Pentax 100mm Macro

I've managed to acquire a nearly new lens at a cracking price - low enough to steer me away from the Tamron or Sigma alternatives. Can anyone give guidance (or point at some) on how and when to use the focus locking device?
Many thanks.

Focus lock is intended to avoid the lens "creeping" if it is pointed downwards or upwards. So, for example, if you were copying documents you could lock the focus point and know that the lens would not gradually creep out of focus due to gravity.

All this implies that the camera is on a tripod and you are making a series of pictures at the same focus point.

In macro work of the kind i have described, manual focus is also very much easier than autofocus.

Thanks for the virtually instant, and very helpful, reply John.

in this review's verdict they talk about the lack of a "focus limiter" how is that different to a focus lock.

http://www.ephotozine.com/article/Pentax-SMC-100mm-f28-D-FA-Macro

I've got the F 100/2.8 Macro version, its Limiter Switch can be set to Full so the AF searches the Full range of the Focus ring or Limit in which case it restricts it's search to the Macro range max 0.6m. See here

shim

...or 0.6m to infinity, depending where the focus is before you switch the limiter in.

Dan

ok got that. so what is the disadvantage of the Pentax 100mm f/2.8 Macro SMC P-D FA not having it.

We've got the FA 100mm f2.8 Macro and the D-FA as well, which is much lighter and much more compact. It probably doesn't need a limiter in the same way as the construction is lighter and less bulky - less work for the AF system to do.

The FA lens is like a tank.

Both are excellent.

I always use it in manual focus anyway and don't usually go in very close. I prefer the 50mm A f/2.8 macro because it's much lighter and has about the best focusing ring of any lenses I've got. The only trouble is the viewfinder of my K10D is a bit off for manual focusing with this lens.

shim

Can you expand on that a bit? A viewfinder can't be a bit off for manual focusing with one particular lens - it would be a bit off for all lenses and your images would be blurred.

If you are finding it difficult to see the point of sharpest focus, the dioptre adjustment may be wrongly set. Point the camera at a blank area of wall or sky (it's better if the lens is out of focus for this) and ajust the eyepiece until the guide lines are critically sharp. That will mean you are now focusing your eye on the ground glass screen and manual focusing should be much easier.

would the 100mm macro lens also double as a 100mm telephoto or does the construction to suit short distance focus mean this is not practical

The D-FA100 makes a sublime portrait / short telephoto lens. Here's a couple of examples:

[IMG]http://i223.photobucket.com/albums/dd37/hefty1969/competition%20stuff/IMGP1614.jpg[/IMG]

[IMG]http://i223.photobucket.com/albums/dd37/hefty1969/competition%20stuff/IMGP1933.jpg[/IMG]

Of course it's pretty damn good at close range too...

One from my Vivitar 100mm f2.8 macro.

Completely baffled why it should be John.... it's even off with the 2x view on the Refconverter M and you can really see what your focussed on with that at 2x.... I'm used to manual focusing and haven't noticed a problem with any other lens. The manual and electric (green light) confirmation agree with one another so I can't just use electronic although I could set a correction. It's only off 5-10mm (back) at about 300mm but it is off. I've tried the PDF 45deg test and that showed it off, but I'm not happy with that test and keep meaning to make something where I can focus parallel with the film plane. I tried a few tests on small boxes, but it was difficult to tell what was off focus because of the depth of field at f/2.8. It seems OK on my *istDS. Shame really because it's such a good sharp lens. I'm going to have to try some further tests and try to get to the bottom of it.



shim

Just carried out a fairly quick test at about 450mm so that I could also check against the Standard 50mm f/1.4 FA lens.

Set a tape up at 45deg. Cut the back corner off an asprin box and trimmed it to fit the tape so that I could focus at 100mm take a pic then move the box to one side and take another pic. Repeated this twice moving the camera slightly.

Setup

[IMG]http://i545.photobucket.com/albums/hh387/penbx/Setup_1.jpg[/IMG]

Results: Top two 50mm A f/2.8 Macro Lens
Bottom: 50mm f/1.4 FA Standard Lens : Left = Manual Focus : Right = AF

[IMG]http://i545.photobucket.com/albums/hh387/penbx/Compare_1.jpg[/IMG]

So it's not just the Macro that's back focusing the Standards doing it as well in both manual and AF. I might try adjusting the AF setting and seeing if it effects the Manual focus. If not it must be the viewfinder screen out of position.

shim