Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
11-08-2019, 12:41 PM   #1
New Member




Join Date: Jul 2018
Posts: 22
AF lens to infinity - lowlight tips

I own a K-70 with kit DA 18-135mm f/3.5-5.6 ED AL (IF) DC WR and the lovely HD DA 55-300mm f/4.5-6.3 ED PLM WR RE.

As a newbie hobbyist photographer, I'm trying to practice estimating DOF and focus distance. I also love to take photos of the moon and night sky.

Because these are entirely autofocus lenses, the focus ring does not stop in either direction - it turns 360 degrees continuously and it is not clear when it has focused at infinity!

I would trade in one or both of these lenses for credit towards the smc Pentax-DA 18-270mm f/3.5-6.3 ED SDM, but the fact that this model is not WR (I shoot by the ocean and from boats frequently) and the cost are both making me debate whether to replac, the 18-135, the 55-300, both, or neither....

1.) tips for focusing my existing zoom lenses to infinity, especially in low light or with manual focus? I have live view zoom set to 8x but it's still difficult to judge for things like the moon and stars.

2.) suggested lenses and/or which of existing zooms to trade in? the 18-270 and macro 100mm 2.8 with an added tele converter are both appealing options. the macro would be the first 1:1 lens I own, I believe one of my zoom lenses currently has the best magnification ratio of just under 1/3 life size.

11-08-2019, 01:15 PM   #2
Site Supporter
Site Supporter
whammerhead's Avatar

Join Date: Aug 2019
Location: Belfast
Posts: 128
What are you shooting from boats/by the ocean? Moon shots would require a long focal length whereas night sky (astrophotography) is better with a wide angle lens which would also cover seascapes/landscapes. What results have you had with your 18-135 (scapes) & 55-300 (moon shots)? I own both but have tended to use the DA300 & D-FA100 macro WR most. Like yourself, I'm a novice. I'm sure a more experienced member will be along to provide a more informed response.
11-08-2019, 01:27 PM   #3
Loyal Site Supporter
Loyal Site Supporter
kiwi_jono's Avatar

Join Date: Dec 2009
Location: Christchurch, New Zealand
Photos: Gallery | Albums
Posts: 2,384
Normally you would need to use manual focus for night shooting.
On some lenses you can focus the lens to the desired distance (infinity or otherwise) during daylight and apply a bit of tape to hold it in position for the night session. However for the DA 18-135 I don't think this is practical as its possible to shift zoom or focus which will both affect focus.

So if it was me, at night, I would select manual focus, and point the lens at the brightest subject you can see (such as the moon). then go to LV and push INFO button until max magnification. Adjust MF until it looks sharpest.
11-08-2019, 01:32 PM - 2 Likes   #4
Pentaxian




Join Date: May 2014
Location: Linz
Photos: Albums
Posts: 2,400
QuoteOriginally posted by Emirena Quote
1.) tips for focusing my existing zoom lenses to infinity, especially in low light or with manual focus? I have live view zoom set to 8x but it's still difficult to judge for things like the moon and stars.
For full moon photography the AF should usually be fine, because the moon is such a bright and big object. When manually focusing on the moon with LV you might get the problem that the moon is shown way too bright in LV no matter your exposure settings, this can be remedied by using either the center weighted or the spot metering option on the camera and positioning the moon in the centre of the frame (for focusing).
For stars it is a little more difficult because this LV overexposing can't be prevented as far as I know (at least I have this problem with my K-3), I usually magnify a star as far as I can and manually focus until the star has the smallest extent on the screen (before infinity and after infinity focus the star will apear bigger on the screen) -> after that I take a test picture and check my focus.

QuoteOriginally posted by Emirena Quote
I would trade in one or both of these lenses for credit towards the smc Pentax-DA 18-270mm f/3.5-6.3 ED SDM, but the fact that this model is not WR

Beside the not existing WR for this lens, image quality will also be worse. It is a good lens for snapshots and the like and when you just watch your picture on a computer screen (not pixel peeping) or on small prints this lens has it's benefits, but if you ever wanna make a large print or crop heavily in your pictures you will very soon get to the limits of this lens (it is always a tradeoff with such consumer zooms).

11-08-2019, 03:00 PM   #5
Pentaxian




Join Date: Jan 2011
Location: New York
Posts: 4,774
You can try liveview autofocus directly on the moon because it's a bright target. If autofocus isn't giving satisfying results, though, try manual focus. Use a tripod, liveview, turn the focus ring to get rough infinity focus, find a bright star, adjust focus until the star is as pinpoint as possible, reaim at the moon. Repeat every time you adjust the zoom ring. Distant water/radio towers with aircraft warning lights can also be good targets for manual infinity focus at night.


I think the 55-300 are optically superior to the 18-270. I would not replace those lenses.

The DFA 100 macro is a great lens especially for macro.No lens is perfect, though, so be advised that the DFA 100's weaknesses are slow-ish autofocus and some CA / purple fringing in highlights.
11-08-2019, 03:26 PM   #6
Senior Member




Join Date: Jul 2017
Posts: 209
When Iím doing astrophotography, I sometimes use a streetlight to focus. I have heard of people using a laser pointer to create a bright stop to focus on. I have also laid down a flashlight on the ground and moved a few hundred feet away to focus. When you adjust your focus, try to make the spot of light as small as possible.
11-08-2019, 03:31 PM - 1 Like   #7
Loyal Site Supporter
Loyal Site Supporter
clackers's Avatar

Join Date: Jul 2013
Location: Melbourne
Photos: Albums
Posts: 15,322
+1 to manual focus, using magnified Live View to make a bright star as small as possible. That's true infinity, ignore whatever it says on the lens ring.



11-08-2019, 11:05 PM   #8
Site Supporter
Site Supporter




Join Date: Jun 2010
Location: Central Coast, CA
Posts: 1,167
I know I'm not the person who asked, but I have to say thanks to clackers. That is why I participate.
11-11-2019, 06:33 AM - 1 Like   #9
Pentaxian
bdery's Avatar

Join Date: Mar 2008
Location: Quebec city, Canada
Photos: Gallery
Posts: 8,778
QuoteOriginally posted by Emirena Quote
1.) tips for focusing my existing zoom lenses to infinity, especially in low light or with manual focus? I have live view zoom set to 8x but it's still difficult to judge for things like the moon and stars.
You've got good advice so far. The only thing I will add is that, if you're using a small aperture (longer DOF), you could also bring a laser pointer with you and illuminate a very distant object that you will then focus on (the laser should be easy to pick up for the AF system of your camera). Then lock focus and shoot away!
12-19-2019, 05:23 PM   #10
New Member




Join Date: Jul 2018
Posts: 22
Original Poster
Thanks for the replies everyone!

I will increase the LV zoom and use that method to manually focus on stars or moon. I have found the full moon is almost too bright (I do always use center metering) and does make the k70 and either zoom Lens hunt and not focus as perfectly as my eye can.

I have had fun doing some light painting with LED flashlights, and just got an IR shutter release so now I can do bulb mode mirror up exposures for minutes

I have a Rokinon 10mm ultrawide that I'll use for true astrophotography with the remote release now. No gps unit for k70 startracer mode, but I'm sure it's not necessary at this point.

Is a couple hundred feet away at f5.6 with 100-300mm sufficient focus point to cover infinity? I did use that method shooting at night a couple months ago but didn't have remote or tripod, so did my best with a stone wall and gentle button presses.

I'll keep my two zoom lenses as recommended and put the macro 100mm on my long term wishlist along with a teleconverter.

I suppose my only remaining nitpick is that i can't appreciate the relationship between DOF, focus point and f stop as a newbie photographer as easily without lens ring labeling.

Come to think of it, I used my mom's k1000 55mm prime today and the marking for 8ft
distance towards infinity was a red tick (I think) instead of white. Any idea what this indicates?
12-21-2019, 08:18 PM   #11
Pentaxian
Just1MoreDave's Avatar

Join Date: Feb 2007
Location: Aurora, CO
Photos: Gallery | Albums
Posts: 5,962
QuoteOriginally posted by Emirena Quote
...
Come to think of it, I used my mom's k1000 55mm prime today and the marking for 8ft
distance towards infinity was a red tick (I think) instead of white. Any idea what this indicates?
It's for using infrared film. When using that film, you had to make a focus adjustment for infrared. I forget exactly how to use the mark to correct focus.
Reply

Bookmarks
  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
af, af lens, autofocus, focus, focusing, infinity, k-mount, lens, lens to infinity, lenses, pentax lens, slr lens, tips, trade, wr, zoom, zoom lens, zoom lenses
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Autofocus inaccurate in lowlight on the K30 Aljech Pentax K-30 & K-50 11 11-28-2016 08:15 AM
Pentax 645Z for lowlight shooting? A.Styrlander Pentax Medium Format 24 05-28-2016 08:46 PM
Whats your toughest lowlight shot? Reportage Photographic Technique 22 08-03-2010 07:59 AM
Lowlight and AF rollin Pentax DSLR Discussion 9 12-01-2009 05:17 PM
fast lens for lowlight yinzerparty Pentax SLR Lens Discussion 14 09-18-2009 11:45 AM



All times are GMT -7. The time now is 07:06 PM. | See also: NikonForums.com, CanonForums.com part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
register
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]
Top