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Old May 11th, 2007, 08:31 PM   #1
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Distance readout accuracy....

Does anyone know, with certainty, just how accurate the distance readout is in Manual focus mode? When it says 95 metres, is that +/- .5, 1.0, 2.0 metres or just somewhere in the general 90's etc?

The reason for asking is that I'm considering mounting one of those laser rangefinders onto the camera to get < 1 metre distance accuracy for focusing, but in order for that to work the readout from the A1 has to be pretty close to spot on.

I wouldn't have believed it before I got the A1, but @ 100 metres even 1 metre or less out is enough to show up glaringly on the big screen, and stopping down to get better depth of field just digs the hole deeper.

Yes, yes, I could lug a 24" HD screen round with me and get the focus that way, tho' a tad impracticle.

Now, before everyone starts ROFL'ing, consider this. @100 metres you have 4 objects in view, rangeing from 98 metres to 106 metres. You twidle focus back and forth and decide on a spot. The camera says it's @ 99 metres. But hey, because I don't know that the furthest objects are @106, I don't push the focus out to 102, it simply can't be seen on that little screen (not with my ageing eyes, anyway).

Yep, could hire a focus puller (but we're back to the accuracy of that readout) and besides, those 4 objects above are exceedingly bad tempered elephant seals who eat focus pullers for lunch (and happen to be on a rock a 50 metre swim away).

All I/P gratefully received.
Thanks.

Last edited by Chris Soucy; May 11th, 2007 at 08:32 PM. Reason: syntax
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Old May 12th, 2007, 01:55 AM   #2
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Now, I might be wrong about this but i have an unproved theory. I believe that the messurement is not actually an accurate reading of how far away you are from the object, but rather the setting of focus. I know regular film camera lenses have notches on the focus listing the distance that the lens is in focus. As for the accuracy, It's really hard to tell because you don't have that physical messurement where you can see where you are, but you just see the digital read out. And the focus ring is a servo, so that's not 100% receptive.

Hope any of that helped.
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Old May 12th, 2007, 06:06 AM   #3
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I don't think it's very accurate. From a tripod shoot at roughly 10 meters on manual focus. Push AF (11.5), push 2 (7.3), push 3 (13) push 4 (9.5), push 5 (10.0), etc, etc. Same subject, no movement, in front of curtain.
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Old May 12th, 2007, 08:30 AM   #4
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Get a long tape measure and check it. I cant vouch for the distances you mention but I checked mine with a tape measure at 10m and less and it was very accurate. It also agreed with my focusing by eye using an HD monitor. By the way, how accurate is the laser range finder you are planning to use?
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Old May 12th, 2007, 09:35 AM   #5
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Yes was about to post the VERY SAME question myself!
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Old May 12th, 2007, 12:18 PM   #6
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Quote:
Originally Posted by Brad Tyrrell View Post
I don't think it's very accurate. From a tripod shoot at roughly 10 meters on manual focus. Push AF (11.5), push 2 (7.3), push 3 (13) push 4 (9.5), push 5 (10.0), etc, etc. Same subject, no movement, in front of curtain.
This test just shows how the Push AF (IAF/Auto focus) works. I think distance readout is quite accurate, but Push AF is not.
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Old May 12th, 2007, 03:32 PM   #7
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Laser Rangfinders.......

Depending on make, model, range required and price (of course) they vary from +/- 2 cm to 200 metres (really!) to +/- 1 metre to 1500 metres. The main problem with the shorter range units (primarily designed for the building trade/ real estate etc) is that the laser doesn't bounce too well off non reflective surfaces.

The models designed for hunting/ golf etc usually give the usable distances for deer, which is as close as I'm going to get to my usual seal targets.

My main problems are in the 50 - 150 metre range where it would be nice to use the more accurate units, but they just don't have the legs for it.

They do have the advantage (the hunting models) that they have an in built tripod socket so can go on the hot - shoe with the appropriate adapter, tho where my Sennheiser receiver is going to shift to is a moot point.

Of course, not even worth trying (cheap they ain't) unless the range figure they provide can be mapped to the lens pretty accurately.

Yes, I have thought of doing the tape measure/ multiple printed targets thing to test the accuracy of the focus myself, but was interested in seeing if anyone "official"/ "In the Know" would stick their heads over the parapet and venture some "facts", saving me an awefull lot of buggering about (it's not all beer and skittles here in the South Island you know!).

Cheers

Chris
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Old May 12th, 2007, 04:29 PM   #8
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Just in case it wasn't clear.........

in my other posts, these rangefinders can pick a target only about the size of a cigarette packet or even smaller. In my original 4 group scanario, given the stated accuracy of the finder (+/- 1 metre), I would get a closest figure of 97 metres and a furthest of 107, ergo, to have the best chance of getting all 4 somewhere in the ball park, setting the focus @ 102 and prey.

However, given that DOF is greater behind the plane of focus than in front and that @ 10 metres (front to back) I know I'm in deep doo -doo with it, I'd probably set the focus for the mid point of the closest two objects and leave the furthest to their own (slightly blurry) devices.

It has occured to me that if Canon built one of these things into the next itteration of the A1/ G1 series and got the accuracy down to +/- .5 metres, AND got the "PLane of Focus Distance readout" to withing the same sort of tolearance, you've got a built in, deadly accurate focus puller that even works in rain, snow, pitch darkness, up cliffs, across water etc etc (go tell that to the Union!).

Cheers

Chris
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Old May 12th, 2007, 05:06 PM   #9
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Canon wrote themselves in the manual that the read-out distance may not be accurate if the temperature of the camcorder changes...

This being said, I seem to recall that the A1 read-out accuracy is 10cm up to 10m, and 1m above that at telephoto, far less in wide-angle. I have been using a laser rangefinder to pick out reference distances at about 20m distance in a theater and that worked. My rangefinder is a Leica disto, which is accurate to +-1.5mm.
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Old May 12th, 2007, 05:35 PM   #10
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Hi Jerome........

Nice to hear from you. I didn't think I could be the only one to have thought of the idea. Yeah, that Leica sure sounds like a nice unit. They also make the ones marketed by Stanley world wide, tho' don't think they have the same degree of accuracy.

It occured to me whilst having my shower this am that if Canon REALLY wanted to stomp the competition into the dust they could easily develop the focusing system to die for using the same technology.

Does anyone remember the Sony FSC - 707 still camera? (or was it the 717?). Whatever, it had a red laser for night shots that scanned the entire field of view (quite something to behold, from reports), then worked out focus, aperture, flash strength etc from the recieved data and click!

Use the same tecnique but using the new invisble lasers and electronics, frame your picture on the A3 (as it will be by then) then hit the "focus assist" button. Laser scans entire field of view,viewfinder ceases showing piccy and switches to an aerial view of the field in front with all detected objects shown as lines of varying thicknesses based on height, the closest at the bottom of the screen, the furthest at the top.

Across the screen from left to right is a solid line depicting the currently selected "plane of focus". Above and below that line are a pair of dotted lines showing "acceptable depth of field" based on zoom position, selected aperture etc etc. The top line slightly further from the POF than the lower. Using the scroll wheel, move POF to selected spot. Want a bit more/ less DOF? Tweak the aperture, and DOF lines move accordingly. Hit "Record".

On the button, first time, every time.

Neat huh? (Just drop the royalty cheque in the post, thanks Canon).

The creepy thing about this system is it's not Star Wars, it's already in use for delivering something a tad less friendly than photons, and it's been around for ages.

Cheers,


Chris
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Old May 12th, 2007, 08:53 PM   #11
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Giving this even more thought...

Wow, the possibilities are endless. How about keepin the picture on screen but reverting to monochrome and highlighting those area @POF in a colour of one's choice, and those within ADOF in a contrasting colour.

Throw in a "self learning" function so that you could map any area of the sensor, pixel for pixel with your on - camera or external display (move it round with a dinky joystick), move it to an area it "thinks" is @POF and if it isn't absolutely tack sharp, hit a "calibrate" button and set it straight. Do this at half a dozen different distances, and hey presto, it's perfectly calibrated. This removes the need for any manufacturers adjustments due to age, component tolerances, wear and tear etc.

Gotta get me one of them! Anyone want to buy a pre - loved A1? I'm getting an A3/ A1s Mega or whatever it's going to be called.
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Old May 14th, 2007, 03:08 PM   #12
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Quote:
Originally Posted by Chris Soucy View Post
Does anyone remember the Sony FSC - 707 still camera? (or was it the 717?). Whatever, it had a red laser for night shots that scanned the entire field of view (quite something to behold, from reports), then worked out focus, aperture, flash strength etc from the recieved data and click!
Not quite: http://www.dpreview.com/reviews/sonydscf707/page4.asp

Anyway, I find that the A1 AF works well enough for my needs, and I am not sure I'll want a laser show.
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Old May 14th, 2007, 09:36 PM   #13
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....and the answer is - maybe!

Decided to put this to the test myself and get some sort of answer.

Printed out 12 A4 sheets with 8 black bars equally spaced on each, the black and white bars the same size. Arranged them on a large piece of cardboard ( 1 metre X .82 metre - 39" X 32") in a cross hatch pattern, 6 with bars vertical, 6 horizontal.

Set this target up parrallel to the plane of focus of the camera @ 10, 20, 30, 40 & 50 metres using a surveyors tape measure. Locked the camera to 1/100th in Manual, zoomed in till the target filled the viewfinder (or the central section above 30 metres) and started hitting the Push AF button in Manual focus mode with the tape running. Between each hit I put the lens alternately at infinity or 1 - 2 metres.

This is probably more info than you want, but here's the results:

The figures (in metres) are those displayed in the viewfinder for focus distance.

Test 1.

10 metres: 12 hits. Lowest 9.1m, highest 9.6m. Largest hits:9.4m with 5

20 metres: 22 hits. Lowest 18m, highest 21m. Largest hits: 19m with 12

30 metres: 20 hits. Lowest 28m, highest 34m. Largest hits: 30m with 7

40 metres: 25 hits. Lowest 37m, highest 46m. Largest hits: 41m with 7

50 metres: 43 hits: Lowest 38m, highest 67m. Largest hits: 52m, 56m, 57m, 58m with 5 hits each.

Test 2.

I then dialed in each of the distances it displayed @ all distances whilst running the tape (so, at 20 metres I dialled in 18, 19, 20 & 21)

Test 3.

I then tried manually focussing using both Peak and Magnify to see what figures I got by eye

10 - 10
20 - 19
30 - 30
40 - 37 - 44 but decided to go with 39 (only 'cause I "knew" what it should be)

50 - 59

At 50 metres I was struggling to decide on any figure whatsoever. The range of displayed distances during which it "looked" sharp in the viewfinder varied from 34 to well over 80 metres. However, even in "Zoom" mode the target was signifcantly smaller than the full lcd, which is where both the camera and I struggled.

I then took the camera back inside and played the tape back on my 46" Bravia.

Conclusion?

Test1.

Having played it back @ 2 X speed I was hard pressed to see any really signifcant difference in the focus,with one exception - the 38 metre setting @ 50 metres. Given the range of figures displayed during the test I was expecting the results to range from "OK" to "Yuck". This leads me to think that the actual focus set by the camera is somewhat independant of the figure displayed, in fact it would seem that it can get the focus pretty well spot on most times (on this type of target, good sun etc using Push AF) but vary the actual readout considerably.

Test 2.

This was interesting. When the previously displayed distance figures were dialled into the lens, it did show differences in the focus, not suprisingly at the top and bottom extremes of the range. Figures on and just either side of the mean were pretty good, the rest weren't.

Test 3.

The fact that both the camera and I were struggling @ 50 metres with such a high contrast target is a worry. However, knowing that the target was @ 50, dialling in anything between 47 & 53 gave pretty good results. It would appear from this that if you manually dial in a set distance, it will give you the right focus (-ish). To that end, the laser range finder trick might just work, tho' as this test was only out to 50 metres ( I ran out of driveway) I guess the only way to know for sure is to try it.

Cheers,

Chris
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Old May 15th, 2007, 07:26 AM   #14
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Thanks Chris,

I've been pushing that AF button much more than I have to apparently.

I very often can't "see" that the camera is out of focus (21" field monitor) but the numbers get scary because I know they can't be right. I end up pushing until the read-out is close to what I think it should be.

Looks like I shouldn't worry as much.
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Old May 17th, 2007, 05:46 PM   #15
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Now, here's a question....

for someone with an absolute gold mine of useless information.....

I've been mulling over the results of my little field test the other day, bearing in mind the comments from Bill Ball about his A1's accuracy @ 10 metres, and it suddenly occured to me that there might, just, be a reason for my 10 metre test to be sooo wide of the mark.

I had (perhaps incorrectly) assumed that the focus distance was, as in film/ digital SLR's, taken from the "film" plane, and had taken my start point for the test at some arbitrary spot on the camera where I "sort" of figured the ccd's might be. But, what if that was wrong? Is it from the lens front element? Could this explain the discrepancy?

Well, it might go some way, but having just measured from the front lens element to where I thought the focal plane should be, it's only .1 of a metre. Bugger! But, as the saying goes, every little bit helps.

Does anyone have a definitive answer to this Q? Front element or film plane? Somewhere in between?

Yes, yes, I realise this is like debateing how many angels can dance on the head of a pin, but now it's popped into my head, it's really bugging me that I don't know.

Cheers


Chris
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