Just had a thought


walkeja

Link Posted 23/07/2012 - 13:16
Dangerous, I know, but reading the topic the other day about adjusting the camera for autofocus and lenses that wern't quite sharp. How did we get on in the days of film when this facility wasn't available? Does this mean that the manufacturing standards of lenses or cameras have dropped?
Pentax K1-ii and MZ6
Pentax Lenses 28-80 F, 300 DA*, 80-200 F, 35 F2.4 AL, M50 F1.7, 28-105 DFA, 20 F4 SMC
ONE UNITED Member

greynolds999

Link Posted 23/07/2012 - 13:21
Because we used to focus manually! Therefore when the image was in focus, it was ... err ... in focus!
My Photobucket

johnriley

Link Posted 23/07/2012 - 14:06
And that's still true. What has been added is the opportunity to pixel-peep, which means that we now attribute focusing errors to the technology rather than user error.

Sometimes adjustment may be helpful, especially with independed lenses, but generally it isn't. I have never adjusted a lens. I know when my lenses are sharp, it's when the pictures are sharp. And also, as greynolds999 states so succinctly, when they they are in focus they are in focus.

When they are not we may be at fault as well and we can always give a tweak using QuickShift in many cases.
Best regards, John

Jonathan-Mac

Link Posted 23/07/2012 - 15:00
Film is also easier to focus on because the light-sensitive layer is thicker than the surface of an electronic sensor.

Plus film is not as sharp as digital and peeping at very fine detail (at least at 35mm) was far less common, because it was harder to do (loupe or large print). So slight focusing errors were not as obvious.
Pentax hybrid user - Digital K3 & K200D, film 645 and 35mm SLR and Pentax (&other) lenses adapted to Fuji X digital
Fan of DA limited and old manual lenses

johnriley

Link Posted 23/07/2012 - 15:06
Some camera tests would comment on the accuracy of collimation, but usually that was just limited to a terse "the point of focus was correct" so it wasn't seen as a major issue.

Film flatness was more of an issue, particularly when film was left in the camera and would gain a "kink" in the transport mechanism. This was true of TLRs, with their complex film path, and 35mm cassettes at the point of exit from the cassette.

With 35mm film eventually it was realised that exiting from the cassette tangentially was helpful. In any event, the first shots made were often less sharp that subsequent shots, when a flat part of the film was reached.
Best regards, John

wvbarnes

Link Posted 23/07/2012 - 18:35
John is right about film flatness. I recall from selling cameras in the Seventies 126 and 110 cartridge film was useless because of the lack of control over film flatness under the pressure plate. Good 35mm cameras controlled the film better by efficient clutches and good pressure plates.

As to the business of adjusting lenses for perceived front and back focus in the digital era I've found professional friends of thirty or more years experience bemused by the notion. Probably because they use very high quality lenses and despite all the gadgets they mostly manually focus.

Personally if I had a new lens that wasn't sharp in use I'd want it replaced as 'not fit for purpose'. I can undertand the facility for very old legacy lenses which surely is Pentaxes intention?

MrB

Link Posted 23/07/2012 - 23:20
Doubtless there are professionals in every sphere who shun some of the modern gadgets but there will be others who rely on them. I can comment only as a relatively inexperienced amateur digital photographer - the view through the 18-135mm zoom on my K-5 is good, but it simply cannot compare with the brightness, clarity, and manual focusing aids of my old ME Super with 50mm prime, and so I prefer to use AF all the time. As the K-5 includes the feature to check and calibrate the AF system, my view is that I might as well make use of it, particularly as it is not a difficult procedure, and any adjustment needed for any attached lens is stored in the camera. I happen to think it can make a helpful difference to AF but, as with many other things in photography, there is a free choice as to whether or not to use the feature. I also think it is evidence of camera quality rather than declining standards.

Philip

johnha

Link Posted 23/07/2012 - 23:32
I agree with much of the above. Contax used a vacuum pressure plate supposedly to minimise film flatness issues. The LX has a longer pressure plate than most Pentax cameras (and longer film guides).

I had a 20x30 inch poster from Kodachrome 25 printed. The detail was amazing but showed up limitations with my gear (MX & M 28/2.8 ) and technique.

If there is a build problem, it is most likely to be in the body, not the lens. The body drives the lens until it has feedback that it is 'in focus' (maximum contrast?) and the only way an 'AF adjustment' would improve things is if the imaging sensor, mirror(s) or AF sensor are out of alignment.
PPG Flickr
Last Edited by johnha on 23/07/2012 - 23:33

Algernon

Link Posted 24/07/2012 - 07:56
Johnha...
I wonder how they held the slide flat to make the print

Unless you sandwiched film between 2 sheets of glass in
an enlarger it was never flat
Half Man... Half Pentax ... Half Cucumber

Pentax K-1 + K-5 and some other stuff

Algi

MrB

Link Posted 24/07/2012 - 20:45
For anyone who is interested, I used the method I described in the other thread at
https://www.pentaxuser.com/forum/topic/rolls-royce-special-edition-36076/p-0
to photo a ruler before and after AF adjustment. These are the crops - the image should be viewed at full size:




The camera was tripod mounted and the shutter was released by the self-timer to avoid camera-shake. The focus point was on the 200mm mark. Before AF adjustment (left) the depth of field lies mostly between 200mm and 210mm (in fact the 2 of 200 is not sharp); after AF adjustment the depth of field is is more evenly distributed either side of the 200mm mark (as it should be, and the 2 is now sharp).

These results are consistent and, therefore, reproducible - by anyone.

Philip
Last Edited by the Moderator Team on 25/07/2012 - 08:27

johnriley

Link Posted 24/07/2012 - 20:51
Interesting Philip. I'm not suggesting this isn't valid, but I do have some thoughts about it. I'm wondering about the fact that if correctly focused there should be more DOF behind the focus point than in front. I wonder if some users are tring to make it equal both sides.

The second thought is that checking focus at one distance, one focal length, what aboput all the other distances and focal lengths? If a zoom, of course. Field flatness at close distances will also be a major factor in this.

I'm not sure that it's worth meddling with unless there is a demonstrable problem.
Best regards, John

wvbarnes

Link Posted 24/07/2012 - 21:04
Hi,

I agree with John.

I recall the rough rule of thumb always was two thirds behind against one third in front, varying with the aperture used of course. i'm sure that's not changed in the digital era.

My maths is terrible but you can Google or Wikipedia all this hyperfocal stuff for fancy formulae.

I'd never heard about all this messing with per lens settings until internet forums started on it. I still say if you buy a lens and its wonky send it back. if it's an old one then fair enough BUT then surely you rely on manual focus.

Magnified live view will of course prove it all to you as that is (supposedly) spot on.

Algernon

Link Posted 24/07/2012 - 21:20
DOF close up is about 50/50

There are plenty online checkers to find it exactly.
Half Man... Half Pentax ... Half Cucumber

Pentax K-1 + K-5 and some other stuff

Algi

MrB

Link Posted 24/07/2012 - 21:24
If you look at depth of field tables you will see that, at shorter distances, the depth of field is about the same on each side of the subject plane. E.g. For a lens on a K-5 at f=50mm and f/5.6 for a subject 100cm away, the depth of field is 4.12cm in front of the subject and 4.49cm behind.

It is, of course, at even shorter distances where accurate focusing becomes much more important. Much less so at greater subject distances - in the scenario above, if the subject were 5 metres away, the depth of field would increase to total almost 240cm, and at 10m DoF would be over 3m in front and over 8m behind.

Philip

(Sorry Algi - didn't see your comment, but this is supporting it!)
Last Edited by MrB on 24/07/2012 - 21:26

johnriley

Link Posted 24/07/2012 - 21:37
Interesting. I've just looked at the DOF tables in my Pentax book and to be honest they are sometimes quite mysterious at very close distances, where even for prime lenses focal length can change as we focus. Goodness knows what strange effects happen with some zoom designs.

The principles remain the same always, but modern lens making does seem to make a nonsense of them sometimes. I suppose when we put aspherical elements in for a start things will change considerably.
Best regards, John
Add a Comment
You must be registered or logged-in to comment.