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08-19-2017, 05:59 PM   #1
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Focusing Points

Why are all the focusing points on DSLR's clustered around the center area of the view finder ? I I mean even if you have 99+ focusing points but they are all clustered in the middle of the view frame what is the point ? Users have been screaming pretty LOUD about this, but manufactures refuse to budge. What is it too expensive, is it technically impossible, is it scientifically not worth it, what is the deal ? Meanwhile Mirror-less cameras have managed to spread their focusing points to the very edges of the view finder. Is there some reason for this ?


Last edited by hjoseph7; 08-20-2017 at 06:55 AM.
08-19-2017, 08:30 PM   #2
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The AF sensor on your DSLR relies on light passing through a transparent part the mirror and being reflected down by another mirror onto the AF sensor. if you had AF sensors covering the entire frame the whole of the main mirror would need to be semi-transparent and the secondary mirror and AF sensor would need to be much bigger. This would increase the size of SLR's significantly.

In addition, AF is optimized to a certain amount of light. If you try to utilise edge of frame AF points together with centre of frame points, you may find not enough light is being captured from the edge of frame points.. Equally, a lens which has weaker edge resolution would also present a problem for the edge AF sensor points.

Mirrorless cameras use a different form of AF... Contrast detection versus Phase Detection for viewfinder AF. Contrast detection relies on the image formed directly on the sensor and can therefore use any point of the frame. You can simulate this if your SLR has the Live View function which also uses Contrast detection AF.

Last edited by pschlute; 08-19-2017 at 08:46 PM.
08-19-2017, 09:44 PM   #3
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"The AF sensor on your DSLR relies on light passing through a transparent part the mirror and being reflected down by another mirror onto the AF sensor

Read more at: Focusing Points - PentaxForums.com

With all due respect where did you get this information from ? As far as I know a DSLR uses the same mirror as an SLR film camera unless its a Sony
08-19-2017, 11:09 PM   #4
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QuoteOriginally posted by hjoseph7 Quote
With all due respect where did you get this information from ? As far as I know a DSLR uses the same mirror as an SLR film camera unless its a Sony
Not quite. The AF unit is in the bottom of the camera and uses a semi-transparent mirror.





See too: https://www.bhphotovideo.com/explora/photography/tips-and-solutions/how-focus-works


Last edited by MarkJerling; 08-20-2017 at 02:39 AM.
08-20-2017, 02:46 AM   #5
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QuoteOriginally posted by hjoseph7 Quote
With all due respect where did you get this information from ? As far as I know a DSLR uses the same mirror as an SLR film camera unless its a Sony
FIlm SLRs with autofocus used exactly similar mirror arrangement as DSLRs. See: Autofocus: phase detection
08-20-2017, 10:27 AM   #6
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QuoteOriginally posted by pschlute Quote
if you had AF sensors covering the entire frame the whole of the main mirror would need to be semi-transparent and the secondary mirror and AF sensor would need to be much bigger.
The whole mirror is already semi-transparent. The center clustering is purely a result of appropriate allocation of a limited resource.

QuoteOriginally posted by pschlute Quote
Mirrorless cameras use a different form of AF... Contrast detection versus Phase Detection for viewfinder AF. Contrast detection relies on the image formed directly on the sensor and can therefore use any point of the frame.
This is true with the exception of mirrorless cameras using recent design image sensors that incorporate PDAF built into the sensor itself.


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08-20-2017, 10:36 AM   #7
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QuoteOriginally posted by hjoseph7 Quote
Users have been screaming pretty LOUD about this, but manufactures refuse to budge.
The screams have been pretty muffled on this site. I could provide a list dozens of points long indicating why things are the way they are, but I believe that the most obvious answer relates the simple "why" question. What is the most obvious use case (pain point to cause the screaming) to support AF at position 1,1 in the image frame?


Steve

08-20-2017, 12:45 PM - 1 Like   #8
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Yes I agree Steve. Most of the time I use the centre 9 points, and move these around if needed. I don't see any need to have a wider spread than what we have already.

I think sometimes the demand for technological improvements ignores the fact that photography is an art form and ultimately the fellow (or Lady) behind the camera is in charge.
08-20-2017, 04:48 PM   #9
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Fellow Pentaxians, dont let them feed you those lines...

Its just because its cheap to have only a few center AF-points.

but it would economically intelligent to make the jump and invest in better AF overall and for all Pentax SLR models to have THE better AF so Ricoh draws customers to the brand, instead of turning them away with last century upgrades.
not that other brands would be THAT much better and give their customers superior AF in all their Cameras, but it could be the dealbreaker for potential Pentax-DSLR buyers, so the brand gets back where the brand once was. on TOP!

BTW: widespread af-points are 100% possible.
link > https://cdn.photographylife.com/wp-content/uploads/2016/01/Nikon-D500-Viewfinder-Coverage.jpg
08-20-2017, 05:44 PM   #10
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The D500 AF points layout does indeed cover a large area. Some over at the Nikon forum have questioned how useful that is. The D5 and the D500 uses the same AF module, but in the D5, of course, the points are centred more because of the larger mirror.
The D750 covers a significantly smaller area.


Of course, I seldom use my camera's 11 focus points, preferring most often to use only one. So, not sure that I need more than a hundred!
08-20-2017, 06:00 PM - 1 Like   #11
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Rather than whining about the lack of AF points, I would like to have camyum and hjoseph7 give some valid reasons why these extra points are needed. After all the image in the link camyum shared above would be best served by either spot AF or in fact manual focusing.
08-20-2017, 06:43 PM   #12
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QuoteOriginally posted by GUB Quote
Rather than whining about the lack of AF points, I would like to have camyum and hjoseph7 give some valid reasons why these extra points are needed. After all the image in the link camyum shared above would be best served by either spot AF or in fact manual focusing.
Gub a few years ago I use to shoot Middle school and Highs School youth portraits. It was more like assembly line photography and didn't lead to much creativity. Usually the kids were about the same height with some variations. With the camera held vertically on a tripod, I would place the outer edge of the focusing point on my subjects eyes and fire away. Of course some kids were much taller or shorter than the other kids so I had to make accommodations to keep that lone focusing point on the subjects eyes, because when it comes to portraiture once the eyes are out of focus the whole portrait is shot. Lucky for me my K-5 IIs also has this outer focusing point that I can select and when the camera is held vertically I can place over a subjects eyes. However not all cameras have this outer focusing point. Look at the new canon 6D Mark II. I would never buy a camera like that because of the focusing points. Although Canon boasts that the camera has 41 or whatever focusing points they are all bunched up in the middle leaving a huge gap between the edge of the frame and the nearest focusing point. Imagine if I had to shoot 300-400 kids with this camera. I would have to use the old Focus-and-Recompose 300-400 times !

Last edited by hjoseph7; 08-21-2017 at 04:19 AM. Reason: vertical instead of horizontal
08-20-2017, 07:20 PM   #13
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QuoteOriginally posted by camyum Quote
Fellow Pentaxians, dont let them feed you those lines...
Which them might you be talking about? Perhaps you might want to volunteer your personal experience with one of these superior systems that provides a full spread of AF points across the frame.

QuoteOriginally posted by GUB Quote
After all the image in the link camyum shared above would be best served by either spot AF or in fact manual focusing.
My (admittedly limited) experience with the generally excellent Nikon AF systems is that the camera would probably have locked on the guard rail in preference to picking up the cyclist unless the user intervened to prevent such.


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08-20-2017, 07:38 PM   #14
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QuoteOriginally posted by hjoseph7 Quote
Gub a few years ago I use to shoot Middle school and Highs School youth portraits. It was more like assembly line photography and didn't lead to much creativity. Usually the kids were about the same height with some variations. With the camera held vertically on a tripod, I would place the outer edge of the focusing point on my subjects eyes and fire away. Of course some kids were much taller or shorter than the other kids so I had to make accommodations to keep that lone focusing point on the subjects eyes, because when it comes to portraiture once the eyes are out of focus the whole portrait is shot. Lucky for me my K-5 IIs also has this outer focusing point that I can select and when the camera is held horizontally I can place over a subjects eyes. However not all cameras have this outer focusing point. Look at the new canon 6D Mark II. I would never buy a camera like that because of the focusing points. Although Canon boasts that the camera has 41 or whatever focusing points they are all bunched up in the middle leaving a huge gap between the edge of the frame and the nearest focusing point. Imagine if I had to shoot 300-400 kids with this camera. I would have to use the old Focus-and-Recompose 300-400 times !
Pretty obscure example. I think you had better come up with a better example than that for Pentax to retool its assembly line!!
08-20-2017, 07:49 PM   #15
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QuoteOriginally posted by stevebrot Quote
the camera would probably have locked on the guard rail in preference to picking up the cyclist unless the user intervened to prevent such.
In which case the shot would have had to be pre-planned and so pre focusing on the correct area of the guard-rail in manual focus would have been just as effective. Funny thing is the shot appears to be misfocussed - isn't focus just in front of the cyclist?
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