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Fast Moderate Wides, Focus Shift and Pixel Peeping
Old 04-12-2015   #1
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Fast Moderate Wides, Focus Shift and Pixel Peeping

I love my 35/1.4 Nokton and 28/2 Ultron lenses, even on the almost new to me M240.

Now, plenty of reviews have questioned their usability on digital Leicas due to "focus shift", after testing the lenses against a tape measure or flat focus target.

Annoying for me, as in 9 out of 10 photos, these tests are not representative of what I do: focus and recompose.

Let me explain: typically, when shooting a portrait, I use the RF patch to focus, and then recompose. Get the eyes out of the center of the picture, or similar.

A picture says more than 1000 words, here is what then happens geometrically:

- I fix the focal plane at the object distance "o"
- I rotate the camera to put the focus object out of center. For example using the 1/3rd rule. Then I shoot.

By doing this, I also rotate the focal plane of the lens (blue vs. red in the picture), and introduce a focus error, "d". This error is roughly proportional to the square of the angle of rotation. For normal and longer lenses, that angle is very small. For very wide lenses, there is no problem, since the DOF covers any error introduced. Well, unless you use a 21 Summilux, maybe, which I don't .

Typically, the largest focus error is introduced for fast (shallow DOF) moderate wides, 35 and 28mm for example. The above picture shows the calculation for both focal lengths. Now consider that a 28/2 at min focus has a DOF of 7.22cm. A 35/1.4 at the same distance has a DOF of 3.23cm (see dofmaster.com). In both cases, the focus error introduced by a simple rule of 3rd composition can be roughly half the DOF for 1/3rd composition, and worse when you rotate the camera more.

How does that look in practice, say, for the 28/2 Ultron ? The following are 100% crops of test photos when my copy of this lens is focused in the center, at f2.0, 2.8 and 4.0:

Clearly the lens shifts, and for this test, it is optimal at f2.8.

Here the same test, but - after center focus - tilting the camera so that the object touches the corners of the Leica's 90mm frame-lines (roughly rule of 3rds):

You see, that by recomposing, the lens is now optimal for f2.0. Which suits me fine.

Note that the 28 Ultron seems to have a nice flat focal plane, so the calculated numbers fit. Lenses with more field curvature will behave differently. I have lenses where the field curves back, and some lenses where the field curves forward, either making the above effect smaller or larger.

The morale ? Numbers are just that, they might not say anything about how a lens performs in the way you use it ... look at real usage examples, or try before you buy.


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Old 04-13-2015   #2
robert blu
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Thanks for the clear explanatione and the related geometric diagram, which helps to understand the phenomenon.
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Old 04-13-2015   #3
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... I struggled with this a few years back shooting a chessboard tiled floor from an elevated position

The rf/viewfinder was so confuse I couldn't hit focus ... when I drew it out later my diagram and test shots looked a lot like yours

Thankfully, in the wild I seldom wander from f8 or so ... so many of my mistakes go completely unnoticed.
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Old 04-13-2015   #4
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thanks for the clear explanation Roland - this also becomes an issue when photographing groups of people such as at social events like weddings; whether to arrange the people on an arc equidistant from the lens, or on a line perpendicular to the lens axis would depend on the lens designer's intent on where the plane of focus lies. I've always gone for an arc to ensure subjects are rendered the same size, without being aware of the dof implications (and I've never shot such pictures at apertures wider than f4 at 24mm at subject distances under about 12', so dof was never such a critical issue).

I cannot remember seeing any mention of this subject in lens performance data (and I think only ever in 1 review).
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Old 04-13-2015   #5
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I have to take cosine error into account when photographing paintings in museums. Especially with impressionist paintings, there isn't anything detailed enough in the painting to focus on! So I focus on the edge where the frame meets the canvas. If I panned to there, I would get cosine error. Instead, I side-step to the left, focus, and side-step back to center, always keeping the lens axis at 90 degrees to the painting. Seems to work well, at least for small paintings.
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