Go Back   Steve's Digicams Forums > Digital SLR and Interchangeable Lens Cameras > Pentax / Samsung dSLR, K Mount Mirrorless

Reply
 
Thread Tools Search this Thread
Old Dec 29, 2008, 1:43 PM   #11
Senior Member
 
snostorm's Avatar
 
Join Date: Jan 2004
Location: Chicago Suburb, IL, USA
Posts: 2,770
Default

mtngal wrote:
Quote:
Is there anything published to say which focus points are cross-type and which only focus on one plane? That would definitely make a difference and if someone was using a non-cross-type then it would make sense that it would be inconsistent. What is interesting is that some would find selecting the center focus point from the selectable menu being more accurate than using the center focus position.

I have noticed that sometimes moving the camera slightly will help the 12-24 get a focus lock when it's struggling. I've always put this down to the fact that as a wide angle, often the lines are smaller and the camera doesn't "see" them as well. Or that the focus spot isn't quite where the dot is (also possible) and moving the camera a bit gets the line to cross the sensor.
Hi Harriet,

With the 11 point SAFOX VII system, the central 9 points are all cross type, with the extreme left and right points having vertical-only sensors. You can see the difference in the viewfinder overlays that light up -- the cross-types show up as squares and the vertical-onlys show up as vertical lines.

Knowing the exact area that the focus point covers in the viewfinder can be very important -- and you are correct that the focus point doesn't necessarily correspond directly with the indicator that lights up. I'm very conscious of this with my bird shooting. More often than not, they tend to pick landing spots that are very confusing to the AF system, and with the lenses I use, DOF can be very thin, so it's important to try to get AF to lock on the right thing. This is one of the reasons why I've taken to using a tripod more than not lately -- more accurate focusing -- with the camera moving around randomly when handholding, it can be frustrating trying to time AF actuation to when the AF point happens to be on the actual subject. The tripod not only gives a steadier hold during exposure, it
allows me to pick the area to focus on much more precisely.

Moving the camera to give it a good contrast area to lock on is a good practice. In a situation there isn't a good vertical or horizontal contrast area, I will even tilt the camera to make an angled contrast area vertical or horizontal relative to the viewfinder to make it easier on the sensor to get an accurate lock, then tilt it back to recompose the scene once I get the lock.

Scott
snostorm is offline   Reply With Quote
Old Dec 29, 2008, 10:20 PM   #12
Senior Member
 
mtngal's Avatar
 
Join Date: Nov 2005
Location: Frazier Park, CA
Posts: 16,056
Default

Thanks for the information about which sensors are which. And I never thought about rotating the camera to change a diagonal line to be a vertical line to the camera - great idea!

P.S. I was reading an article or something in a book recently that talked about the sun - apparently bright sunlight really plays havoc with AF mechanisms, apparent if you try to get the sun shining through trees.
mtngal is offline   Reply With Quote
Old Dec 30, 2008, 9:56 AM   #13
Senior Member
 
snostorm's Avatar
 
Join Date: Jan 2004
Location: Chicago Suburb, IL, USA
Posts: 2,770
Default

Hi IO,

One more thought -- Are the batteries fully charged? . . . or if you're using Lithiums - are they new? A lot of users have reported increased focusing issues (both speed and accuracy) when the batteries (on any model) get a bit low. It might be that the lens/filter combo might like more juice to AF reasonably.

Scott
snostorm is offline   Reply With Quote
Old Dec 30, 2008, 1:55 PM   #14
Senior Member
 
Illuminati's Avatar
 
Join Date: May 2006
Location: California, USA
Posts: 583
Default

Just wanted to add a couple more details.

In AF-S, the camera will not take the picture until AF has successfully focused on something. The blinking hexagon indicated that the camera was unable to focus, which explains why you could not trigger the shutter.

If you used AF-C, you would be able to trigger the shutter at any point during the AF. Whether your picture is in proper focus at the time of exposure is another matter, however.

- Jason
Illuminati is offline   Reply With Quote
Old Jan 1, 2009, 9:59 PM   #15
Senior Member
 
tacticdesigns's Avatar
 
Join Date: Sep 2008
Posts: 998
Default

Hello,

With my Sigma 18-200 f3.5 - 5.6 lens, I notice it has a hard time focusing in low light, especially when I have it near 200mm. [ie. Won't acquire AF & fire.]

Since it is dark, I find it hard to focus manually, so . . .

I zoom out to 18mm, and the camera is able to get an AF lock when I press the trigger half-way. I then zoom in a bit to, let's say 50mm& again, probably since the focus is pretty close, it is able to get an AF lock. Then do this again at 100mm, etc. Then I move it to 200mm & if it's not too dark, then it eventually acquires AF lock at 200mm. Then I take the picture.

My assumption why this works with a variable f-stop lens is that zooming out lets more light in and the camera is able to get the lock. Then zooming in, the camera is so close on focus that it just makes minute adjustments from there.

Not sure how applicable this would be with such a wide lens, but I find it useful to get around the "Won't fire on AF-S" problem when shooting indoors and the flash just doesn't help AF.

Take care,

Glen



tacticdesigns is offline   Reply With Quote
 
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



All times are GMT -5. The time now is 9:00 PM.