Pentax-M 85mm portrait lens

I wonder if anyone can advise me on this.

I have a K100D and an old SMC Pentax-M 85mm portrait lens.

It is a beautiful lens. When I really hit the focus, then the clarity is superb.

The difficulty I am having is getting the focus right.

My eye is telling me that I'm focusing correctly. So is the focus check on the camera. (Obviously the Autofocus as such won't work with this lens.)

Half the time, however, despite being quite certain that I have focused correctly, the focus will turn out to be wrong - usually an inch or so closer than I expected.

Now, maybe my eyes just aren't what they were, and it's all to do with lack of recent practice etc. On the other hand, is anyone out there aware of a specific problem which might relate to this combination of lens and camera?

Thanking you all in advance,


William

I have an 85mm softfocus lens. That's a bugger to focus! make sure your diopter is set right, and trust the green confirmation circle.
if yours is the "soft" version like mine, you have to remember that the images get softer towards the edges, so you cannot focus and recompose your shot. You focus on what you want sharp...then recompose, by cropping the image in post...

a quick search on these forums will yeild lots of info on adusting the diopter....

Yours evidently isn't, Don...

he,he,he!
not exactly a circle is it?
chalk that one up as a "Brain Fart"!

Thanks to both of you for responding.

Not sure, however, that I've really got to the bottom of this.

I have tried photographing several objects, getting the focus just right at the exact centre of the lens. Both I, and the autofocus were in perfect agreement. And, on the photo, the focus was just "out".

The dioptre, by the way, is set perfectly for my eyes - so that's not the issue - I don't think.

Setting the aperture ring to f2 plays complete havoc with the focusing. I suppose that that is to be expected; it is obviously going to be more critical. If, however, having set the focus to what my eye and the autofocus agree is correct, I then just nudge the focus a tiny bit too close, then the focus is correct.

Does this indeed suggest a fault on the camera, or is it an incompatibility between the lens and the camera? I have a 35-105 zoom, with autofocus and an “A” setting for the aperture. Whilst the overall quality of my zoom is less good – and it’s a lot less bright; f4, as opposed to f2 for the portrait - it certainly doesn't seem to have the same sort of focus problems.

I would dearly love to solve this problem. The portrait is a wonderful lens. It cost me about £85 20 years ago; replacing it with something as good is clearly going to cost me an awful lot more than that.

Any thoughts?

Your camera could be front or back focusing. Have you tested it with a focus chart. :

Try this chart.

link

Umm - tell me more. I am, incidentally, very new to digital cameras. You never guessed that, did you?

In addition to testing the focusing, will a firmware update help me?

Let's go right back to basics. Focusing is done at open aperture where the image is brightest and the depth of field is smallest, so the slightest movement of the ficusing ring will snap in and out of focus. The eye is only fairly good at this so tests have revealed that the focusing position that we see can vary a fair bit from attempt to attempt. However, the DOF when the image is taken is usually more than sufficient to cover any slight errors.

If you are saying that you are shooting at f2 and the focus is in the wrong place then the first thing to do is get a stationary subject and put the camera on a tripod. Now try the focusing. If it is now OK then the problem could well be both the subject and the camera moving after you have focused.

Models move about a fair bit and so do photographers and at f2 there is no room for this. Also, don't forget that with a portrait the eyes are the place to focus. It's all in the eyes.

With an 85mm lens at faitly close distances f2 may well be too wide to get enough of the face in focus, so don't hesitate to use f5.6 or even smaller apertures, depending on the photographic style you want.

My suspicion with this is that you are shooting hand held and simply losing the point of focus by the time you have pressed the shutter release.

Dear John,

This is all helpful stuff, and I'll certainly try it.

There are a few things, though, that might be worth pointing out:

I regularly used this lens with my old ME-Super and never had this problem;

I tend to use multiple exposures when taking portraits - a better chance of getting good shots in general, and less chance of camera shake (certainly on the second or third shots);

I appreciate the problems with depth of field. The issue has been that this problem came up in the aftermath of an photoshoot where I was making a particular point of using higher F numbers, precisely in order to improve the depth of field. The light was good and bright - too bright for one model; we had to go indoors, where, ironically, I got a much better series of shots, though under much less good light conditions;

I always focus on the eyes. It has rather felt as though the more accurately I am focused, the less good the end result.

I shall now get out my trusty tripod and see what happens.

A few minutes later:

I suspect that your comment on DOF is probably correct - though I still have doubts.

I've tried a few shots on the tripod, using f2, and the results are not entirely consistent. What is disconcerting is that, with the tripod to hold the camera steady, and the autofocus telling me that all is well, I would expect consistently well focused results. This is not what is happening.

I took a look, incidentally, at the focus chart you pointed me at. Can you explain how it works? It might still be worth doing some tests.

(And thank you!)

William

William. Generally with the focus chart you lay it flat on a table and set the camera on your tripod, pointing it at a 45degree angle to it.
Get the camera as close as you can and focus on the centre line.
(i would focus directly on the numbers running down the side of the chart)

Fire off a few shots at different f-stops and this should tell you if its back or front focusing.

Others may have a different way of doing it, and i'm sure you will get a few ideas from those more in the know than I. Good luck with it.

Hi William, firstly the lens you're using is a great piece of glass; I only changed mine when I acquired the faster f1.4 version. You’re right it would cost a lost more to get a comparable lens now, something like the FA 77mm f1.8 Limited (₤529 @ SRS).

I have found certainly a lot easier to get good consistent focusing once a changed the standard focus screen to a Katz eye screen on the K10D. This gives a much more familiar screen like the LX and if I remember correctly the ME Super, a split circle and micro prism ring. This for me gives much more critical focus, especially when combined with viewfinder magnifier O-ME53 (magnification of 1.2).

All that having been said, I don’t know if it is possible to change screens on the K100D, just a thought for you.

I have found certainly a lot easier to get good consistent focusing once a changed the standard focus screen to a Katz eye screen on the K10D. This gives a much more familiar screen like the LX and if I remember correctly the ME Super, a split circle and micro prism ring. This for me gives much more critical focus, especially when combined with viewfinder magnifier O-ME53 (magnification of 1.2).

Sorry about that - hit the wrong button.

Yes, I miss the split prism - I had both an ME Super and an LX, and both had it. OK, so it could distract a little from the shot, but it was more helpful than not.

Tests with the chart have proved rather inconclusive. Best results seem to come at around f5.6 to f11. Hard to gauge quite why or how. I'l play with it for a bit longer before I decide that it's undependable, or not worth the hassle.

Thank you, all, for your suggestions - and I'll happily take note of anything that anyone else has to suggest.

The focus chart should be used with the lens wide open. My K10D had a backfocus problem and these are my "before & after" shots of the chart:

Before

[IMG]http://i223.photobucket.com/albums/dd37/hefty1969/IMGP5284.jpg[/IMG]

After

[IMG]http://i223.photobucket.com/albums/dd37/hefty1969/IMGP5312.jpg[/IMG]

As you can see in the first shot, despite the autofocus (and my eyes) confirming the thick black line was in focus the camera was actually focussing on a point around 14mm behind it (you can measure this by looking at the mid-point between where the image comes into focus and where it fades out again).

The second shot was taken after I'd calibrated the camera and shows the sort of image that you want to find - the black line perfectly central to the area in focus.

Try the test with several lenses and under different lighting conditions, all of them at their widest. Use a remote to trigger the camera if you have one and use the 2 second timer to give you mirror lock - eliminate all chance of shake basically.

If all your lenses display a similar pattern (mine did!) then it's safe to calibrate the camera (we can point you towards directions if it comes to that). If your lenses have different characteristics then at least you'll have a clear indication of how much compensation you need to apply to a wayward lens to bring it into line.

Good luck anyway