How Far Should You Zoom In with that Lens?

Just zoom that lens to the max whenever you want, right? Not so fast. If you want the best resolution with the biggest subject size, there might be better way.

Probably the majority of zoom lenses are at their worst resolution when zoomed to their maximum focal length. Many lenses perform much better at modestly less zoom. If you’re willing to do a bit of cropping with your editor, you might like the result much more.

Nikkor 55-300mm f/4.5-5.6G AF-S ED VR DX

Let’s take a look at the Nikkor 55-300 DX zoom lens as an example.

The 55-300mm zoomed to 200 mm

MTF50 resolution plots at f/5.6 for 300mm

The resolution plots shown above for the lens zoomed to 300mm yield a peak center resolution of 33.9 lp/mm at f/5.6, which is the brightest aperture at this focal length. The best it could do (upper right with this lens copy) was 38.6 lp/mm, in the sagittal direction. Pretty unimpressive numbers.

This lens copy appears to have a slight problem with ‘tilt’, which is usually the culprit in lenses having different resolutions when comparing opposite sides of the frame.

MTF50 resolution plots at f/5.0 for 200mm

The resolution plots shown above for the lens zoomed to 200mm yield a peak center resolution of 55.8 lp/mm at f/5.0, which is the brightest aperture at this focal length. The best it could do (upper right again) was 61.2 lp/mm, in the sagittal direction. This is a really good result, especially considering the modest cost of this lens.

Zoom resolution comparison at all apertures

If you change the zoom from 300mm to 200mm, it’s a decrease of 33% in focal length. The resolution change in the center goes from 33.9 lp/mm to 55.8 lp/mm, or a 64.6% increase! For a focal decrease of one third, the resolution jumps by two thirds! If you then crop the 200mm shot to match the 300mm shot, you get a net resolution gain of one third in the final photo!

Even stopping down this lens while at 300mm, the resolution is never competitive with the 200mm setting. It's better to cut your losses and just zoom in less.

If you can’t gain more resolution zooming out than you subsequently lose by cropping to regain the field of view, then this lens isn’t a good candidate for this technique.

Switching over to the peak resolution measurements, the 300mm-to-200mm zoom change gets a resolution change from 38.6 lp/mm to 61.2 lp/mm, or a change of 58.5%. This resolution increase is a bit less than two thirds, but still a huge improvement that is well-worth the crop to yield a field of view that matches the 300mm shot.

Let’s take a look at some other gains that accompany decreasing the focal length to try to get more resolution. Both the center and edges get higher resolution. You gain a slightly brighter maximum aperture. The lens focuses a bit quicker, since it moves the glass a lesser distance with a brighter aperture.

What’s the downside to decreasing the focal length? The background won’t be quite as out-of-focus, assuming that you leave the aperture setting alone.

Summary

This technique won’t apply to all zooms, of course. The most costly pro zooms are usually so good that you don’t need to bother zooming out to gain resolution. Many amateur zooms, however, benefit greatly by a modest focal length reduction.

It’s worth taking a closer look at your zoom lenses to see if you might get a big step-up in resolution by avoiding your zoom’s longest focal length and then cropping modestly. This technique isn’t a cure-all, but on lenses that are weak at the longest focal lengths it might get you much better pictures.

To evaluate your own lenses, you might consider using the free MTFMapper program. This program is what I used to measure the lens mentioned in this article. MTFMapper was written by Frans van den Bergh. His software was used by NASA to evaluate the lenses on the Mars Rover.