Pentax-M 28mm F3.5 scale markings

*wiseman* · 03-02-2009, 12:26 PM

Can anyone tell me why the "8" on the aperture ring and the "10" ft/"3"m on the focus scale are in red? The few other M series lens I have do not have this marking.

*X Man* · 03-02-2009, 12:54 PM

It's a "hyperfocal" mark. Basically, if you set the lens to F8 and focus at 10 ft, lining up all of those markings "everything" from 10' to "infinity" will/should be in "acceptable" focus.

Regards,
Mike

Douglas_of_Sweden · 03-02-2009, 01:46 PM

Originally Posted by X Man

It's a "hyperfocal" mark. Basically, if you set the lens to F8 and focus at 10 ft, lining up all of those markings "everything" from 10' to "infinity" will/should be in "acceptable" focus.

Regards,
Mike

Not only from 10 foot to infinity. With 10 foot in the midle of the DOF, it goes the other way too. That would be from 1.5 foot to infinity (from 8 to 8 on the DOF scale). This red marking is basically a recommended setting from Pentax how to set the lens to get the best compromise of settings with a wide depth of focus with the idea that if you walk around without a specific target in mind you should keep the camera on this setting because it will work OK in most conditions. This is how we prepared for instant focusing before autofocus. When it works it is faster than any autofocus system since you just have to lift the camera and shoot, zero time for focusing.

ftpaddict · 03-02-2009, 01:47 PM

How about setting the lens @ f/22 and adjust focus ring accordingly? Then you'd have just about everything in focus.

*X Man* · 03-02-2009, 02:31 PM

Thanks for the correction, I figured I only had it half right.

Regards,
Mike

Originally Posted by Douglas_of_Sweden

Not only from 10 foot to infinity. With 10 foot in the midle of the DOF, it goes the other way too. That would be from 1.5 foot to infinity (from 8 to 8 on the DOF scale).

Ben_Edict · 03-02-2009, 02:38 PM

As Douglas and Mike wrote, it is a mark for hyperfocal distance. You will find this on many old wide angle lenses, where this was called the "snapshot setting", as it allowed very quick reactions, without achieving focus first. Indeed it is even faster than any AF system and still useful today, if you know how to use it. And the old aperture markings around the focus indicator were very helpful for this.

Ben

*wiseman* · 03-02-2009, 03:18 PM

Thanks to all for the helpful information.

Douglas_of_Sweden · 03-02-2009, 03:20 PM

Originally Posted by ftpaddict

How about setting the lens @ f/22 and adjust focus ring accordingly? Then you'd have just about everything in focus.

Except that your exposure time will be much longer than at f8, most likely too long to take a sharp photo. This feature is something you use for snap shooting, not landscape on tripod, where you could use f22 if you like to (for the moment forgetting that most lenses does not perform at their top at the smallest apperture).

*Andi Lo* · 03-02-2009, 03:53 PM

How about that red mark between the focusing scale and the aperture ring? The one above the mark for f/4? =) Thanks for all the previous information! I was wondering what those marks were on all my takumars

*SOldBear* · 03-02-2009, 04:34 PM

Originally Posted by Andi Lo

How about that red mark between the focusing scale and the aperture ring? The one above the mark for f/4?

That's for infrared film.

After focusing with the viewfinder, you transfer the distance pointed to by the center mark (for visible spectrum) to the red mark before releasing the shutter.

DSLR sensors supposedly do not respond to infrared so that mark does not apply.

*Andi Lo* · 03-02-2009, 05:57 PM

Wow, that's an interesting tidbit. Thanks for the info Soldbear =)

Douglas_of_Sweden · 03-03-2009, 01:43 AM

Originally Posted by SOldBear

That's for infrared film.

After focusing with the viewfinder, you transfer the distance pointed to by the center mark (for visible spectrum) to the red mark before releasing the shutter.

DSLR sensors supposedly do not respond to infrared so that mark does not apply.

Correct, it is beause light of different wave length takes somewhat different path through the glass (or actually at each glass/air interface) resulting in a little bit different focal point for each wavelength. This contributes to optical problems even in visible light. For infrared the difference has gone so large that one need to focus at a different point.

DSLRs have an infrared filter on the sensor (and the whole thing about converting DSLRs for infrared shooting is to remove that filter and then put on another filter that only let through infrared light (or part of it)).
When shooting infrared with a film camera you need to use special film that are sensitive to infrared but not (or to a small extent) to visible light. It is quite difficult since the film is sensitive to heat sources which radiates infrared, so you can easilly shadow the film if you place it near a heat source. I used it in my military service. Quite nice effects, which makes me thinking of converting my *istDS for infrared.