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09-13-2012, 06:26 PM   #16
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Err...will this knowledge make me a better photographer, or did you guys happen to be photographic equipment engineers? If this knowledge will make me a better photographer - I wanna get in on it. Soon as I understand how knowing what aperture my AF sensor is at will help me with my pics....until I stumbled into this thread i didnt even know there is an aperture value for AF?

09-13-2012, 07:14 PM - 1 Like   #17
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QuoteOriginally posted by D4rknezz Quote






Err...will this knowledge make me a better photographer, or did you guys happen to be photographic equipment engineers? If this knowledge will make me a better photographer - I wanna get in on it. Soon as I understand how knowing what aperture my AF sensor is at will help me with my pics....until I stumbled into this thread i didnt even know there is an aperture value for AF?
I suppose you could not read such threads

Anyway, it is useful to know how the AF systems work, at least in terms of the light levels they are designed for and how slower lenses might not focus in as low a light level as faster ones, no?

I am quite sure that many low light focus 'errors" and complaints stem from folks who do not understand the limits of the system.

Anyway, stumble on....

Ray
09-14-2012, 05:52 AM - 1 Like   #18
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QuoteOriginally posted by D4rknezz Quote


Err...will this knowledge make me a better photographer, or did you guys happen to be photographic equipment engineers? If this knowledge will make me a better photographer - I wanna get in on it. Soon as I understand how knowing what aperture my AF sensor is at will help me with my pics....until I stumbled into this thread i didnt even know there is an aperture value for AF?
I asked the original question, and I will try to give an explanation as to why I would bother about understanding the technicalities of the auto focus system. It does not have much to do with trying to be a better photographer at all. There were mainly two reasons why I asked the question:
1) I am curious and I want to know how everything works, whether I happen to have some practical use for that knowledge at the moment or not.
2) I wanted to be able to better judge the implications of introducing more precise F/2.8 sensors and claiming a two-stop increase in light-sensitivity.

Before, I could see two alternatives. Either, the improved light sensitivity would only be there when using large aperture lenses, or it would be there for all lenses. If what people have described in this thread is correct, then the case seems to be that indeed the auto focus light sensitivity should likely have been improved for all lenses, and the addition of F/2.8 sensors only had to do with increased precision for large-aperture lenses, and not the light sensitivity.

I find this information valuable, because now I can make a more informed assumption about how the new auto focus system has been improved. Naturally, it cannot replace tests (which will be published soon, when the testers can get hold of cameras).

When the question on how this thread helps anyone being a better photographer was asked, I interpreted it as if there was an underlying assumption that all threads have to be about improving ones photographic skill. I think that both discussions on how to improve yourself and discussions about understanding technical things should be OK -- in fact, if it would only be about improving your own skill, it should be a photography forum rather than a brand-specific forum.

I do not see any problem in being technically interested and wanting to know how stuff works and also wanting to create beautiful pictures. The technical part does not necessarily have to be just a means towards better pictures, but can be interesting in itself.

This does not mean that I only care about the technicalities, and such things. I take pictures and try to improve, also by studying books about composition, light and "telling a story with the picture" and so on, and at the same time I like to understand the technology. Wanting to understand how stuff works and wanting to improve my own technique are not mutually exclusive! I very much am of the opinion that photographers take pictures, and that cameras are tools that are used -- I am very sure that a good photographer could make better pictures with some old compact camera in many situations than I could with my K-5, for example. I know several photographers who I consider better at taking pictures than I, although they know almost nothing about the technology that makes their cameras work.

Lastly, if you think that all of the above seems very strange: try to think of something that you would be curious about, without having any practical use for the information. For example, wouldn't it be fun to know how an airplane can fly, how a loudspeaker works to produce sound or how a light bulb works (substitute something that you find interesting)? That is something similar to how I feel about understanding more about how cameras (and most other things as well) work.

I hope that I was able to clarify a bit.
09-14-2012, 10:12 AM   #19
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QuoteOriginally posted by Ray Pulley Quote
Sorry. Read it again.

What you posted has nothing to do with the light sensitivity of the sensors. Baseline is EXACTLY what I was describing earlier and just like whether a point is cross or not, has nothing to do with the sensitivity of the AF sensor to light or the amount of light reaching the sensor. Every point is actually a pair looking at rays split into two. How far apart they are sets the baseline, f2.8 point pairs being further apart than f5.6 pairs. This is why the f2.8 pairs cannot see anything with a f5.6 lens attached as I described earlier. The further apart the points are, the more accurate the focus calculation.
Sure but you comment on my post and i wasn't talking about the base-line

Sorry for being unclear but i clearly said points.


QuoteQuote:
Generally, an aperture value is associated with an AF line sensor. The terminology usually used is "f/number-sensitive", e.g, you may have an f/5.6-sensitive line sensor, or an f/2.8-sensitive line sensor. The f/number refers to the maximum aperture of the lens, because AF is performed with the lens wide open (i.e. the aperture you choose for the shot does not matter, only the max aperture of the lens). The use of 'sensitivity' in this context implies that light levels matter, because that's what we think of when we normally use f/numbers. In this case, though, a wider aperture simply means a wider baseline for the rangefinder system is required for that line to function. Personally, I think better terminology might be to use threshold instead of sensitivity, so an f/2.8-threshold line would require an f/2.8 lens to function, and if you mounted an f/4 lens, that sensor line would not operate. An f/5.6-threshold sensor would work with any lens having a max aperture of f/5.6 or wider.



Last edited by Anvh; 09-14-2012 at 10:21 AM.
09-14-2012, 10:14 AM - 1 Like   #20
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QuoteOriginally posted by D4rknezz Quote






Err...will this knowledge make me a better photographer, or did you guys happen to be photographic equipment engineers? If this knowledge will make me a better photographer - I wanna get in on it. Soon as I understand how knowing what aperture my AF sensor is at will help me with my pics....until I stumbled into this thread i didnt even know there is an aperture value for AF?
Knowing your tools is half the work right?
If you know what works and what doesnt work and know why then yes it might help you a bit to get more out of your camera.
Not saying you have to know this or all the details though but basic understanding never hurts.
09-14-2012, 10:31 AM   #21
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QuoteOriginally posted by Ray Pulley Quote
This is a bit hard to explain without a diagram, but I will do my best:

First the K30 improvements -

The AF systems of every Pentax DSLR prior to the K5 suffered from focus shift due to chromatic aberrations in the optics of the AF system. CA is a wavelength issue where the light rays of different colors do not focus all in the same focus plane. CA causes the AF system to think the light rays of certain colors are in focus when they really are not. The K5 introduced a color sensor to try and offset this problem. The k30 uses a diffractive optic design that optically corrects the CA in the AF system. This should be a better solution than the K5 color sensor, IMO.

As for f2.8 sensors -

The f2.8 sensor is not what you think it is. The sensitivity of the AF array is not directly related to the aperture of the AF system.

The AF array has a mask over it that blocks off all but very specific portions of the light coming from the lens. If you think of the AF array as looking back at the lens through these slits, it can only see points located on a very specific part of the lens (there are two matching slits, one on each side).

The points are located on a circle which corresponds to an aperture. In the case of the current AF system, the circle is the size of an f5.6 aperture. If the slots were located further apart they would allow the AF array to "see" a point on a bigger circle on the lens let's say f2.8.

The f5.6 sensors will work on faster lenses because the faster lenses are bigger and will always allow the AF array to "see" the smaller f5.6 circle through the mask.

The f2.8 sensors will not work on the slower lenses, however, as the slower lens isn't big enough to allow the sensors to "see" any lens glass at all. They will simply be blocked as there is no glass out there to "see".

The upside to f5.6 sensors is that all lenses from f1.0 to f5.6 will focus. The downside is that they are closer together, which means they are less accurate, so while the faster glass will focus, it will not be as accurate as it could be.

I hope this helps

Ray


Ray,

I too would want to "see" what this means on a diagram of some sort.
Not that it would change my "abilities" to take shots but I thought it is a good idea to understand how his look like.
I am very "visual" and need some "visual info" ... any chnace to dig something somewhere? I've browsed the NET but I don't exactly know what I should be looking for.

Cheers.

JP
09-14-2012, 04:23 PM   #22
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wow a lot of information. I hope it brings the K-5-II a lot further then the K-5.
09-14-2012, 05:53 PM   #23
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QuoteOriginally posted by Anvh Quote
Sure but you comment on my post and i wasn't talking about the base-line

Sorry for being unclear but i clearly said points.
Sorry for the misunderstanding.

Digging a bit further, it does appear that Canon has hybrid AF sensors that have some elements that are more light sensitive than others.

Here is the sensor that is pictured in the article you referenced, which looks to me to be an array of pixels that are all the same type:



This is a bit older camera, I believe.

Here is a link to a white paper on a bit newer Canon camera, and if you scroll to page 13, you can see an image of the AF module. It has 4 arrays mounted to a sub-board, 2 pairs, and in a system like that, the array pairs could be more or less sensitive one to another (only in pairs though):

http://downloads.canon.com/CDLC/EOS1D_MarkIV-WP1.pdf

So, while the term f2.8 points generally refers to the location of the points as described above and does not necessarily mean they are more sensitive to light, they could be so. It is interesting that Canon makes no distinction in the specs for the AF range for faster or slower lenses, but just lists the range for this camera as -1 to 18ev.

So, at the end of the day, Pentax may have a more light sensitive total AF array, or just a pair of more sensitive arrays for just the f2.8 points.

Ray

09-14-2012, 05:54 PM   #24
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QuoteOriginally posted by jpzk Quote

Ray,

I too would want to "see" what this means on a diagram of some sort.
Not that it would change my "abilities" to take shots but I thought it is a good idea to understand how his look like.
I am very "visual" and need some "visual info" ... any chnace to dig something somewhere? I've browsed the NET but I don't exactly know what I should be looking for.

Cheers.

JP
Try the PDF linked in my post above.

Ray
09-14-2012, 09:06 PM   #25
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A Slightly Different K5II AF Question

Changing the subject slightly:

Has anyone heard whether the K5II has the DO optic in the AF module like the K30?

The reason I ask, is that I am more than a bit convinced that a significant portion of the issues with the K5 AF are related to the color sensor part of the AF system. It seems clear to me that the initial tungsten FF issue was related to this system as it was fixed with a firmware update. Given that the color sensor was introduced to deal with CA shifts in the physical optics of the PDAF module that cause FF in tungsten light, it seems reasonable to conclude that the firmware fix was made to correct a programming problem in that system.

I think the DO optic is a much better solution, and so far seems to be working.

I have two k5s, and do not have many AF issues to complain about, but I have seen some misses that seem to be related to color. If you think about how inaccurate AWB can be, you can see that determining actual overall scene lighting color temperature and then making a precise adjustment to the AF might be a hit or miss thing.

Otherwise, there are other sort of vague claims about K5II AF improvements, but I would probably not be tempted by the K5II just for the low light improvements without the DO optic. Almost all of my lenses are f2.8 or faster, so the improved accuracy if the f2.8 sensors might be a good reason to consider the K5II, but I am not having any significant AF accuracy issues, so that isn't looking like something to tempt me to look at the K5II.

I guess time will tell, and I am certainly tired of Pentax reading AF complaints, so if the K5II corrects most of the things users complain about, I'm all for it even of I do not buy one.

If nothing else, the changes to the K30, and the emphasis on AF improvements for the K5 clearly show us Pentax is listening and tackling the things that users complain about most (well, except SDM...)

Ray
09-15-2012, 06:06 AM   #26
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Thanks Ray but sadly i can't open the PDF

I would love to know if Pentax has those dual points so for f/5.6 and f/2.8 lens in the middle of the sensor?
It seems you can take up what they read that way but there is room for speculations.
09-15-2012, 07:03 AM   #27
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QuoteOriginally posted by Anvh Quote
Thanks Ray but sadly i can't open the PDF

I would love to know if Pentax has those dual points so for f/5.6 and f/2.8 lens in the middle of the sensor?
It seems you can take up what they read that way but there is room for speculations.
You should be able to select it and download from this page:

http://www.learn.usa.canon.com/resources/articles/2011/eos_white_papers.shtml

Ray
09-15-2012, 09:23 AM   #28
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Diagram

With credit to "Hans from Denmark" on the "Photography on the Net" Canon forums, here is a simplified view of the f2.8 versus f5.6 autofocus aperture:



This diagram does not show all of the optics and even the aperture masks, but you can see why the f2.8 sensors cannot be used for a slower lens.

There were a few other points made in this thread about the Canon f2.8 points, and I found one to be interesting in thatthe more light sensitive pixels in the array are usually bigger, which means that they have less resolution, which gives back some of the gains made from the larger baseline.

In the specific case being discussed at the Canon forum, the larger, more light sensitive pixels in that array are used at f4.0.

I wish Pentax would give us some whitepaper type descriptions of things like the AF and flash systems.

Ray
09-15-2012, 09:50 AM   #29
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Ah, because the smaller maximum aperture will just block the 2.8 sensor...
09-18-2012, 12:50 PM - 1 Like   #30
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QuoteOriginally posted by hjb981 Quote
I asked the original question, and I will try to give an explanation as to why I would bother about understanding the technicalities of the auto focus system. It does not have much to do with trying to be a better photographer at all. There were mainly two reasons why I asked the question:
1) I am curious and I want to know how everything works, whether I happen to have some practical use for that knowledge at the moment or not.
2) I wanted to be able to better judge the implications of introducing more precise F/2.8 sensors and claiming a two-stop increase in light-sensitivity.

Before, I could see two alternatives. Either, the improved light sensitivity would only be there when using large aperture lenses, or it would be there for all lenses. If what people have described in this thread is correct, then the case seems to be that indeed the auto focus light sensitivity should likely have been improved for all lenses, and the addition of F/2.8 sensors only had to do with increased precision for large-aperture lenses, and not the light sensitivity.

I find this information valuable, because now I can make a more informed assumption about how the new auto focus system has been improved. Naturally, it cannot replace tests (which will be published soon, when the testers can get hold of cameras).

When the question on how this thread helps anyone being a better photographer was asked, I interpreted it as if there was an underlying assumption that all threads have to be about improving ones photographic skill. I think that both discussions on how to improve yourself and discussions about understanding technical things should be OK -- in fact, if it would only be about improving your own skill, it should be a photography forum rather than a brand-specific forum.

I do not see any problem in being technically interested and wanting to know how stuff works and also wanting to create beautiful pictures. The technical part does not necessarily have to be just a means towards better pictures, but can be interesting in itself.

This does not mean that I only care about the technicalities, and such things. I take pictures and try to improve, also by studying books about composition, light and "telling a story with the picture" and so on, and at the same time I like to understand the technology. Wanting to understand how stuff works and wanting to improve my own technique are not mutually exclusive! I very much am of the opinion that photographers take pictures, and that cameras are tools that are used -- I am very sure that a good photographer could make better pictures with some old compact camera in many situations than I could with my K-5, for example. I know several photographers who I consider better at taking pictures than I, although they know almost nothing about the technology that makes their cameras work.

Lastly, if you think that all of the above seems very strange: try to think of something that you would be curious about, without having any practical use for the information. For example, wouldn't it be fun to know how an airplane can fly, how a loudspeaker works to produce sound or how a light bulb works (substitute something that you find interesting)? That is something similar to how I feel about understanding more about how cameras (and most other things as well) work.

I hope that I was able to clarify a bit.

Ya i agree I wasnt questioning the validity of the thread, I was really wondering out loud whether I should care about this particular bit of information - or not - because I am going on information overload these days . It seems to me that there are members of the board who are legitimate engineers, and I can try and join the conversation, but I will be left behind pretty quickly, with no improvement to my photography, and maybe a little disappointment at why I dont understand how a specific number of blades is important to my lens, or something like that .

This particular information seems to be a little more practical in nature, and from what I get from the discussion, it seems to me that while the autofocus of K52 may be similar to K5 on say an EV0 and a 18-55 3.5-6 lens, if I put a 2.8 lens in front of the K52, I may get a better autofocus because it will switch the AF system to the 2.8 ..err...arrays.

Right?
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