Measuring focus for video

[Frank Barrera]

About backfocus.

 

I know about wide open f-stops to do it accurately while keeping the exposure correct as well (use a higher shutter setting if necessary to do this as well), but what is best, zooming the lens into infinity at object far aways, or, is there a magical distance once should settle on when adjusting backfocus and use a chart?

 

If a chart is not available, does the distance criteria change for setting backfocus?

 

Backfocus charts are all designed to be about the same size and since you need to zoom into them all the way with different focal legnths from the various zoom lenses out there; the answer to your question is that the "magical distance" is not fixed. The distance must change due to the length of your particular zoom. In other words the chart would have to be much further from the camera when using a 22X7.8 versus a 9X5.5.

 

Infinity or not on the focus ring is not at issue. It's about the size of the object in your frame that you are using as a test.

 

When no chart is available and you use something like a dollar bill the detail is much smaller and you would need to bring the bill closer to the camera in order to accurately judge focus.

[Alessandro Machi]

Infinity or not on the focus ring is not at issue. It's about the size of the object in your frame that you are using as a test.

 

When no chart is available and you use something like a dollar bill the detail is much smaller and you would need to bring the bill closer to the camera in order to accurately judge focus.

 

With an E.N.G. lens, I'm not sure If I tried to adjust back focus on a dollar bill at say 4-6 feet away that my adjustment would actually carryover correctly to all other distances.

[chuck colburn]

Backfocus charts are all designed to be about the same size and since you need to zoom into them all the way with different focal legnths from the various zoom lenses out there; the answer to your question is that the "magical distance" is not fixed. The distance must change due to the length of your particular zoom. In other words the chart would have to be much further from the camera when using a 22X7.8 versus a 9X5.5.

 

Infinity or not on the focus ring is not at issue. It's about the size of the object in your frame that you are using as a test.

 

When no chart is available and you use something like a dollar bill the detail is much smaller and you would need to bring the bill closer to the camera in order to accurately judge focus.

 

Hello Frank,

 

Yeah, what you said. Old timers would sometimes use the front page of a newspaper. I guess that was before the 1951 Airforce resoultion target and Siemens stars. LOL

By the way gotta love your member number, 1440, does that mean your always in sync? http://www.cinematography.com/forum2004/st...icons/icon4.gif

[Brian Drysdale]

With an E.N.G. lens, I'm not sure If I tried to adjust back focus on a dollar bill at say 4-6 feet away that my adjustment would actually carryover correctly to all other distances.

 

When you're setting the backfocus all you're doing is ensuring that the more critical wide end of the zoom lens is focusing on the same plane as the telephoto end. Once correctly set up it has no effect on the distance you're focused on in front of the lens because the images from both ends of the zoom range are now focusing on the same plane regardless of the object distance.

 

The macro selector changes the backfocus, so that you can focus closer, however, you can't zoom because it's only in focus at one focal length, not the entire range of the zoom lens

 

If you have any doubts the best way is to check it for yourself. Certainly people have been setting their ENG lens backfocus by using a chart/dollar/newspaper like this for many years.

[Frank Barrera]

Old timers would sometimes use the front page of a newspaper.

What is this "newspaper" you speak of?

 

By the way gotta love your member number, 1440, does that mean your always in sync?

Usually I run at 1440 rpm but somedays... well I just drift ever... so... slightly...

[chuck colburn]

What is this "newspaper" you speak of?

Usually I run at 1440 rpm but somedays... well I just drift ever... so... slightly...

 

Newspapers Frank Newspapers!

 

You know those things printed on paper and sold in little vending machines out in front of the liquor store. Yeah those vending machines right next to the other vending machines selling other printed matter that you don't want anybody else see you buying.

I know what you mean by that slight drifting problem... mayby my "crystal" is starting to wear out?

 

Chuck

[chuck colburn]

Back to the subject at hand. Changing the back focus has a slight effect on the front focus. So be sure to do it a least twice. That is zoom in tight on your target, focus the lens, zoom wide and set back focus. Now zoom tight and refocus. Zoom wide again and see if your back focus has held. If not, reset the back focus this should take care of any front focus shift that might have occured the first time.

 

Chuck

[Alessandro Machi]

Three scenarios to consider.

 

Zoom into infinity (do this outdoors), focus, zoom out to wide, adjust backfocus, zoom back into infinity, check sharpness, zoom back out, check sharpness , readjust back focus if necessary

 

Or...

 

....Do the exact same thing, but do it in a medium size or large room, zoom in to the back wall, set focus, zoom out, adjust back focus, zoom back into wall, check sharpness, zoom out to wide, recheck sharpness, readjust back focus if necessary. ..

 

Or...

 

....use the chart and do the identical procedure.

 

My question is.... If this technique is properly done in all three scenarios, does the backfocus get locked down in the exact same position for all three set-ups? If the answer is yes, then does that mean that all that really matters is achieving maxium sharpness via back focus in the wide position irrespective of where the focus ring is set to?

[chuck colburn]

Three scenarios to consider.

 

Zoom into infinity (do this outdoors), focus, zoom out to wide, adjust backfocus, zoom back into infinity, check sharpness, zoom back out, check sharpness , readjust back focus if necessary

 

Or...

 

....Do the exact same thing, but do it in a medium size or large room, zoom in to the back wall, set focus, zoom out, adjust back focus, zoom back into wall, check sharpness, zoom out to wide, recheck sharpness, readjust back focus if necessary. ..

 

Or...

 

....use the chart and do the identical procedure.

 

My question is.... If this technique is properly done in all three scenarios, does the backfocus get locked down in the exact same position for all three set-ups? If the answer is yes, then does that mean that all that really matters is achieving maxium sharpness via back focus in the wide position irrespective of where the focus ring is set to?

 

The answer to your question is yes.

It's just easier to do with a focus chart no matter where you are.

Just do it twice at least if you had to move the back focus adjuster the first time.

Chuck

[Alessandro Machi]

The answer to your question is yes.

It's just easier to do with a focus chart no matter where you are.

Just do it twice at least if you had to move the back focus adjuster the first time.

Chuck

 

Would it be logical to assume that the middle point of the focus range is probably the ideal distance at which to both focus and set back focus at?

[Brian Drysdale]

Would it be logical to assume that the middle point of the focus range is probably the ideal distance at which to both focus and set back focus at?

 

The answer is the distance that you can achieve the a good resolution on the chart you're using when at the wide end of the zoom lens. If you go too far away the chart can be too small for you to make a judgement on how sharp it is. With a wide angle zoom you need to set the chart up closer than with the standard zoom, otherwise the chart can be too small in the frame to make critical use of the star.

[chuck colburn]

Would it be logical to assume that the middle point of the focus range is probably the ideal distance at which to both focus and set back focus at?

 

Hello Alessandro,

 

As are favorite Vulcan would say, "That is illogical" !

And I'll tell you why. A zoom lens, any zoom lens, only comes to best focus at three points. These are its longest, shortest and some point (appx. half way inbetween) focal lengths. This is due to the fact that all zooming optical devices suffer from what is known as the "S" curve. That is to say as you zoom from the longest focal length to the widest the lens goes (albeit a small amount) out of optimum focus on one side of the film plane and then comes back to best focus (that point inbetween the longest and shortest focal lengths) and begins to go out of optimum focus on the other side of the film plane. Untill it reaches its shortest focal length where all things being equal it once again has reached it's best focus.

So if you begin at any point besides the longest focal length you are not going to get the best result possible from a given lens. I hope this makes some kind of sense. It's the reason why when you take your zoom lens in for overhaul it entails more then just a lube job and some lens mount shimming. Thus resulting in that wallet busting bill you get upon its return.

 

Chuck "thirty years of doing the above repair work" Colburn

[chuck colburn]

Alessandro

 

Disregard the above (except as some info on how zooms work) I misread your question as asking if one should start at a FOCAL LENGTH in the middle of the range not as you asked as a FOCUS RANGE. You can pretty much do it at any distance you want though I would avoid the absolute closest focusing distance.

Gotta cut back on my speed reading! LOL

 

Chuck