DSLR auto focus....overide?

kinetic (Steve) · #1 17-03-2010, 05:47 PM

A quick question for the DSLR terrestrial gurus....
Do you, as a default, ALWAYS rely on the auto focus
function of your lens combo to choose for you OR

switch it to manual and refine even better?

I have noticed lately that the auto function can get it SO
wrong...and a manual tweak is needed , almost without exception.

Sorry for stating the obvious for DSLR naturals but I'm
curious to know what they think.

Even with centre weighted or spot focus etc, it can be way out.

Steve

bojan · #2 17-03-2010, 06:06 PM

Steve,
you simply have to (at least when Canon DSLR's are in question)..
The image in finder is almost totally useless for focussing Unless you use RA microscope for that).
From time to time I am using modified 50mm F1.4 FD on my EOS (it is manual lens), and I needed to apply focus confirm circuit (http://cgi.ebay.com.au/Focus-Adjusta...item35a797c77f), otherwise it is no go.

adman (Adam) · #3 17-03-2010, 06:45 PM

On Canon's there are multiple focus points in the viewfinder, and if you leave it up to the camera to choose, you need to be very aware of the point that it has chosen to focus on. Their autofocus is very very good, but often on auto it is easy to overlook where it has decided to focus on. In practice, I always force the camera to focus on the centre square. Press the shutter release halfway with the central square where you want to focus (it helps to have a vertical edge on the subject) then reframe the shot with the shutter release still held halfway in. Then fire!

I almost never focus manually - unless there is a good reason - like low light/low contrast subject.

Adam

Paul Haese · #4 18-03-2010, 11:15 AM

Auto focus can sometime have trouble when there are lots of long lines in the frame. This is most likely what is going on here.

Auto focus is so superior to your own eyes being able to pick sharp it is best to leave it in auto and change your focus point with the selector.

It can be common for lenses to hunt and this is particularly so for slow lens such as f5.6. Fast lenses don't often hunt but can sometimes get the focus point wrong. Changing the focus point helps that. To find out how to change the focus point of your camera see the manual.

troypiggo (Troy) · #5 18-03-2010, 11:28 AM

I almost always use AF. Exceptions would be and should be macro, astro, landscapes if you're using hyperfocal length focusing techniques, low light situations where the AF doesn't get enough light to work (a Speedlite can focus assist here), and if you're using a teleconvertor on a lens sometimes the AF won't work if the widest aperture is smaller than f/5.6.

If you're having trouble with the AF being off/inaccurate, check what AF points you're using. I nearly always just use a single point for whatever composition I'm shooting. I don't trust the multi-point one, because you don't know what you're going to get. Also check if you're in AI-servo etc focusing modes.

I was having trouble with my 40D front-focusing - I would point at something and the lens would AF, but the actual shot was focused in front of the objects. I sent the body away to get calibrated, cost $80, and it's spot on now. Couldn't be happier.

Lenses can also need calibration. Test all of your lenses, if your shots are consistently focusing the same (ie all in front or all behind), then maybe the body needs calibrating. If you only have one or 2 lenses "off", or some are focusing in front, some behind, then maybe it's the lenses that need calibrating?

bojan · #6 18-03-2010, 12:00 PM

Quote:

Originally Posted by troypiggo

Lenses can also need calibration. Test all of your lenses, if your shots are consistently focusing the same (ie all in front or all behind), then maybe the body needs calibrating. If you only have one or 2 lenses "off", or some are focusing in front, some behind, then maybe it's the lenses that need calibrating?

This should not be the case... unless there is overshoot or undeshoot (some users reported things like this).
Body should AF correctly on any lens (even manual ones if Focus confirm chip is used), because focus sensors are at the bottom side of the mirror box.. and what could go out of cal is the position of secondary mirror that directs the image towards sensors.

troypiggo (Troy) · #7 18-03-2010, 01:24 PM

I've never had a problem with this myself, but have heard of others reporting so. Makes you wonder why the newer Canons like the 5D MkII have a per lens auto-focus micro-adjust functionality.

Nightshift · #8 18-03-2010, 01:27 PM

This is one of the reasons I bought a Nikon D5000 not a Canon, Canon IMHO has never had ideal focus capabilities in their DSLR's, on the Nikon I can let it multi point focus and then with my thumb I can "joystick" around through the eyepiece and choose the focus object which may be way off to the side of the FOV, very intuitive and accurate, Obviously I can simply touch a switch on the lense itself and overide AF all together if I want.

Cheers, Dennis.

troypiggo (Troy) · #9 18-03-2010, 01:59 PM

Not sure when you did your research, or what models you looked into, but my Canon 30D and 40D have that AF point joystick feature, also very intuitive.

Getting a little OT here, sorry Steve.

Octane (Humayun) · #10 18-03-2010, 02:35 PM

As Troy mentioned, I also use hyperfocal distance focusing for my landscapes and have had great success.

I typically shoot when it's dark and auto-focus can have a really difficult time, especially when trying to hunt through dark foliage.

I'm quite lucky as I haven't had to calibrate any of my L-series lenses through the lens microadjustment feature in the 5D Mark II.

Dennis, your statement implies that you've used (or tried out) every single DSLR Canon has ever made. Yes, there were major issues with the 1D/1Ds Mark III, and it took Canon a while to acknowledge the problem and resolve it. The 1D Mark IV is receiving a lot of praise for having quite possibly the finest auto focus mechanism around today.

H

Terry B · #11 18-03-2010, 03:12 PM

I almost always use autofocus. Be aware that the diopter correction on your finder might be incorrect making the image appear out of focus when you use the autofocus. This makes you think that the autofocus is wrong when it isn't.
The only time I don't use autofocus is when taking pics of my aquarium. It likes to focus on the glass rather than the fish.

kinetic (Steve) · #12 18-03-2010, 05:18 PM

Quote:

Originally Posted by troypiggo

Not sure when you did your research, or what models you looked into, but my Canon 30D and 40D have that AF point joystick feature, also very intuitive.

Getting a little OT here, sorry Steve.

Not at all Troy, and thank you for everyone's input.
I suffer from the diopter problem that Terry mentions and
it was really the reason behind my question.
As I wear prescription glasses now with the 40+ deterioration in
eyesight, focusing the DSLR is such a pain, both for daytime
and night work.
Manually focussing is very hard with or without glasses on.
It's a really hit and miss affair.

Thankyou everyone for the feedback...love you guys!

Steve