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Focusing technique: zoom in + focus + zoom out?

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R_Medvid

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Hi all,

I used to have 10D and used to focus this way: (I split focusing and shooting into 2 buttons: * button in rear and shutter button respectively) -- zooming in fully, then focusing on the subject with * button, then zooming out till your needed frame -- and SHOOT.

Now as I changed to 350D I'm reading in the manual that focusing should be done AFTER you zoom in or out, as using zoom ring may CHANGE focusing setting :(

Why is that? Does anybody else employ the focusing technique I use? I find it effective since it enables me to focus on small details...

BTW, no such requirement is stated in 10D manual.
 
It works with some lenses (called parfocal lenses). Other lenses (I beleive the majority of them) will change their focal distance during zooming and it won't work (whatever camera body you are using). And no I don't have a list of which lenses are parfocal and which aren't, sorry! But if a lens worked with this technique on one camera then it should with another.
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Some of the least worse of my photos:
http://www.flickr.com/photos/susans/
 
fine for you if it worked with a specific lens, but it just won't work at all with most others. Even my 70-200 L IS (1800 euros, L lens, etc) isn't parafocal... focusing at 200 gives a blurry image at 70. Your best bet is to have a camera that focuses properly at whichever focal length you use... or get a split circle focusing screen and do the work yourself :)

The technique is supposed to work on the Tamron 28-75 2.8. Personally most of the time I don't have the luxury (=time) to AF at 75 and zoom back to 28.

Guillaume
http://www.at-sight.com
 
Thanks! This is the first time I've been pointed to this specific feature of lens. Never thought of it...

However, my question then transforms into this:

Since the focusing distance ring is not moving while you zoom in amd zoom out -- and the value of that distance remains unchanged -- this means that you can not trust the focusing ring....

Am I lost?...
 
Guillaume paris2 said:
fine for you if it worked with a specific lens, but it just won't
work at all with most others. Even my 70-200 L IS (1800 euros, L
lens, etc) isn't parafocal... focusing at 200 gives a blurry image
at 70. Your best bet is to have a camera that focuses properly at
whichever focal length you use... or get a split circle focusing
screen and do the work yourself :)

The technique is supposed to work on the Tamron 28-75 2.8.
Personally most of the time I don't have the luxury (=time) to AF
at 75 and zoom back to 28.
Click to expand...
hmmm.. it comes out ok if i focus at 28mm, but a little blurrier if I focus at 75 and then zoom back to 28mm with the Tamron, so either it is not parfocal or mine, unlike most claims, focuses a tiny bit worse at the long end.
 
Guillaume paris2 said:
fine for you if it worked with a specific lens, but it just won't
work at all with most others. Even my 70-200 L IS (1800 euros, L
lens, etc) isn't parafocal... focusing at 200 gives a blurry image
at 70. Your best bet is to have a camera that focuses properly at
whichever focal length you use... or get a split circle focusing
screen and do the work yourself :)
Click to expand...
Er, the 70-200/2.8L is supposed to be parfocal. I'll try and remember to check mine tonight, but sources certainly list it and the non-IS version as parfocal, as was the 80-200 before it.

http://www.rogercavanagh.com/helpinfo/30_parfocal.stm

There may of course be focal distances where this does not hold true maybe?
Guillaume paris2 said:
The technique is supposed to work on the Tamron 28-75 2.8.
Personally most of the time I don't have the luxury (=time) to AF
at 75 and zoom back to 28.

Guillaume
http://www.at-sight.com
Click to expand...
--
http://public.fotki.com/wibble/public_display/

 
...that while the meter of focusing distance says 3 m it actually may change to 2.8 m (for example) as I zoom out. WEIRD
 
well, you got it right, the focusing ring can't really be trusted anyway. Even when I look at the "meters" scale, most (if not all) of my lenses show a 2 when my subject is like over 3m away... or maybe the other way around, but still, the readings are pretty vague.

The distance isn't changing, obviously. The elements inside the lens might not move that dramatically. But even a slight defocus (because of the moving elements) will give you a bad image.

As I see it, the current AF lenses and their focusingg rings aren't nearly detailed enough for real manual focusing. Most of the lenses have too short a ring to really make a difference while focusing. Of all my lenses, the least worse is probably my 70-200. the ring is smooth, and has a long "throw". Plus MF is easy as it's a telephoto with huge aperture. All my other lenses are pathetic, that's why I have to rely on AF. I have a split-circle focusing screen installed, but I find it hard to precisely focus msot of my lenses.

Guillaume
http://www.at-sight.com
 
My EF-S 17-85 is "parafocal" - though until I read this thread I didn't know what to call it. Up to now I've just viewed it as a rather convenient feature of the lens.

My 75-300 IS isn't, nor is my 55-200
 
than my misfocusing 10D, then! I might need to do more testing now that I've got myself a tripod... but hey, even with a parafocal lens: misfocus at 200 is still misfocus at 70 LOL

and I believe the opposite is true, if the camera is ok at 200 then it should also be ok at 70.
There seems to be less and less parafocal lenses these days.

Guillaume
http://www.at-sight.com
 
Yeah, my 17-85 is parfocal and that technique works superbly. I only use that technique on the 17mm end, as the AF (f4) at wide angle is horrible. Once I use the zoom in, focus, zoom out, shoot method, the picture is very sharp. This method helps the AF by providing a zoomed in picture for contrast evaluation and focusing. However, focusing on a strong contrast background (eg. black on white) helps eliminate the need for the parfocal technique.

The only downside is that I sometimes forget to use that technique and I get soft pictures.
 
Unfortunately is struggles with backfocus at the wide end, but parafocal use with it give accurate wide angle focus.

I think the narrower DOF of the tele end of zooms helps the camera more accurately focus there. Seems mose wide angle lenses struggle a bit on the wide side, thankfully that huge DOF helps hide the struggles.
 
One thing I would highly recommend is figuring out where the infinity focus point is on your lense, by looking at the scale when you have something in focus at hundreds of yards away. That way you can set the camera to infinity focus accurately when AF doesn't work for whatever reason.

I'm doing a lot of night photography now and you just can't do auto focus in lots of situations with things at a distance. The infinity focus point is not exactly where it's marked on the scale, but it's close.

I saw a hasselblad recently that had an amazingly detailed focussing scale... I drooled over it a bit.

I wonder if anyone has taken to modifying the focussing scales of Canon lenses... all it should require is opening up the lense, calibrating it on a bench, and then carefully applying a more detailed sticker over the old one.
 
not even a bit?
R_Medvid said:
Hi all,

I used to have 10D and used to focus this way: (I split focusing
and shooting into 2 buttons: * button in rear and shutter button
respectively) -- zooming in fully, then focusing on the subject
with * button, then zooming out till your needed frame -- and SHOOT.

Now as I changed to 350D I'm reading in the manual that focusing
should be done AFTER you zoom in or out, as using zoom ring may
CHANGE focusing setting :(

Why is that? Does anybody else employ the focusing technique I use?
I find it effective since it enables me to focus on small details...

BTW, no such requirement is stated in 10D manual.
Click to expand...
 
quite the opposite.

of course it only works for single shot composition.
pentool said:
R_Medvid said:
Hi all,

I used to have 10D and used to focus this way: (I split focusing
and shooting into 2 buttons: * button in rear and shutter button
respectively) -- zooming in fully, then focusing on the subject
with * button, then zooming out till your needed frame -- and SHOOT.

Now as I changed to 350D I'm reading in the manual that focusing
should be done AFTER you zoom in or out, as using zoom ring may
CHANGE focusing setting :(

Why is that? Does anybody else employ the focusing technique I use?
I find it effective since it enables me to focus on small details...

BTW, no such requirement is stated in 10D manual.
Click to expand...
Click to expand...
--
http://public.fotki.com/wibble/public_display/

 
Sorry for yelling, but this is great info. I have a 70-200 F4L that I have been having some problems with at the short end. It is tack sharp at 200, but at 70 is is back focusing terribly. I looked it up at http://www.rogercavanagh.com/helpinfo/30_parfocal.stm and sure enough it is parfocal so I made a little experiment where I first focus it at 70 and take the picture and another picture where I focus at 200 and then zoom to 70 and take the picture. Here are the results (100% Crop):



So this leads to a few questions:

1. Why does Canon say not to do this in their manual(70-200F4) ? It says "Be sure to finish zooming before focusing. Changing the zoom ring after focusing can affect the focus"

2. Is it safe to say that when a zoom lens can focus at one range and not the other it is the lenses fault and not the cameras? (I have had a 300D and a 20D and my 70-200F4 behaves similar on both cameraes.

3. I guess a simple test to see if your parfocal zoom is a dud is by setting it up on a tripod perpendicular to a brick wall and seeing if the focus range changes any when you focus at the short vs the long end. Funny this hasn't been mentioned in any of the "my 70-200F4 can't focus threads"
 
Parfocal is only valid one way - long to short.

You cannot expect good results focusing at the wide end, then zooming in - DOF issues would make it totally unreasonable.
runboy27 said:
Sorry for yelling, but this is great info. I have a 70-200 F4L that
I have been having some problems with at the short end. It is tack
sharp at 200, but at 70 is is back focusing terribly. I looked it
up at http://www.rogercavanagh.com/helpinfo/30_parfocal.stm and
sure enough it is parfocal so I made a little experiment where I
first focus it at 70 and take the picture and another picture where
I focus at 200 and then zoom to 70 and take the picture. Here are
the results (100% Crop):



So this leads to a few questions:

1. Why does Canon say not to do this in their manual(70-200F4) ? It
says "Be sure to finish zooming before focusing. Changing the zoom
ring after focusing can affect the focus"

2. Is it safe to say that when a zoom lens can focus at one range
and not the other it is the lenses fault and not the cameras? (I
have had a 300D and a 20D and my 70-200F4 behaves similar on both
cameraes.

3. I guess a simple test to see if your parfocal zoom is a dud is
by setting it up on a tripod perpendicular to a brick wall and
seeing if the focus range changes any when you focus at the short
vs the long end. Funny this hasn't been mentioned in any of the "my
70-200F4 can't focus threads"
Click to expand...
--
http://public.fotki.com/wibble/public_display/

 
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