topic Re: Autofocus woes in EOS DSLR & Mirrorless Cameras

Autofocus woes

davetong — Thu, 28 Feb 2019 01:46:28 GMT

Canon D70, 85mm lens, 1/200 at f2.8

It's not sharp, even though the autofocus claims to have a good lock on.

Judging by the tiles on the floor it's focused a good 1m too short.

Does the camera need servicing?

Also, what's with "the camera reports the distance as being between 2.88m and 11.9m" - that's a massive range given the 0.78m depth of field

Original image at https://www.dropbox.com/s/agk1qtakqh9j4iz/_MG_6750.DNG?dl=0

Re: Autofocus woes

Waddizzle — Thu, 28 Feb 2019 03:11:10 GMT

@davetong wrote:
Canon D70, 85mm lens, 1/200 at f2.8
It's not sharp, even though the autofocus claims to have a good lock on.
Judging by the tiles on the floor it's focused a good 1m too short.
Does the camera need servicing?

Also, what's with "the camera reports the distance as being between 2.88m and 11.9m" - that's a massive range given the 0.78m depth of field

As far as the camera reporting a distance between 2.88mm and 11.9m, that is called DOF, or Depth of Field. One explanation for the wide range of distance is that it is reporting the focusing range of all of the active AF points. That is a fairly old LR add-in, so it could be reporting incorrectly with more recent releases of LR.

[The report shows 31 AF points on a camera that only has 19 AF points.]

Judging from the number of active AF points, combined with the fact that you are in focusing/ in AI Focus mode, it seems likely that the camera dial was set to Green [A], Automatic, to take the photo. Or, maybe not. In either case, experienced users avoid AI Focus like the plague, because it does not always lock focus before firing the shutter. Stick to P mode, and One Shot focusing mode.

BTW, when an entire photo is evenly OOF throughout the entire frame, that points to one of two causes. One, the cause is camera motion blur, or even blur from a moving subject. Two, the camera/lens combo might need a focus calibration.

However, there is also a third more probable cause, IMHO. It is the lens. The EF 85mm f/1.8 USM does not focus very consistently. It does not focus consistently enough to make bothering with an AFMA adjustment worth the time.

Re: Autofocus woes

davetong — Thu, 28 Feb 2019 19:29:51 GMT

Thanks for the response.

The shot isn't OOF throughout the frame; as I noted, some of the tiles in front of the subject are in focus, which would

appear to rule out camera shake, rather it indicates to me that the camera had misjudged the distance by approx 1m.

I'm not convinced that it's the lens; I've experienced similar problems when shooting with the 50mm 1.8 at low stops
I haven't had problems with the kit zoom lenses, but that's not surprising since they don't go wide enough
to create a shallow enough depth of field. Some other shots were pin sharp, which tends to rule out focus calibration.
If there is a problem it's either with the camera or the operator.

The shot was taken in manual mode, not automatic. I used to use AI Servo mode; I switched to AI Focus

because of the visual indication of focus lock (hollow laugh) - I'll try switching to One Shot and see if that helps.

I've never fully understood why, but Canon claims that the D70 has "up to 31 AF points" in Live View.
https://support.usa.canon.com/kb/index?page=content&id=ART142752
I wasn't shooting in Live mode so it's irrelevent, but I expect that's why the plugin reports the number.

Regarding depth of field, the plugin explicitly reports that as 7.02-7.80m, which is what I'd expect when shooting at f2.8

so that doesn't explain why the EXIF data reports the distance to the subject as being between 2.88m and 11.9m

Re: Autofocus woes

ebiggs1 — Thu, 28 Feb 2019 22:43:53 GMT

Stop using AI-focus mode and you will be fine.

Re: Autofocus woes

ebiggs1 — Thu, 28 Feb 2019 22:45:59 GMT

"...some of the tiles in front of the subject are in focus..."

I don't think so. They have more contrast but still look OOF to me.

Re: Autofocus woes

davetong — Fri, 01 Mar 2019 20:25:28 GMT

Here's a better example:

https://www.dropbox.com/s/ius8xoh48ntrzv0/_MG_7079.DNG?dl=0

This was taken in Manual mode, using One-Shot focus. Same body and lens.

The leaves and the tree in the upper left and the gravestones on the right are sharp,

but the focus locks are supposed to be on the crypt in the distance.

I'm going to try cleaning the AF sensors

Re: Autofocus woes

Waddizzle — Fri, 01 Mar 2019 19:40:02 GMT

Do not clean it. Your photo is in focus. But, there multiple AF points indicating a potential to lock.

You cannot accurately determine which AF point was used to focus the lens. My guess says the AF point over the nearest object to the camera, which would be the tree on the left.

Re: Autofocus woes

davetong — Fri, 01 Mar 2019 20:35:40 GMT

It really is not in focus. Here's a closer look. The tree with the focus point is out of focus.
None of the supposed focus points are actually in focus.

The tree to the left of it, which is a good few meters closer to the camera, and the gravestone beside it are sharp.

You can download the full DNG file from the Dropbox link

https://www.dropbox.com/s/ius8xoh48ntrzv0/_MG_7079.DNG?dl=0

Re: Autofocus woes

davetong — Fri, 01 Mar 2019 20:34:18 GMT

Re: Autofocus woes

CaliforniaDream — Fri, 01 Mar 2019 22:20:26 GMT

The first pic you showed us did not have the tree on the left, did you crop it out? Did it have a focus point also. It appears to be the closest item in the pic, which the camera may have "correctly" focused on.

Re: Autofocus woes

Waddizzle — Sat, 02 Mar 2019 00:27:17 GMT

@davetong wrote:
Here's a better example:
https://www.dropbox.com/s/ius8xoh48ntrzv0/_MG_7079.DNG?dl=0

This was taken in Manual mode, using One-Shot focus. Same body and lens.
The leaves and the tree in the upper left and the gravestones on the right are sharp,
but the focus locks are supposed to be on the crypt in the distance.

I'm going to try cleaning the AF sensors

I suggest using just the center AF point when using One Shot mode. The center AF point is the most sensitive and accurate Af point in that camera. I also suggest looking at the AF points in Canon’s DPP software, just as a benchmark comparison. DPP can only show you the active AF points,

I will also suggest trying a different lens. I am not the world’s greatest fan of that lens. My copy does not seem to focus consistently at f/2.8 and wider.

Are you using any lens filters? If so, remove it. I have also noticed that CPL filters can seem to confuse phase detection AF systems. UV filters tend to be unnecessary because most digital sensors have UV built into the image sensor assembly.

Re: Autofocus woes

davetong — Sat, 02 Mar 2019 00:57:16 GMT

Here is the full, uncropped screenshot. And as I said before, the full, raw image is on Dropbox:

https://www.dropbox.com/s/ius8xoh48ntrzv0/_MG_7079.DNG?dl=0

I recently purchased this 85mm f1.8 lens as a portrait lens. Unfortunately I don't currently have the budget

for the high-end lenses, and the kit 18-50 and 55-250 lenses don't seem to show the problem, probably because they are so slow.

I have had similar issues with the 50mm f1.8 lens.

The frustrating thing is that while it's not consistent I do find that several of the photos in a particular set will show the problem.

Re: Autofocus woes

Waddizzle — Sat, 02 Mar 2019 01:06:16 GMT

“I suggest using just the center AF point when using One Shot mode. The center AF point is the most sensitive and accurate Af point in that camera. I also suggest looking at the AF points in Canon’s DPP software, just as a benchmark comparison. DPP can only show you the active AF points,

I will also suggest trying a different lens. I am not the world’s greatest fan of that lens. My copy does not seem to focus consistently at f/2.8 and wider.

——————————————————

Try the above, and get back to us.

Re: Autofocus woes

ebiggs1 — Sun, 03 Mar 2019 16:40:38 GMT

"The frustrating thing is that while it's not consistent I do find that several of the photos in a particular set will show the problem."

David,

I think you gear is working properly. I don't think you understand how it works, though. The camera will always try to put the closest thing in critical focus. The rest may exceed your DOF. If I read your EXIF correctly you shot at f2.5 using an 85mil.. Pretty small DOF at that aperture. Do this for me, turn off all the focus points except the center one. Use only the center point. Use One-shot. Put the camera in P mode. Take the very same photo if you can and focus precisely on what you want on sharp focus. I bet it will be fine.

DO NOT TRY TO CLEAN ANYTHING INSIDE THE MIRRORBOX OF YOUR CAMERA. Put nothing smaller than a football in there.

Tip: If you want more of the shot in focus try a smaller aperture like f11 or f16. Your SS was way too fast for a shot like this. It isn't necessary.

Re: Autofocus woes

davetong — Mon, 04 Mar 2019 02:59:32 GMT

Well the entire point of the test was to shoot at a wide apeture for narrow DOF, with a fast shutter to rule out camera shake, and to exercise the 19 point AF. It seems odd that the camera would focus on objects outside the range of the autofocus points and still report that those points were in focus, so clearly my understanding of how autofocus is supposed to work is different from Canon's.

From what you guys are saying the only autofocus mode that actually works reliably is one shot with the centre AF point, so I'll stick to that for now.

Re: Autofocus woes

Waddizzle — Mon, 04 Mar 2019 03:26:02 GMT

@davetong wrote:
Well the entire point of the test was to shoot at a wide apeture for narrow DOF, with a fast shutter to rule out camera shake, and to exercise the 19 point AF. It seems odd that the camera would focus on objects outside the range of the autofocus points and still report that those points were in focus, so clearly my understanding of how autofocus is supposed to work is different from Canon's.

From what you guys are saying the only autofocus mode that actually works reliably is one shot with the centre AF point, so I'll stick to that for now.

Your LR plug-in cannot determine which AF point was used to lock focus at the moment the shutter was fired. It is only able to display which AF points were capable of locking focus. Also, AF points are little larger than the squares you see in the viewfinder or the LR plug-in display, not a whole LOT bigger, but they do fill in the empty space between squares.

Sorry, but your understanding of DSLR AF systems is a little flawed. It is not a question of how you feel it should work. It is a matter of how it actually works. When you have all AF points active, what critieria should the AF system use to select an AF point for focusing? Many more times than not, the user is trying to focus on the nearest object to the camera.

At this point we cannot conclude that the camera focused on a tree at the left edge of the frame. THIS is why you are being asked to use only one AF point, so that a determination can be made which AF point locked focus. This is why you were asked to use the Canon DPP software to read the AF points.

A determination needs to be made as to whether or not your camera is focusing accurately. It could be experiencing what is known as “back focusing” or “front focusing”. We would like to help you and guide you in making this determination.

Additionally, you should only be using the viewfinder for focusing and composing your shots for these tests. Do not use the Live View method, which uses the rear LCD screen as a viewfinder. The LCD uses a separate focusing system, which is not as accurate as the AF system employed when you use the viewfinder.

Re: Autofocus woes

Waddizzle — Mon, 04 Mar 2019 03:37:44 GMT

Here is a layman’s explanation of AF systems.

https://en.m.wikipedia.org/wiki/Autofocus

The viewfinder uses the phase detection method. The LCD screen uses the contrast detection method, but with a major twist that is not important right now. Suffice to say, the two systems are very different because they are designed to serve two entirely different purposes.

Basically, the viewfinder AF system is FAST, and is designed for photos. The LCD AF system is slower, whose primary role is for focusing during video recording. The shutter speeds used by the two recording methods vary widely. Of course, it is not as simple as that, but it is not far from the mark.

Re: Autofocus woes

Waddizzle — Mon, 04 Mar 2019 03:57:37 GMT

Here is another link to the puzzle. This is what happens when light enters a camera lens.

https://en.m.wikipedia.org/wiki/Digital_single-lens_reflex_camera

The illustration at the link is lacking.

That illustrates an SLR film camera. A DSLR, digital SLR, camera includes a couple more pieces.

Item #2 is the reflex mirror. When you fire the shutter, it swings up and out of the way to allow light to reach #3 and #4, the shutter and the 35mm film. There are two critical components, which are not shown, an AF sensor and a metering sensor. Light entering the camera is actually split in two directions, straight up and straight down.

The illustrations shows the reflex mirror reflecting light upwards. Some of the light is also reflected straight down, more or less towards where the number 2 in a circle is located. The AF sensor is located at the bottom of mirror box, looking upwards at the reflected light from the reflex mirror.

The other missing component is the metering sensor, which measures the intensity of the light so that the camera can set an automatic exposure. The diagram shows light being reflected straight up, but not the metering sensor. It would be located at the top of the mirror box, on the surface between the #7 and the #8.

When the camera is in Live View or Movie mode, the mirror swings up, and stays there. Now, the image sensor, #4, is used for metering, focusing, and image capture. Because it serves all of these roles, metering and focusing is much slower compared to when the mirror is in its’ normal “down” position for photography.

Re: Autofocus woes

ebiggs1 — Mon, 04 Mar 2019 15:09:28 GMT

"... clearly my understanding of how autofocus is supposed to work is different from Canon's"

All the above info is nice to have but all you really need to know is, for the shot you displayed you used the wrong focusing pattern, too wide of aperture and unnecessarily fast SS. The camera will try to grab the closest object. When you have may points for it to select from this is what you get. In certain circumstances it is beneficial to use this. A bird in flight for instance. For a landscape type shot it isn't so great. Close the aperture down to f11 or f16 and select a SS around 1/100. Try again.

With the aperture around f11 or f16 the multi-AF point may work, OK, too. Remember the DOF from an 85mm lens at a wide aperture is narrow.