[Nayu]

Hey everyone,

When I adjust focus with my Pentax-A 50mm f/2, the image through the viewfinder as well as in live view seems to visibly shift. I recorded a video to show the problem:

You Tube

You Tube

What is going on with my lens? Is it possible to fix this behavior? I have never seen this happen before with this lens in my 1-2 years of owning it. Also, it does not happen with any of my other lenses except my 18-55 WR, where the issue is visible with autofocus as well.

Thanks for reading.

[kiwi_jono]

I don't have that lens but:
1. Its not unusual for angle of view to change a bit when changing focus,
2. The shifting up / down / left / right looks like barrel wobble to me. The A series of lens are known a bit for this (I have A 50 f1.4 that has a little barrel wobble but not this bad). Have a look at the barrel as you turn the focus is it moving up / down / left / right?

Theres probably not much you can practically do (someone else may have an idea if you are brave enough to operate on it) other than buy another lens. If the wobble bothers you, then you could always look at buying a nice A50 f1.7 which is still pretty cheap and better IQ than the f2.

[Nayu]

I don't have that lens but:
1. Its not unusual for angle of view to change a bit when changing focus,
2. The shifting up / down / left / right looks like barrel wobble to me. The A series of lens are known a bit for this (I have A 50 f1.4 that has a little barrel wobble but not this bad). Have a look at the barrel as you turn the focus is it moving up / down / left / right?

Theres probably not much you can do other than buy another lens. If the wobble bothers you, then you could always look at buying a nice A50 f1.7 which is still pretty cheap and better IQ than the f2.

The shifting up/down/left/right is the problem I meant, thanks for putting it concisely. After checking, it looks like it is indeed barrel wobble, and the intensity increases when I change focus directions closer to the short focus distance side. I (think) I can feel the barrel hit each side as I change focus directions through the camera.

It does bother me as I find the A 50/2 more fun to use every day compared to my DA 50/1.8, but I can work with the DA 50 instead. Maybe I'll grab an A 50/1.4 one day .

Thanks for the help.

[UncleVanya]

Grab an M or K series... EDIT: IF it is wobble they are a bit better made.

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Last edited by UncleVanya; 12-15-2017 at 05:36 AM.

[stevebrot]

What I am seeing in the video is something called focus breathing. Magnification with external focus primes increases as the lens is focused far-to-near. FOV also decreases resulting in a change of the of relative position of points within the frame. There is also a small change of point-of-view due to extension. I also noticed some camera movement.

I have a short video done on my K-3 with A 50/1.7 that shows the effect on a regular grid with a fixed point at mid-frame and various objects. If will share it when I figure out how.


Steve

[stevebrot]

I have a short video done on my K-3 with A 50/1.7 that shows the effect on a regular grid with a fixed point at mid-frame and various objects. If will share it when I figure out how.

Figured it out...

You Tube

You Tube

The pencil tip is at center of frame. Note the eraser and silver spinner and the dot marks and their relative positions as focus shifts.


Steve

[Digitalis]

What I am seeing in the video is something called focus breathing.

Exactly. Internally focusing lenses are particularly well known for suffering from it.

[UncleVanya]

I had thought it was focus breathing myself until the OP said he was seeing barrel wobble. I was going to suggest using s Target that was farther away to see if the problem was only noticed close up.

[clickclick]

Exactly. Internally focusing lenses are particularly well known for suffering from it.

Just curious as trying to learn more, what is it about internal focus that would cause this?

Thanks

[Ontarian50]

Of course, the 50mm lenses are unit focusing (the whole group of glass moves back and forth) not internal focus (where only one inside lens group moves).

Still, unit focusing lenses do breathe a bit, because the focal length increases slightly as the lens moves forward, so the magnification increases that little bit.

Internal focus lenses often lose focal length as you focus closer, and it's often more noticeable - especially on wide range zooms at their long end.

However, looking at the OP's video, I can't determine if we're talking about focus breathing as he focuses closer, or the little sideways jiggle as the focusing is moved.

Is it just the tripod shifting slightly as the lens is touched? Or is the camera super solidly mounted, and the lens does indeed have some slop in its focusing mount, and needs re-greasing to snug it back up again?

[stevebrot]

Just curious as trying to learn more, what is it about internal focus that would cause this?

Thanks

Focus breathing in the strict sense is the change of effective focal length that accompanies the in-out movement of external focus (block focus) lenses. To demonstrate, simply put a traditional prime on an SLR and rack the focus ring. The FOV decreases as the lens focuses nearer. We tend to ignore it because we are used to it even though it can affect composition.

Where focus breathing has gained more recent notoriety is with internal focus lenses where focus is attained by changing the relative position of lens elements with little or no extension.* As a result, the effective focal length stays the same with the side effect that FOV increases when focusing closer from infinity. FOV will be wider than expected for a given focal length at moderate distances. The lens breathes "wide" rather than "narrow".

An example that has pained me is my Sigma 17-70/2.8-4 Contemporary. I have been very pleased with its performance, but was disappointed to find that its "reach" at 70mm and portrait distance was somewhat less than expected. In fact, the FOV at 70mm and five feet is about the same as with a 50mm lens at the same distance.

Edit: The above is a simplified discussion. In the real world, a particular lens may use a combination of extension and floating elements to accomplish focus with the result that both lens extension and focus breathing (either direction) is minimized. It should be noted that lenses such as the DA 50/1.8 where the elements move as a block within the outer shell (no increase in overall length) are not internal focus. Similarly, lack of front element rotation also does not mean internal focus.


Steve

* This is a very poor explanation of what actually happens, but I could not find the reference I was looking for that explains internal focus in terms of primary planes.

[stevebrot]

Is it just the tripod shifting slightly as the lens is touched? Or is the camera super solidly mounted, and the lens does indeed have some slop in its focusing mount, and needs re-greasing to snug it back up again?

Any of the above might be contributing, though my vote is camera motion during the focus pull.


Steve

[stevebrot]

I had thought it was focus breathing myself until the OP said he was seeing barrel wobble. I was going to suggest using s Target that was farther away to see if the problem was only noticed close up.

I was thinking that maybe a direct comparison with the OP's DA 50/1.8 might help sort things out as well.


Steve

[clickclick]

So if I'm following this right, the difference in the internal focusing lenses is how the elements are moved in relation to each other versus just moving the whole group fore and aft? It's interesting, and having picked up a 60-250 recently, this has percolated up the awareness chain for me. I also have a DA 50/1.8, but honestly haven't shot with it enough to notice. I ought to play with it more, but when I reach for something 50ish, I tend to grab either my A50/1.4 or a CZJ Biotar 2/58, which I really love.

Thanks for the input.

[Digitalis]

Just curious as trying to learn more, what is it about internal focus that would cause this?

Internal focusing systems in lenses are utilized to keep lenses compact and reduce the number of externally moving parts. They are designed with stationary* front and rear cells with a focusing group that moves internally and non-linearly with focus distance**. The focusing group is typically in the optical center of a lens, though there are cases where the focusing group can be situated close to the front** or rear cell. The non-linear motion of the focusing group gives lens designers freedom to compensate for aberrations over a wider range of focus distances. Another advantage internal focus brings is the AF drive module doesn't have to move the entire optical assembly to focus: instead it only has to move the focusing group which, even in lenses with very wide apertures will have considerably less weight and mass, so AF gains considerable speed and responsiveness.

The downside is that internal focus designs can often exacerbate focus breathing and this is particularly noticeable in the case of macro primes and lenses with a zoom ratio approaching or greater than 1:3. Also Internal focusing can alter the effective aperture of the lens itself due to loss of focal length at close focus distances.

This is a very poor explanation of what actually happens, but I could not find the reference I was looking for that explains internal focus in terms of primary planes.

There is a lot of reference material out there but a lot of it seems to have trouble making the distinction between floating elements and internal focusing.


*Though in some cases IF lenses do utilize forward and rear extension the degree of extension is far less than any equivalent non IF lens could manage and maintain the same functional purpose.
**As opposed to common block focus lenses which move in a linear fashion. Though to further complicate things: there are lenses that feature floating lens elements which can have individual lens elements that can be completely stationary, or move in contrary motion to the primary optical groups - and these lenses themselves may have internal focus groups in them as well.
*** this is a very common trend with retrofocus wide angle lenses which are already a headache to design.