If you are using a Nikon D800, chances are that you want to make big prints or crop aggressively. To get the kind of sharp images that can stand up to these stresses, you need a tack-sharp image. Focus is a big part of getting sharp images.

As I mentioned yesterday, camera shake is a critical factor in determining image sharpness. For best results, you want to use either a very fast shutter speed or a tripod to eliminate camera shake from softening your images. Today, I’ll take a quick look at focus accuracy and how you can maximize it with your Nikon DSLRs.

Modern camera autofocus systems are amazing both in terms of their speed and accuracy. The Nikon D800 and D4 cameras use an updated version of the 51-point Multi-CAM 3500 focus module that debuted with the Nikon D3. Despite the fact that this focus system is amazingly fast and sensitive, it is still prone to small errors. In fact, a small error must be built-in to any AF module by design; otherwise the camera would continuously “hunt” for focus. Focus is also affected by lens design and normal lens sample variation.

Methodology

In these tests, I used my Nikon D800e on a tripod with mirror lock up (MLU) to eliminate camera shake. I used the 24-70mm f/2.8 AFS G Nikkor lens set at f/3.2 (just slightly stopped down from wide-open). I compared image sharpness under three focus conditions:

• Live View Focusing
• CAM 3500 focusing
• CAM 3500 focusing with AF Fine-Tuning

I compared in-camera JPEGs with Standard Picture Control and Sharpening set to +4 for a standard comparison. If you shoot RAW, you can get even sharper images by using custom sharpening on your D800 RAW files.

Live View Focus Test

For my first test shot, I used Live View to focus the camera. Live view focusing is different in that the image is focused using the camera’s image sensor with a contrast-detection system. This is not as fast as the normal AF module, but it’s definitely the most accurate form of focus. I like to use Live View focusing to test the sharpness of my lenses, because it’s the least prone to focus error.

My test shot (full-frame):

100% (1:1) crop

As you can see, there’s a fair bit of detail here that really comes out. This image is very sharp.

Autofocus with the CAM 3500 module

Next, I used the CAM 3500 focus module with the center point. Here’s the result and 100% view:

Full Frame Image:

100% crop:

It’s painfully clear that this image isn’t tack-sharp. It might be fine for a small print, but it has nowhere near the sharpness of the Live View reference image. Given that I used a tripod and MLU, this image isn’t showing camera shake. Nor is my house moving (although California residents might be prone to seismic activity). The culprit here is simply a small focus error in the AF module.

AF Fine-Tuning

My next step was to use my LensAlign tool and dial in AF Fine-tuning on the D800e body (located under the Settings/wrench menu). I came up with a setting of -10, and got the following image:

Full Frame Image:

100% Crop:

This image is at least as sharp as the Live View image, and it would stand up to large prints or aggressive cropping.

Conclusions

If you want big prints from your D800, you need sharp images. The same is true if you want to crop your images aggressively. With its 36 megapixel resolution, you can get incredible details from the Nikon D800/e– provided that you start with accurate focus. Live View focusing is a great way to focus your images, but it’s fairly slow and doesn’t work well for hand-held images. However, you can use images captured with Live View focusing to see if your camera requires any focus tuning. If your shots captured from a tripod with normal AF seem soft, you might want to try the AF Fine-Tuning option. If you do use AF Fine-Tuning, make sure you run the calibration steps in a controlled setting. It can be easy to introduce focus errors if you don’t set the calibration correctly. I recommend using a target like the LensAlign system.