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12-23-2010, 05:08 PM   #1
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Join Date: Oct 2010
Location: TN
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What is back focus?

New term to me for my 2010 photo vocabulary- Back Focus

What is back focus?

The reason I ask, is that I was contemplating getting the DA* 55MM SDM F1.4
(for my K-7)

In the reviews, they say this lens has back focus issues and that the SDM is slow compared to the drive AF on the FA 50mm.

I have seen SDM on Canon and Nikon (don't kill me, the local pros I work with let me borrow their cameras) It is so quiet! It will help when shooting pets and also people in a quiet venue like a classy restaurant.

Is slow SDM focus really a problem for indoor settings?
But the back focus everyone commented on might determine if the $600 is worth it.

I hope everyone has a wonderful Christmas!

12-23-2010, 05:14 PM - 1 Like   #2
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Join Date: Jan 2010
Location: Seattle
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I can't answer your SDM questions as I don't own the lens, but back focus is when the camera focuses the lens such that the focus is locked on something behind its intended target. The result is what you thought you were focusing on will be slightly out of focus, whereas something behind it will be much crisper. There are numerous causes of backfocus, which can be caused by the lens or the camera itself, and sometimes may be exacerbated by bad lighting conditions (at least in my experience).

Fortunately your K-7 has settings that you can use to compensate for back (or front) focus. In severe cases the range of possible adjustments might not be able to compensate for the problem.
12-23-2010, 07:52 PM   #3
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Join Date: May 2010
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DA*55 is not a slow focus lens at all... if you do not shoot very fast moving objects, such as sports, sdm is fast enough, even for infoor settings.

I didn't hear much about DA*55 back focus problem, but if it does back focuses, it's really annoying. Fortunately the k-7 has focus adjustment function, so if the back-focus problem is not very severe, you can alway adjust it yourself, especially considering that DA* 55 is not a zoom.
12-24-2010, 02:49 AM   #4
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Join Date: May 2009
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Posts: 62
I've used my DA* 55/1.4 on both the K10D and K-7.

On the K10D there used to be a constant back focus in almost every shot. I had to hack the firmware to adjust for this, bacause the K10D doesn't normally support this.

On the K-7 the focus is much better. It hardly ever back focusses, and only in very bad light conditions wide open. This does make it hard to adjust for though, despite the K-7 supporting this, because it's not constant. I have setup the adjustment level and only turn on the adjustment when in a situation experiencing back focus. This only happens sporadically though.

The focussing speed is the slowest of all my lenses (see my signature). The DA 35/2.4 is a lot faster, but the focussing speed of the DA* 55/1.4 is still very acceptable. It's still my favorite lens.


Last edited by -Mous-; 12-24-2010 at 03:09 AM.
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