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09-07-2013, 06:59 AM   #1
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Adjust focusing

Hi,
I have k30, i am using selective focus af point to shoot mostly the subject when not in centre point, but many photographers are using central focus point to lock the focus point on the subject that not in the centre of viewfinder then recompose it and shoot. Which technique is more accurate ? or central focus point is more accurate then other rear focus point? Thanks

09-07-2013, 07:44 AM   #2
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With recomposing it depends on the photographer, some are really good at it. But its not a super reliable technique by itself.
Using select point should be more accurate, but AF points on the sides are often not as accurate as the centre point (depends on camera model). With the K-30 the off-centre points should be pretty good, though. They are cross-type, but a little smaller than the centre one. Keep in mind the AF points are actually bigger than the little red lights in the viewfinder.

I usually use select point myself, but I suggest you just try it yourself. Each technique can work, but they both depend on how well the photographer can use the camera. I mean, heck, sometimes I use centre AF without recomposing and still manage to misfocus everything.
09-07-2013, 08:51 AM   #3
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I also use select focus, where the focus point used is manually selected by the photographer. I leave it 'parked' on the center focus point. The issue is whenever you use a focus point and then turn the camera in an arc to recompose (as opposed to moving your feet in a line parallel to the subject), you are changing the sensor to subject distance.

'Focus and recompose' is a safe technique where you have adequate depth of field (DOF) to cover this change, and where focus does not have to be absolute. 'Focus' is one specific distance between the subject and the sensor, whereas DOF is a distance range of 'acceptable focus'.

If my DOF is not deep enough, or focus is critical, I compose my image first, then move the focus selection point before focusing and taking the picture.

Many current AF lenses no longer have DOF scales on the lens barrel. So the only two ways to check DOF are using a DOF calculator (like an app for your smartphone) - OR, reprogram your RAW/Fx button to stop down the lens so you can visually check DOF. Otherwise, experience will tell you when you are stopped down far enough that adequate DOF is not a concern.

Personally, I shoot RAW+, so leaving my RAW/Fx button in its default programming to store a RAW image is a waste. I use the button quite frequently to check DOF.
09-07-2013, 03:46 PM   #4
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DOF is important for me because i use DA 50mm f1.8 prime lens. what the problem is that when i focus on people faces i get the blur pics (face) generally, but sometimes i get sharp pics (faces). I use Aperture f1.8 - f2 - f2.4 - f.2.8 to get good bokeh but i am not satisfied sharpness of my images. how can i fix them or how can i guaranteed to get sharp images(portraits)?

09-07-2013, 05:47 PM   #5
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We need samples to demonstrate your issues. Keep in mind that AF 'points' are anything but. AF 'regions' is a better description. The area considered by the camera is much bigger than the little red squares you see in the viewfinder. And if there is anything within the chosen AF area that is more tempting to the camera than your intended subject, that is where the AF will go. For example, it is difficult to shoot a squirrel on a tree branch a short distance away. The focus point is the squirrel’s eye. And if the eye isn’t in sharp focus – well you ain’t gonna win any awards. It’s just another nice snapshot. The problem is all those little twigs and branches near the squirrel are much tastier targets to the AF system – even if you are only using one focus point. It is almost guaranteed you will have to manually tweak focus for a shot like this.

Have you tried using Live View and the K-30's Focus Peaking function?

And by the way have you tuned the K-30 AF for this lens? There are a variety of methods; I like this one:
PENTAX DSLRs: Front or Back Focusing Problems? Free test (Lens Alignment) charts for Pentax, Nikon, Canon, Sony, Olympus.
09-08-2013, 06:20 AM   #6
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Oh, and one more thing, the in-focus field is actually not square. It can be bent in odd ways and thinner on the sides. And some lenses have poor edge performance. This can lead a person to think the camera AF is wrong, but its just the overall design.
And I agree with the suggestion of trying live view with focus peaking. Even if you don't end up using it, it is a great tool to figure out how the DoF generally works. With your DA 50mm f1.8, focus peaking should be pretty good, especially for portraits.
The other thing I would mention is that people often don't like super sharp portraits, because it can show all the wrinkles, blemishes, etc. In the past, photographers often used "soft focus" or special soft lenses (like this one). As long as the eye nearest to the viewer is mostly in-focus, its good
Everything else depends on lighting (overexpose a little, use diffused light) and post processing (so the skin tones look natural and warm)
09-08-2013, 07:01 AM   #7
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I checked my lens according to yvon bourque chart and i haven't found any back or front focus problem. I also use live view focus peaking and it is pretty good, i notice that focus peaking is more accurate and catch the faces easily. Specially In low light conditions, focus system (af point) have difficulty in lock position. It locks anyway but take sometime. Manual focus is also alternative method but it required some experience i think.
Another question is what is the correct shutter speed for sharp images for portraits.
Also i dont know my lens performance, may be it have poor performance for sharp images, is there any review about it?
09-08-2013, 10:31 AM   #8
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As I understand from your opening question you are concerned with focus points and how it affects composition, and then there is also a concern on image sharpness. Two separate topics...

1) With the focus points, I use all modes in different scenarios but most of the times, I use AF Active Area where I can select the focus points. Before getting a Pentax I'm only used to 3 focus points so having 11 focus points is a luxury already. It only makes framing subject faster as it saves time to recompose a subject from a center point. I use multi-segment metering most of the time because it's good enough when you you don't have a gray card. So when do I use center point? Only when I use spot metering. I have a lot of strobist friends who rely on spot metering (and therefore center focus) because they use gray card. They say getting the right exposure is very predictable and repeatable with this method especially when you are in a controlled environment i.e. studio, and you want a correct skin tone. I have setup my AF/AE-L button to be just "AE Lock" so that I can lock my exposure when I press it then frame my subject. Very helpful when you use spot metering and center point.

There is an option to make the focus point biased for the focus point and it is found in "Link AE and AF point" in customization so when you activate this and you use multi-segment metering you'll get something like a spot metering wherever you place the focus point. I don't use this because it is not clear how much "bias" is the exposure using this method. Spot metering is more predictable and reliable in studio situations, IMHO. Just experiment to know what works best for you.

2) On image sharpness the first thing you want to avoid is motion blur by using a fast enough shutter speed. You can freeze most human movements at 1/250 second. Having a shallow depth of field is also tricky when your subject moves a lot. You mentioned your lens has no front/back focus issue so it must be the movement that is spoiling your shots.

If you are just interested about the technical spec of the lens then you can find DxOMark helpful when choosing a sharp lens subjectively. For example, you will know that DA 50mm f1.8 is very sharp from f2.8 to f8.0 so you may want to use f2.8 and up if sharpness is your top priority. Just go to "measurements", then "sharpness" and finally "field map" to visualize the sharpness from center to edge. I just hope they test more lenses because they just started adding more Pentax lenses for reviews.

09-08-2013, 03:30 PM   #9
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I looked dxomark tests, my lens is best overall performance at f2.8 as you sad like.
If sharpness is concern i will try to shoot at f2.8
I havent used gray card method before, i will try this method also.
I will try to pay attention focus point,shutter speed and light conditions when i shoot the subject. I hope that my pics will be better.
Thanks all....
09-08-2013, 05:21 PM   #10
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QuoteOriginally posted by Fuhrercyp Quote
Another question is what is the correct shutter speed for sharp images for portraits.
So long as your subject is still (they usually are for portraits), shutter speed has no impact on image sharpness. Shutter speed impacts how movement is recorded.
Aperture impacts depth of field (the range of 'acceptable focus') and therefore has some impact on sharpness. Each lens has its own 'sweet spot' - the aperture where you achieve the maximum sharpness for that lens.

And to round out the trio, you have ISO, or sensor sensitivity, which impacts the amount of digital noise. The greater the noise, the lesser the perceived sharpness.

You can't just twiddle these three items any way you want. Image exposure is controlled by a combination of all three. For every change you make to one of the trio, you have to make offsetting changes to one or both of the other two.

I will suggest the following site as a way to play with the controls of an advanced camera and better learn their interaction:
CameraSim simulates a digital SLR camera - SLR Photography Demystified
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