What are the main factors that will affect the sharpness of my images?

Question

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 length can affect my image and I am 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 a 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 consistency into 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?

Answer 1

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.

Answer 2

• Shutter speed. Rule of thumb for 35mm film or FF digital is 1/focal length. That means, for a 50mm focal length set on your lens use 1/50s or shorter. This is just a starting point and smaller format cameras (APS etc.) will need shorter. You can compensate with image stabilizer (allows 1x - 4x longer shutter speeds for stationary subjects - IS does not help with moving stuff, only with shaking photographers), or a tripod. With tripod, avoid so called critical shutter speeds. These are speeds usually around 1/10 sec where the tripod doesn't work as well as at shorter and longer exposure times. If you have a DSLR, you can also make your camera steadier if you engage the mirror lockup feature. This prevents mirror slap. When shooting fast movement (e.g. sports), you may need shutter speeds as short as 1/500s or more. A flash can help.
• Aperture. Lenses are usually best when stopped down a little. Wide open lenses are often less sharp, especially towards borders of the frame. Lenses also become less sharp when extremely stopped down (f/16 or more) due to diffraction. Wide open lenses have smaller depth of field and small focusing errors may be visible more than when stopped down.
• Camera (image file) resolution. You need to start with high enough resolution. Sometimes people resize the image by mistake to a small size, say 640x400 and then use them for printing or for large images on the web. But the image information is already gone and the images look blurry. High resolution is especially needed for printing. You will need around 300 pixels for every inch of width. So for printing a 10x15" you need 3000x4500 pixels.
• JPEG compression. Images that are too compressed may look blurry.
• Any glass layers in front of your lens (like low quality filters) may add a little bit of blur.
• Sharpening. Image processing, lens, user errors and other factors takes away some sharpness and this can be compensated by sharpening either in camera or in photo editor. Skilful sharpening can make a big difference and it is part of regular photo processing workflow for many professional photographers.
• Lens quality.
• Focusing errors. The AF may be front- or back-focusing. This can be compensated through the camera menu or by professional service. Look up "focus and recompose" on the internet for an explanation of the technique whereby photographers use the center focusing point, and then recompose causing focusing errors. With fast lenses with very shallow depth of field there may be slight movement of the photographer or the subject between achieving focus and taking the picture. It can be big enough to make the image out of focus. If you suspect that this is a problem, consider using servo focusing (sport) mode that does not stop focusing after half pressing the shutter button.

We could go on and on with this. It helps taking a closer look at the un-sharp photograph to determine the kind of blur. Are only people blurred? Is everything blurred in one direction? Is the focus plane before or behind? Does the image look pixelated? These are clues that can help find the cause of the blur.

Answer 3

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).

Answer 4

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.

Answer 5

Assuming you have eliminated camera motion (which you probably have if you are shooting at the speeds you cite), you may find one other variable not listed in your factors affecting depth of field: Distance to subject.

The closer you are to the subject, the shallower the perceived depth of field will be. You can test this yourself by setting up a row of dominoes or similar object receding from the point of the camera and spaced equally and photographing them (use a tripod), varying only one of the factors you list above. Try using, for example f/5.6 and varying the focus. You'll probably notice that the number of dominoes you can keep in focus close to the camera is smaller than the number of dominoes you can keep in focus, say, 10' away.

This is kind of a thought exercise. If you want to shoot pictures of dominoes, then by all means knock yourself out. In the absence of actually doing it, believe me, the DOF gets much shallower when focusing on near objects.

People who really know focus are more likely to focus just a bit behind the subject. If you focus on a point about ⅓ of the way into your scene, you are more likely to get good depth of field. The reason for this is that depth of field is a range. Using the domino example again, if you are focusing on the 5th domino and want that to be tack sharp, you will see that the 4th, and probably the 3rd are also sharp. So... what to do with that extra depth of field? If you don't care about dominoes 3-4, focus behind the 5th domino to bring that domino and more of the ones behind it into focus.

This is a common technique in tabletop product shooting, where the camera is often pointed across the setup. Getting most of the setup in focus requires monkeying around with the focus such that an are behind the beginning of the setup is the sharpest with the lens wide open.

If you are more interested in landscapes and maximizing depth of field, look at articles regarding hyper focal distance. This article is quite illustrative.