T2i Lens not holding focus through zoom range

F

FatBird19

Well-known member
Hey everyone, need some advice.
I'm having some problems with the stock lens.


In picture mode, when I auto focus the camera (beep noise) at the wide end of the lens the image is in focus, but when I zoom into 55m the lens is very out of focus.

Then if I autofocus at 55mm and then zoom out to 17mm the image is out of focus again (albeit not as bad as when I focus wide and zoom in).


What's wrong here?
It seems to me that the lens is defective. It seems that the focal point changes through the zoom range.

Where do I go from here? Who should I contact to get this resolved?

What other tests might I do to understand what's happening?

Thanks,
-Justin
 
Still camera lenses don't act like video zoom lenses. You cannot focus them the same way. Your lens is not defective, they way you are using it is. Most will not keep the same aperature either. You must first get to the distance you want on your zoom and then focus.
 
boulder said:
Still camera lenses don't act like video zoom lenses. You cannot focus them the same way. Your lens is not defective, they way you are using it is. Most will not keep the same aperature either. You must first get to the distance you want on your zoom and then focus.
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I don't remember having this problem though when I had my XTi. Maybe I just didn't notice at the time...

As long as I set the aperture to 5.6, it maintains the same exposure throughout the zoom range. I thought the focus would have worked similarly; If I set the focus at 4'6", shouldn't it maintain that same focus distance whether the zoom is at 17mm or 55mm?
 
dude, you can blow over $1000 on a band new L zoom and it still won't hold focus over the zoom range

thats why cine lenses cost more than still lenses
 
FatBird19 said:
As long as I set the aperture to 5.6, it maintains the same exposure throughout the zoom range. I thought the focus would have worked similarly; If I set the focus at 4'6", shouldn't it maintain that same focus distance whether the zoom is at 17mm or 55mm?
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http://en.wikipedia.org/wiki/Zoom_lens

There are two kinds of lenses that are called "zoom lenses," parfocal and varifocal. Parfocal lenses are designed to hold focus throughout the range. Varifocal lenses are not designed that way. Most zoom lenses for SLRs are varifocal, not parfocal. Parfocal lenses are much more difficult to engineer, and thus they're more expensive. So you should expect kit lenses to be varifocal.
 
... and here is another kicker. A lot of still zooms don't hold exposure through the zoom range
even if they do have a fixed aperture. for example you might keep the aperture a f2.8
take a still at widest, zoom all the way in and take a still also at f2.8

then when you compare the 2 stills you might see a light loss of a quarter or third stop
 
ydgmdlu said:
http://en.wikipedia.org/wiki/Zoom_lens

There are two kinds of lenses that are called "zoom lenses," parfocal and varifocal. Parfocal lenses are designed to hold focus throughout the range. Varifocal lenses are not designed that way. Most zoom lenses for SLRs are varifocal, not parfocal. Parfocal lenses are much more difficult to engineer, and thus they're more expensive. So you should expect kit lenses to be varifocal.
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*The more you know!*

Last time I had an XTi was probably 3 years ago, and I've had a lot more production experience since then, which is probably why I didn't notice at the time.

Now I have another reason to appreciate why people pay so much more for cinema lenses.
 
Personally the ability to hold the focus through the zoom is way down in my needs list. First, I NEVER zoom in a shot, I find my focal length and hold it. Second I use the magnify button to "zoom" into the image for focus. To me the speed of the lens is where I would put the money. The technique of zooming in and focusing is probably not long for this world with the direction things are taking.
 
boulder said:
Personally the ability to hold the focus through the zoom is way down in my needs list. First, I NEVER zoom in a shot, I find my focal length and hold it. Second I use the magnify button to "zoom" into the image for focus. To me the speed of the lens is where I would put the money. The technique of zooming in and focusing is probably not long for this world with the direction things are taking.
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hey boulder, may I point out to all that are three distinctly different things that you are talking about

• focus assist is the function in the DSLR camera that represents a small portion of the image at x5 or x10 on the LCD in order for the user to get better focus, no focal length change involved (no change to the lens at all in fact)

• also you mentioned the technique of using the zoom of a lens (ie changing focal length) to get a focus and zooming back out to where you want your frame to be (which is an old camcorder trick) before pressing record

• and then the topic of the thread is about a still lens losing focus as you change focal length which negates the ability for a DP or director to have a zoom shot while recording.

Zoom shots are another tool in the toolkit. Just look at Kubrick's Barry Lyndon. Academy award for cinematography. And the specialized zoom shot in that. A x20 lens!

edit: a 12-240mm Angenieux 16mm with 1.6 extender for a 35mm format
 
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20x zoom shot from Barry Lyndon

4483356022_61c7356f52_o.jpg


edit: special thanks to George A for the Lyndon grabs and the info on the Angenieux x20 lens
 
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[quoteJ • focus assist is the function in the DSLR camera that represents a small portion of the image at x5 or x10 on the LCD in order for the user to get better focus, no focal length change involved (no change to the lens at all in fact)
[/quote]

Yes I am aware of this, just saying it is simular to the way videographers use the zoom for focusing.

And yes, I'm also aware that Kubrik used a zoom, Penn did too in Bonny and Clyde. Sure there are rare instances of a zoom being used, but name a hollywood movie in let's say say 2009 that used one...

Come on now...that's a challenge!...and using Google is cheating
 
boulder said:
[quoteJ Sure there are rare instances of a zoom being used, but name a hollywood movie in let's say say 2009 that used one...

Come on now...that's a challenge!...and using Google is cheating
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A lot of films still use the "dolly zoom" effect, so there's at least that. And I agree that zooming isn't used as much, but it still has its place.

Can't pick out a specific shot off the top of my head in recent films, but I could probably say the same about a lot of different shots. Unless it really stands out, you're not going to remember it.
 
handsome pete said:
A lot of films still use the "dolly zoom" effect, so there's at least that. And I agree that zooming isn't used as much, but it still has its place.

Can't pick out a specific shot off the top of my head in recent films, but I could probably say the same about a lot of different shots. Unless it really stands out, you're not going to remember it.
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Ok...I'll open it up...since 2000
 
boulder ... that's quite a challenge, especially since I don't think I've seen any films from 2009 yet other than avatar.

As far as unique memorable zoom shots then I can't think of any other than my favourites which are older movies.

BUT !

May I point out that zoom shots are used in a subtle way in many many modern productions. I'm talking about when the actor is framed head and shoulders and they start saying something really important and there is a slow creeping zoom ever so subtle that adds gravity to what they are saying. So to answer your question, probably most productions from 2009 use a zoom shot and certainly every TV narrative or series I've seen has one.
 
boulder said:
Ok...I'll open it up...since 2000
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Frank Darabont's The Mist. Possibly other movies that heavily use handheld footage for that "documentary" look, such as The Hurt Locker and Paul Greengrass's Bourne sequels, though I don't remember for sure if they had zooms. But definitely The Mist.
 
ydgmdlu said:
Frank Darabont's The Mist. Possibly other movies that heavily use handheld footage for that "documentary" look, such as The Hurt Locker and Paul Greengrass's Bourne sequels, though I don't remember for sure if they had zooms. But definitely The Mist.
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A winner!

Usually a zoom is used to simulate quick handheld footage, present a voyeristic feeling or make the viewer aware of the camera. My point is when I usually see zooms (and I see lots) in non professional productions, they are more often than not misused...zoom shots are the bane of the amature cinematographer. They should be used wisely and sparingly
 
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FatBird19 said:
Hey everyone, need some advice.
I'm having some problems with the stock lens.


In picture mode, when I auto focus the camera (beep noise) at the wide end of the lens the image is in focus, but when I zoom into 55m the lens is very out of focus.

Then if I autofocus at 55mm and then zoom out to 17mm the image is out of focus again (albeit not as bad as when I focus wide and zoom in).


What's wrong here?
It seems to me that the lens is defective. It seems that the focal point changes through the zoom range.

Where do I go from here? Who should I contact to get this resolved?

What other tests might I do to understand what's happening?

Thanks,
-Justin
Click to expand...

Better consider the 18-55mm a good 18mm f3.5 for $100. I was shooting some timelapes tests today and when I zoomed in to repo my shot it was focused on another freakin planet. I never expected much from this lens anyway, but this gave me a good laugh at least.
Spend more, get more. Do your research, and buy quality glass for the long haul, it pays off in the end.
 
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