Brothers (Perth, Australia): D800 70-200mm shot at 200mm, f/2.8, 1/60sec and ISO 100. This was focused on my youngest son’s eyes and is tack sharp in that area.
There are a few basic tenants of photography that cannot be ignored and one of the most absolute is that something in the image needs to be in focus. I do not mean more or less in focus…I mean it has to be TACK sharp.
I love an image that has great Image Quality and whose subject is absolutely sharp. The image can be of questionable artistic value but I can appreciate the technical aspects involved in getting such an image. Even looking at all the images I have shot the ones that I consider tack sharp are in the minority.
I have decided to write up a three post special focused on how to ensure your images are tack sharp. To be clear there is very little “new” information in photography and going through various sites I have found several that have written on this topic very well. I will give references whenever appropriate to document where you can go for more information or better help.
So, in proper Transit fashion vill up the glass with Merlot, kick up the feet and settle in for some photography shop talk.
Lesson 1: Autofocus Calibration
Every camera and lens manufacturer builds to a certain specification. They build their internal quality standards in order to deliver consistent output. Obviously they build within a certain tolerance which means that each lens or camera that comes off the assembly line are not EXACT copies of one another. They are nearly identical and how “nearly” depends on their tolerance.
But what happens when you get a camera or lens that is on one extreme of the tolerance? The result can be (although not always) a lens that does not focus where it is supposed to be.
When you pick up a lens you might feel that the lens is a bit “soft” and you may hunt for a properly focused image. When this happens you need to go back to basics. You need to remove as many variables that can affect your camera’s ability to focus (see the next couple of posts on this topic). If you still feel that a lens is soft then you need to get a little more serious and do some Autofocus Calibration First Aid.
I have seen plenty of forums that tell everyone that their camera and lenses are fine and the issue is with the shooter. This might indeed be the case but since Autofocus Calibration is so easy to do I feel that deterring someone from testing their lens is lazy. If you want to test your lens then go for it! It might even be fun.
Prepare the Tools:
Autofocus Calibration can be done at home but you need to prepare a little bit. I have fun doing this part as it reminds me of the art projects I did as a boy in school.
- Download an Autofocus Test Chart. I use Jeffrey Friedl’s test chart found here. I like it because it is simple and he does a great job explaining how to properly print it.
- Get matte adhesive paper. Trust me on this one. Matte paper will prevent reflections of light and will help ensure your camera has the best chance to autofocus. Follow Jeffrey’s instructions on how to print.
- Stick the test chart to some stiff cardboard. Make sure it is FLAT and stuck on well.
That is it. You are set up. You have your reusable test chart and are ready to go.
Autofocus Calibration Stand:
You will need to set up your test chart on a stable platform. You need it at a 45 degree angle. I found that the best was to borrow my daughters music stand that allows you to adjust the angle at which the music sits. I then taped it into place to ensure the wind would not blow it over.
Please note that the shot below was taken in some uneven lighting. This is only for the picture. All the tests need to be in consistent lighting.
Please note that the dappled lighting here cannot be used when testing lenses. Find constant light.
As you can see in the picture below it is easy to put it on a 45 degree angle.
The final bit of kit you will need is a tripod. The make, model or color really does not matter. Just make sure it is sturdy.
Calibration First Aid:
Now would be a great time to refill that glass. Everything up until now has been fun. Now we have to do some fairly repetitive tasks and most importantly we have to trust our eyes! So believe me it is a good time for a refill of Merlot.
Place your camera and the lens you wish to test on the tripod. Make sure it is secure. Put is between 2 and 8 meters from the test chart. How much will depend on the focal length of your lens. If it is 24-70 two meters should be fine. If it is 70-200 then take it out a bit more.
This picture is drawn as if looking from the top. The camera must be square with the test chart.
Make sure your camera is level on the tripod. Then ensure that the test chart is completely square with the camera. The only angle you want is the 45 degree tilt of the chart.
Once everything is physically set up and then prepare your camera.
- Set your camera to aperture priority and set it for the widest aperture (the smallest F-Stop number). For example my Nikon 24-70mm was set at f/2.8.
- If your lens is a zoom lens then zoom so that the paper takes up the majority of the frame.
- You need to have a fast shutter speed. Anything over 1/60 of a second should be ok but try to get it a bit faster if possible.
- Set it to self timer and set that at 2 seconds. You can do this or use a remote trigger. The idea is not to move the camera when it is taking its shot. Movement can easily be confused with poor focus.
- The focus point should be on the central line.
Ensure the focus point does not touch anything but the middle line.
Now manually move the focus to infinity and press the shutter release allowing autofocus to grab the focus and take the shot. Manually move the focus to the other extreme and press the shutter again. The idea is to have autofocus move the focus from one extreme to where it believes the focus should be.
After taking several shots (to see if the shots are repeatable) then switch to live view and focus manually. This is the control image that you will compare the other images to. Download the pictures and compare them side by side in Lightroom. They should be the same. If they are not you will need to apply a manual calibration for that lens. Look to your camera manual to find out how.
This is what I call the real world test. Here you want to find different distance objects to capture to see how your focus is doing.
Here I chose some objects on a string. I got low and shot at an angle to show some depth. I focused right where the red box is. On lightroom I zoomed in and found that the focus was dead on.
What did I find when I tested my own lenses? Well it turned out my gut feeling was right all along. My 70-200mm lens was tack sharp on focus. My 85mm was about right. But my 24-70mm was way off. I ended up putting a -12 adjustment and found that to work. This is a pretty large adjustment considering that my Nikon allows for up to a +20 to -20 adjustment.
This is a simple project for a lazy Sunday and the rewards are amazing. Even if you do not need to adjust your lenses at all it is worth the effort to know that your lenses and cameras are calibrated together.