on the K10d:



this is from the K5:




this, too:



100% crop:




thoughts?

Bret

It may be an obvious question, but could the focusing ring have slipped a little? What is the closest the lens will focus to? I believe it should be 0.3m or 1 foot.

If it is out at the short end, then the ring may have slipped. The ring is only held in place by 3 screws which are accessible once the name plate ring is removed.

Loosen the screws by no more than half a turn, move the ring by around 3mm, retighten and then recheck. For wide angle lenses I find setting the focus at minimum distance more accurate, due to the large depth of field at infinity.

Regards - Steve

Are you using the cameras in manual mode with the custom function for aperture set for aperture ring use. Otherwise you will only shoot at f2.8 and have a shallow depth of field.

I assume from what you say, that you set to the infinity symbol on the lens and the pictures taken at this setting are blurred. If this is the case then you would have the same result on a manual film camera. Or are you focusing the lens by viewing through the viewfinder.

If it is not a depth of field problem, then suspect a lens assembly fault (as John suggests), or a displaced mirror in your camera.

I once dropped a SLR and my next films were all blurred. The mirror had come loose from its seating.

Totally off topic, but I just wanted to comment on the first photo: Nice shot!

Back to topic: I seem to have the same issue with my SMC M 28 2.8. Although I thought it was just because it was old, this thread seems to indicate that I should be expecting more from it.

Might need to look up a place here in London who can do quick effective repairs on this lens.

I haven't *proven* that there's a problem on both bodies. I set the focus at infinity and have assumed that (considering that hyperfocal appears to work very nicely) everything else was good. It's a 28/3.5 and the hyperfocal is right at the right-hand side of the infinity symbol at the moment, for f8. Which, thinking about it, is a bad thing - as that implies the focus point is over 3m and it should be 2.6.

I think I'll take a look at minimum focussing distances next and work from there. At a risk of sounding stupid, how do I take the name ring off?

thanks for the help so far!

Bret

The finishing ring is screwed into the filter thread and unscrews in the same way. This can be fun and games however - I've had some which won't unscrew! A drop of lighter fluid will sometimes unstick them and there are a few techniques to persuade them to unscrew without doing any damage. You need some sort of fairly solid rubber block which you can press against the ring to get a grip and turn it.

I definitely think something has happened with your lens, although I've noticed that hyperfocal distances don't seem to work terribly well on digital. I found that with the K 28/3.5 shots taken using the hyperfocal setting at f8 weren't as sharp as those taken with the lens set to infinity at f8.

oookay, looking at it yesterday;
- it's the same on both bodies
- it's only infinity that's out
- the minimum distance is about the same as a sheet of A4 from the object to the back of the camera....

Bret

Assuming you're looking at it from the camera side, it sounds like the focus ring has definitely slipped (or been put back together wrongly) as I'd expect the hyprefocal to be to the left of infinity.

See the comment:

I have lenses where the focus ring can be turned so that the Infinity mark can be turned beyond the Index mark. Everything is then slightly out of focus. Not much, you would not notice anything on a 6x4 print, but obvious when enlarged - like your two examples of the Silver Birches.

Moving the focus ring on its mount could correct this, but the markings could then be out at closer distances.

This depends on which way the focus ring rotates to go from close to distance. Quite a lot of my lenses do not have any mark for the hyperfocal distance.
Geoff

From what I understand (in "The Asahi Pentax Way"), all lenses have a hyperfocal distance if you align the infinity mark with that for the aperture in use on the DOF scale. Objects between infinity and whatever distance lines up with the other aperture mark should then be in focus at that aperture. I suspect Pentax only bothered to put a specific set of orange marks on the wide angles because they thought it'd be handy for landscape shooters, whereas for normal subjects people are likely to want the increased sharpness of focusing on the subject rather than letting DOF fill the gaps.

True and, indeed, how well you can tell between left and right! (which never was my strong point - different dominant eye and hand)

I had a look at my M28/3.5 yesterday evening and the hyperfocal marking is on the right hand side of the infinity symbol, as expected (not the left, as per my previous post).

However, it does sound as if the lens is not focussing to infinity.

I think that checking the minimum focus distance is a good start, although due to the non-linear focus scale, a small error in location of the ring at minimum focus distance would translate into a massive difference at infinity.

Definitely sounds like lens problem, then, which is good news.

Another easy check would be to mount it on your K-5, on a tripod, aim at an object a long way away (I use a telegraph pole about 300m away from my window), use live view zoomed in to its fullest extent on a portion of the image where there is some detail. Move the focus throughout the range, starting from near, moving towards infinity.

If the image gets progressively sharper until you hit infinty, then the focus ring hits its stop before you reach infinity.

If the image gets sharper, sharpenss peaks and then falls off again as you move towards infinity then the lens is focussing past infinity.

I second that - Live View at 10x magnification (available when the body is set to MF) is like a microscope for focussing issues. It should be easy to see the difference between the visual appearance of correct focus and that of 'nearly' focussed.

I also tried a "ray of light" shot last night from the balcony at 1/1250. With my 135/3.5, the aerials on the roofs over the other side of the road are sharp and delineated - with the 28 they aren't. They should be as was f8 on both lenses and around 50m. No settings were changed.

The 28 also has very slowly reacting aperture blades. Could this have something to do with it? I'm coming to terms with the fact that this is an economic loss, but I will probably ask someone to take a look at the lens anyway.

I'm used to using LV for correcting video focus and confirming absolute focus on many different objects and agree it's rather useful.

Bret