question regarding Canon 7D live view vs viewfinder
wheresdavid
Registered Users Posts: 297 Major grins
Question, the other day when I was testing my 7D using the moire technique - Ie focusing on my laptop screen using live view and then turning it off and using the shutter release button something "odd" happened. When I manually focused using live view and then turned live view off and then looked through the view finder the center focus point was way off as in not even on the laptop screen. I wasn't zoomed in using live view or if I was only 5x. I was using my 240105 set at about 50 or 70. If I first used my viewfinder and using the center focus point and focused it on the middle of my screen and then switched to live view I didn't even see the laptop, what I saw was above the laptop. I order to use live view to "see" the center of the screen I had to loosen the ballhead and tllt the camera down ("quite a bit"). I thought my camera was at the same height as my lap top screen and was square to it.
I never use live view and I thought that if I zoomed in with live view and then turned live view off and looked through the view finder I should be seeing the same thing depending on if I zoomed in with live view. The area of focus for live view was "in the center". Is this normal?
Cheers
I never use live view and I thought that if I zoomed in with live view and then turned live view off and looked through the view finder I should be seeing the same thing depending on if I zoomed in with live view. The area of focus for live view was "in the center". Is this normal?
Cheers
0
Comments
Unfortunately, using a ruler or "focus chart" to try to detect front or back focus issues may induce those issues.
Even selecting a single focus point is not as finite as the display might indicate. The actual region affected is quite a bit larger. Because of this a lens/camera may choose a mark other than what you might think or intend.
I suggest using a single target with a sharp "cross" mark oriented at roughly perpendicular to the camera and lens. The background around and in front and behind the target should be as non-distinct as possible.
The idea is to give a single distinct target so that the AF sensor does not get confused by marks before or after the target. If you want to supply a scale, make sure that it is well to the side of the target and away from the marked regions of AF in the viewfinder.
I know this goes against a number of tools and sites that exist to supposedy measure front and back focus, but frankly, they haven't researched the impact of what they do.
Assuming that you do properly construct a single target like I suggest, front focus is the tendancy of a lens and camera to place the prime focus at the focal plane in front of the target.
Back focus likewise tends to consistently focus behind the target.
Be sure to take into account the minimum focus distance (MFD) for the lens involved, which can change in many zoom lenses depending on the focal length chosen.
I prefer to use twice MFD for testing front and back focus. Most lenses do not do their best at MFD, the exception being true macro lenses.
If you want to use a test chart, this is the one I recommend:
http://regex.info/blog/photo-tech/focus-chart
Be sure to read the entire article before testing.
Moderator of the Cameras and Accessories forums
With regards to my post - it wasn't regarding the moire method per se. I guess using the Moire method you use live view first and then turn it off and press the shutter button half way to see if the focuses changes ... but to put it totally in laymen's terms - my live view wasn't showing me what I was seeing through the view finder. In live view I would see my laptop screen. When I turned off live view and looked through the viewfinder I was looking at the space below the table the laptop was sitting on. hope that makes a little more sense. I was assuming that live view and the viewfinder would be "showing" the exact same thing (depending on whether or not I was zoomed in using live view) I wasn't hand hold the camera, it was mounted on my tripod.
Cheers,
Dave
So this is a science experiment?
I suggest doing a similar test with a more static subject to see if the same effect occurs.
If it does occur then it would appear that the live view and viewfinder view are different. Possibly the live view has the ability to magnify and center around the actual area of focus? I don't have a 7D to test so I can't do more than speculate at possibilities.
If the phenomenon does not occur with a static subject then I would speculate a possible interaction with the LCD display.
I would caution against the use of an LCD display for AF testing at any rate because there is a known interaction between flickering light and AF. The new Canon 1D MKIV has both hardware and processor algorithms to try to detect and counter the problem. An LCD display has a "refresh" cycle that would probably be interpreted by the camera's AF sensor and processor as flicker and could couse inaccurate AF as a result. The only focus method I would recommend of a computer display at any rate is manual focus, or possibly a live view with manual focus.
Moderator of the Cameras and Accessories forums
Cuong
correct...if I zoom in with LV and move the focus point around, then exit LV, then re-enter LV, that adjusted focus point will remain where it last was set. this threw me off the first time, but then I just zoomed out and reset it.
Cheers