Other Focus Shifting

[Kamoshika]

I keep reading about how a certain lenses can suffer from focus shifting to a certain degree. Being ignorant about several aspects of photography, I tried to come up to speed on the subject. From what I gleaned, it seems this is something that occurs when you focus on a subject wide open, then stop down the lens--without refocusing--which results in the subject being out of focus. Have I got this right? It seems that just refocusing when using a different aperture takes care of the problem. This doesn't seem to be anything too cumbersome for me to do.

So, a last question: Has focus shifting been a problem for anyone here? And, if so, in what situation?

 

[ctitanic]

As far as I know, focus shifting affect DSLR but do not affect Mirrorless. This is the main reason why in DSLRs you will find a focus microadjustment option while we do not have that in our R systems.

 

[Kamoshika]

As far as I know, focus shifting affect DSLR but do not affect Mirrorless. This is the main reason why in DSLRs you will find a focus microadjustment option while we do not have that in our R systems.

Actually, one of the lenses I was reading about that has this "problem" is the RF 100mm f2.8L.

 

[Bryan Conner]

From what I understand, focus shift is something that is inherent in some lens designs and the problem is completely independent from whether said lens is mounted on a DSLR, a mirrorless camera, or even a film camera. Here is a pretty geeky explanation of the issue: Focus Shift Article

I think that you would have to focus the lens with the lens stopped down to your desired aperture instead of the normal process of focusing wide open and then the lens is stopped down when the image is captured.

It is a very noticeable problem in macro photography because the depth of field of focus can be extremely narrow. This is why some photographers, when shooting handheld macros, will only focus with the lens stopped down.

 

[ctitanic]

Actually, one of the lenses I was reading about that has this "problem" is the RF 100mm f2.8L.

Well, then your only option is to send it back to Canon to calibrate the lens because the R systems do not have a micro adjustment option like the DSLR did. But I may be confusing two different issues here.

 

[Archibald]

Well, then your only option is to send it back to Canon to calibrate the lens because the R systems do not have a micro adjustment option like the DSLR did. But I may be confusing two different issues here.

Yeah, you are not understanding the issue.

It is fairly common for the focus of a lens to shift when stopping down. Usually the effect is minor. But it reportedly is significant with the RF 100mm macro lens. However, if you shoot at f/11 or narrower, DOF will cover you. I always shoot stopped down, so not an issue for me.

 

[Kamoshika]

From what I understand, focus shift is something that is inherent in some lens designs and the problem is completely independent from whether said lens is mounted on a DSLR, a mirrorless camera, or even a film camera. Here is a pretty geeky explanation of the issue: Focus Shift Article

I think that you would have to focus the lens with the lens stopped down to your desired aperture instead of the normal process of focusing wide open and then the lens is stopped down when the image is captured.

It is a very noticeable problem in macro photography because the depth of field of focus can be extremely narrow. This is why some photographers, when shooting handheld macros, will only focus with the lens stopped down.

Great technical article, but I guess I'm still not getting it completely. For macro, I have my camera set to magnify while holding down my back-button focus and turning the focus ring (One-Shot→enabled (magnify)). I find I can really fine tune the focus this way. But with regard to focus shifting, why would it matter what aperture I choose if I'm manually focusing--or even using the AF--each time before a shot?

 

[Archibald]

Great technical article, but I guess I'm still not getting it completely. For macro, I have my camera set to magnify while holding down my back-button focus and turning the focus ring (One-Shot→enabled (magnify)). I find I can really fine tune the focus this way. But with regard to focus shifting, why would it matter what aperture I choose if I'm manually focusing--or even using the AF--each time before a shot?

If you are focusing with a magnified view at the shooting aperture, then focus will be accurate.

Lenses that have some residual spherical aberration will shift their focus slightly as you change the aperture. I think of it as the outer glass having a slightly different focus than the inner glass. Stopping down excludes the outer glass and you are left with the focus of the inner glass.

With the lens elements all fixed and everything stationary except for the aperture, the focus plane will shift slightly as you change the aperture.

It's been a while since I read about the RF 100 L, but believe it is an issue if you focus wide open and then shoot at f/4 or 5.6. The focus will shift a bit and it seems it is noticeable. If you stop down to f/11 the focus also shifts but now the DOF is deep enough that it covers the error.

For the 100L, it is something to be aware of when shooting at those wider apertures.

 

[Bryan Conner]

If you are focusing with a magnified view at the shooting aperture, then focus will be accurate.

Lenses that have some residual spherical aberration will shift their focus slightly as you change the aperture. I think of it as the outer glass having a slightly different focus than the inner glass. Stopping down excludes the outer glass and you are left with the focus of the inner glass.

With the lens elements all fixed and everything stationary except for the aperture, the focus plane will shift slightly as you change the aperture.

It's been a while since I read about the RF 100 L, but believe it is an issue if you focus wide open and then shoot at f/4 or 5.6. The focus will shift a bit and it seems it is noticeable. If you stop down to f/11 the focus also shifts but now the DOF is deep enough that it covers the error.

For the 100L, it is something to be aware of when shooting at those wider apertures.

Exactly, well said.

 

[Only RF]

Yes two different things being discussed. I'm not expert on this but I think shifting that refers to the lens is also known as breathing.

What is Focus Breathing and How You Can Mitigate It

photographylife.com

Shifting for DLSR's is due to the nature of the mirror assembly. It can be corrected using MFA, but only in one direction. MFA corrects for front or back focus only, not both. Even after you properly MFA there will always be a little back and front focus drift. Again this is due to the nature of the mirror assembly.

ML bodies are not supposed to suffer either both consistent back or front focus or the drift between back and front focus. Some people have stated that Canon should have left MFA on the LR bodies because they experienced consistent front or back focus. ML's are not supposed to do that but what do I know. 3rd party lens casing it? For Canon lenses the only option would be to send your gear to Canon.

Since ML's have no mirror the assembly and focus on the sensor I'm still puzzled as to why some get consistent back or front focus.