Weird lens behaviour

[madeinlisboa]

I took this image and noticed later that the bottom is kind of auto focus, but it is at the same distance as the subject. This never happened before and did not happen again, but I'm curious to know why

 

[Lance B]

I took this image and noticed later that the bottom is kind of auto focus, but it is at the same distance as the subject. This never happened before and did not happen again, but I'm curious to know why

Not really. If you focused on the face/head, the bottom of the image is much closer to the camera than where you focused on the subject. Not only that but the camera may have inadvertently focused on the leaves/branches just to the right/left of the face/head which would mean that the foreground is even further in front of the plane of focus.



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[madeinlisboa]

Thank you for your answer, but that just couldn't be possible. The aperture was 5.6 and the legs suddenly get out of focus below the knee

 

[Tord S Eriksson]

Thank you for your answer, but that just couldn't be possible. The aperture was 5.6 and the legs suddenly get out of focus below the knee

The lady's face isn't particularly sharp either, while the bushes to the left are very sharp, or at least sharper!

So I think Lance got the answer!

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tordseriksson (at) gmail.....
Owner of a handful of Nikon cameras. And a few lenses.
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[briantilley]

Thank you for your answer, but that just couldn't be possible. The aperture was 5.6 and the legs suddenly get out of focus below the knee.

I'd say that Lance's answer is exactly right.

Because the camera is angled upwards, the distance to the subject's face is quite a bit greater than the distance to her feet. Coupled with the apparent focus point being a little behind the face, that will put the feet and the closer rocks outside the depth of field, even at f/5.6.

 

[WildlifePhotog]

Thank you for your answer, but that just couldn't be possible. The aperture was 5.6 and the legs suddenly get out of focus below the knee.

I'd say that Lance's answer is exactly right.

Because the camera is angled upwards, the distance to the subject's face is quite a bit greater than the distance to her feet. Coupled with the apparent focus point being a little behind the face, that will put the feet and the closer rocks outside the depth of field, even at f/5.6.

Bingo! It’s due to the camera being angled upwards, which means the focus plane is not flat, like it would be if you had taken it straight on. However I will say that f/5.6 on a wide angle, I’m surprised that her legs are out of focus. Maybe you also missed focus a little bit or the lens back focused?

 

[madeinlisboa]

Thank you for your answer, but that just couldn't be possible. The aperture was 5.6 and the legs suddenly get out of focus below the knee.

I'd say that Lance's answer is exactly right.

Because the camera is angled upwards, the distance to the subject's face is quite a bit greater than the distance to her feet. Coupled with the apparent focus point being a little behind the face, that will put the feet and the closer rocks outside the depth of field, even at f/5.6.

Bingo! It’s due to the camera being angled upwards, which means the focus plane is not flat, like it would be if you had taken it straight on. However I will say that f/5.6 on a wide angle, I’m surprised that her legs are out of focus. Maybe you also missed focus a little bit or the lens back focused?

I have a lot of photos with this lens. It's perfectly calibrated. This is the first time it happened. What I found weird is the sudden lost of focus to her feet. Should be gradual but suddenly everything below is out of focus

 

[WildlifePhotog]

Could also be motion blur, she may have just had shaky legs or moved slightly. Test your lens out on a flat object with lots of details, or a test target...maybe even a fence if its straight. Or a wall, and make sure it's uniformly sharp, well I mean the focal plane/area of focus is sharp. Obviously on the 24-120mm VR the corner's wont be as sharp as the center, when you focus on the center. People move, almost all of the time, so they are not great test targets!

 

[j_photo]

Could also be motion blur

That's what it looks like to me. The shutter speed was quite slow.

 

[madeinlisboa]

Could also be motion blur, she may have just had shaky legs or moved slightly. Test your lens out on a flat object with lots of details, or a test target...maybe even a fence if its straight. Or a wall, and make sure it's uniformly sharp, well I mean the focal plane/area of focus is sharp. Obviously on the 24-120mm VR the corner's wont be as sharp as the center, when you focus on the center. People move, almost all of the time, so they are not great test targets!

Nope. Everything lost focus below her, even the rocks and the background is blurier below her feet.

I almost get always sharp shots with this lens. I find the behavior on this particular shot very weird.

 

[WildlifePhotog]

Could also be motion blur, she may have just had shaky legs or moved slightly. Test your lens out on a flat object with lots of details, or a test target...maybe even a fence if its straight. Or a wall, and make sure it's uniformly sharp, well I mean the focal plane/area of focus is sharp. Obviously on the 24-120mm VR the corner's wont be as sharp as the center, when you focus on the center. People move, almost all of the time, so they are not great test targets!

Nope. Everything lost focus below her, even the rocks and the background is blurier below her feet.

I almost get always sharp shots with this lens. I find the behavior on this particular shot very weird.

It could also be motion blur on your part, just like with a panning shot, it’s possible to have part sharp and part not, but it’s unlikely. Have you looked into field curvature? It might be possible in this image that field curvature played a big role in part being sharp and part not. Since the camera was tilted, it’s definitely possible to get this result due to field curvature.

 

[madeinlisboa]

Could also be motion blur, she may have just had shaky legs or moved slightly. Test your lens out on a flat object with lots of details, or a test target...maybe even a fence if its straight. Or a wall, and make sure it's uniformly sharp, well I mean the focal plane/area of focus is sharp. Obviously on the 24-120mm VR the corner's wont be as sharp as the center, when you focus on the center. People move, almost all of the time, so they are not great test targets!

Nope. Everything lost focus below her, even the rocks and the background is blurier below her feet.

I almost get always sharp shots with this lens. I find the behavior on this particular shot very weird.

It could also be motion blur on your part, just like with a panning shot, it’s possible to have part sharp and part not, but it’s unlikely. Have you looked into field curvature? It might be possible in this image that field curvature played a big role in part being sharp and part not. Since the camera was tilted, it’s definitely possible to get this result due to field curvature.

Not so sudden, specially at 5.6. It's like I had applied a blur in photoshop to the lower part of the photo with almost no feathering

 

[WildlifePhotog]

Well if you're sure that nothing moved, including yourself than you absolutely need to test your lens as I already recommended. Read up on things like field curvature and de-centered lens elements. There are ways to test for that sort of thing, including available online test charts. Another good method is to take some images at or very near infinity. Could be distant buildings or trees, the ocean, whatever. Take an image at f/8-f/11 with the center focus point, and then for the subsequent images move the focus point to the top left, top right and bottom left and right corners. Check all corners are sharp! If something is wrong with the lens you should find it when checking it on test charts and the other method I mentioned.

 

[davescliches]

It doesn't look like motion blur to me ! ............ also, the rocks and feet look blurred while the legs look sharp even below the knee ... (too rapid of a blur/sharp transition in my book) .............. on a 24mm lens at F5.6 DOF would have normally cover that small change of distance ........

.......... I've had that sort of behaviour with the SR system used by Pentax ......... all of a sudden an inexplicable blurred area at the bottom of the frame ......... after a while I realized I was not giving sufficient time to let the SR/IBIS start up and stabilize the sensor .........

.......... normally I would have doubted the lens's centering but not as the user is not seeing this issue in other photos .......... so I'm not here ...

......... I would check by taking similar images with the "same settings" ........ one with no pre- focusing at all ........ just press the shutter button directly .......... so at the moment the shutter button is pressed the IBIS has yet to engage ........

........ and another where the IBIS has had enough time to fully engage before the shutter is fired ...........

......... and make comparisons between the two images .......

........ IBIS is fairly new to Nikon and it isn't instant ....... the sensor has quite some mass and will need a certain time to be effective ......... we have been used to the in lens VR stabilization ........

........ my thoughts

dave's clichés

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