Does point of focus change due to zoom?

[Deuce 1]

I put together a new setup to allow me to shoot flash photos with my 10D. I'll get some pictures up after the American Boogie, but I'm finally to the last bit, adjusting my ringsite.

I have a concentric site on this helmet (DeuceCam Mark II ). In order to get everything sited in (In parallel, see Quade's articles) does zooming the lens on the PC120, and putting the zoom lens on my 10D increase the accuracy of the siting?

I use a laser site that is aligned with the video camera. So if I zoom in on the dot with the video, and then zoom in on the dot with the 10D with the 28-200 at 200 am I increasing my site accuracy?

It really seems to be the case when I back everything out to the .5 and the 24mm. But sometimes what seems obvious is wrong, so I'm looking for feedback.

JP

Edited to add, I focus on the laser dot at least 60 feet from the camera. (Infinity setting on the lens)

[LouDiamond 1]

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am I increasing my site accuracy?

What your doing in effect is making the "staw" diameter smaller( Picture a big gulp straw and a coffee stir straw) Any sighting error on your part during filming will be more apparent than your normal settings. It's kind of like the guy who wants to dial the magnification on a scope all the way in so he can see the target. Also be sure that your point of aim is indeed your point of impact with both cameras as a small deviation can mean someones heads are cut off for a whole dive.

"It's just skydiving..additional drama is not required"
Some people dream about flying, I live my dream
SKYMONKEY PUBLISHING

[Deuce 1]

I knew I could depend on you for a nice weapon analogy I could understand

So, if PC120's and my 28-200's optics function the same as a Leupold 3.5-14, Everything will remain in the X ring regardless of the power setting.

It seemed too obvious.

[quade 3]

When I first read your subject line I thought we might be talking about "back focus".

Back focus is the setting/ability of a zoom lens to maintain sharp focus over the entire zoom range without having to actually refocus the image. On consumer video cameras this is mediocre at best, but generally acceptable since most folks are using auto-focus anyway and it just doesn't matter.

However, what I really think you're talking about is what I call center tracking. Again, on consumer video cameras this is pretty abismal and I've demonstrated this to several folks over the years with my camera and its luma-key function using my reticule.jpg.

You can also show this by hooking up the camera to a monitor and placing a dot on the monitor.

Zoom all the way in and place the center of the picture on a very visible point (I use a flashlight in the distance). Then as you slowly zoom out, the center of the picture -should- track and stay on the target object, but with most consumer video cameras it has a tendancy to drift off center. Adding additional wide angle lenses may make the situation better or worse depending on their construction. In consumer cameras, there's not a hell of a lot you can do about any of this drift.

If I get a chance tonight I'll upload some examples of what this center tracking problem looks like.

With your 10D, the exact same things happen, however to what degree I can't say without testing. If the center does drift, then zooming in does nothing to increase your accuracy except at that specific zoom setting.

quade -
The World's Most Boring Skydiver

[Deuce 1]

So upload the reticule to my memory stick, use it instead of that silly curtain jpeg and 'speriment.

Just doing it by hand, the center track seems to elevate. That is, with the video , the dot seems to move upward as I zoom in. There is no movement in windage.

Q, I suspect that this is because the laser is above the camera, and that distance becomes visible as the zoom exaggerates it.

I'm thinking out loud here. Just as when a subject moves closer, the differences between the parallel lenses becomes apparent (sidemount vs. topmount), when zooming, you're gonna get the same issue, because you are in effect moving the subject closer.

?

[quade 3]

No.

It's because the optical elements in the zoom on your camera are shifting around and not tracking in a straight line.

Use something not attached to your camera, like a flashlight, pretty far away. That'll prove it ain't any parrallax issue.

quade -
The World's Most Boring Skydiver

[Deuce 1]

Check.

[vdschoor 0]

So is the best way to set it up to make sure that everything is setup perfect within the range that your subject is going to be in freefall?

I need to complete my setup tonight (add the still camera to the helmet) so it is ready for Davis this weekend!

[quade 3]

My recommendation would be to set the zoom(s) to whatever focal length (field of view -- "wideness") you plan on using in freefall and then align everything to a point at least 30 feet away if at all possible.

In my experience, zooming the lens in for alignment and then zooming it back out afterward for actual usage isn't the optimal alignment because of what I previously referred to as an error in center tracking on consumer video cameras.

See http://futurecam.com/sightingIn.html

quade -
The World's Most Boring Skydiver

[quade 3]

As promised . . . here's what the issue looks like on -my- camera.

The camera isn't moving -- only the zoom setting -- the camera is locked down on a tripod.

quade -
The World's Most Boring Skydiver

[Deuce 1]

Thanks for that.

JP

[Zenister 0]

Quote

When I first read your subject line I thought we might be talking about "back focus".

Back focus is the setting/ability of a zoom lens to maintain sharp focus over the entire zoom range without having to actually refocus the image.

is this the same as "depth of field?"

____________________________________
Those who fail to learn from the past are simply Doomed.

[quade 3]

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is this the same as "depth of field?"

No, back focus and depth of field are unrelated issues.

Back Focus is ONLY an issue for zoom lenses. It's an uncommanded and unwanted change in focus while zooming the lens due to the physical characteristics of the lens. Most zoom lenses have an adjustment for this.

Depth of Field is the amount of acceptable focus in front of and behind the point of sharpest focus. This happens with all lenses including "prime" (non-zoom) lenses. Depth of Field is something you intentionally control to achieve a desired effect.

More HERE.

quade -
The World's Most Boring Skydiver

[Deuce 1]

Hey Q, isn't that illustrated best by Hitchcock in the moving the camera while zooming thing?

(Expecting a "no", it's a "*")

• Quote

[quade 3]

Ya know, I'm not sure if that has a specific term other than "dolly & zoom, ya know, like in Vertigo?"

See IMDB Glossary.

It's also been used in modern movies like "Raiders". Typicially to show a main character being disoriented or in shock at something he's seen.

The camera lens zooms in (or out) while the camera dolly pulls back (or pushed in). To do it "correctly" without robotics is kind of a hit and miss artistic / choreography thing. You want the character's head to remain the same size on the screen while the background drastically changes. So, the dolly guys have to be in sync with the lens pullers and everything has to line up. Real pain in the ass to get just the right effect. With robotics it's quite a bit easier to set up.

quade -
The World's Most Boring Skydiver

[czechbase 0]

I have a question: when using a Sony cam, does one need to turn on manual focus and focus before flying; turn on infinity or just the normal auto focus thingy??

Thanks!
Matthew

www.motavi.com

[quade 3]

I think you'll find the best bet is to set the focus on infinity and then lock it down. I use a rubber band, other folks use gaff tape.

On the Sony consumer cameras you'll know you're at the infinity setting when you see a little pictogram of a mountain in the viewfinder.

quade -
The World's Most Boring Skydiver

[czechbase 0]

What do you mean by lock it down? Once it has been activated it stays that way until you turn it off no?

Also with the Sony cams, you need to pull out the viewfinder a little, what's the best way to make sure that the viewfinder doesn't slip back into the camera?

I notice that when you put the viewfinder back into the camera, you can't see anything!

Thanks for your help
Matthew

www.motavi.com

[quade 3]

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What do you mean by lock it down? Once it has been activated it stays that way until you turn it off no?

On the older model Sony cameras the infinity setting is a temporary setting activated by holding a switch. Touching the focus ring while the camera was on afterward would then disturb this. By locking down the focus ring with a rubber band or piece of gaff tape, the setting would stay set.

quade -
The World's Most Boring Skydiver