One of the qualities photographers most want to improve in their photographs is image sharpness. I think subject matter, composition, and lighting are equally or important, but let's address image sharpness since so many people ask about it. Although these image sharpness tips are for underwater photographers, many of these sections apply equally for land photography also.
Most common causes for lack of image sharpness:
1) Not being in the plane of focus. Every photo has a plane of focus where sharpness is maximized. Obviously the subject must be in this plane to be sharp. With close-up macro photography, small movements or surge can put the subject out of the plane of focus. Look over the depth of field tests.
2) If photographing underwater, there is loss of image sharpness from too much distance through murky water. A underwater subject needs to be close to the lens to be sharp, since sharpness falls off rapidly as you put more water in between the lens and the subject. This is especially true when the visibility is not great. This is a large advantage of ultra-wide fisheye lenses, they allow you to get very close to a large subject for a shot. 12 inches or closer to the lens is ideal. Even in clear water, you can lose sharpness quickly. For a very sharp photo, you'll usually want at least 40 feet (12 meters) of visibility.
3) Incorrect camera settings let in too much ambient light. Usually a strobe, firing at 1/1,000th of a second or faster, will freeze any subject movement or camera shake. However, people often have their camera on auto mode, P mode, or A/S mode with no exposure compensation. All of these choices can cause the camera to expose for ambient light, which will usually result in a slow shutter speed, un-sharp photos, with washed out colors due to a very weak strobe being fired. Shoot in full manual mode unless you really know what you are doing. Read more about underwater camera settings.
4) Not using a strobe. When shooting ambient light, it can be difficult to get shutter speeds necessary for very sharp photos with ambient light, unless they are taken in bright light at large apertures. However, absolute sharpness is not necessary for a great photo. When shooting ambient light, keep the sun behind you, or shoot silhouettes, unless you are going for a moody wreck shot.
5) Having the improper dome, extension and diopter for a wide-angle lens. For example, the nikon 12-24mm lens will need a large dome port, a 40-60mm extension, and a +3 or +4 diopter. Read about dome port optics
6) Shooting at too small an aperture, like F40. Diffraction will start to blur the details. But don't get too hung up on this, it's only noticeable at very small apertures. Use F8-F11 for optimal image sharpness. See the results of my diffraction tests on the 105mm lens, and read about aperture and depth of field in underwater photography
Even Sharper images
Once your photo is sharp, let's look at how we can make it sharper:
Shoot with a quality prime lens (F2.8) at optimal aperture (F5.6 - F13). These lenses usually have great image sharpness. Every lens has a "sweet spot" where sharpness is maximized. This is usually when the lens is stopped down 1-2 stops, but if you want to be sure, test it out yourself. But make sure you have enough of depth of field, especially when shooting very close-up.
Get very close to your subject, within a few inches, to minimize loss of sharpness from shooting through water.
Fill the frame with your subject, so no cropping is needed.
Shoot in RAW or at the highest quality, largest size JPEG
Using a large dome port with the proper extension
For areas of the photo not lit by the strobe, make sure the shutter speed is fast enough to freeze the motion
Look at your lighting and contrast. Side-lighting will bring out detail, making a photo appear to be sharper, as will higher contrast. See types of lighting underwater
Make sure the photo is properly exposed, especially the area in which you want to see detail. watch out for lost shadow detail, or blown out highlights in 1 color spectrum.
Use spot focus. If your camera has the ability to move the focus point with the cursors, move the focus point over the area of the subject that you want in focus (e.g. - eyes).
Check your histogram, and make sure all 3 color channels are properly exposed.
Post-process your photo, adjusting levels to get the proper amount of contrast. To slightly sharped a photo, use the unsharp mask, and view your photo at 100% magnification to see the results.
This photo has excellent image sharpness. It's a colmani shrimp on a sea urchin, taken in Anilao, phillipines. Note the texture on the spines of the urchin. This texture is also caused by side-lighting the photo, and sufficient depth of field. Image sharpness on the shrimp would appear to be lost if the photo was over-exposed.
Even blown up to 100% magnification, this chameleon is very sharp, especially the point of focus at it's eye. 105mm VR lens, F10, 1/400th, ISO 320, daytime. Good depth of field, steady handholding, focus point on the eye, vibration reduction turned on, and a shutter speed faster than 1/160th (35mm equiv of 105mm is 157) all helped create a sharp image. Shooting "topside" above water, it is much easier to get a sharp imageness, because there is no water to suck the sharpness out of your photo.
Getting sharp images in a wide-angle photo underwater can be difficult, especially when there are fine details in a subject like sea fans. I'm getting good at it but there is always room for improvement. It helps to have clear water, get very close with a wide-angle fisheye lens (30cm away), expose the image properly (I slightly overexposed this one in some parts), and shoot at F11 when possible. Nikon d300, Tokina 10-17mm lens, F9, 1/160th, ISO 320
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