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03-18-2012, 01:33 PM   #1
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DOF - Help? Just aint gettin it -

Hi everybody, let me explain my situation and my problem. I have a KX with two tamron lenses, a 70-200 2.8 and a 28-70 2.8 - Now here is my problem or my bad thinking, whenever I shoot sports, or a parade, or people even a large group with several rows I will put it on center focus and 2.8. Now I am thinking 2.8 is needed because the group may be inside and i need to let light in, or maybe candid shots. But when I do this it seems not sharp, well it is not sharp.

My sports shots, seem sharp at times but should I just adjust my thinking and shoot a different aperature, I think it is just my bad thinkin that say hey you have a 2.8 it is good through all zoom points so use it?

Maybe I just dont have a understanding of DOF?

Please and thank you for all your help

Also maybe these tamron lenses are slower to focus on the KX as compared to say the K5?

03-18-2012, 01:45 PM   #2
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If the lenses are not correctly focused, your shots will be out of focus and thus won't be sharp where you wanted them to be,

The K-x would focus slower than the K-5 - that is one of the advantages of a higher end body - better AF.

Both your lenses should appear sharp when used correctly. They are good lenses.

Some things to try:

- If you don't need f/2.8 (you get fast enough shutter speeds at other apertures), use f/4 or f/5.6 - this way, focusing errors will be less dramatic
- Use AF point selection to select the AF point to use
- Use manual focus - it's not that hard and you may find that you get more reliable results with it than with AF
03-18-2012, 01:47 PM   #3
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Sorry, but what do you mean by "use af point selection to select af point to use"?
03-18-2012, 01:47 PM   #4
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Can you upload some pics, with EXIF attached?

03-18-2012, 01:52 PM   #5
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Well let's start by stating that you have two excellent lenses - the 70-200 especially is a razor sharp lens but the 28-75 is no slouch either. When you say not sharp .. do you mean a whole group or a single subject ? All of the single subject or just part ? Or is nowhere sharp ? As Lufluss said, it'd be easier to pinpoint the problem if you can uphold 2 or 3 shots.
03-18-2012, 02:14 PM   #6
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Stop down a bit. Especially outdoors. F8 and be there.
03-18-2012, 02:26 PM   #7
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It could be both lenses suffer from heavy front-focusing problem; as other posters suggested, a sample picture with EXIF information would help to determine the issue.
03-18-2012, 02:30 PM   #8
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QuoteOriginally posted by geoles2 Quote
Maybe I just dont have a understanding of DOF?
There is an inverse relationship between depth of field and aperture. In simplistic terms at f/2.8 your DOF may be only inches, or even less than an inch depending on your distance to the subject. It is more complicated than that but as a quick explanation if you are at f/2.8 and focus on someones eye, their nose could easily be OOF, thus making your picture look 'soft' or OOF. The solution is to stop down a bit, the more you stop down the greater the DOF.

Of course everything is a compromise, as you stop down you get less light and then shutter speed has to slow or the ISO has to go up to compensate. And if you stop way down, say f/22 as an example, you get lots of DOF, but little light and you also start to run into problems with something called diffraction.

As noted above, "f/8 and be there" has been a mantra for generations of photographers. Start with f/8 and adjust to either side depending on the situation.

Your lens does go to f/2.8, but that does not mean it is the best aperture for every shot. The f2.8 does get you a nice bright viewfinder since you are looking through the lens wide open, but you need to stop down to get a good depth of field. Take a look here for a calculator that will help explain DOF and distance.

03-18-2012, 03:08 PM   #9
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geoles2, have a look at this online depth of field calculator. You can select your camera, focal length, aperture and focus distance. If you spend a little bit of time playing with the different variables, you'll have an easier time understanding how they all interact.

It seems that you automatically assume you have to use f/2.8. If you're shooting a group indoors, bump up your ISO a little bit, stop down to maybe f/4, and if your shutter speed is still going to be slow, put the camera on a tripod and tell everyone to hold still.

There are all kinds of possible reasons why your photos aren't sharp. We're just guessing until you show us some.
03-18-2012, 03:11 PM   #10
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Time to once again repeat this mantra, the rules of DOF:
* For thicker DOF, use a shorter focal length and/or tighter aperture and/or further camera-subject distance

* For thinner DOF, use a longer focal length and/or wider aperture and/or closer camera-subject distance
Some implications: A long fast lens has thin DOF except at great distance, while a short slow lens has thick DOF except at close distance. To thicken DOF with a long lens, stop-down the aperture and/or move further away, and increase ISO for a fast-enough shutter. Sharpness depends not just on DOF and lens IQ, but also on shutter speed and SR. Your Kx has good high-ISO performance, and ISO noise can be fixed in PP, while motion blur can't be so fixed. So stop-down, crank-up, and GO!
03-18-2012, 03:14 PM   #11
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Please post pictures - I have a k-x and 70-200 and can give tips if you wish.
03-18-2012, 04:50 PM   #12
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QuoteOriginally posted by aleonx3 Quote
It could be both lenses suffer from heavy front-focusing problem
It would be a mistake to jump ahead to explore FF/BF as an issue before the OP has even properly come to terms with DOF. Good DOF management will nuke any possible FF/BF anyhow, especially for distant sports subjects.
03-18-2012, 09:01 PM   #13
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QuoteOriginally posted by geoles2 Quote
Sorry, but what do you mean by "use af point selection to select af point to use"?
Press Info to bring up the settings menu. Then look for the Auto 11 option (it's right in the center of the screen), select it, and switch to Select AF Point. You'll see a screen showing you all the AF points and you can move with the wheel to select the one you want to use. Your camera will then only try to focus in that area. This helps when the camera has more areas on which it can focus and it chooses a different one than the one you want. Unfortunately, the K-x doesn't display these points in the viewfinder, so you'll have to remember where they are in the viewfinder. Practice helps.

If you cannot follow the above steps, check your camera manual - they cover these settings.
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