SAF Front Focus and CAF/SAF Focus Comparisons with the 300mm f4 – E-M1s.

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drj3 Forum Pro • Posts: 10,918
SAF Front Focus and CAF/SAF Focus Comparisons with the 300mm f4 – E-M1s.
7

The target was a photograph of a butterfly decal (Image #1) displayed on my computer monitor. The monitor allows very precise location of the focus point by viewing the clarity of the individual pixels.

The monitor was photographed at an angle (Image #2) so that actual focus location could be easily determined with a high degree of accuracy (the center of the DOF) and the focus errors computed based on the EXIF focus steps.

The camera settings were - Small single focus point, AF Scanner = Mode 3, C-AF Sensitivity = +2, with Aperture Priority.

I evaluated focus accuracy and variability using two different photographic techniques.

In the first condition I started with an unfocused image, half pressed the shutter button to focus, and then fully pressed the shutter button after focus confirmation (with a 2 second delay). This is labeled Normal Focus on the images and tables.

The second condition was to Pre-Focus on the image and then refocus before exposure. This is my typical photographic technique for stationary birds. It the same as Normal Focus, but I stop after the half press of the shutter button with focus confirmation. I release the shutter button and check the focus point position and initial focus. I then half press the shutter button again and with focus confirmation, I expose the image. The camera is Pre-focused on the target before I focus for exposure.

Image #3 shows the location of the focus box on the target.

Image #4 is a crop of the images with a focus distance equal to the median for the CAF – Normal Focus, SAF Pre-Focus, and SAF Normal Focus conditions. (The CAF –Pre-Focus median was essentially identical to the CAF-Normal Focus with only ≈ .012 inches difference in focus distance).

Image #5 is a crop of the center of image #4. You may need to click on Original at the bottom of the image to see the difference in sharpness with the DPR jpegs. A well focused image would have the pixels in the two vertical lines in the center of approximately equal in sharpness with the actual focus distance between the two lines (CAF – NF with FS = 8488). The two SAF images are both front focused. The line on the left is sharp on the SAF images and the line on the right is unfocused. The detail on the left line is sharper than the detail on the right line in these two images since the DOF is shifted to the left, closer to the camera.

The results shown in the table in Image #6 and are typical. The Median CAF focus for both Normal and Pre-Focus is always very close to the best focus for all evaluations I have conducted. The Median SAF focus is almost always front focused, though the amount of front focus will vary with the particular target, the focus point and the amount of illumination.

The Focus variability (SD) with Pre-Focus CAF is typically considerably less than the other three conditions, which are often similar in size. Simply maintaining the half press of the shutter for a longer period of time after focus confirmation in the CAF Normal condition, giving the camera more time to determine correct focus, might result in focus variation similar to that of the Pre-focus CAF condition. However, I have not tested that possibility. This would probably require the AF scanner to be set to mode 3 and C-AF Sensitivity to +2 to keep the camera continually checking focus.

Image #7 is one of the images from a similar test with the MC20+300mm where I photographed one of the decals on the window with both high and low illumination. The angle was not as great so you see less difference in sharpness at the focus point, but can more easily see the front focus shift producing greater sharpness on the closer (left) side of the image.

The table in Image #8 summarizes the results of the MC20+300mm test. The low illumination/contrast condition, summarized in the lower half of the table, often required multiple half presses of the shutter button to achieve focus confirmation with CAF when the image was unfocused. CAF was still more accurate even with this focus difficulty. If this was a real target, I would have switched to the larger single focus point for easier focus, but that would probably result in more focus variability.

Image #1

Image #2

Image # 3

Image #4

Image #5

Image #6

Image #7

Image #8

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Chris DC Contributing Member • Posts: 502
Re: SAF Front Focus and CAF/SAF Focus Comparisons with the 300mm f4 – E-M1s.
1

Doc:

This is really interesting work. Well done. I will try CAF  against SAF in the field and see if it yields better critical focus.

I tend to weave and bob a little after my half press and this can lead to out of focus results. Your approach should help.

Thanks!

CDC

Lotus Cat Forum Pro • Posts: 10,525
Which E-M1?

Please forgive me if the answer is obvious in your excellent post: Which E-M1 did you use?

Jim Pilcher
Bonita Springs, Florida, USA
Life is good on the Gulf

Gary from Seattle Veteran Member • Posts: 5,405
Re: SAF Front Focus and CAF/SAF Focus Comparisons with the 300mm f4 – E-M1s.

I have noticed that with 840mm on the EM-1X that with distant birds there is a tendency to front focus; though not uniformly so. This on water birds, so with texture in the water it is fairly easy to see where exactly the camera focused. The error is large enough to render some images useless. However, I also need to factor in the fact that I am probably moving the camera some in a burst sequence. Also, I may at times be shooting when the green box is no longer green - on focus.

When the eye of a bird is large enough for AI to lock in on the eye, the camera tends to do much better.

With the numbers you have here and a telephoto lens, the indication is that a slightly greater aperture is likely to keep the focus within DOF. Also the DOF may well be such that it is very narrow anyways, and for instance it makes a difference whether or not the bird turns it's head.

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OP drj3 Forum Pro • Posts: 10,918
Re: Which E-M1?

James Pilcher wrote:

Please forgive me if the answer is obvious in your excellent post: Which E-M1 did you use?

Jim Pilcher
Bonita Springs, Florida, USA
Life is good on the Gulf

I used the E-M1.3 for this test, however, tests with the E-M1.2 have given similar results.

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Interceptor121 Senior Member • Posts: 2,831
Re: SAF Front Focus and CAF/SAF Focus Comparisons with the 300mm f4 – E-M1s.

Something is going on with the programming of the functionality here. I doubt it is a hardware problem

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OP drj3 Forum Pro • Posts: 10,918
Re: SAF Front Focus and CAF/SAF Focus Comparisons with the 300mm f4 – E-M1s.

Gary from Seattle wrote:

I have noticed that with 840mm on the EM-1X that with distant birds there is a tendency to front focus; though not uniformly so. This on water birds, so with texture in the water it is fairly easy to see where exactly the camera focused. The error is large enough to render some images useless. However, I also need to factor in the fact that I am probably moving the camera some in a burst sequence. Also, I may at times be shooting when the green box is no longer green - on focus.

With CAF, the cameras first focus and then confirm focus. The delay is significant only when the target is moving.

With stationary targets I wait for the focus confirmation, with moving targets, I ignore the green box.

It is important to wait on the focus confirmation if you have AF Focus Adjusted the lens. The correction will not be applied until focus is confirmed.

At least 80% of my flying swallow images will not have focus confirmation, even with a fairly long sequence of images which are in focus.

When the eye of a bird is large enough for AI to lock in on the eye, the camera tends to do much better.

With the numbers you have here and a telephoto lens, the indication is that a slightly greater aperture is likely to keep the focus within DOF. Also the DOF may well be such that it is very narrow anyways, and for instance it makes a difference whether or not the bird turns it's head.

The front focus will generally be noticed by the area in focus, more than the sharpness at the focus point.  It is important when you are trying to get the best feather detail for closer targets or want to balance focus on your target.  With more distant targets with a greater DOF and less detail, it would be far less important.

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OP drj3 Forum Pro • Posts: 10,918
Re: SAF Front Focus and CAF/SAF Focus Comparisons with the 300mm f4 – E-M1s.

Interceptor121 wrote:

Something is going on with the programming of the functionality here. I doubt it is a hardware problem

I am convinced it is the CDAF algorithm.  It has a slight bias for the closer area within the focus points.  As long as the user is aware of this issue, it is easy to avoid the problem.

It would rarely be a problem when one uses a single or limited focus area.  I almost always use SAF and a single focus point with my 12-100mm f4 lens unless I am photographing a moving target or a small target at minimum focus distance.

It can be a problem when you have a very shallow DOF with very long telephoto lenses when photographing close targets or a very fast lens of any focal length photographing close targets.  It is typically a difference in focus around the chosen focus point since the focus point would generally be within the DOF.

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Gary from Seattle Veteran Member • Posts: 5,405
Re: SAF Front Focus and CAF/SAF Focus Comparisons with the 300mm f4 – E-M1s.

drj3 wrote:

Interceptor121 wrote:

Something is going on with the programming of the functionality here. I doubt it is a hardware problem

I am convinced it is the CDAF algorithm. It has a slight bias for the closer area within the focus points. As long as the user is aware of this issue, it is easy to avoid the problem.

It would rarely be a problem when one uses a single or limited focus area. I almost always use SAF and a single focus point with my 12-100mm f4 lens unless I am photographing a moving target or a small target at minimum focus distance.

It can be a problem when you have a very shallow DOF with very long telephoto lenses when photographing close targets or a very fast lens of any focal length photographing close targets. It is typically a difference in focus around the chosen focus point since the focus point would generally be within the DOF.

I have thought about this with S-AF and CAF. I wonder though if the chosen array/single target pattern makes a difference with CAF+Tr? I am currently using a five target cross on CAF+TR. Would I do better if I used a single small target? I haven't been doing that as I am not sure that I can consistently be that accurate in targeting; or would it make no difference in CAF+Tr because the camera essentially takes over in matching bird shapes and eyes in AI?

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Stizzu68 Regular Member • Posts: 278
Re: SAF Front Focus and CAF/SAF Focus Comparisons with the 300mm f4 – E-M1s.

Interesting comparison. Can be the differences related to the different size of the phase-detection VS contrast detections focus points?

I never did tests but I with the OMD 1MKii I just realized that the CAF it is first faster to lock onto the subject and it is then very precise, sometimes more than the SAF.

As a result, I now use nearly always the CAF even with static subjects and the results are good.

Stefano

OP drj3 Forum Pro • Posts: 10,918
Re: SAF Front Focus and CAF/SAF Focus Comparisons with the 300mm f4 – E-M1s.

Gary from Seattle wrote:

drj3 wrote:

Interceptor121 wrote:

Something is going on with the programming of the functionality here. I doubt it is a hardware problem

I am convinced it is the CDAF algorithm. It has a slight bias for the closer area within the focus points. As long as the user is aware of this issue, it is easy to avoid the problem.

It would rarely be a problem when one uses a single or limited focus area. I almost always use SAF and a single focus point with my 12-100mm f4 lens unless I am photographing a moving target or a small target at minimum focus distance.

It can be a problem when you have a very shallow DOF with very long telephoto lenses when photographing close targets or a very fast lens of any focal length photographing close targets. It is typically a difference in focus around the chosen focus point since the focus point would generally be within the DOF.

I have thought about this with S-AF and CAF. I wonder though if the chosen array/single target pattern makes a difference with CAF+Tr? I am currently using a five target cross on CAF+TR. Would I do better if I used a single small target? I haven't been doing that as I am not sure that I can consistently be that accurate in targeting; or would it make no difference in CAF+Tr because the camera essentially takes over in matching bird shapes and eyes in AI?

With CAF the camera would typically be focusing on the most distinct feature in the target area with no real focus bias.  I doubt there would be much difference in the five cross and a single point with CAF+TR, since the camera will be trying to focus on specific target characteristics.  Once focused, the shallow DOF will generally result in your target being more distinct than background features.

Where it would make a difference is for situations where the background is essentially at the same distance as the target and just as detailed.

For example, I always use CAF with a single point for photographing hummingbirds and hummingbird moths eating from flowers with the flower petals, leaves and stems all very close to the bird/moth where the focus can quickly jump to the background.  One tenth of an inch is the difference between feather detail and a deleted image.

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Stizzu68 Regular Member • Posts: 278
Re: SAF Front Focus and CAF/SAF Focus Comparisons with the 300mm f4 – E-M1s.

drj3 wrote:

Gary from Seattle wrote:

drj3 wrote:

Interceptor121 wrote:

Something is going on with the programming of the functionality here. I doubt it is a hardware problem

I am convinced it is the CDAF algorithm. It has a slight bias for the closer area within the focus points. As long as the user is aware of this issue, it is easy to avoid the problem.

It would rarely be a problem when one uses a single or limited focus area. I almost always use SAF and a single focus point with my 12-100mm f4 lens unless I am photographing a moving target or a small target at minimum focus distance.

It can be a problem when you have a very shallow DOF with very long telephoto lenses when photographing close targets or a very fast lens of any focal length photographing close targets. It is typically a difference in focus around the chosen focus point since the focus point would generally be within the DOF.

I have thought about this with S-AF and CAF. I wonder though if the chosen array/single target pattern makes a difference with CAF+Tr? I am currently using a five target cross on CAF+TR. Would I do better if I used a single small target? I haven't been doing that as I am not sure that I can consistently be that accurate in targeting; or would it make no difference in CAF+Tr because the camera essentially takes over in matching bird shapes and eyes in AI?

With CAF the camera would typically be focusing on the most distinct feature in the target area with no real focus bias. I doubt there would be much difference in the five cross and a single point with CAF+TR, since the camera will be trying to focus on specific target characteristics. Once focused, the shallow DOF will generally result in your target being more distinct than background features.

Where it would make a difference is for situations where the background is essentially at the same distance as the target and just as detailed.

For example, I always use CAF with a single point for photographing hummingbirds and hummingbird moths eating from flowers with the flower petals, leaves and stems all very close to the bird/moth where the focus can quickly jump to the background. One tenth of an inch is the difference between feather detail and a deleted image.

I also like to have the focus really nailed. With the 300F4 and the OMD1Mkii I also often use the single small point if the subject is quite  enough, but I get better results with the CAF than with the SAF. Never managed instead to get anything useful from the CAF + TR with the 1MKII. Is it the Mkiii better?

OP drj3 Forum Pro • Posts: 10,918
Re: SAF Front Focus and CAF/SAF Focus Comparisons with the 300mm f4 – E-M1s.

Stizzu68 wrote:

drj3 wrote:

Gary from Seattle wrote:

drj3 wrote:

Interceptor121 wrote:

Something is going on with the programming of the functionality here. I doubt it is a hardware problem

I am convinced it is the CDAF algorithm. It has a slight bias for the closer area within the focus points. As long as the user is aware of this issue, it is easy to avoid the problem.

It would rarely be a problem when one uses a single or limited focus area. I almost always use SAF and a single focus point with my 12-100mm f4 lens unless I am photographing a moving target or a small target at minimum focus distance.

It can be a problem when you have a very shallow DOF with very long telephoto lenses when photographing close targets or a very fast lens of any focal length photographing close targets. It is typically a difference in focus around the chosen focus point since the focus point would generally be within the DOF.

I have thought about this with S-AF and CAF. I wonder though if the chosen array/single target pattern makes a difference with CAF+Tr? I am currently using a five target cross on CAF+TR. Would I do better if I used a single small target? I haven't been doing that as I am not sure that I can consistently be that accurate in targeting; or would it make no difference in CAF+Tr because the camera essentially takes over in matching bird shapes and eyes in AI?

With CAF the camera would typically be focusing on the most distinct feature in the target area with no real focus bias. I doubt there would be much difference in the five cross and a single point with CAF+TR, since the camera will be trying to focus on specific target characteristics. Once focused, the shallow DOF will generally result in your target being more distinct than background features.

Where it would make a difference is for situations where the background is essentially at the same distance as the target and just as detailed.

For example, I always use CAF with a single point for photographing hummingbirds and hummingbird moths eating from flowers with the flower petals, leaves and stems all very close to the bird/moth where the focus can quickly jump to the background. One tenth of an inch is the difference between feather detail and a deleted image.

I also like to have the focus really nailed. With the 300F4 and the OMD1Mkii I also often use the single small point if the subject is quite enough, but I get better results with the CAF than with the SAF. Never managed instead to get anything useful from the CAF + TR with the 1MKII. Is it the Mkiii better?

I have not really checked the E-M1.3 with CAF+TR for any moving target.  It never worked well for me with the E-M1.2.  Recent posts appear to indicate it is better with the E-M1X.

The only time I use CAF+TR is for a larger stationary target where I want to focus and recompose when I might also want to change distance.  It works reasonably well, if I focus the single center point on my target and then half hold the shutter button while I recompose and move my position.

I did a little example previously with my FTs 12-60 and my cat, where I focused on the cat's eye and then moved and recomposed the image using CAF+TR.

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Stizzu68 Regular Member • Posts: 278
Re: SAF Front Focus and CAF/SAF Focus Comparisons with the 300mm f4 – E-M1s.

drj3 wrote:

Stizzu68 wrote:

drj3 wrote:

Gary from Seattle wrote:

drj3 wrote:

Interceptor121 wrote:

Something is going on with the programming of the functionality here. I doubt it is a hardware problem

I am convinced it is the CDAF algorithm. It has a slight bias for the closer area within the focus points. As long as the user is aware of this issue, it is easy to avoid the problem.

It would rarely be a problem when one uses a single or limited focus area. I almost always use SAF and a single focus point with my 12-100mm f4 lens unless I am photographing a moving target or a small target at minimum focus distance.

It can be a problem when you have a very shallow DOF with very long telephoto lenses when photographing close targets or a very fast lens of any focal length photographing close targets. It is typically a difference in focus around the chosen focus point since the focus point would generally be within the DOF.

I have thought about this with S-AF and CAF. I wonder though if the chosen array/single target pattern makes a difference with CAF+Tr? I am currently using a five target cross on CAF+TR. Would I do better if I used a single small target? I haven't been doing that as I am not sure that I can consistently be that accurate in targeting; or would it make no difference in CAF+Tr because the camera essentially takes over in matching bird shapes and eyes in AI?

With CAF the camera would typically be focusing on the most distinct feature in the target area with no real focus bias. I doubt there would be much difference in the five cross and a single point with CAF+TR, since the camera will be trying to focus on specific target characteristics. Once focused, the shallow DOF will generally result in your target being more distinct than background features.

Where it would make a difference is for situations where the background is essentially at the same distance as the target and just as detailed.

For example, I always use CAF with a single point for photographing hummingbirds and hummingbird moths eating from flowers with the flower petals, leaves and stems all very close to the bird/moth where the focus can quickly jump to the background. One tenth of an inch is the difference between feather detail and a deleted image.

I also like to have the focus really nailed. With the 300F4 and the OMD1Mkii I also often use the single small point if the subject is quite enough, but I get better results with the CAF than with the SAF. Never managed instead to get anything useful from the CAF + TR with the 1MKII. Is it the Mkiii better?

I have not really checked the E-M1.3 with CAF+TR for any moving target. It never worked well for me with the E-M1.2. Recent posts appear to indicate it is better with the E-M1X.

The only time I use CAF+TR is for a larger stationary target where I want to focus and recompose when I might also want to change distance. It works reasonably well, if I focus the single center point on my target and then half hold the shutter button while I recompose and move my position.

I did a little example previously with my FTs 12-60 and my cat, where I focused on the cat's eye and then moved and recomposed the image using CAF+TR.

Never worked also for me.

But I am really impressed by the 1x with the bird autofocus.

It works really well. It is not perfect but it makes  much easier to focus on composition but need to get used to it.

I did some tests with small birds on branches and I can now get many perfectly focused shots with all autofocus points active. And it tracks perfectly also other animals including my crazy dog that is a very challanging target.

Before the new firmware the AF was my main complain when I compared my camera with the Nikon D500 +500 F5.6 of my father. Now I think th1x is better and easier to use

Interceptor121 Senior Member • Posts: 2,831
Re: SAF Front Focus and CAF/SAF Focus Comparisons with the 300mm f4 – E-M1s.

However the benefit you mentioned is subject detection that doesn’t use any af features it just informs AF of which focus area to use

this is more important to the average shooter that is going to significantly increase the number of shits taken

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Stizzu68 Regular Member • Posts: 278
Re: SAF Front Focus and CAF/SAF Focus Comparisons with the 300mm f4 – E-M1s.
1

Interceptor121 wrote:

However the benefit you mentioned is subject detection that doesn’t use any af features it just informs AF of which focus area to use

Yes, that is true. But in the end, the result is what matters. Personally, I do not care about the technical aspect. I only need to understand how it works to make the best use of it.

What you write is correct when you use bird-AI it clearly first try to detect the bird even before you start the AF. Once you start to focus the AF "know where to focus. and this is working quite well in many situations and that is what matters to me.

this is more important to the average shooter that is going to significantly increase the number of shits taken

Not sure what you mean by this but in my experience, AF is what most matters to wildlife photographers even the most experienced one, more than noise or megapixel. I think "experienced" photographers care more about how good AF than others. And of course that you must be able to use it the proper way. That is why the big F2.8 - F4 lenses are so popular despite the price and why many wildlife photographers still use DSRL or start now that especially Sony and Canon have excellent autofocus, to switch to mirrorless.

For me the more 'experienced' I become, the more I care because I spend more time photographing and want to have results I am happy with. When I go out photographing I hope to come back with some good pictures, not to come back and tell how good I was at getting a good shot despite the second Liga AF. And this was the only aspect that made me consider several times to use another system since I know that with a Nikon, that I know less about than the Olympus, I have less difficulty to nail focus.
Now it seems the opposite so I am very happy.

To go back to the topic that is why I read often this forum because within tons of nonsense there are very useful posts made by people that take time to "explore" their camera features and share they finding like the OP.

Stefano

Interceptor121 Senior Member • Posts: 2,831
Re: SAF Front Focus and CAF/SAF Focus Comparisons with the 300mm f4 – E-M1s.

What I means is that subject detection makes more a difference to someone who is struggling to keep a bird in the centre of the frame than how quick AF works on a single dot

Computers are faster than human in most cases subject detection has been lagging but now is catching up as processing power goes up

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Stizzu68 Regular Member • Posts: 278
Re: SAF Front Focus and CAF/SAF Focus Comparisons with the 300mm f4 – E-M1s.
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Interceptor121 wrote:

What I means is that subject detection makes more a difference to someone who is struggling to keep a bird in the centre of the frame than how quick AF works on a single dot

That is clear now.

But personally, I do not want to keep the subject in the center of the frame. I try to compose as well as possible when shooting and with wildlife can be challanging.

Keeping the subject in the center is much easier but the cropping capabilities of m43 are not enough for me to photograph that way.

Pictures I keep must have enough pixel for a good print.  A good AF or good skills in manual focusing are then necessary to get a good shot.

Stefano

Interceptor121 Senior Member • Posts: 2,831
Re: SAF Front Focus and CAF/SAF Focus Comparisons with the 300mm f4 – E-M1s.

I am totally with you. The intelligent subject detection also means you are quicker to snap a picture if your subject is moving instead of having a matrix in the centre. And most times you do not want the subject in the centre

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Doxa750 Contributing Member • Posts: 599
Re: SAF Front Focus and CAF/SAF Focus Comparisons with the 300mm f4 – E-M1s.
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This is an awesome post Drj3.  Much appreciated for sharing your works.

Cheers

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