Live view autofocus vs. Viewfinder autofocus point

tecboy · Feb 5, 2015

I have met a photographer who prefers live view autofocus instead of using a viewfinder autofocus point, because it is better and get a sharper image. Really?

Tinderbox (UK) · Feb 5, 2015

Most dslr uses contrast focusing based during live-view, contrast detection is more accurate than phase detection for static subjects, but phase detection is better for moving subjects especialy when they are moving towards or away from the camera, some Canons use phase detection in live view as it`s built into the sensor.

John.

Big Mike · Feb 6, 2015

Typically the live view (contrast) based auto focus is slower. This is one of the reasons that many of us prefer a DSLR camera over smaller type of digital camera...The DSLR is capable of faster AF, which often relates to less 'shutter lag'. The biggest complaint about P&S cameras is that they take to long to snap the photo, and in many cases, that is related to their slow AF.

Case in point, some DSLR cameras that offer live view, give you the choice to use 'Quick Mode' AF, while in live view. In this mode, the camera flaps down the mirror, uses the phase detection AF, then flips the mirror back up. They offer this because it can be quicker/faster than using the contrast detection AF.

As for one or the other giving you a sharper image....I wouldn't put it like that. I might say that one mode or the other might struggle to reach focus in time...but if they are both working correctly, then they should both be able to achieve accurate focus, and thus the resulting sharpness of the photo should be the same.

tecboy · Feb 7, 2015

I don't understand the logic to take the time to switch to live view and digital zoom in to get accurate autofocus.

Overread · Feb 7, 2015

I honestly don't think that guy knows quite what he's doing in terms of autofocusing and regular focusing (that said its a very typical "shop" tutorial in that it is so quick and glossed over it really presents very little information nor guidance).

If you want to use live-view for focusing the way most people do it is to :

1) Use normal shooting mode AF to get the focus roughly in the right spot.
2) Select live-view mode and enable manual focusing (if needed - some lenses have all the time manual focusing, but if using one of those make sure to use back-button AF otherwise the halfshutter press when taking the shot will cause the AF to engage and might shift the focus).
3) Magnify the photo over the key part you want in-focus
4) Adjust the manual focusing ring until you get the focusing you want - take the shot.

I'm unsure if he's really doing anything in that tutorial except using the regular live-view AF (which in all honestly likely means his regular AF mode is setup wrong if its leading him astray more so)

nerwin · Feb 8, 2015

I use live view only when I'm using a tripod because I like that fact that I can zoom in digitally and fine tune the focus to get a super sharp photo. Its only good for static subjects though.

Dave442 · Feb 8, 2015

And in Live View you can move the focus point anywhere on the screen.

Buckster · Feb 8, 2015

There are times when I prefer focusing with live view so that I can zoom in for a more precise focus, but usually only when shooting macros, and then, using a larger screen by tethering my 10" tablet to the camera for it, and both camera and tablet are locked down on a tripod.

TCampbell · Feb 8, 2015

One live view settles in and focuses on your chosen point, it will maximize the best possible focus on that point. It can do that mathematically by identifying edges of contrast and then adjusting focus until it maximizes the contrast difference between adjacent pixels along that edge.

The downside to live view is that it's slow and works it's way to better focusing by trial and error... it samples, adjusts, re-samples, compares the difference to determine if things are getting better or worse (if worse it has to reverse the focus direction), and continues to take a few more samples until it can work out when it's finally optimized focus.

Phase detection focus (through the viewfinder) is VERY fast. But the downside to phase detection is that the sensors are not on the image sensor (Canon has a few models for which this is no longer true) so it's possible for focus to be bang on accurate on the focus sensor... and off on the actual image sensor. The sensors can be shimmed (requires factory service -- this isn't something we can do ourselves) to get this right (and part of factory production is to test and adjust each camera... but odds are one will sneak out that wasn't quite up to spec.) SOME camera models (a common feature of higher end DSLRs) allows you to apply some micro-adjustment to the focus. If you detect your camera consistently front focuses... you can tell the camera to focus back a few "units". Also... accurate focus for one lens might not be accurate for another.

So basically when phase detect focus is working accurately, then it's speed gives you a tremendous advantage. If you don't need the speed... then contrast detect focus is more accurate.

If battery power is a consideration, phase-detect takes substantially less power. When you use live view, you're basically using your sensor "like a video camera" in that you are opening the shutter, turning on the sensor and streaming the video to the camera's LCD screen... that takes a noticeable amount of power. When you use the viewfinder, the sensor is technically not active. If you spend a whole day shooting exclusively on "live view" and spend another whole day shooting exclusively via the viewfinder, you'll probably notice a remarkable difference in how much live view draws down the battery power. If you have spare batteries then this might not be a big deal to you.