Auto-focus help Lens or Camera?

[Brepan]

Alright guys thanks for your time to read this.

I shoot using a Canon 550D and recently bought a Sigma 24mm 1.8 and it arrived today, I'm very happy with the lens over all but I have one thing bothering me. I dont know if its just the lens or my camera.

So my problem is the auto-focusing. I have it set up on manual mode and keep the same settings therefor the image should be the same. When I auto-focus using the viewfinder it looks sharp and then when i view the image close up its noticeably blurry. Then when I switch it to the live view and take the same shot with no changes, it comes out a lot sharper. The focusing does take longer but i cant see how there should be a difference.

So has anyone came across the same problem or know how to fix this?

Thanks.

[ccting]

Originally Posted by Brepan

So my problem is the auto-focusing. I have it set up on manual mode and keep the same settings therefor the image should be the same. When I auto-focus using the viewfinder it looks sharp and then when i view the image close up its noticeably blurry.
.

That's why i don't recommend noobs like me to get a manual focus lens. You can always get an autofocus lens and manually focus, but not fully manual focusing.

Grace, what is your advice for manual focusing?

[ravncat]

Possibly a bit of both.

Modern cameras are optimized for f2.8 lenses - and faster lenses and thin depths of field are tricky, especially since modern optical viewfinders are also optimized for f2.8 performance. This means it is incredibly hard to really accurately see the depth of field changes at faster apertures, like 1.4 or even 2. You are also looking at a small image in the viewfinder that has yet to be magnified, making it appear sharper (more accurately focused) than the later large image.

There's also a difference in method of focusing, live view uses Contrast Detect Autofocus, while your normal AF system is Phase detect. Phase Detect Autofocus is faster, but not always as accurate as Contrast Detect (for the time being). The PDAF system also needs your lens to be at the perfect distance from your sensor - there are some small tolerances in lenses and sensor positions and even the mount positions due to manufacturing. Many cameras now include a way of fine tuning your autofocus to compensate for this (See products like lens align, and look up front and back focus). These errors are usually fairly small, so small it doesn't usually matter - until you get into very thin depth of field. (Occasionally something can be badly misaligned and need calibration by the factory) My guess is you're having those issues with Narrow DOF and the optimization for a slower (2.8) lens.

Contrast Detect Aufotocus on the other hand, is also done through the lens, but is about finding edges - and it tends to be slower and more accurate (for the time being) This accounts for why your images focused in live view tend to be sharper.

(As a point of note, AF fine tune, should be checked against the live view Contrast Detect AF. And I'd recommend CDAF (live view autofocus) for focusing precisely. One of the compromises to make is AF speed vs AF accuracy... I personally like having both a fast and a precise way to AF as an option.

[Aegea]

PDAF and CDAF use different sensors in different locations in the camera. If the light path to the PDAF sensor is not exactly the same length as the light path to the CDAF sensor (which is actually in the image sensor) then PDAF will misfocus a bit - perhaps only visible with a fast lens wide open. Similarly, if the light path to the the viewfinder matte screen is a bit different from the path to the image sensor, then manual focusing will be a bit off. Here's a link that explains the situation for Nikons, and I think Canon is much the same. (I wouldn't recommend fiddling around inside the camera as the author does unless you have very steady hands and know exactly what you are doing.) As ravncat says, more modern cameras can do the adjustment electronically.

Nikon D70 Focusing Problem Workaround