ISO focus problem..

JezSutton

JezSutton

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Jez
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I have a tamron 18-200mm lens and if I focus on an object, at ISO 100 the focus is correct if I change the ISO to 400 the focus moves back about 3inches !! other than the ISO everything is the same.. F6.3 - Speed 1/30 at ISO 100 and 1/125 at ISO 400 focal length 200mm.

can anyone help..

Jez
 
ISO is not related to focus, if you change it then your shutterspeed or aperture will change.
 
If you are increasing the ISO your making the sensor more sensitive to light and the f number will decrease thereby narrowing the depth of field, what mode is the camera set too?
 
I am guessing S/V or shutter priority, so the depth of field will narrow when
you increase ISO thereby giving the impression of focusing 3inch closer or further away.
 
sbowler579 said:
If you are increasing the ISO your making the sensor more sensitive to light and the f number will decrease thereby narrowing the depth of field, what mode is the camera set too?
Click to expand...

Actually the reverse is true. If you change from ISO 100 to ISO 400 and the shutter speed remains the same, then the aperture size must decrease (bigger f number) to maintain the same exposure value.

A smaller aperture should give a greater DoF, which should result in an increase in the amount of sharpness, not less.
 
The camera is set on full manual with single point centre focus, I put a tape measure 7ft away running diagonally and focussed on a number half way along..

the focus point moves 3 inches up the tape measure..
 
Yes, perfectly true. What am I like, DOH.
 
After you increased the ISO isnt the shot overexposed?
 
sbowler579 said:
After you increased the ISO isnt the shot overexposed?
Click to expand...

Well spotted, just checked shots and I should have written that the shutter speed was changed manually from 30th on ISO 100 to 125th at ISO 400

DOH ! must have had my weekend brain in !!
 
Focus point thingy is strange though? Well I am off to order the book, "Understanding exposure".:LOL::LOL:
 
JezSutton said:
I... at ISO 100 the focus is correct if I change the ISO to 400 the focus moves back about 3inches !!
Click to expand...
This doesn't make any sense at all.

I think it would help if you described your test procedure in detail and showed us some pictures.
 
Here are some shots..

ISO 100


ISO 400


The camera's focal point was not changed during the test's and the shots were taken on a timer so the focal point could not move due to hand shake. it was mounted on a tripod on concrete.. and the following shot with the ISO back at 100 the focal point moved back to around 37"

Weird !!!
 
second photo is on program auto, at what distance were the photos taken, I'm thinking minimum distance focusing for that lens ?
 
JezSutton said:
The camera's focal point was not changed during the test's and the shots were taken on a timer so the focal point could not move due to hand shake. it was mounted on a tripod on concrete..
Click to expand...

Yet the framing is different...
 
mmmm....... i would have focused once clicked it onto m/f and then taken the two shot
 
You cant test it like that anyway.

If you focus half a dozen times on that tape measure with the same settings you will find that it varies as the AF point is not small enough.

What you need to do is focus on an object parallel to the sensor (but leave the tape measure in the frame so you can see where it is focusing). You should also take 3 shots at each ISO (if thats the test you want to do) and manually put it way out of focus between each shot.
 
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