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FORUMS Photography Talk by Genre General Photography Talk 
Thread started 04 Jan 2019 (Friday) 03:38
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Focus Points

 
digital ­ paradise
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Jan 06, 2019 13:02 |  #16

With Canon cameras ETTL 2 flash is not focus point based. ETTL was focus point based but Canon changed that in version 2 so you could focus and re-compose and flash exposure would not be effected. In ETTL 2 Flash exposure is based on the reflected light from the pre-flash. I used to have an excellent Canon Master Craft video about ETTL 2 that explained it but like many other articles, blogs, etc that link is also broken by Canon.

You can lock flash exposure but it only meters within the spot metering circle. I found it extremely annoying at events because the flash would fire and I had to tell people that wasn't the actual shot so I just stopped using it.


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Post edited 1 month ago by Pictoraider. (4 edits in all)
     
Jan 06, 2019 15:05 |  #17

digital paradise wrote in post #18785863 (external link)
With Canon cameras ETTL 2 flash is not focus point based. ETTL was focus point based but Canon changed that in version 2 so you could focus and re-compose and flash exposure would not be effected. In ETTL 2 Flash exposure is based on the reflected light from the pre-flash. I used to have an excellent Canon Master Craft video about e-TTL 2 that explained it but like many other articles, blogs, etc that link is also broken by Canon.

You can lock flash exposure but it only meters within the spot metering circle. I found it extremely annoying at events because the flash would fire and I had to tell people that wasn't the actual shot so I just stopped using it.

The pre-flash is emitted after pressing the shutter release to the end : at this very moment, the active focus point is not on the main subject in case of composition changing. And problem occurs.

Below are pictures with the built-in flash of the EOS 300D which built-in flash works exclusively in evaluative e-TTL II mode with no permitted flash correction by user in one-shot AF mode.
The ambient light metering has been made in M mode and 1/200e s shutter speed has been used in order to eliminate all influence of existing ambient light at f5 & 100 ISO.
The armchair and the two pillows are at same distance of the camera, and so far at same distance of the built-in flash.
The 40-70mm EF is capable to transmit working distance to the camera inner computer.

The flash exposure in e-TTL II is not the same...as it should be according to theory.
It varies a lot : and thus in relation with the position of selected focus point at the very moment of taking picture by pressing the shutter release to the end,
if not memorized by pressing the appropriate button * on the back of the camera with the thumb.

Regards,

Canon evaluative e-TTL II flash exposure and focus point (with and without memorization of flash metering) displayed in DPP sofware, levels untouched :


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Post edited 1 month ago by CyberDyneSystems. (2 edits in all)
     
Jan 06, 2019 15:25 |  #18

Important to me.

- More points helps with subject tracking.

- Location of those points is important to me. I only tend to "select" about 5 or 6 of the many different points as my active starting point, these want to be towards the outer edges. The top most center point, and the right most center point (or top when the camera is turned "portrait" orientation) are my go to for keeping the AF point on the subjects eye. For shooting a moving subject, full bodies, focus recompose would not be the best solution by a significant margin. My keeper rate would plummet.

All of this importance however is subject based, but even static can use off center AF points to make things easier. Focus recompose is not helpful when you are locked down on a tripod.


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Jan 06, 2019 16:02 |  #19

I use the centre point AF almost exclusively. For a stationary/perched subject in AI Servo I use back button focus to recompose with excellent results.

Admittedly I have rarely experimented with the Auto-tracking features of my body. What I did test, I did not like the results. I don't think I have yet read a review that favours the roaming AF points for tracking, they all seem to say it is technology needing more work - so I use the centre or sometimes the expanded centre focus points.

I have not read 1Dx 2 reviews for its tracking system, perhaps it works well.

I know members here who say they use tracking (I presume the type where AF points attempt to follow the subject around the viewfinder) and they like it.

I can experiment more with tracking since it is winter and I only have Canada Geese to shoot. I did set an option for the "Center Zone" the other day...


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Post edited 1 month ago by Wilt.
     
Jan 06, 2019 16:53 as a reply to  @ Pictoraider's post |  #20

Read this by Canon Europe...

https://cpn.canon-europe.com …ion/technical/E​-TTL_II.do (external link)

Your Canon 300D is one of the very first digital cameras by Canon and it has ETTL, but not ETTL-II


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Jan 07, 2019 12:36 |  #21

I did a search and found this.

To counter this issue E-TTL II takes two precautionary measures. The first is to make a careful comparison across 17 metering zones. Using readings from these zones in both the ambient light and the light reflected from the pre-flash, the camera is able to understand which area of the image is likely to contain the subject, independently of the AF point in use. Overlaying the relative luminance levels of all 17 zones in ambient and pre-flash exposures the camera determines the area least affected is most likely to be the subject.

https://cpn.canon-europe.com …ion/technical/E​-TTL_II.do (external link)

Canon also introduced distance information in the newer lenses which assists the exposure system and basically uses the good old GN calculation. Distance info is disabled if you tilt or swivel the head. I actually called Canon about this may years ago and the told be the pre-flash was the primary in determining flash exposure while the distance info was a second reference.


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Jan 07, 2019 12:50 |  #22

digital paradise wrote in post #18786570 (external link)
I did a search and found this.

To counter this issue E-TTL II takes two precautionary measures. The first is to make a careful comparison across 17 metering zones. Using readings from these zones in both the ambient light and the light reflected from the pre-flash, the camera is able to understand which area of the image is likely to contain the subject, independently of the AF point in use. Overlaying the relative luminance levels of all 17 zones in ambient and pre-flash exposures the camera determines the area least affected is most likely to be the subject.

https://cpn.canon-europe.com …ion/technical/E​-TTL_II.do (external link)

Canon also introduced distance information in the newer lenses which assists the exposure system and basically uses the good old GN calculation. Distance info is disabled if you tilt or swivel the head. I actually called Canon about this may years ago and the told be the pre-flash was the primary in determining flash exposure while the distance info was a second reference.

I am aware of these informations.
In Canon notice, e-TTL II assumes the area in the frame that shows most significant difference before and after pre-flash is likely the main subject because the user is likely supposed to position himself face to the main subject with nothing between them two.

My problem is as follow with the EOS 20D which is supposed to have e-TTL II function :
hope you can help for these issues...


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digital ­ paradise
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Jan 07, 2019 12:54 |  #23

Pictoraider wrote in post #18785918 (external link)
The pre-flash is emitted after pressing the shutter release to the end : at this very moment, the active focus point is not on the main subject in case of composition changing. And problem occurs.

Below are pictures with the built-in flash of the EOS 300D which built-in flash works exclusively in evaluative e-TTL II mode with no permitted flash correction by user in one-shot AF mode.
The ambient light metering has been made in M mode and 1/200e s shutter speed has been used in order to eliminate all influence of existing ambient light at f5 & 100 ISO.
The armchair and the two pillows are at same distance of the camera, and so far at same distance of the built-in flash.
The 40-70mm EF is capable to transmit working distance to the camera inner computer.

The flash exposure in e-TTL II is not the same...as it should be according to theory.
It varies a lot : and thus in relation with the position of selected focus point at the very moment of taking picture by pressing the shutter release to the end,
if not memorized by pressing the appropriate button * on the back of the camera with the thumb.

Regards,

Canon evaluative e-TTL II flash exposure and focus point (with and without memorization of flash metering) displayed in DPP sofware, levels untouched :
thumbnail
Hosted photo: posted by Pictoraider in
./showthread.php?p=187​85918&i=i83834340
forum: General Photography Talk

That is ETTL for you. As pointed out you are working with ETTL version 1. I don't know that much about version 1 but as you can see appears it makes a difference which pillow it is on.


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Post edited 1 month ago by Wilt. (5 edits in all)
     
Jan 07, 2019 12:58 |  #24

digital paradise wrote in post #18786570 (external link)
Canon also introduced distance information in the newer lenses which assists the exposure system and basically uses the good old GN calculation. Distance info is disabled if you tilt or swivel the head. I actually called Canon about this may years ago and the told be the pre-flash was the primary in determining flash exposure while the distance info was a second reference.

Unfortunately, the distance info provided by the camera seems pathetically 'coarse' in its precision, at least in terms of the testing I had done a number of years ago, with the EXIF information vs. actual (tape measure) increments.

https://photography-on-the.net …showthread.php?​p=18342828
https://photography-on-the.net …showthread.php?​p=16743446

Insufficient for flash calculations to be useful.

My understanding of 'distance' information used by ETTL-II, is that merely the reflectiveness of 'same distance' zones are compared with the primary focus point, and zones which are exceptionally reflective are ignored for ETTL flash computations (mirrors/shiny metal reflections of the pre-flash))

The failure of 'guide number computations' as seen in aTTL (precursor to eTTL) might merely be the hobbled reflection of the coarseness of focus distance reported by the lens!


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Jan 07, 2019 13:41 |  #25

That is probably why distance info is just a verification. Besides it does nothing unless the flash head is pointed forward. I have a feeling it was an afterthought and really meant to assist built in flashes.


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Jan 07, 2019 15:08 |  #26

Here is a series shot with 7DII set in M at ISO100 1/125 f/4 with flash metering in Evaluative mode. In the scene is a sofa which is same brightness as an 18% gray card, with ambient light intensity which measures ISO100 1/10 f/4. The shots were taken with ETTL-II flash (pointed straight ahead, not bounced). Shot 1 has focus point centered, Shot 2 has focus point on white pillow (which is +2.2EV brighter than sofa), and Shot 3 has focus point on the large square darker pillow to the right which is -0.5EV darker than sofa.

IMAGE: http://i69.photobucket.com/albums/i63/wiltonw/Principles/ETTL2_zpsl7vtccix.jpg

As one can see, there is not a significant shift of brightness based upon the object at the point of focus. Shot 2 is a tiny bit darker than Shots 1 & 3, but even in Evaluative flash metering there is not a major bias caused by the white pillow at the point of focus.

Then I changed flash metering from Evaluative to Average and shot the series again, in the same sequence of focus point position

IMAGE: http://i69.photobucket.com/albums/i63/wiltonw/Principles/ETTL-IIAvg_zps7gs4d4kp.jpg

Not a significant observable difference in overall behavior from the first series, apart from the fact that shot 2 seems in this case to be less biased by the white pillow AF point than in the first series.

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Jan 07, 2019 15:23 |  #27

That is a well balanced scene. Also that is where ETTL 2 has improved not being focus point orientated.


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Jan 07, 2019 15:42 |  #28

Here I insert various brightness objects into the foreground, put the AF point at that position and shoot with flash metering set to Average

IMAGE: http://i69.photobucket.com/albums/i63/wiltonw/Principles/ETTL-IIAvg%20graycardfocus_zps6jvrnnrk.jpg

Lastly, same sequence but flash metering set to Evaluative

IMAGE: http://i69.photobucket.com/albums/i63/wiltonw/Principles/Eval%20focusgraycard_zpsebrhbyf4.jpg

Again we see that the brightness of the object at the point of focus biases more strongly the exposure than when the metering is set to Average.

So, between the two different scenarios tested in my two recent posts, this seems to corroborate what was published on POTN nearly a decade ago...
Set the flash metering on Eval when shooting outdoors with flash used as fill, but set the flash metering on Average when shooting with flash as primary source of illumination.

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Jan 09, 2019 11:43 |  #29

Wilt wrote in post #18786584 (external link)
Insufficient for flash calculations to be useful.

My understanding of 'distance' information used by ETTL-II, is that merely the reflectiveness of 'same distance' zones are compared with the primary focus point, and zones which are exceptionally reflective are ignored for ETTL flash computations (mirrors/shiny metal reflections of the pre-flash))

Thanks for your e-TTL II experience sharing. The main goal of e-TTL II is about preventing blowing out of potential white dresses, whites tones and highlights. It's not about providing correctly exposed pictures nor an easy-to-use consistant automation for flash photography beginners.

I now fully understand that e-TTL II flash exposure is not based on focus point. My e-TTL II experience provided erratics errors which were the impredictable combination of versatile ambient light exposures due to evaluative method ponderated by focus point and of erratics flash exposure due to elements of diferent brightness positioned on the foreground of.

Specular flash reflects in mirror or in hight reflective objets depend mainly of flash angulation : so far underexposing these possible hotspots is not a solution. Flash angulation management is.
White dresses, elements of white tones positioned on the foreground between main subject and camera are overexposed due to inverse square law : this is solved by the management of the distance flash to subject and/or the use of off-camera flash which is placed over and/or after all these foreground elements. And should not be solved by underexposing main subject.

Face to average scenes to be photographed, the solution with e-TTL as the e-TTL of my old EOS 300D is to use all the bunch of focus point : doing so, you have good approximate ambient exposure via evaluative metering and accurate flash exposure via taking in count working focus plan. So far, you get predictable consistant flash photos. If white tones elements are to come in the frame, it is up to flash photographer to wait and postpone his photos taking. Imho,

Below is an exemple of numerous recomposed framings taken with EOS 300D with a ambient-flash ratio of 1:1 face to an average scene using out-to-date e-TTL : the e-TTL doesn't benefit of working distance but does be aware everything in working focusing plan is worthy,...


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Jan 09, 2019 12:51 |  #30

Use the histogram. Shooting events I'd use white purses, shirts, sweaters and even table cloths if available. You may want to use RGB mode to be more precise because the reds will clip more and won't show up in the normal mode.

https://neilvn.com …am-to-determine-exposure/ (external link)


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