Sharpness of a still shot using back button focus

[cauger61]

If a still shot is taken under continuous auto focus (AF-C) using back button, will that be as sharp as it is taken under single servo (AF-S) using half-pressed shutter?

My attempt to capture shots using back button focus under AF-C, turned out to be soft when I re-focused to another figure using BBF again.

Actually, I was testing the focusing accuracy of my 35mm lens as to whether it was back focusing or not. I used tripod and aperture priority mode at f/2.8 with auto-iso.

In doing so, I placed five different bottles at an angle side by side with little space between them. Initially I focused on the middle bottle (3rd); it was sharp. But, when I focused on the last bottle, that appeared soft as if the focus was held locked on the previous shot i.e. on the 3rd bottle.

[LenR]

The attached link may be of some help..........

https://www.usa.canon.com/internet/p...ocus-explained

[William W]

I expect that AF-C is to be used for moving Subjects, and if used for a still Subject is prone some to error: not that the error would be less sharp, simply not locked focus where the user might expect it to be focused.

I expect that Nikon's User Manual would make a note of this: Canon do, the incorrect use of the 'continuous' autofocus mode is a common cause of randomly OoF images.

WW

[LenR]

Bill is correct - AF-C is used to track subjects in motion.
For the focus test (back/forward) AF-S would be preferable as would using the centre focus point rather than a peripheral one.

[Manfred M]

As someone who uses BBF with AF-C on Nikon cameras virtually for 100% of my shooting, I can say that I have never had a problem with sharp focus outside of the very occasional "user error".

To use this technique, you also need to set your camera to use a single focus point and use a technique called "focus and recompose". Select the place in the frame you want the focal plane to fall and use BBF to capture focus. Then release the back button and frame up the shot and then click the shutter release. If you use any other focus mode, you can get focusing errors as the camera, rather than the photographer, determines where the focal plane lies.

The only times that I do not use BBF is for night photography, where I manually focus and for action shots where the subject is moving towards (or away) from me, in that case I use a 3D tracking mode with AF-C.

[William W]

As someone who uses BBF with AF-C on Nikon cameras virtually for 100% of my shooting, I can say that I have never had a problem with sharp focus outside of the very occasional "user error".

To use this technique, you also need to set your camera to use a single focus point and use a technique called "focus and recompose". Select the place in the frame you want the focal plane to fall and use BBF to capture focus. Then release the back button and frame up the shot and then click the shutter release. If you use any other focus mode, you can get focusing errors as the camera, rather than the photographer, determines where the focal plane lies.

The only times that I do not use BBF is for night photography, where I manually focus and for action shots where the subject is moving towards (or away) from me, in that case I use a 3D tracking mode with AF-C.

OK - Thanks.

Re-iterating and a question to clarify.

If you have AF-C selected and BBF selected, then, is it always that pressing BBF activates Auto Focus and releasing BBF will Lock Focus?

Asking another way, is there a selectivity to choose how pressing and releasing BBF will fucntion in respect to activating focus and locking focus when in AF-C Mode?

WW

[Stagecoach]

If a still shot is taken under continuous auto focus (AF-C) using back button, will that be as sharp as it is taken under single servo (AF-S) using half-pressed shutter?

As a Nikon shooter, from experience, I would say yes.

Based upon the assumption that you are using the same focus point and that you are activating the 'focus command' (whether by shutter button or AF/ON button) from the same de-focused position of the lens to take account of the deadband.

My attempt to capture shots using back button focus under AF-C, turned out soft when I shifted my focus point to another figure.

The accuracy of focus points varies, the centre point generally being the most accurate. Some cameras have had significant discrepancies with outer points.

Actually, I was testing the focusing accuracy of my 35mm lens as to whether it was back focusing or not. I used tripod and aperture priority mode at f/2.8 with auto-iso.

In doing so, I placed five different bottles at an angle side by side with little space between them. Initially I focused on the middle bottle (3rd); it was sharp. But, when I focused on the last bottle, I appeared soft as if the focus was held locked on the previous shot i.e. on the 3rd bottle.

When undertaking any of these rear/front focus test routines it's suggested to take a number of shots each time, de-focusing the lens in different directions prior to activating AF before coming to conclusions.

I can think of no reason with a Nikon camera in AF-C why the AF accuracy would not be as good as when in AF-S other than the always present focus deadband.

[William W]

OK

Hi Manfred,

I think I have found the answer to my question...

Do you use AF-C with the selection of Focus Lock activated?

Bill

[William W]

. . . My attempt to capture shots using back button focus under AF-C, turned out soft when I shifted my focus point to another figure. . .

Hi Grahame,

I did not interpret this sentence as I thin you did.

I though he meant that he shifted the same selected focus point to a new figure in the scene.

Bill

[William W]

A few years ago, in concert with a fellow in the US, we each did a range of testing of Canon's Auto Focus, in Continuous Mode and comparing/contrasting those results to using AF in One Shot Mode .

The tests were specific, using a Sturdy Tripod and Head, shooting in good light using Centre AF Point only, focusing on an hard contrast edge of a stable and still Subject (red front hard cover of a book set on a black background) at about 2000mm Subject Distance.

The tests were simple:

The lenses (as per Grahame's comments) were set to various/random out of focus settings on the focus turret and then AF was initiated and locked, then the shot was made.

We took about 20 shots per lens and I tested 3 lenses and Michael tested three or four.

There was a significant difference in the AF error rate when using Continuous Mode, from memory recall 30/70 for continuous and something like 3(errors)/97(correct) for One Shot. Noted that the AF error was not large, but is was significant, sometimes in the range of 20mm off focus.

I interpreted the OP's question to be similar, in fact almost identical to this test I did a few years ago.

Noted I have not used Nikon gear for several years, but it was my belief that the OP's situation was so similar to my tests with Canon that I think, (without further clarification or correction from Sanjib) that the comment is relevant and perhaps the key to the issue.

In any case I am very interested in Sanjib's comments and also my reading further from Nikon users here their experiences and the nitty-gritty of the Nikon AF system's workings.

My experience is that nuances of slight OoF are not necessarily noticed when general shooting, yet will likely be picked up when one is doing more specific and somewhat clinical testing.

WW

[Manfred M]

OK

Hi Manfred,

I think I have found the answer to my question...

Do you use AF-C with the selection of Focus Lock activated?

Bill

The only time I use focus lock is when I use a camera that is set to focus through the shutter release, as using that technique adds an extra step into the process; focus, focus lock and shutter release. With BBF and AF-C mode the AF runs continuously until the back button is released. The activation of the autofocus functionality of the shutter release has to be disabled for the technique to work.

I shoot with my thumb on the BB focus button and use my index finger to activate the shutter release. When I am happy with my focus point, I release the BBF button, recompose and shoot. The main advantage of the technique is that I have total control of the focus plan and don't have to worry about what the AF points are doing. An example where this works very well for me is when using a DoF / hyperfocal distance approach. I tend to focus in front of the subject, and let the broader DoF setting to get a sharper image.

[William W]

The only time I use focus lock is when I use a camera that is set to focus through the shutter release, as using that technique adds an extra step into the process; focus, focus lock and shutter release. With BBF and AF-C mode the AF runs continuously until the back button is released. The activation of the autofocus functionality of the shutter release has to be disabled for the technique to work.

OK Thanks, I understand.

The highlighted sentenced makes the functionality clear to me.

Noted also and importantly this condition - "The activation of the autofocus functionality of the shutter release has to be disabled for the technique to work."

Thanks again.

Bill

[Stagecoach]

Hi Grahame,

I did not interpret this sentence as I thin you did.

I though he meant that he shifted the same selected focus point to a new figure in the scene.

Bill

Hi Bill,

Reading it again I believe it could be interpreted both ways, hopefully Sanjib will confirm.

If using the same 'focus point', and re-positioning the camera when in AF-C we don't know if he re-focused by taking his finger off and pressing again, or, kept it pressed?

[cauger61]

Hi Bill,

Reading it again I believe it could be interpreted both ways, hopefully Sanjib will confirm.

If using the same 'focus point', and re-positioning the camera when in AF-C we don't know if he re-focused by taking his finger off and pressing again, or, kept it pressed?

Yes, after taking the first shot with BBF, I lifted my fingers off AF-ON button and then refocused by pressing once again for the second shot i.e. targeting my camera on the last bottle. However in both the cases, the camera was on tripod and I rotated the camera on a ball-head.

One thing, that I failed to mention is that the shots were taken under fluorescent lights. I shall perform the tests once again under natural light and post those images with exif data for further clarification.

[cauger61]

Bill is correct - AF-C is used to track subjects in motion.
For the focus test (back/forward) AF-S would be preferable as would using the centre focus point rather than a peripheral one.

Yes I tried with AF-S and centre focus point. Those were quite sharp!! My point is, if BBF helps toggling between AF-C and AF-S without changing the autofocus mode, does that come at the cost of sharpness?

[Stagecoach]

Yes, after taking the first shot with BBF, I lifted my fingers off AF-ON button and then refocused by pressing once again for the second shot i.e. targeting my camera on the last bottle. However in both the cases, the camera was on tripod and I rotated the camera on a ball-head.

Ok

One thing, that I failed to mention is that the shots were taken under fluorescent lights. I shall perform the tests once again under natural light and post those images with exif data for further clarification.

If using the 5 bottles again I would ensure that their centres are exactly the same distance apart.

In addition take shots of the centre bottle and the ones next to it (not the last). The reason for this is that for each case you will also be able to see not just the sharpness of the target bottle but also compare how 'out of focus' the bottle is in front and behind the target one.

[cauger61]

The only times that I do not use BBF is for night photography, where I manually focus and for action shots where the subject is moving towards (or away) from me, in that case I use a 3D tracking mode with AF-C.

Yes Manfred, I understand, here lies the hunch!! The BBF performs better under natural lights. My tests were under fluorescent lights. You have rightly pointed out, shots taken using manual focus were sharper than those taken with AF.

[Stagecoach]

shots taken using manual focus were sharper than those taken with AF.

What is your 'subject to camera distance' during your tests with the 35mm at f/2.8 ?

[cauger61]

Hi Bill,

Reading it again I believe it could be interpreted both ways, hopefully Sanjib will confirm.

If using the same 'focus point', and re-positioning the camera when in AF-C we don't know if he re-focused by taking his finger off and pressing again, or, kept it pressed?

Yes you are very correct to be confused. I am very sorry not to explicate the point. Yes, I misplaced the word "shifted". That should be re-focused. Thnx u for pointing out.

[pschlute]

One thing, that I failed to mention is that the shots were taken under fluorescent lights. I shall perform the tests once again under natural light and post those images with exif data for further clarification.

Hi Sanjib

My experience when calibrating individual lenses to the AF system is that the lighting you conduct the tests under can make a difference.

I always perform AF tests under natural light. Of course if you are always going to be shooting under a specific light source (or more accurately....auto-focussing under a specific light source), then you would be better off calibrating under that source.