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02-06-2012, 10:30 PM   #1
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aperature settings

Just tried out my 50 mm 2.8 macro on my K-5 (using at that time as a regular lens) at my son's birthday party, noticed that at 2.8 aperature (no flash) pictures close up of just him turned out very sharp and nice but pictures of a group of 3 or more center sharp or one person in middle but those on the sides very soft and some even out of focus. It was even worse when farther away. The next day I was experimenting trying to correct this by setting aperature to 3.5 then 4.0 and up to 6. (got my kids to pose) I noticed that the higher the aperature (less opening) or more depth of field pictures became sharper throughout meaning everyone in the group came in focus. Is this a true finding or the way it should work? If so what is the advantage of having a lens with a greater max aperature such as 2.0 or 1.8 if it won't give good results across the entire picture. Could others there with much more experience help me out with this and hopefully give some teaching on how to better manage these settings as I take a lot of family pictures as above. Perhaps you could point me to some articles if helpful. Thanks

02-06-2012, 10:38 PM   #2
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Yes, that's how it's supposed to work. Higher aperture number = smaller opening = more depth of field.
The advantage of a wide aperture is it lets more light in, so you can shoot in darker conditions. Or, perhaps you want shallow depth of field -- if you're shooting one flower in the garden you don't want all the twigs in the background in focus.
Choosing the right exposure is a balancing act between shutter speed & aperture. They work inversely... the setting you choose, depends whether you want to stop action, or shoot in low light, or have a lot of things in focus farther away from you, or very little in focus.
If you type "understanding aperture" into Google, you'll find a lot of articles that should help you understand how it works.
Good luck & have fun photographing.
02-06-2012, 11:02 PM   #3
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Play with this DOF calculator and you will gain some idea of how it works.
Jake
Online Depth of Field Calculator
02-07-2012, 02:32 AM   #4
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It would be helpfull if you could post some photos so that we can actually see the effect.

There are 2 different issues here which should not be mixed:

1. Depth of field (i.e sharpness of the things close and those that are far away). The DOF depends on the apperture and focal length. Simplified it means that regardless of focal length infinitly big apperture (big opening) gives inifinitly shallow DOF. On the other hand infinitly small apperture (small opening) gives inifinitly deep DOF (everything seems to be in focus).

2. Center vs. corner sjarpness. In theory if you point your camera perpednicularly to a flat surface than the whole projection (picture) should be eqaully sharp regardless of apperture. In theory this is true regardless of the distance to the object. Now theory and practice are not necesserely the same thing. So if you want to check the corner sharpness try to find a fairly uniform brick wall, put your camera on tripod (or at least try to hold it straight) and take a photo of the wall from coupe of difference distances and with different apperture. It is important that the camera is perpendicular to the wall (no tilting) so that one flat surface (wall) is projected to another flat surface (sensor). In this case you can actually estimate both center vs corner sharpness and vigneting (light intensity falloff towards the edges). Now every lens has better sharpness than in corners and virtually all lenses have some vigneting (with brick wall you could actually estimate the barell distorsion as well but more of that some other time). Note also that closing the aperture helps to get better corner sharpness as well.

02-07-2012, 06:34 AM - 1 Like   #5
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You descibed the way it works very well... You don't appear to have a problem ta all... In fact I'd suggest you've made a breakthrough...

Enjoy that 50mm macro... Which version... I'm a tad jealous...
02-07-2012, 06:19 PM   #6
Ric
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some pictures both taken at f 2.8

here are some photos that hopefully will better illustrate what I have described (good call stanislav)
thanks so very much for all the useful input so far
as you can see some people sharp others out of focus all shot at f 2.8.....will a smaller aperature fix this problem thereby increasing the DOF or is this a problem with not compensating for the distance, or both?

Last edited by Ric; 03-04-2013 at 01:04 PM.
02-07-2012, 08:51 PM   #7
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QuoteOriginally posted by Ric Quote
here are some photos that hopefully will better illustrate what I have described (good call stanislav)
thanks so very much for all the useful input so far
as you can see some people sharp others out of focus all shot at f 2.8.....will a smaller aperature fix this problem thereby increasing the DOF or is this a problem with not compensating for the distance, or both?
I’m not sure how far the camera is from the focus point but assuming its 15 feet away, which is probably the distance you need to fit this many subjects in one frame with a 50mm lens, at f2.8 your DOF is about 3 feet, with near limit of 13ft and far limit of almost 17ft. But from your samples I think the problem might be the focus point. It seems too far back putting your front subject outside of the near limit. Assuming the lens is fifteen feet away, I’m guessing for photo number one had you focused around the woman wearing stripes the overall results would’ve been much more acceptable. Same for number two, outcome might have been better had the focus locked on the woman sitting on the floor. Cheers!
02-07-2012, 09:17 PM   #8
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not sure what happened in the first image, where there does not appear to be anything in sharp focus, the second image seems to indicate that you focussed too far back (on the black shirt?). Stopping down (to a larger f number) would would increase the Depth of Field. Focussing on the sitting girl's ear or the guy's elbow would also help.

smaller maximum f-number lenses (aka faster lenses) will have a wider range of f-stops where the resolution will be at its best (approx. 2 stops down from the widest f-stop to about f-11 or f-16). For an f 2.8 lens there are about 3 stops of greatest resolution, an f-5.6 lens only one! Resolution is the ability to reproduce or resolve fine detail. Above f11 or 16, resolution drops (though Depth of Field and the amount of the image, front to back, increases) due to light refraction through the small aperture opening. See also "modulation transfer function" if you want to make your brain hurt!

regards,

02-07-2012, 11:11 PM   #9
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This is rather normal behaviour with wide aperture.

If you take a look at the first photo you took a picture at the slight angle with respect to windows and the background wall, so the most distant part of the wal (with respect to camera) was on the left hand side. The sharpest part of that photo is the lamp and the sun-blinds closest to lamp on the left side. Then everything else is getting out of focus the closer the subject gets the more blurred it is. So it seems that the biggest issue with this photo was bacfocus.

On the second photo the sharp part (or plane) is part of person sitting to the right the fornt of stand where you have your lamp, the armrest of the sofa (just behind the girls head) and the rigt leg of guy in glasses sitting on the sofa. If you take a look all those subjects (or parts) are on the focus plane perpendicular to your camera. Everything in front and behind that plane is out of focus, more so as the distance is growing.

So this is a clasical DOF effect. So you can either close the aperture or perhapse use wider lens (since doff is deeper with wide lenses) or arrange the photo so that the interresting parts are on the focal plane (hope you understand what I mean by that).

Keep up the good work
02-08-2012, 07:06 PM   #10
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It's not just depth of field that's making the edges soft. Most lenses only gain corner sharpness several stops above maximum. For a 2.8 lens that's around F 6.7-8
02-08-2012, 11:20 PM   #11
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None of the faces in these shots are close enough to the edges for corner sharpness to be an issue, though. The softness here is a simple matter of DOF being too shallow - the inevitable results of shooting at large apertures and short distances. Plus in one example, the camera having actually chosen to focus on something behind the subject. It is important to try to control where the camera focuses. But also, when shooting groups at large apertures, you need to get everyone in the same plane of focus.
02-09-2012, 02:39 PM   #12
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Marc:
I do concur. Line-ups are good for a lot of reasons I guess! I am surprised the Auto Focus chose the blinds on the one image. This is why I still like manual focus for many shots! But I'm old school and a control freak.

Ric- On the K-5 you can set the focus point to SPOT measurements (the white box with a dot). I use this quite a bit to target the specific focus area. First, make certain the camera is not set to "C", "Catch in focus". Now, point the center spot at the area you choose for the focus point, push the shutter button half way (you will hear/see the focus routine), hold it there, recompose and push the rest of the way. Try to pick a focus point a little closer to you than further away as you have more depth of field behind the point of focus than in front of it.

Give it a try on some test subjects and see how you do!

Regards,
02-14-2012, 09:20 AM   #13
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Thanks much everyone for the very helpful comments
02-14-2012, 09:24 AM   #14
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I haven't read through the entire thread, so I don't know if this has been mentioned already. But one little tip when dealing with DOF and group shots: Focus on the person closest to the camera, and then adjust your aperture accordingly to account for the depth of field that you need. It makes it easier to get the entire group in focus when focusing on the closest person and then switching to a smaller aperture for greater DOF.
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