Front and rear focusing test

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GEH007

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My Sigma 50-150 2.8 has arrived and although I've not tested it thoroughly as yet, I think the front focus issue that some suffer from looks to have escaped me.

However, to be certain, I'd like to take a little time to test it properly to ensure all is well, can anyone recommend a straightforward test of some kind and also what settings would you use when testing?

Many thanks.
 
Line up 3 AA batteries staggered in distance. Focus on the middle one at close of min. focus distance.

Or you could google focus chart you'll find loads of charts you can print and test with.
 
pxl8 said:
Line up 3 AA batteries staggered in distance. Focus on the middle one at close of min. focus distance.

Or you could google focus chart you'll find loads of charts you can print and test with.
Click to expand...

Sounds like a good one thanks, what aperture would you suggest testing at? As wide as possible to arrow in on the focus point?

Thanks
 
This lens is well known to front focus at 150mm with close objects.

This was addressed somewhat with the MKII version.

Do not use an angled test chart. Never ever test a lens with anything other than a perpendicular target - never slanted.
 
Focus tests should always be done wide open so you have the min. DoF to show any errors more precisely.
 
puddleduck said:
This lens is well known to front focus at 150mm with close objects.

This was addressed somewhat with the MKII version.

Do not use an angled test chart. Never ever test a lens with anything other than a perpendicular target - never slanted.
Click to expand...

Thanks Andy, would you mind elaborating a touch. If the target is perpendicular, say 3 batteries lined up, there is nothing in front of the focus point but fresh air isnt there? So how can you tell in that instance if it front focuses? Do you mean use a larger 3d target from straight above so you can tell which point on the target the focus is on? Thanks


pxl8 said:
Focus tests should always be done wide open so you have the min. DoF to show any errors more precisely.
Click to expand...

Yes, thought so, thanks for clarifying.
 
thompo said:
Would say having the camera tripod mounted point downwards @45 degrees on to a ruler set on a table work well for this? been meaning to test my 70-200 sigma to check its ok
Click to expand...

As puddleduck said, angles for focus tests are bad.

The AF sensor is larger than you think and will lock on to the closest point it can find which can lead to false results. Having the focus target flat means the AF is either right or not rather than right for just a little bit.
 
thompo said:
Would say having the camera tripod mounted point downwards @45 degrees on to a ruler set on a table work well for this? been meaning to test my 70-200 sigma to check its ok
Click to expand...


a 70-200? I wouldnt use something on a table near by ... for a long lens I found that even the slightest front/back focus doesnt show up on close objects... you need something with a bit of distance..

On my 300 i used crowds at the opposite end of a football pitch.. aimed at one row and looked to see if people front or back was more in focus... anything reasonably close looked good anyways.
 
GEH007 said:
Thanks Andy, would you mind elaborating a touch. If the target is perpendicular, say 3 batteries lined up, there is nothing in front of the focus point but fresh air isnt there? So how can you tell in that instance if it front focuses? Do you mean use a larger 3d target from straight above so you can tell which point on the target the focus is on? Thanks




Yes, thought so, thanks for clarifying.
Click to expand...

Yes, batteries work well. Actually if you have 3 CF cards, I find those excellent and you can measure focus accuracy down to the thickness of the card :)

As long as your target is perpendicular with nothing that infront or behind that can "distract" your AF sensor you'll be OK.

Just avoid test charts :)
 
Ideally, you want to set the distance at greater than 50x the focal length you've selected. Aberations in the lens (and they all have them) can cause slight variations in AF performance but these will be negligable as you focus further away.

Bob
 
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