24-105mm Canon... Focussing Questions, lens specific and general queries!

captures.in.time

captures.in.time

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Hi all

Was off togging at the weekend with my 24-105mm. Not really had a chance to look at the photos yet but have a few general questions regarding focusing... before this I knew the answers but after this weekend im confused!

Firstly where is infinity! I often like to manual focus when shooting landscapes and if it was a scene with no real foreground such as at Scotts view or on night shots I would always simply set the lens to f11 and then focus to infinity... But where actually is infinity? I know that's such a stupid question as it's the far extremity of the anticlockwise twist of the focussing... but is it??? The reason why I say this is when I auto focus on such a scene it rarely goes to that far end... favouring the left end of this mark (L__) on the focus ring. Now also when I looked through the viewfinder it appeared to be clearer at this end too... However i'm half blind and have always trusted the sophisticated autofocus over my rubbish eyes!

Secondly I found that ocasionally if I focussed on a forground object, then forused on another, then went back and refocused on the original object that the reading the lens was giving for focus distance on the ring was different from my first reading... I kinda dont understand this... although both pictures seemed ok!

I guess I just long for proper focus scales like you got on old manual focus lenses... maybe because it's what I learned on and now struggle with the modern focus rings which to me seem a token gesture!

Mark
 
oh and appologies for my poor spelling... just re read my message... a few typos... but hey ho... I think you get the idea!

Mark
 
Using a DOF calculator it would seem that anything over 275 ft would be effectively infinity at F4. This being the Hyperfocal distance for for 105mm end of the lens. ( For a Canon 5D camera)
 
Thanks guys... I'm more on about on my focus scale where is inifinity in relation to this symbol (L__) on the lens focus scale?
 
I think you're confusing infinity with the hyperfocal distance. Where you need to focus, using this method, depends on format, focal length, f-stop and required print size.
 
No im not... I know about hyperfocal distance... I use this ocassionally if I feel I wont get enough DOF to get my extreem foreground in focus... I just want to know when my lens is focused to infinity... before it was simply when it was at the infinity (00) symbol... now there is this (L__) symbol before it... so simply my question remains where is infinity... is it anywhere from the start of the (L__) symbol or is it only at the far right... the full anticlockwise turn.
 
well as stated it depends on lots of things. But at the hyperfocal distance everything from (a varying amount in front of) that point to infinity is in focus. The DoF is maximised.
 
In my canon book it says, Infinity compensation mark. To compensate for shifting of the infinity focus point that results from changes in temperature. The infinity position at normal temperature is the point at which the vertical line of the L mark is aligned with the distance indicator on the distance scale. So at normal temperatures it should be the left hand end of the L mark.
 
Thankyou!!!!

So when focusing manualy I should focus to the point in the L__ mark!

So what is this all about as i've never even heard of this before... And I bet all the people above didn't either!

What tempratures would constitute abnormal then... and def need to look when im autofocusing if it always picks this point in the L when at these normal tempratures... off to look at the manual for the lens now... maybe I should have looked there first... LOL

M
 
Huss1100 said:
I would like to know if it means abnormally high temperatures, low temperatures or both ?

Kev.
Click to expand...

Just read my lens manual and it says exactly the same as yours... but no hint to what are abnormal temps?

M
 
As you have already deduced, the focus mark is the upright of the L. It has been said that lenses have movement beyond this point to compensate for temperature variations (and hence the lens varying in size slightly), but under most normal conditions there won't be any significant change.

Probably the main reason for this extra range of movement is to allow the autofocus to work properly. For the lens to be able to focus on infinity it has to be able to sample the focus either side of it. You'll now see that older manual focus lenses don't need this requirement, hence infinity was always just cranking it round as far as it would go.
 
MisterE said:
Probably the main reason for this extra range of movement is to allow the autofocus to work properly. For the lens to be able to focus on infinity it has to be able to sample the focus either side of it. You'll now see that older manual focus lenses don't need this requirement, hence infinity was always just cranking it round as far as it would go.
Click to expand...

This probably explains it!
 
A few small facts to add to this....

Lenses are calibrated at 20 degrees celcius (an accepted standard temperature benchmark for calculating expansion and contraction of bodies)

All lens/body combinations will be degraded by manufacturing tolerances. These deviations will affect the position of the lens barrel when focussed to a specific point and this is termed collimation.

Therefore, the ability to rotate beyond the marked infinity position is to cater for thermal changes and manufacturing tolerances....in reality, they both account for actual collimation.

HTH

Bob
 
Interesting post Bob - and I think you can safely remove the "too little knowledge" line on your footnote!!
 
markelliot said:
Secondly I found that ocasionally if I focussed on a forground object, then forused on another, then went back and refocused on the original object that the reading the lens was giving for focus distance on the ring was different from my first reading... I kinda dont understand this... although both pictures seemed ok!
Click to expand...

I found this with the 17-85 4-5.6, at longer distances, when the lens would often stop at slightly different focus positions.

I think it's because the system works within a tolerance of depth of focus (just to be correct, this is not the same as depth of field, but no matter) and when you're focused on infinity at f/4 that tolerance is quite large. So depending on which direction the lens is coming from (or simply on a random whim ;) ) it will stop anywhere within acceptable focus tolerance. I too found it made no difference to the sharpness of the finish picture.

The AF system doesn't work like you might manual focus, hunting back and forth slightly before deciding on the optimum mid point. The camera knows what acceptable sharpness is and when it detects that, it stops. It's relevant to note here that f/2.8 lenses trigger the extra accurate mode available with the centre point, which means they get a bit closer to that optimum point.

To check your infinity mark, have a look at the moon using live view on max magnification :)
 
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