Please help me understand this ...

Started Jul 9, 2013 | Discussions thread
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Hardy Steiner Senior Member • Posts: 2,442
Please help me understand this ...

I recently acquire the old non DG version of the 28-70mm EX f2.8 lens. I tested the lens in my local park and at 28mm the results were not quite as expected ... the foreground was a bit too soft in some cases. I have a 28mm prime as well so I could compare it to this one on another day.

All pics blow are at f8:

So here the 28-70mm with AF (focusing point on the tree in the centre)

Notice how soft the fence gets in the right hand corner. Now I also used the DOF online calculator and then kind of made sense, but I remembered the my 28mm prime behaved differently.

So next thing I did was use the MF and focused a bit more closer (so according to the online DOF calculator I should still have the tree sharp, but also the fence):

28-70mm with MF

So yes, a lot better.

Now comes the interesting part:

28mm prime lens with AF

So how come the prime lens with AF renders the fence in the right hand corner sharp, whereas the zoom with AF doesn't (but with MF it comes a bit closer)?

As we are on the topic, I have another question: As with all my wide angle lenses, I took a test shot of this old building ... ok, this is only f4.5 ... but how come the right hand side is way better than the left hand side (and I was standing parallel to the building):

28-70mm AF on building

Now according to the online DOF calculator the DOF should be between 6.77m and infinity (I was about 30m away from the building). Now is there something wrong with this lens?

The picture looks fine at f8.

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