50mm f/1.8G backfocus

Started Dec 1, 2012 | Discussions thread
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Phillip S
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50mm f/1.8G backfocus
Dec 1, 2012

MY COMMENT: Dec 1, 2012

Feeling that this would be a good general purpose lens for the new Nikon D600, I purchased the Nikon 50mm f/1.8G a few weeks ago, but I have not had the occasion to use it very much to date.

I read positive reviews, but, as of this week, I think that I have some understanding of Thom Hogan's (byThom.com) lackluster appraisal, and, I think that my lens behaves the way that Photozone.de describes (and shows on ruler) the back-focus problem.

Once stopped-down to f/4, f/5.6, f/8 (maybe even at 2.8), the lens really does back-focus enough to give the result that you will wind up focusing on a person's ears even when you autofocus right on a person's nose.

I did some tests with my D600 with both the 50mm f/1.8G and compared it (for reference) with the autofocus with the Nikon 70-300mm VRII lens.

Note that I shot in RAW and viewed the results of my test on a 21” iMac desktop with Nikon View NX.

Testing the 70-300mm f/4.5;5.6 VRII lens at 70mm, 135mm, 200mm, and 300mm, and authenticating the proper focus-point by using the Live View (with both manual and autofocus) to compare with autofocus through the viewfinder eyepiece, I found that the 70-300 lens will autofocus perfectly at all the focal lengths listed.

The positive, perfect, and happy results of the 70-300 VRII were not so with the 50mm f/1.8G --

Doing Live View confirmation of accurate focus (again with both manual settings and Live View autofocus) with different apertures, I would follow each confirmation by using the viewfinder autofocus with the 50mm f/1.8G –

The results were that when the aperture was at f/1.8 and f/2 then the viewfinder autofocus tended to be accurate, but from f/4 and f/5.6 and f.8 (I did not check f/11) the lens would back-focus enough to do what I said above:

If you focused on a person’s nose, the lens would actually be focused on the person’s ears.

Now, my focus target was obviously not a person

(although there are some good reasons for testing with people as well, but that is a long discussion),

as my test was done with a tripod locked on a single locked target that had a flat point (for focus) along with both a curved area and a deeper area/point for reference in order to give some sense of depth

(and let me say that I used a target that has meaning to me, as having pulled focus years ago on several motion pictures, I’m not a big fan of focus charts).

Now, I also tested the 50mm f/1.8G lens with the AF Fine Tune feature of the D600 camera, and I found that a setting of -20 (minus 20) seemed to work for me

(but I would not take that as locked in stone at this point, although my results with that have been good, as I will explain).

With the AF Fine Tune feature set to “ON”, the results with both the target tests as well as other (wider and tighter) “real-world” shots show that this is both good and bad,

but, ultimately, this is the way that I will use this lens.

The bad:

At f/1.8 and f/2, with AF Fine Tune turned ON and at -20, the results are that the lens will not focus properly at f/1.8 and f/2 (i.e., you will not be in proper focus).

The good:

At f/4, f/5.6, f/8 (I did not test f/11), with the AF Fine Tune turned ON and at -20, the results seem to be almost perfect (in fact, maybe perfect, and regardless of how close or how wide you were focusing).

So, what does all this mean?

1.

Well, I think that it means that there are no autofocus problems with both my Nikon D600 camera nor my 70-300 f/4.5;5.6 VRII lens

(unless their ‘problems’ cancel each other out, but I seriously doubt that is the case).

2.

That the 50mm f/1.8G lens will be of some aggravation to me –

For the type of shots that I take, I will almost always have to remember to turn AF Fine Tune to “ON” in order to use the lens.

Why?

Well, even in focus, I think the lens looks poor at f/1.8 and f/2.

At f/4 and f/5.6 (where I have taken the most photos and the apertures that I most commonly use), I think the images from the lens look very good to excellent for a 50mm focal length.

3.

Still to do is to test the lens more at f/2.8, as that is also an aperture that I find myself using on many occasions, but I have not yet determined (nor experimented) to see if f/2.8 autofocuses best with AF Fine Tune “ON” at -20 or simply with AF Fine Tune set to “OFF”. I tend to think that it will be best set to “ON”, but I cannot say at present.

This Thread is not intended to be a discussion of this lens bokeh or color quality or any such other, but it is only about using the viewfinder autofocus and the resulting issue with the 50mm f/1.8G lens back-focusing.

As most of us can focus properly if we have the time and the situation that allows for Live View, but that is not the case for many/most situations, and putting the camera viewfinder up to your eye and achieving perfect autofocus is the goal of most of us.

I would be interested to hear if others have a similar experience with autofocusing with this lens.

Nikon D600
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