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I am relatively new to DSLR photography and I am mainly learning by taking photos and enjoying doing so. One of the curiosities with my photos are how the level of sharpness varies from photo to photo.

I do understand how Aperture/Exposure/ISO/ShutterSpeed/Focal lengh can affect my image and I aware of this when shooting, but I have yet to find the "Butter Zone" where I know my images will come out sharp.

That is, I will take an shot with settings that have previously yielded sharp results only to find that it is not as sharp as before.

I appreciate that there are compromises to be made and in all honesty most of my pics will print OK on paper, but I have taken very sharp images with the kit I have before and want to build some consistancy in to my photo taking.

I have a D5100 and 55-200mm VR lens (VR is on permanently and I mainly hand hold) I try to shoot at 1/250-400+ @200mm where possible. ISO is usually 100-1600 (although rarely as high as 1600) Aperture I glue to f8 unless I need more light and I shoot in varied conditions.

Are there any rules of thumb that I should consider?

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Examples always help if you can post two with the same settings - one sharp and one not. We'd better be able to isolate your issue. Its a great question as is though. –  rfusca Jun 8 '12 at 16:54
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Although your question is not identical (it's nicely worded in a general way, while this one is specific), some of the answers to How I could take crisp sharp shots without expensive lens? basically apply. –  mattdm Jun 8 '12 at 16:54
    
You seem to have a fairly OK list to start with. Answer on question referenced by mattdm interleave with them. || You know all this but: Do not use IS with tripod - it can ADD noise. Lens centre and edge sharpness can vary appreciably. Diffraction onset varies somewhat with lens used. Down to about f/22 can be good but not always. 1/250th at 200mm meets classical rqmnt and add IS, Ninja breath holding, conscious bracing and you should be many stops better than needed. Contrast and sharpness are "entangled". Subject motion matters. Atmospherics matter usually only at distance. ... –  Russell McMahon Jun 8 '12 at 21:52
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@RussellMcMahon Just to be correct, IS (VR) cannot add noise but motion blur if it thinks it should track something that moves inside the image although it is on a tripod. –  Simon A. Eugster Jun 9 '12 at 5:49
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@SimonA.Eugster - I'm an engineer. Motion blur IS noise :-). And even for non-engineers it's a valid concept. An IS / antishake system that can impart 2D motion to a sensor or lens system at a frequency that is useful to overcome real world motion, can also ADD apparent real world motion under adverse conditions. But, I agree with the principle of the point that you were making is that the nature of the degradation is different from many other noise sources. –  Russell McMahon Jun 9 '12 at 16:27
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3 Answers

Possible issues

For Shutter Speed shoot a few images hand-held without VR/IS and with different shutter speeds. Generally pople say to use the reciprocal, so 1/50 s for a 50 mm lens, but since we have more and more pixels you may already notice it with even faster shutter speeds. Try to find which factor still works for you (also try 1/100 s and more just to see the difference, or for 200 mm even 1/1000 s). With VR enabled a lower shutter speed should then do.

Shutter speed is maybe the most important reason if the camera just «does not want to make sharp images».

(Note: Using the reciprocal works more or less because the motion blur is small enough for not being seen. The D5100 uses a crop sensor, so the 50 mm looks like a 85 mm – but actually it is only a crop from a full frame 50 mm image. Compared to this one you make yours larger for viewing, and motion blur is better visible. In a nutshell, reciprocal is a rough guide, but needs to be adjusted to your lens and camera resolution.)

Funnily Auto Focus can be a source of problems too, which is especially noticeable with faster lenses. Open the aperture as far as possible (on a tripod), let the camera auto-focus on something, then enter Live View and zoom in as far as possible. For large apertures (f/2 e.g.) you will often notice that the focus is not correct, and adjusting it in Live View by hand can improve the situation greatly.

Sometimes it is enough to adjust the autofocus system, can be done in the menu on a D7000, for a D5100 you maybe need to visit the Nikon support. I have some lenses that always focused behind the subject.

Then obviously the Lens Quality matters too, but this did not change in your case. Some good links for this are: Lens comparison images, Lenstip, slrgear

Motion or lens blur?

Compare some images you shot with slower shutter speed and with deliberate slight defocusing. The blur has a different quality, and after a while you can easily tell if images are not sharp because of motion blur or because of focusing problems (or bad lens quality).

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I would recommend to not enable VR unless you really need it.

If VR is on, then you must wait to engage. Half-press the shutter and wait a little time. If you press directly the shutter till the end to take the photo, the VR will not have time to set up and you will have a blurred image.

Also you must understand very good how the Phase Detection AF system works and what are its capabilities (ie. low light, AF point expansion etc.) - sometimes it focus where you think it will focus, sometimes not. For ex. a common beginner mistake is to have AF always on AF Servo.

Aside of that, see the clasical answers to the question which @mattdm referenced in his comment to your question.

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I recommend trying this handholding practice exercise: go in manual mode, fix ISO and aperture, and take a photo of a single object over and over again, slowly increasing the shutter time every few shots. There will usually come a point where you can only hand hold the shots successfully and get sharp output about 50% of the time. It just depends how much you happen to move during the exposure - this might begin to explain your inconsistent results despite using the same settings. If your handholding is too erratic, even the 1/focal length rule will not save you from motion blur - this exercise can help you identify your limits and improve.

You can usually visually distinguish motion blur from out-of-focus issues by just examining your photos closely. If out-of-focus is the issue, then experiment with different autofocusing modes and manual focus on stationary objects. Note that if depth of field is narrow, autofocus may make strange choices about which part of an object to focus on, making parts that you expect to be sharp end up out-of-focus - manual focus or smaller aperture can avoid this.

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