Using Tape Measure For Precise Focus
 

After some slightly soft results (fully zoomed out on an interview subject who's standing approx 4 feet away) having used Magnify and Peaking, I've resorted to a tape measure to keep my mind at ease. I tried a 'laser tape measure' today but the results weren't completely accurate - the instructions also say it's not to be used where precise measurements are required.

I've just filmed some tests on my XHA1 using a normal tape measure and am looking forward to seeing how sharp they are when I play them back shortly.

However, I'd just like to ask, does anyone else here use a tape measure with their XHA1 for interviews or filmmaking?

If so, what's the ideal spot to place the beginning of the tape measure (I'm thinking that perhaps the tape measure might need to begin a bit further back on the A1)?

At the moment I'm placing the beginning of my tape measure immediately just behind the little screw that holds the sensor at the front of the camera.

I'll describe my findings once I watch my footage.

http://www.dvinfo.net/conf/canon-xh-...e-readout.html

I wondered that too in post #3 in above thread. Do let us know if you find out where to measure from.

Once you have measured the distance from the subject to the camera, are you going to set the focus based on the distance indication on the display? Because if you are, be aware that the distance displayed is not that accurate. I have two identical XHA1s that provide distances as much 6 feet different from each other when focused at a distance of approximately 60 feet.

Measure from the front of the glass on the lens. Yeah, I know it's weird, since with older cameras the measurement is from the film plane, but I've heard (more than once) to use the front glass.

Interesting - where did this information come from?

With ENG style video lenses you measure from the front, with Cine style video lenses you measure from image plane like a film camera.

If you're planning to use tape measurements the Cine style lenses are much better because they have the stretched focus scale that film camera lenses use. The ENG style video lens usually only rotate a 1/3rd of the way and have a more compressed focus scale.

I doubt you can get any Cine style lenses for 1/3" cameras, although they're available for 2/3" cameras and are commonly used on dramas etc.

Of course, you can make your own focus scale for your ENG style lens, in fact that would be good idea because the manufacturer's original markings may not be accurate. You also need a manual zoom lens, rather than one of those continuously rotating focus ring jobs found on most consumer/prosumer cameras.

And?...
 

Brian, given that we are stuck with the lens supplied with the XH series (should have gone to Specsavers and bought an XL series), where do we measure from - the glass or the focal plane (and if the latter where is it)?

It's easy enough to find out.

For the front element, using a tape measure set up a flat chart that easy to eye focus on at a distance that matches a measurement on your focus scale. With the lens aperture wide open, zoom in and eye focus it - if the scale's distances matches the measured distance from your front element, that's your reference point. I suspect this will be the case, since it's pretty standard on video cameras.

However, if it's not you can find your focal plane marking by using the tape measure. If you set up the chart so that it in focus at a locked off focus marking on the lens and then measure that distance back to the camera you should find your focal plane. You can put a reference mark on your camera body and ideally a tape hook.

Also, I'd check that tape measurement works with a range focal lengths. The focus rings on the software controlled varifocal length zooms found on 1/3" cameras often tend not to hit repeatable marks. You should also check that the focus scale on the lens is accurate, the markings can be pretty rough settings and they won't have witness lines that give you the exact position on the focus scale, only numbers.

Have you tried getting in really tight on your subjects face, setting the focus and then zooming out? When you are in tight, you can use the magnification and peaking to really get a sharp focus. I typically set the focus using my subjects nose.

The eyes are better, they are the main points of interest in the typical frame sizes used in interviews.

Think this may be of interest.........
 

http://www.dvinfo.net/conf/open-dv-d...an-i-sure.html.

Sure works for me.


CS

I never have time for tape measures or the magnify button (which requires you to stop shooting) and I don't find the peaking button to be very helpful either.
I zoom in and centre the image on what I want in focus, press the instant focus button and zoom out.
It seems to work.

Dosen't the XH-A1 have an electronic focus mechanism rather than mechanical?

I would think an electronic focus mechanism would require a qualitative focus adjustment, as on an external monitor.

A true mechanical focus would offer repeatable focus results and quantitative focus measures would actually work well there.

Sometimes the XH-A1 focus can be very frustrating. Shots you make every possible effort to focus later appear to be soft for no apparent reason. I believe it's a very infrequent image processing snafu of some kind that causes this occasional poor focus. I say that because it sometimes looks like there is a little more wrong with the image than just mis-focus if you look very closely at the image.

This is affected a lot by how you set the exposure. If you use small apertures you are likely to get a soft image even when the focus is spot on. Try shooting everything at F4 for a while to see the XH-A1 at its best, (in terms of sharpness).

Richard

Peter, if you look at your lens there is a red line around it as I think there is on all canon lenses I Think jvc rings are green and I am sure that that is your focal plane and that is where you take measurements from but don't take that for gospel I have never used a tape to focus.

Alan