Front or Back Focusing Possible with EVF?

[Jamie Pillers]

Recently I've read comments on the web where someone is complaining that such-and-such lens back-focuses or front-focuses on their EVF-equipped digital camera. I don't understand how this is possible and maybe someone here can explain.

I assume that if you focus with an EVF system, you are seeing exactly what the camera's sensor sees. So if the image is in focus in the EVF, doesn't it have to be in focus on the sensor, and therefore in the final image file?

 

[f16sunshine]

I assume that if you focus with an EVF system, you are seeing exactly what the camera's sensor sees. So if the image is in focus in the EVF, doesn't it have to be in focus on the sensor, and therefore in the final image file?

The answer is yes if one focusses and takes the image the aperture desired.

If for example you focus at f1.5 with a C sonnar 50mm and then quickly stop don to f4 to take without checking focus again... you may be surprised to find the focus point has shifted.
If you focus at f4 to begin with you will have correct focus for f4.

Cheers!

 

[sebastel]

and then we have the uses of EVF with autofocus. but that was not included in the original question, i guess.

 

[GaryLH]

Yep what Andy said and the problem.

By f4 or greater, dof is wide enough, unless u have a really high resolution evf, it is hard to judge precise focus even w/ mag active (for my bad eyes that is). I have found that most of my lenses that have this problem, the issue disappears by f2.8.. So for those lenses I will focus at a specific fstop wider than f2.8 or use f2.8 to get a close to the precise focus point before I stop down to the desired fstop.

Gary

 

[Jamie Pillers]

The answer is yes if one focusses and takes the image the aperture desired.

If for example you focus at f1.5 with a C sonnar 50mm and then quickly stop don to f4 to take without checking focus again... you may be surprised to find the focus point has shifted.
If you focus at f4 to begin with you will have correct focus for f4.

Cheers!

But you would see that in the EVF, correct? I think most people using EVF-based cameras are looking through the EVF as they change aperture in order to see what change is made to exposure, depth of field, etc..

 

[GaryLH]

But you would see that in the EVF, correct? I think most people using EVF-based cameras are looking through the EVF as they change aperture in order to see what change is made to exposure, depth of field, etc..

If u focus at f1.4 and stop down to f8 for example, unless your evf is either high enough resolution to c it or u happen to hit mag focus again, I doubt if most people would notice it.

Gary

 

[Jamie Pillers]

Yep what Andy said and the problem.

By f4 or greater, dof is wide enough, unless u have a really high resolution evf, it is hard to judge precise focus even w/ mag active (for my bad eyes that is). I have found that most of my lenses that have this problem, the issue disappears by f2.8.. So for those lenses I will focus at a specific fstop wider than f2.8 or use f2.8 to get a close to the precise focus point before I stop down to the desired fstop.

Gary

Gary and Andy,
I guess I'm missing something here. If one focuses with the lens wide open, say f2, and then changes to a smaller aperture, say f4, the depth of field will be bigger. How can the scene then be out of focus?

 

[Sarcophilus Harrisii]

At distance you would probably be right with the above comment about DOF. But at close range some lenses shift focus when stopping down by a greater margin than the increased DOF can accommodate. And DOF provides a zone of "acceptable focus", a problematic topic in itself. The only truly sharp point of focus is the one set, and if this changes when the lens is stopped down, it changes...

 

[GaryLH]

The results depend on a lot of factors. The most obvious times it is easy to notice focus issues of this type is when u are relatively close to the subject, dof is thin, and u can spot it.

In most cases dof covers up the glaring issue..but in most cases when u look at it closely it resembles more a zone focus shot then a precisely focus shot that had ample dof or it can be like the perverbial nose vs eyes in focus one. If u focused on the eyes but the nose was where the best sharpness is and dof covers it up mostly when u look at it from the evf..but when u look at it blown up bigger on your computer monitor, u can c what happened. The resolving power of most evf's is really not good enough to easily spot a lot of these issues.

Gary

 

[Jamie Pillers]

The results depend on a lot of factors. The most obvious times it is easy to notice focus issues of this type is when u are relatively close to the subject, dof is thin, and u can spot it.

In most cases dof covers up the glaring issue..but in most cases when u look at it closely it resembles more a zone focus shot then a precisely focus shot that had ample dof or it can be like the perverbial nose vs eyes in focus one. If u focused on the eyes but the nose was where the best sharpness is and dof covers it up mostly when u look at it from the evf..but when u look at it blown up bigger on your computer monitor, u can c what happened. The resolving power of most evf's is really not good enough to easily spot a lot of these issues.

Gary

Gary, if I'm focusing on someone's eye wide open, I'd be using the magnification feature of my EVF. Wouldn't I be seeing EXACTLY what the sensor sees?

 

[f16sunshine]

Focus shift is also quite dependent on the lens and distance as has been said.
The further away the subject is the less problems will occur. In close range shift will be more appear ant.
Some older lenses shift quite a bit and you may never notice if you are shooting beyond mid distance.
Some others shift very little and are of wider focal lengths. The Previous version from current FLE Summilux 35mm asph for example.
I've read it shifts but I never saw an example of that in my images. It was too wide and too little to see.
Copy variation is also a factor. In retrospect I should have kept the copy of that lens I had as it was maybe exceptionally well corrected compared to other asph 35's (or maybe it's all bs😛).
A fair amount of internet FUD exist as well. I noticed shift with a Zeiss OPton f1.5/50mm lens on my M8. Otherwise I have never had a problem with it with many different lenses over the years.
Best bet is to use your EVF at desired aperture when using a lens with a reputation of focus shift.

 

[GaryLH]

Gary, if I'm focusing on someone's eye wide open, I'd be using the magnification feature of my EVF. Wouldn't I be seeing EXACTLY what the sensor sees?

If u are focusing at the shooting aperture it is no problem. This type of problem only comes into play when u are not focusing using the desired fstop. If u are using a f1.4 lens and focus using f1.4 but shot at f4 at the lenses closest distance for example, a lens w/ this issue would show severe mis-focus because the dof is to razor thin. If u now move out to middle focusing distance, the dof is a bit deeper front and back of the focus plane, it now feels in most cases like a zone focused shot or u may not notice it at all.. Move it out further toward the longer distances of the lenses focusing range and u may not ever notice the focus shift.

A lot depends on the focal length of the lens and the amount of focus shift that particular lens exhibits.

Gary

 

[Jamie Pillers]

If u are focusing at the shooting aperture it is no problem. This type of problem only comes into play when u are not focusing using the desired fstop. If u are using a f1.4 lens and focus using f1.4 but shot at f4 at the lenses closest distance for example, a lens w/ this issue would show severe mis-focus because the dof is to razor thin. If u now move out to middle focusing distance, the dof is a bit deeper front and back of the focus plane, it now feels in most cases like a zone focused shot or u may not notice it at all.. Move it out further toward the longer distances of the lenses focusing range and u may not ever notice the focus shift.

A lot depends on the focal length of the lens and the amount of focus shift that particular lens exhibits.

Gary

Then maybe I should ask: Assuming we're talking about a modern lens here, is this a problem that shows itself only if one is looking at image files blown up to the 100% level, or is this an issue that if a print was hung on the wall, a viewer would say something like "Geez, at least couldn't the photographer have focused the d**n camera??" 🙄

 

[GaryLH]

Doesn't need to be 100% blow up, if it is there will c it. The issue is really related to the evf of camera x in terms of resolving quality and the visual acuity of your own eyes when using the evf of that camera.

As Andy has stated so long as u focus at the intended fstop, it will be fine. My old eyes are terrible (really bad stigmatize thus visual acuity is poor), so I need mag view to spit the ones that are not so obvious when looking thru evf. That same shot when looking at my computer screen is much easier for me to c the problem, if it is there.

Gary