October 20 2011 - Photo Jottings

October 20 2011


Accurate focusing is crucial for sharp shots using fast, telephoto lenses.

I added the images and text below to the Minolta AF 135mm F/2.8 review, not because this lens is different from other telephoto fast lenses, but to show people how critical it is to focus correctly for the sharpest shots.  The cropped images below were all taken at F/2.8, ISO 100, -0.70eV and 200sec.

Accurate focusing is crucial for telephoto lenses when set to wide apertures, such as the 135mm F/2.8 here.  In the top crop, the camera focused slightly behind the house, you can see this by looking at the background foliage above the house, it’s sharper than the other two crops.

The middle crop was focused slightly in front of the subject, although this shots appears better because of the lack of magenta color fringing, it’s really not quite as sharp as it could be.

The bottom crop is focused correctly on the house, you can see the foliage out front and the metal window guard on the left is sharper here.

The range of perfect focus is super slim, and the differences in focusing accuracy in these crops highlights the limits of the camera’s AF system.  I used the Sony A580 for this test, but even using the A900 with micro adjustments enabled still isn’t perfect all the time.  The camera’s AF system chose the focus for the above crops, each time slightly different.  The best way to see if you have your subject in perfect focus is to use live view, focus check and magnified to the max.  At smaller apertures, focusing isn’t so critical, that’s why cheap cameras come bundled (kit) with slow lenses, it’s not just the lower cost, it’s mostly because the AF system doesn’t have to be very good to produce
sharp shots.

This lens focused correctly when set to the infinity hard-stop, but if you’re at shorter distances, you’ll want to do a focus check to be sure of getting the sharpest shots.  The above 100% crops were taken almost 200 yards (200m) away from the camera, so infinity for this lens at F/2.8 is somewhere beyond that distance.

Focused slightly behind the subject
Focused slightly in front of the subject
Focused correctly
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