Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
08-06-2009, 10:24 AM   #1
Veteran Member
WMBP's Avatar

Join Date: Dec 2006
Location: Dallas, Texas
Photos: Gallery
Posts: 2,496
focus with zoom, then change focal length?

I've had this question on my mind for a long time and may even have asked it here before but I searched and I can't find it.

The question is, if I'm using a zoom lens (as I usually am), does it make sense to zoom all the way in to focus, and then zoom back out to compose and take my shot? In other words, say I've got the Pentax 18-250 lens and I'm shooting my pet cat. I may want to take the shot at 75mm. But I can zoom in to, oh, 200mm to focus on those whiskers, and then pull back to 75mm to shoot. Another example: I zoom in on the subject's eyes to focus, then zoom out to get the whole face. Does it make sense to do this or is it a bad idea?

It should be obvious, but let me say explicitly that I am either manually focusing or using the AF button - NOT allowing the camera to auto-focus when the shutter button is pressed.

One other clarification: I'm NOT talking about macro photography here. Just "normal" stuff, photographing people, pets, wildlife, etc.

Finally, I guess I should say that I have been doing this for a long time and I don't SEE anything wrong with it. But I'm not very good at measuring subtle technical differences in pixels. I'm asking the question here hoping to here from those of you who know more about the technicalities involved than I do.

Thanks in advance,

Will

08-06-2009, 11:01 AM   #2
Veteran Member




Join Date: May 2009
Location: Knoxville, Tennessee
Photos: Albums
Posts: 842
That would depend on the specific lens you're using. There are two types of zoom lens:
  • Parfocal lenses - these maintain focus when the zoom is changed (the focus may actually vary, but only within a small enough range that it can effectively be considered unchanged).
  • Varifocal lenses - these change the point of focus when the zoom is changed.

I believe the Pentax 18-250 is a varifocal, incidentally. There's some useful reading in this thread.
08-06-2009, 12:44 PM   #3
Veteran Member
WMBP's Avatar

Join Date: Dec 2006
Location: Dallas, Texas
Photos: Gallery
Posts: 2,496
Original Poster
Thanks, Knoxploration! I love it when I ask a question and get exactly the KIND of response I was hoping for.

I have heard these terms - parfocal and varifocal - before but never paid attention to them. Now I know. How do you tell for sure whether a lens is one or the other? There seems to be some uncertainty about whether the Pentax 18-250 is varifocal or not.

Will
08-06-2009, 01:00 PM   #4
Senior Member
deludel's Avatar

Join Date: Jan 2008
Location: Durham, CT
Posts: 196
QuoteOriginally posted by WMBP Quote
It should be obvious, but let me say explicitly that I am either manually focusing or using the AF button - NOT allowing the camera to auto-focus when the shutter button is pressed.
Isn't using the AF button the same as letting the camera auto-focus w/ a 1/2 press of the shutter?

(I have the k10, in case it matters)

Great question overall - and thanks!

08-06-2009, 01:09 PM   #5
Veteran Member
WMBP's Avatar

Join Date: Dec 2006
Location: Dallas, Texas
Photos: Gallery
Posts: 2,496
Original Poster
QuoteOriginally posted by deludel Quote
Isn't using the AF button the same as letting the camera auto-focus w/ a 1/2 press of the shutter?

(I have the k10, in case it matters)

Great question overall - and thanks!

Well, I mentioned how I focus because I thought it might be pertinent in the context of this question.

With auto-focus disconnected from the shutter-button and tied instead to the AF button, I can do the following:
  1. Zoom to 200mm
  2. Press AF button to focus on subject's eyelashes
  3. Zoom back to 70mm
  4. Press shutter and shoot

If autofocus were tied to the shutter button, I'd have a problem at step 4 above. It's been a while now since I last used the half-press-to-focus option, so I don't remember it well. Perhaps you can half-press to focus without taking a shot (in step 1 above) and then press fast to shoot without refocusing (in step 4). But it seems to me that you get into habits and I suspect I'd pause halfway down while shooting, causing a refocus, and thus possibly losing the benefit of the whole exercise.

But obviously, I'm a little unclear about the relationship between zooming and focusing, which is why I posted the question. It's possible that, if the lens is a varifocal lens, then refocusing is something I would want to do one way or the other. But if it's a parfocal lens then I'm thinking that refocusing would not be a good idea. My assumption is that focusing at a higher focal length - where the camera has more contrast to lock on to - makes for a more accurate focus.

By the way, there are other, real advantages to disconnecting autofocus from the shutter button. I disconnected autofocus from the shutter button sometime in the last year and won't go back.

Will
08-06-2009, 01:12 PM   #6
Senior Member
deludel's Avatar

Join Date: Jan 2008
Location: Durham, CT
Posts: 196
Of course!! I was not thinking about changing zoon my using the 1/2 press. I've been trying to improve my focus lately, and always forget to try the AF button. I never realized I could disable the shutter/focus .. I'll try that, thanks!!
08-06-2009, 01:31 PM   #7
Veteran Member
krypticide's Avatar

Join Date: Dec 2007
Photos: Gallery
Posts: 1,079
I do the zoom-focus thing with my Sigma 70-200mm. For whatever reason, my K20D can miss the focus at 70mm when shooting at large apertures (thus small DOF), so if possible I will zoom in first, focus, then zoom out and take my shot.

A little off topic, but I would keep my finger on half-pressed on the shutter when using the AF button. I do the same thing the OP does, but if I don't keep my finger press down, I end up doing a full-press of the shutter without leaving enough time for SR to activate.
08-06-2009, 01:35 PM - 1 Like   #8
Veteran Member
WMBP's Avatar

Join Date: Dec 2006
Location: Dallas, Texas
Photos: Gallery
Posts: 2,496
Original Poster
QuoteOriginally posted by deludel Quote
Of course!! I was not thinking about changing zoon my using the 1/2 press. I've been trying to improve my focus lately, and always forget to try the AF button. I never realized I could disable the shutter/focus .. I'll try that, thanks!!

Lori,

Give it a try, you might like it.

I shoot weddings and other similar events (Catholic Confirmations and First Communions, etc.) and often have to shoot in low light. Using AF to focus means that I can focus once, and if I don't move, I don't have to keep focusing. And this helps a lot because in low light the camera sometimes has trouble locking focus. I started using the AF button last year after missing a number of shots at a First Communion due to the camera's autofocus kicking in unnecessarily and getting confused. It might lock focus quickly on a girl wearing a white dress with flowers on the front, but when a boy with a dark suit stepped to the same spot the camera wasn't sure where the focus was.

Using AF button to focus has other advantages most of which comes simply from the fact that I now focus more deliberately, more purposefully. Out of focus shots were never a huge problem but I am sure I get even fewer now than I used to.

The ONLY drawback is when I lend the camera to a stranger who has kindly volunteered to take a photo of my wife and me, say, while we're hiking or something. Everybody expects focus on half press, and I have to quickly explain, no, press AF to focus, then press the shutter to take the photo.

Will

08-06-2009, 01:37 PM   #9
Veteran Member
WMBP's Avatar

Join Date: Dec 2006
Location: Dallas, Texas
Photos: Gallery
Posts: 2,496
Original Poster
QuoteOriginally posted by krypticide Quote
I do the zoom-focus thing with my Sigma 70-200mm. For whatever reason, my K20D can miss the focus at 70mm when shooting at large apertures (thus small DOF), so if possible I will zoom in first, focus, then zoom out and take my shot....
Do you know if your Sigma 70-200 is parfocal or varifocal?

I'm getting the impression that most of the modern zoom lenses designed for digital cameras must be varifocal - but since I only learned the term "varifocal" an hour ago, I'm not yet sure I'm right. :-)

Will
08-06-2009, 02:27 PM   #10
Pentaxian
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 15,399
QuoteOriginally posted by deludel Quote
Isn't using the AF button the same as letting the camera auto-focus w/ a 1/2 press of the shutter?

(I have the k10, in case it matters)

Great question overall - and thanks!
actully by pressing the AF button you may not change metering? This might be important if you have exposure lock
08-06-2009, 02:31 PM   #11
Pentaxian
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 15,399
QuoteOriginally posted by WMBP Quote
Do you know if your Sigma 70-200 is parfocal or varifocal?

I'm getting the impression that most of the modern zoom lenses designed for digital cameras must be varifocal - but since I only learned the term "varifocal" an hour ago, I'm not yet sure I'm right. :-)

Will
Will

the only thing I would wonder about is whether the manufacturers are relying on AF to correct for slight focusing errors induced in the zooming process.

In principle, you should be able to tell if the design intent is parfocal or varifocal by looking through the viewfinder. Most zooms are parfocal, except for some one touch zooms, some of those are varifocal and the argument was with a single zoom plus focus, you could visually track this,
08-06-2009, 03:17 PM   #12
Veteran Member
WMBP's Avatar

Join Date: Dec 2006
Location: Dallas, Texas
Photos: Gallery
Posts: 2,496
Original Poster
QuoteOriginally posted by Lowell Goudge Quote
In principle, you should be able to tell if the design intent is parfocal or varifocal by looking through the viewfinder.
You mean, I focus at 250mm, then zoom back and if the subject as viewed through the finder appears to stay in focus, then it's a parfocal lens?

That seems like an unreliable test to me. Even with a Katz Eye installed, I have some trouble telling whether a subject is perfectly focused. I generally get good results and generally trust the camera's autofocus, but I never really know if I nailed it until I see the image on my computer screen.


QuoteQuote:
Most zooms are parfocal, except for some one touch zooms, some of those are varifocal and the argument was with a single zoom plus focus, you could visually track this,
I've read that most zooms are parfocal, except for those on fixed-lens (compact) cameras. But I didn't read this in an authoritative source. I can't find authoritative clear info on this subject anywhere. My impression is that we're in "is it a duck?" territory here. If it walks and squawks like a duck, it's probably a duck, but that's about as confident as we can be. Would be nice if we had an ornithologist handy, though.

My impression with the Pentax 18-250 is that if I get the focus to lock at 200mm, the focus won't change MUCH if I try to focus again at say 70mm. And it makes sense to me that it would try to adjust the focus ever so slightly precisely for the reason that makes me want to focus at 200 in the first place - contrast will be better if I zoom in on something contrasty.

Will
08-06-2009, 04:42 PM   #13
Senior Member
deludel's Avatar

Join Date: Jan 2008
Location: Durham, CT
Posts: 196
QuoteOriginally posted by WMBP Quote
Lori, Give it a try, you might like it.

Using AF button to focus has other advantages most of which comes simply from the fact that I now focus more deliberately, more purposefully. Out of focus shots were never a huge problem but I am sure I get even fewer now than I used to.

Will
Will, I just gave it a try. My thumb needs to get used to going for that AF button (and not the +/- button) but other than that, I can see how this will be a great tool for me! That 1/2 press AF technique was causing more harm than good, I suspect.

A couple of follow-up questions regarding Custom Menu Settings. What are your settings/input on the following:

- AF Button Function:

(1) Enable AF ... this is where I have it set now
(2) Cancel AF ... this has something to do w/ the shutter button, but I am not sure what
(3) Center of AF Point .. I think it auto-sets AF point at center when the dial is on "SEL"

- AF but Press Halfway

...thanks to you, I shut this to 2/Off

- AE-L with AF locked

.. I have this set to "off", I dont think I want AE locked like this (I use the AE-L button when I need to)

- Link AF Point and AE
.. Again, I have this turned "off". I feel like this is almost the same as spot-metering,which I would rather consciously turn on when I want to
08-07-2009, 03:48 AM   #14
Veteran Member




Join Date: Dec 2007
Posts: 923
Zooming in , Auto focus, then zoom out is a valid method where the focus subject is a fine detail which the AF is likely to get wrong if you just do the AF at the wide end. It only works if its a non-moving subject.

Most zoom lenses have their focus position vs. distance indication fixed, independent of focal length, so they are intended to maintain focus when the zoom is changed.

However, I have noticed that most consumer zooms actually vary slightly in focus point throughout their zoom range, usually most accurate about the middle of the range. With my kit lens DA 18-55mm, I found it impossible to calibrate the focus perfectly even with the K-7's focus adjustment - I compromised by adjusting it precisely at 35mm, and the error at the extremes 18mm and 55mm is quite acceptable.
08-07-2009, 05:15 AM   #15
Site Supporter




Join Date: Jan 2008
Location: NC
Posts: 787
QuoteOriginally posted by WMBP Quote
Lori,

Give it a try, you might like it.

I shoot weddings and other similar events (Catholic Confirmations and First Communions, etc.) and often have to shoot in low light. Using AF to focus means that I can focus once, and if I don't move, I don't have to keep focusing. And this helps a lot because in low light the camera sometimes has trouble locking focus. I started using the AF button last year after missing a number of shots at a First Communion due to the camera's autofocus kicking in unnecessarily and getting confused. It might lock focus quickly on a girl wearing a white dress with flowers on the front, but when a boy with a dark suit stepped to the same spot the camera wasn't sure where the focus was.

Using AF button to focus has other advantages most of which comes simply from the fact that I now focus more deliberately, more purposefully. Out of focus shots were never a huge problem but I am sure I get even fewer now than I used to.

The ONLY drawback is when I lend the camera to a stranger who has kindly volunteered to take a photo of my wife and me, say, while we're hiking or something. Everybody expects focus on half press, and I have to quickly explain, no, press AF to focus, then press the shutter to take the photo.

Will
i tried this technique and it works great. you just need to get used to it.
when i started using this, i quickly forgot why my camera was not auto-focusing when i pressed the shutter half-way.

and will is right in saying that when you give this to someone to use, you'll probably get a blurred picture because the camera won't AF.
this almost happened to me last tuesday when i asked someone to take my group picture (me part of the group ). i remembered that i had the AF button set and had to reset it to half-press.

regards,

jordan
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!
75mm, button, camera, dslr, focus, lens, photography, question, sense, shot, time
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
What focal length for a manual zoom? janstew Pentax SLR Lens Discussion 24 01-09-2011 08:26 AM
Focus distance / focal length chadci Pentax SLR Lens Discussion 5 10-31-2010 09:12 AM
How Can You Easily Tell How far a Given Focal Length will Zoom? jaieger Troubleshooting and Beginner Help 20 07-18-2010 09:47 AM
Need to Change Lenses for Complete Focal Length? 68wSteve Pentax SLR Lens Discussion 7 07-08-2009 08:00 AM
Auto focus, aperture and focal length xandy Pentax DSLR Discussion 4 12-07-2007 08:08 PM



All times are GMT -7. The time now is 08:03 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