Chromatic aberration & focus accuracy

sasar

How do you think, is it possible that inaccurate autofocus (significant front focus) of the lens has a negative impact on the amount of chromatic aberration?

DaireQuinlan

Never read of any connection between the two.

OTOH, significant spherical aberration can seem to make a lens seem to not focus correctly. My Nikkor 50mm f/1.4 has a noticeable focus shift as you stop down, to the point where I shoot it at either f/1.4 or f/4 and up, never anything in between. You're focusing wide open so at f/1.4 it'll be as accurate as it can be. If you're stopping down then by the time you get to f/4 the focal point has shifted but stopping down that couple of stops helps overcome this.

sasar

As a matter of interest I contacted Sigma repair centre and they are saying it is possible.

charybdis

Chromatic aberration is produced by different wavelengths of light passing through the lens not converging on the same point and the effect becomes more pronounced when an area of high contrast is slightly out of focus. So if you are in a situation where there is shallow depth-of-field, a slight mis-focus on your subject can result in more pronounced chromatic aberration.

sasar

charybdis wrote: »

Chromatic aberration is produced by different wavelengths of light passing through the lens not converging on the same point and the effect becomes more pronounced when an area of high contrast is slightly out of focus. So if you are in a situation where there is shallow depth-of-field, a slight mis-focus on your subject can result in more pronounced chromatic aberration.

And what happens if focus is micro-adjusted in-camera?

charybdis

sasar wrote: »

And what happens if focus is micro-adjusted in-camera?

Micro-adjust or autofocus doesn't really matter, if the subject is slightly out-of-focus chromatic aberration is likely to be more pronounced. If your camera is consistently mis-focusing, the subject will probably be slightly out-of-focus.

sasar

Now, you see, I do understand what CA is, however based on what you are saying (if I understand correctly) - if I shoot at f1.4 I am supposed to get CA.

My question is, if my lens has a strong front focus will that have an impact on the amount of CA?

Front focus does not mean my image in not in focus, it means that focus is not where I wanted it to be.

charybdis

sasar wrote: »

Now, you see, I do understand what CA is

I don't think you do.

sasar wrote: »

however based on what you are saying (if I understand correctly) - if I shoot at f1.4 I am supposed to get CA.

Using larger apertures will result in shallower depth-of-field which is more likely to evidence imperfect focus and chromatic aberration.

sasar wrote: »

My question is, if my lens has a strong front focus will that have an impact on the amount of CA?

"Front focus", in the sense you are using it in, refers to the propensity of a lens to autofocus on a point beyond the intended focus point. If your lens is autofocusing inaccurately, then it is likely chromatic aberration will be more apparent because the intended focus point is probably slightly out-of-focus. This doesn't mean there is necessarily a problem with the optical assembly, it means there is probably a problem with the autofocus mechanism or the user.

sasar wrote: »

Front focus does not mean my image in not in focus, it means that focus is not where I wanted it to be.

Then, by definition, your image is not in focus.

sasar

charybdis wrote: »

Then, by definition, your image is not in focus.

Lets say there are two people in my photo, I intended to have focus on one of them, but due to front focus issue actual focus is on the second person. Do you call this - out of focus image? AND, will this result in having more CA in the image?

charybdis

sasar wrote: »

Lets say there are two people in my photo, I intended to have focus on one of them, but due to front focus issue actual focus is on the second person. Do you call this - out of focus image?

Yes.

Conveniently so, but yes.

sasar wrote: »

AND, will this result in having more CA in the image?

It depends.

sasar

charybdis wrote: »

Yes.

Conveniently so, but yes.

I disagree.

charybdis wrote: »

It depends.

On what?

charybdis

sasar wrote: »

I disagree.

OK.

But if you were trying to photograph a small flower and you totally messed up the focus and took a very sharp and technically good photograph of a mountain in the background and the flower was so out-of-focus that it was invisible, would that be an in-focus image of a flower?

sasar wrote: »

On what?

On what's in the image, where the plane of focus lies, the placement of high contrast areas of the image (if any), the design of lens you're using, the size of your depth-of-field; there are many factors.

sasar

charybdis wrote: »

OK.

But if you were trying to photograph a small flower and you totally messed up the focus and took a very sharp and technically good photograph of a mountain in the background and the flower was so out-of-focus that it was invisible, would that be an in-focus image of a flower?

Yes, that would be an out-of-focus 'image of flower', but an in-focus 'image of mountains'.

charybdis wrote: »

On what's in the image, where the plane of focus lies, the placement of high contrast areas of the image (if any), the design of lens you're using, the size of your depth-of-field; there are many factors.

Lets say I have two similar lenses - one with accurate AF, the other with front focus problem.

I take identical photo with each of the lenses:

Situation #1: I used microadjustment function on the problematic lens to correct front focus issue and images from both lenses are right in focus.

Situation# 2: Knowing that lens has front focus issue, I focus further away, so that the actual focus is right where I need it.


SO, will there be more CA coming from the lens with AF issue in example #1 or #2?

charybdis

sasar wrote: »

Yes, that would be an out-of-focus 'image of flower', but an in-focus 'image of mountains'.

By that logic an out-of-focus photograph can't exist because, no matter where the focus was set, there will always be a plane of focus; a photograph will never be out-of-focus, it will only be a photograph focused on a different plane than the intended one.

It's a reasonable argument, but its definition of "out-of-focus" isn't a useful one.

sasar wrote: »

Lets say I have two similar lenses - one with accurate AF, the other with front focus problem.

I take identical photo with each of the lenses:

Situation #1: I used microadjustment function on the problematic lens to correct front focus issue and images from both lenses are right in focus.

Situation# 2: Knowing that lens has front focus issue, I focus further away, so that the actual focus is right where I need it.


SO, will there be more CA coming from the lens with AF issue in example #1 or #2?

Not necessarily.

And situations #1 and #2 are effectively the same thing: in both cases the focus of the lens is corrected, in #1 by the camera, in #2 by the user.

sasar

charybdis wrote: »

Not necessarily.

And situations #1 and #2 are effectively the same thing: in both cases the focus of the lens is corrected, in #1 by the camera, in #2 by the user.

This is what I wanted to know, however I don't think you're right. Sigma repair service is saying that more CA can be caused by a lens with front focus issue.

charybdis

sasar wrote: »

This is what I wanted to know, however I don't think you're right. Sigma repair service is saying that more CA can be caused by a lens with front focus issue.

It can be caused by a lens with a front focus issue, but chromatic aberration can also appear in slightly out-of-focus areas even when the same lens is correctly focused. Correcting the focusing accuracy of a lens won't necessarily eliminate the appearance of chromatic aberration in photographs taken with it.