Recomposing & keeping focus.
- Thread starter Fujiowski
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retinax
Well-known
Cosine error is definitely a thing. You can see it happen live in front of you with an SLR and certain lenses.
olifaunt
Well-known
Just don't change the focus ring after focusing and you're good to go.
peterm1
Veteran
If you change the orientation or tilt the camera in the manner described in the image you run the risk of getting cosine error. If on the other hand instead of tilting the lens you take a step sideways to recompose the image while maintaining the same distance from and orientation to the plane of focus you will not. Tilting the lens is a faster way to recompose and also most of us are too lazy so standing in the same place and tilting the camera is ingrained in our behavior. At least it is with me.
And so I take my chances with focus error - as someone else pointed out, some lenses (many older lenses) do not have a flat plane of focus - it is curved due to uncorrected spherical aberration, so theoretically the plane of focus with these lenses should remain on the main subject as you tilt the lens. I read somewhere for example that the old Pentax M42 Takumars have this tendency but many other lenses do too, which I guess is why many (especially) older lenses are relatively poor in the corners when shot wide open by pixel peepers and others who are inclined to shoot photos of brick walls ( 🙂 ). And I am guessing that modern super corrected lenses with a flat field are likely to have the tendency to show cosine error when use the way this thread described for this reason ??
And so I take my chances with focus error - as someone else pointed out, some lenses (many older lenses) do not have a flat plane of focus - it is curved due to uncorrected spherical aberration, so theoretically the plane of focus with these lenses should remain on the main subject as you tilt the lens. I read somewhere for example that the old Pentax M42 Takumars have this tendency but many other lenses do too, which I guess is why many (especially) older lenses are relatively poor in the corners when shot wide open by pixel peepers and others who are inclined to shoot photos of brick walls ( 🙂 ). And I am guessing that modern super corrected lenses with a flat field are likely to have the tendency to show cosine error when use the way this thread described for this reason ??
Lucadomi
Well-known
In my experience, with manual rangefinder you focus on what you main subject is and than recompose. Recompose by just moving your main subject away from the center of the image, at your preference. Things around it willl be in focus or out of focus depending on the aperture/depth of field.
DougFord
on the good foot
Whether using manual focus or AF, once you've established focus or rather a focus plane, you can then move in any direction, parallel to the established focus plane. Give 13.8 billion light years to the right or left a try, keep us updated on your findings.
rbiemer
Unabashed Amateur
fujiowski,
Out of the two dozen replies, so far, the ones I quote here are actual answers to the question you actually asked.
The rest are interesting discussion points and probably good info but for the cameras (and their lenses) that you asked about, go with these and you'll be fine.
Rob
Out of the two dozen replies, so far, the ones I quote here are actual answers to the question you actually asked.
The rest are interesting discussion points and probably good info but for the cameras (and their lenses) that you asked about, go with these and you'll be fine.
Rob
I'm feeling a little sorry for the OP wading through this mix of contradictory replies to his question. If the question sounds somewhat confused, that's understandable if posed without knowing the answer. But this has resulted in some replies not so relevant to how I interpret the question, some accurate and some inaccurate! IMO the best answer is this one (don't even need to get into the math):
Technically, most good modern lenses have a nearly flat focal plane, which means that when you recompose after focusing in the middle, you get backfocus. It's called cosine error. Forum member ferider has explained it here: https://www.rangefinderforum.com/forums/showthread.php?t=148485
with this diagram:
In practice, and evidenced by people here who don't believe there's a problem, you need to have pretty thin depth of field, a wide lens and rather extreme recomposing to notice. Just do your own tests, theoretical reasoning won't help much figuring out the practical limits because we usually don't have data about how flat the focal plane of a lens is. A lens with focal plane curved such that the camera is in the middle of the circle would eliminate cosine error. Focal plane curved away from the camera makes it worse.
Rob-F
Likes Leicas
I have found this to be true in actual practice. Cosine error is real. Let me give my own example. I'm photographing a painting in the museum. It is an impressionistic painting, with no good edges to focus on. So I focus on the frame instead. Now, when I move the camera through an arc so as to center the picture in my finder, the flat plane of the painting is not at the same distance from my lens as the frame had been when I focused. Why not? Well, for one thing, look at the diagram above. It shows how the plane of focus has shifted when the camera was moved.
Or, imagine a rigid extension ruler that extends from my lens to the picture frame. It has a red tip where it touches the frame. It's the exact length of my focusing distance, which isn't going to change as I now swing the camera to center it on the painting. Now, where is the red tip? Is it in the same plane as the painting?
NO! It is not! Why not? Because it didn't move in a flat plane, it swung through an arc. So where is it now? It is behind the painting--maybe even behind the wall the painting is on. So now I am back-focused. Visualize that: it makes it easy to see. My point of focus is behind the painting. That's Cosine error.
So what to do? I don't swing the camera in an arc. I side-step until the frame is centered in my rangefinder patch. Focus. Then I side-step back to center the painting, and take the picture without changing focus. Done.
seany65
Well-known
The contradictory and complicated answers this question received, is just typical of the interwebnet.
Rob-F
Likes Leicas
Unhelpful. The OP's question is legitimate. Some of us are trying to answer it helpfully.
Dan Daniel
Well-known
Yep. And there are a range of answers, depending on how involved the OP wants to get. Sure, the 'don't worry, be happy and shoot away' approach will work much of the time. But if/when it doesn't work, the OP might have no idea what went wrong. So people provide more info to give a more thorough explanation of the issues at hand. Again, the OP can read these or can decide to ignore it. I don't see the problem here. Lots of voices, lots of approaches. About the only annoying ones are the one who denigrate others' attempts to help.
seany65
Well-known
Hello Rob. I understand that people are trying to help, but as a "victim of interwbenet answers" to some of my own questions (mostly on other forums), I know what it's like to be confused at some of the answers one can get.
I was just pointing out, in a freindly and non-derogotary way, that it's the usual thing to expect.😉
