Pentax 100mm Macro

Made a few enquiries this morning and it seems that these focus problems exist right across all manufacturers and to make matters worse the manufacturers won't accept that they exist. Canon and Nikon actually use different set ups/sensors (3x more sensitive Canon) in their Pro Cameras than their Consumer Cameras, but do state that the focus should fall within the depth of field on the consumer stuff. Fleet St. etc. actually send their New Pro cameras to a specialist firm to try to have everything aligned correctly before they use them. David Kilpatrick (Minolta/Sony)had a problem on one assignment where everything looked OK through the viewfinder when he took the pictures, but later found they were out of focus. Then there is a problem that some lenses actually shift focus when stopped down. So it looks like I could be wasting money trying to find something that just isn't manufactured.

Quite a good thread on Canon focusing problems link

Point of Focus also changes under different types of lighting.

shim

I had that problem the other night coming back from the pub.

AF tests with the subject at 45 degrees are unreliable because the camera's focusing point may be slightly off centre - not precisely where it is indicated in the viewfinder.

You're better off focusing on a flat subject, making AF adjustments via the Debug/Setup menu, and seeing if they make the results better or worse.

That's why I stood a box up vertical and focussed on that. If you shine a torch onto something flat/horizontal all the light bounces away from you. It has to be something vertical for it to bounce back. I would just about trust a laser coming back off something horizontal, that's the way scanners work but infra-red is a bit dodgy.

I've just realized as I was typing this.... we don't know how wide the AF beam is, on a vertical surface it won't matter it just bounces back.... on a horizontal surface assuming that it's circular, it spreads out and becomes eliptical... making it more inaccurate and we dont know which part of that eliptical beam bounces back.... it would bounce back stronger off any bumps rather than flat surfaces. So it is very inaccurate and that's why I never used it.

Noticed on another site that all the AF points in the viewfinder can give different readings. I only ever use the centre one anyway.

shim

But you're doing something with a tape measure at 45 dgrees. Forget that because the centre focus point may not really be *precisely* in the centre, is my suggestion.

What AF beam? The camera doesn't have one.

When we used film cameras this sort of testing would be done by no-one as the delay in processing meant it never crossed our minds.

These days the immediacy of digital means that we are free to become paranoid about something that has probably always been a problem but we never knew.

I am happy if my shots look sharp. If they look sharp then they are sharp.

I've carried the tests out parallel to the film plane as described above. To make it easier to understand, imagine that you made a simple dolls house out of cardboard with just a cutout for the door. You focus on the front of the house, but can see inside where it's focused on the stairs/floorboards etc. I've just basically rested the tape on the flooboards at an angle to read a distance. I could also have run it up at 45deg. up the stairs and it would be the same. I was also only testing manual focusing.

It's all irrelevant now it's been established that DSLR's only focus within the depth of field.


Sorry I thought they'd moved on from phase-matching. A laser beam would be the best, but as you can't really shine one of them in someones eye I assumed that they would use infra red in a similar way that those cheap DIY measuring devices do.

It was never really needed John, because the manufactures went to great lengths to ensure that the film plane was exactly where it should have been and that the film was held as flat as possible. I think it was Contax who actually put a vacuum plate in one of their cameras that sucked the film absolutely flat. The BJP and AP also did extensive testing of cameras and lenses. Dr. Stewart Bell at AP and other mags. spent years resolution testing hundreds of lenses and if there were focusing problems they quickly show up on resolution tests. I can only ever remember him saying that there was a focusing problem with one lens and that was of a strange design. He used to comment on the flatness of field and sometimes actually measured it. I'm also sure they have to focus properly to do MTF tests.

We just want to know where we stand and how the camera works. If the manufactures are telling porkies or keeping quiet about something we want to know. If they put a DOF button (which I would never use) on a camera that might only be focusing close to the limits of the DOF, making the DOF button totally useless we want to know about it.

If it weren't for people being curious about focusing problems. I doubt if the debug menu fix on the K10D or the inclusion of a database with lens corrections on the K20D would have ever happened. The same goes for whatever fixes other manufactures include with their cameras.

One porky that ALL manufactures got away with years ago was bringing out 50mm lenses instead of 55mm lenses. It sounded like a big improvement but the 50mm were actually 52mm which wasn't such a big deal.

From AP Standard Lens Tests 25th May 1991

Pentax 50mm AF f/1.4 measured focal length 52mm
The f/1.7 was 52.1mm A f/1.2 52mm
Out of 8 manufactures listed the only one that was
not about 52mm were the Nikon f/1.4s at 51.1mm and
51.2mm. The Nikon f/1.8s were 51.9mm
Helios 58mm = 58.8mm

These aren't exactly 2 inch lenses either (as you
sometimes see reported)that would be 50.8mm

shim

OK, I thought you might have been focusing on the tape. Sorry for not studying your pics and posts better. (I haven't been following the thread from the beginning).

Re beams: I can't imagine an IR beam being good for any subject more than a few metres away.

Phase-matching is good enough, I'm sure. Typical problems are due to alignement of the sensors etc rather than there being anything fundamentally inadequate with the system.

This isn't a lie, it's just an approximation. Apart from not really seeing why anyone would regard a 50mm lens as an improvement over a 55mm one....Is a 28mm lens an improvement over a 30mm one?

Basically lenses are put into categories, so a 50mm lens will be around 50mm in focal length. It might well be 52mm, or 48mm, bit it's still intended to be that category. We all know what to expect from it, and virtually all the time the precise focal length doesn't really matter. I don't want my lenses labelled as 51.67mm and suchlike - it would be totally confusing.

The interesting bit comes when we take zoom lenses. Here there might be an argument for better description if your 28-90mm zoom turns out to be 33-84mm, and it just might. You'd sell more zooms than labelling such a beast 35-80mm. In practice, Pentax zooms are quite close and the cheapest independant zooms can be way out.

There are always approximations in life, but they aren't necessarily a conspiracy, just a convenient shorthand.

They are used in surveying instruments, Pentax even make them, that's why I thought they might use them in cameras. Works better with something to reflect off though. This one from Leica can measure up to 3km with a single reflector (further with multiple reflectors) with an accuracy of +-1mm. Visible lasers which don't need a reflector can be used up to 1km +-2mm accuracy.

shim

That Leica one is around £10,000. Bit pricey to put that in a camera I think.

And can you imagine the complexity of trying to get that to work with 11 focus points? You'd have to adjust the angle of the laser, the angle being dependent on the distance to the subject... Catch 22? Or you could just have one laser per focus point. Oh hang on, that's bumped the cost up a bit! Made things a bit bulky too...

Most of the digital 'tape' measure things you mentioned ("cheap DIY measuring devices") use ultrasound. The ones that use lasers are not cheap, at least the ones I've seen aren't.

I got the impression (and I imagine iceblinker did too) when you said IR beam, that you weren't refering to IR lasers, but just the sort of thing that's used for IR triggers.

Dan

Just gives you an idea what would be possible Most of the Leica instruments have a sort of camera already built in. You just point them roughly where the target is and it goes looking for it. Once it gets it in the CCD it calculates how many degrees it has to move horizontal and vertical to get to the exact center of the CCD. It then moves and does the reading and calculation again. If it's set to lock onto the target it follows it around the field as you walk and all the instrument controls can be done from the target end.

Imagine a camera like that you could be on every shot.

Should be enough parts available with GPS total stations are now old technology.

Pentax actually make AF Surveying Levels that recognize the staff and then focus on it so that you can read it quicker. Good idea.... World first I think? Only trouble is Leica make one that does the reading for you (off a barcode on the staff) and will save it to memory for download later to a computer.

shim

I just wonder if shim is barking uo the wrong tree. If the attached link is trustworthy, the whole focus adjustment is based on contrast calculations inside the camera. No beams at all.

I think we've already established that the camera doesn't use beam, just musing about the possibility now.

How would a beam system behave in rain or snow, by the way?