Pentax DA 21mm Limited Lens

pez · Sep 12, 2008

I love this lens. Although there is a thread about a back focus prob with this lens, I have not found this to be the case with mine- hmm. What a great focal length in a compact, lightweight prime for walking around in "stealth" mode, city scenes, etc. I find the focus to be super fast, so I can power up, bring the camera to my eye, and shoot in a fluid motion with zero impediments. Here it is on my kind of dusty K10D (shot with an NV11, lol):

Bifurcator · Sep 12, 2008

Nice!

pez · Sep 12, 2008

That thread about back focus problems started bugging me, so I just did a quick test. This map is laminated and tacked loosely on a wall here at work. I set the camera and 21mm at close distance ~1.5M and about f5.6, ISO100. Here is the auto-focus frame:

And here is a detail from that frame:

Here is the same basic shot and crop, but manually focused as best I could:

OK, so neither mode looks super sharp here, but it was fluorescent lighting and I am tired. However, I'm satisfied that there is no serious problem (unless I don't understand what "back focus" means...)

Peter_pan91 · Sep 12, 2008

geez, i want that lens, badly!

Iron Flatline · Sep 12, 2008

You'll typically notice focus issues at the widest aperture, because your DOF will be shallowest. I don't think you'll ever have a problem with this lens. Focus issues are usually found in super-fast lenses (f/1.4 etc) because at that aperture, if you're shooting a portrait, you might well end up with an out-of-focus shot - and if it's only slight you won't notice on your LCD or through the viewfinder, you'd see it at home on the computer.

That lens rocks, I wish someone made pancakes for the Canons - I'd love to do some street photography with my Canon - though my Leica is quite good, thank you very much

.

If you want it even faster for street photography - keep the camera on, set it to f/8 (lots of DOF), turn off AF, and focus it around 1.5m out in front of you... then almost everything will be in focus. Set the ISO a little higher (ISO 400). You could probably set it to full manual, select a shutter speed, and assume you'll get most of the shots + or - half a stop.

This way, all you have to do is compose the shot, and the camera is out of your way technically.

pez · Sep 12, 2008

Iron Flatline said:
You'll typically notice focus issues at the widest aperture, because your DOF will be shallowest. I don't think you'll ever have a problem with this lens. Focus issues are usually found in super-fast lenses (f/1.4 etc) because at that aperture, if you're shooting a portrait, you might well end up with an out-of-focus shot - and if it's only slight you won't notice on your LCD or through the viewfinder, you'd see it at home on the computer.

That lens rocks, I wish someone made pancakes for the Canons - I'd love to do some street photography with my Canon - though my Leica is quite good, thank you very much .

If you want it even faster for street photography - keep the camera on, set it to f/8 (lots of DOF), turn off AF, and focus it around 1.5m out in front of you... then almost everything will be in focus. Set the ISO a little higher (ISO 400). You could probably set it to full manual, select a shutter speed, and assume you'll get most of the shots + or - half a stop.

This way, all you have to do is compose the shot, and the camera is out of your way technically.

Thanks- I actually do this already in some cases, although not at 400. In good light, I can get away with F8 and 100 or 200, with antishake on. It seems to power up so quickly that I feel comfortable flicking the switch as I bring it up.
I used to use those settings- F8 with the DOF stretching from 1.5 to infinity- with my old Pentax MX (and ME Super), 28mm lens, and Tri-X for street photography. Great fun. :thumbsup:

Bifurcator · Sep 12, 2008

Use this for resolution: http://www.graphics.cornell.edu/~westin/misc/res-chart.html <- can be printed to four A4 sheets for wider lenses.

and this to test autofocus:

You can either print it out or you can focus at it right on your LCD as displayed here. This graphic will cause your AF system to nail the distance PERFECTLY and then you can see about BF issues or whatever. Anyone can try it right now. Works with all AF systems that I've tested. It looks cooler printed out tho.