My focus change in MF mode!! at DVinfo.net

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Old July 1st, 2004, 11:22 AM   #1
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My focus change in MF mode!!

hey!

i just wondering.
if i zoom in a little bit on a close subject (about 1m), set focus manually, and zoom back, the focus change in MF mode! (the image is blurred when i zoom wide). Why? This is also the limitation of the professional canon lens?
If the subject is farther (3m or more) there is no problem.

thx
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Old July 1st, 2004, 12:06 PM   #2
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You need to zoom in all the way to set focus - it won't hold it otherwise.

Robin.
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Old July 1st, 2004, 12:11 PM   #3
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It's your aperture that changes

Here's what is likely happening.

You're shooting in a somewhat dark environment that
causes you or the camera to open your iris to 1.6

Then you zoom in all the way and set focus. Problem is
that your f.stop can remain at 1.6 when zoomed in all the
way. That's just the way the lens is made.

So now you have focused with an fstop of 2.6.

As you zoom back out your fstop goes back to 1.6 throwing
off your focus. What I do when trying to really avoid this
is to set my aperture to 2.6 so that it stays constant.
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Old July 1st, 2004, 02:10 PM   #4
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Re: It's your aperture that changes

Cesar,

i know all of this lens limitation.

But, i test in manual mode (S1/50, F2.8, 0dB), zoom to a close subject in MF, set focus, zoom back, and the picture is again BLURRED!
Interesting is, that with farther subject this doesn't happen.

Is my XM2 bad? :(

Test you guys, with very close subject please!
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Old July 1st, 2004, 03:38 PM   #5
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Can you actually zoom all the way in to the close subject and focus? If not, the problem is that you are within the minimum focussing distance for the lens. Since you can focus on objects further away, this is most likely the answer.

Robin
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Old July 2nd, 2004, 02:13 AM   #6
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No i can't zoom in all the way, because the subject is too close to me. (0,5m)
I can set sharp focus at about 3/4 of the full zoom, so i guess 14-16x.
But this is normal? When i zoom out, the picture is out of focus.
When i zoom back, the subject is again in focus.
If the distance is more than 2-3m away, there is not such a problem...

.just an interesting thing.
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Old July 2nd, 2004, 03:17 AM   #7
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As I said, the problem is because you are within the minimum focus for the lens to the subject. There's nothing wrong with your camera - it's simply the shortcomings of cheap lens systems in this price-range of camera.

Robin.
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Old July 2nd, 2004, 03:40 AM   #8
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OK. But this problem never happened with my sony D8.

So my XM2 has a cheap lens system?
"Professional fluorite lens" is written in the manual..

Anyway, i'm happy with my XM2 :)
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Old July 2nd, 2004, 03:55 AM   #9
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I'm happy with mine as well - up to as point. There are items which I would like to have, such as a manual iris ring as well as a zoom ring, but it's not possible in this price-range. It would be really good to be able to pull focus during a zoom as well, but this lens system will not allow it. Although Canon use the word "professional" when referring to the Fluorite lens, it's not to say that the lens iteself is professional - rather than that the Fluorite elements are used in their high-range lenses.
"You pays your money ..."

Robin
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Old July 2nd, 2004, 07:34 AM   #10
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Is Manual Focus Manual?

We THINK it is a manual focus . . but I recently got hold of info which suggested it is a mortorised setup . . bit like manual White Balance .. remember Robin? Now "manual" zoom would be neat .. and manual iris too . . where would the motor bit give way to the manual . .? And WHERE would we see the difference? . . I aint saying there wouldn't be, but if the irs was "motorised" I'd have that instead of the stepping stones we got .. yeah?
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Old July 2nd, 2004, 08:46 AM   #11
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You're right - it's not strictly manual focus at all. It's a clever servo device which interacts with the zoom function as well Hence the problem that the original poster presented. By manual, I mean fully manual. You turn the ring and a mechanical connection adjusts the iris, zoom, focus - whatever. It's the sort of lens that you would use on the XL1s. Okay, it ain't ever going to happen on the XM2, hence my "money" rider.
I shouldn't really gripe. If I'd saved a bit more money. I could have got the DSR570...or DVW790.....or .... (no end to this, is there?)

Robin
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Old July 2nd, 2004, 09:34 AM   #12
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Will the "Real Manual" please make themselves known?

So, and this IS the serious part of the point I'm trying to make, how much control do we need to have and to KNOW that we have knowing something IS manual to get the results we THINK we should be getting?

I've come to realise that some of the "problems" I/we face are because we think that we have done the correct thing with this cammie because we believe it is all MANUAL - meaning one function is NOT interfering with another . . Well now I want to know exactly what items DO and what items DON'T?

Now let's get tough . . . I've had it with "dancing" around this subject . . just give me the truth . . .the truth I can live with . . I can accomodate for the shortcomings of machines as long as I know "where" they lie . . .

. . . Let the Games Begin!

Grazie
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Old July 2nd, 2004, 10:03 AM   #13
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OK Graham, the control that I’d like to have on my camera, which are only what I expect on the cameras I use in the “day job”:
For a start, Full Manual Focus. A focus ring with metres/feet markings, so you can do controlled focus pulls – and repeat them, without the lens having drifted out of calibration because you moved the zoom a tad.
A Full Manual Zoom, so you can start and stop smoothly and have instant “feedback” as to what you’re doing.
A Full Manual Iris, so that you can change exposure smoothly without the “click-stop” effect.
This isn’t rocket science, it’s available on all proper lenses – it’s just a wish list for me than can never happen with the XM2 because it was never meant to fulful any other share of the market other than where it is right now, namely a “prosumer” camera.
The Canon zoom and focus system on our camera is a very sophisticated device that has to work out the focal length setting on the zoom and then calculate the focus accordingly. It has its shortcomings however – have you ever tried to pull focus as you’re zooming? Don’t bother, you can’t. You’ll have to finish one function first. It’s all down to the cost-cutting design of the lens. Canon would have to push the price up a notch to encompass these design changes – and they’d rightly say that one should therefore consider buying the XL1s – or whatever is due to replace it…

Robin
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Old July 2nd, 2004, 10:19 AM   #14
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Robin, truly thank you for your candour . . I think THIS should be read and re-read by our chums here so that the message sinks in - yeah?

The XM2 is not a manual cammie? . . When it says Manual Mode, beware ? . . Filming in "manual" mode gives some control . .. more than auto, but not full manual. . . the non-manual mechanicals I can really understand not appearing on a cammie costing £1.5k . . the other stuff iris and shutter speeds affecting each other while in manual that's worth exploring . . .

Do I love my XM2 ? . .TRY and take it off me . . . if yer think yer hard enough! Is the zoom control still got a mind of itself? - You betcha! . . .

Thanks again Robin, what you say will hopefully set the record straight about the XM2's limitations. .. . Oh the combined Rack Focus and Zoom thingie .. oh yeah . . .

Grazie
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Old July 2nd, 2004, 11:02 AM   #15
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Prech:
The issue you've observed is normal. The minimum focusing distance of the lens is technically 1m. At its full-wide position the lens can focus at 1cm. But there's a bit more to the story. All lenses are composed of many optical elements arranged in groups. Zoom lenses, like your GL2's, must move those groups dynamically to change the image magnification and focus. When you are focused on a close object, say 10cm, the lens automatically reconfigures itself into a macro-focusing mode. This usually requires that a rear element group be repositioned for close focusing. As you pull the zoom lens wider you will quickly reach the boundary of what this configuration can resolve. Since you have the lens on manual focus it cannot automatically reconfigure this element group back into normal focusing mode. Hence, you reach an out-of-focus condition.

The GL2's lens is designed to accommodate the shooter with a minimum of technical fuss, as are most lenses in this camera's class. But sometimes in trying to be slick, and in not documenting the design's characteristics, they can confuse owners into thinking that the product is defective.

Robin:
The lens of your dreams exists today, although only for the XL1 (within this prosumer realm). The 14x Manual lens is completely manual. Manual (marked) focus, manual (marked) zoom ring, and a stop-less manual aperture ring all mounted on a black barrel suitable for white pencil. Unfortunately, Canon discontinued these lenses in favor of the 16x Manual lens which replaces the wonderful iris ring with the on-camera (stopped) control. (See my article.)

BTW, both the 16x and 14x manual lenses for the XL1 make this close-focus matter clearer. They each feature a "Macro" focus switch on their barrels which enables you to manually change the position of that rear optical group to pull close focus. Reaching the boundary of that configuration produces precisely the same effect as Prech has observed on the GL2.

I hope this helps to explain this observation.
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