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Identifying front- or back-focus: An excellent target
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PostPosted: Thu Jan 18, 2018 11:00 pm    Post subject: Identifying front- or back-focus: An excellent target Reply with quote
I thought I'd post this due to my recent trials with such an issue.

As with most instructions, print the target, lay it flat and light it well,
place camera at 45-degree angle, take shot(s), view, adjust.
This works very well, and objects can easily replace a printed target.

But one point which always escaped me for some reason was,
"How are the adjustments for either situation dealt with?"

This sums it up very succinctly:
Code:
If you focus the subject in the viewfinder to be sharp, but the image is focused closer to the foreground, this is FRONT focus. To correct this you need to fit a THINNER shim in place in between the prism and the focus screen.

If you focus the subject in the viewfinder to be sharp, but the image is focused further to the backround, this is BACK focus. To correct this you need to fit a THICKER shim in place in between the prism and the focus screen.

This got me on a clearer understanding of the process, as opposed to the sometimes
confusing or contradictory information I had found in the past.

My results:
Consistent front-focus:

Focus achieved:


I like the chart because it's much more than a graded scale of lines.
It's available with various tones of the greyscale, includes character fonts.
There's actually more to see with this one, unlike the many others which
appear to be nothing more than a ruler on paper.
Well-written detail of the process and the charts can be found here:
http://regex.info/blog/photo-tech/focus-chart


PostPosted: Fri Jan 19, 2018 2:17 am    Post subject: Reply with quote
Like 1

EDIT: Best used with fast wide angle lens!


PostPosted: Fri Jan 19, 2018 11:42 pm    Post subject: Reply with quote
visualopsins wrote:
Like 1

EDIT: Best used with fast wide angle lens!


Actually, use it with any lens which can offer a shallow depth of field.
I dialed it in while using my Tamron SP 28-80 @ f/3.5; I'm sure I could have
done the same with any other lens wide-open.

My next test will be an autofocus Sigma 10-20, to see how it compares,
then try a couple of others.


PostPosted: Fri Feb 23, 2018 11:18 pm    Post subject: Reply with quote
Update to show other lenses and test results.
100% crops of test images are shown.

The Zuiko 50mm f/1.8 is spot-on:


The Vivitar 28mm f/2.8 is also spot-on:


Both are Olympus-mount, coupled with a cheap adapter to my 50D.
The adapter is the same, used on both lenses.
Mild sharpening applied to better demonstrate the results.

The Sigma 10-20mm f/5.6 didn't fare as well in this test scenario as I'd hoped,
but I didn't really expect such an ultra-wide angle lens to deliver at close range anyway,
with AF having an issue focusing on greyscale patterns. I'm keeping the 10-20 anyways,
as I love the ultra-wide it achieves. Wink


PostPosted: Sat Feb 24, 2018 12:15 am    Post subject: Reply with quote
I use a precision metal ruler, works great (and also shows if there is CA)


PostPosted: Sat Feb 24, 2018 12:38 am    Post subject: Reply with quote
kds315* wrote:
I use a precision metal ruler, works great (and also shows if there is CA)

Interesting, Klaus.
How do you identify CA with such a test?
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