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Tip of the Month

Use a shorter lens for depth of field
and crop!

November 2009

cheetah chase

The cheetahs were close, and were stalking a mixed herd of impala and reedbuck. I had a choice -- I could use my 500mm lens and risk keeping the cheetahs and their prey within the frame and getting one of the players sharp or be conservative and safe by using a shorter zoom lens where I'd be able to catch all of the action, albeit with a smaller image size.

cheetah n reedbuck

I chose to play it safe, reasoning that with three cheetahs and the potential of a chase going anywhere I had a better chance of catching the action with my zoom. The lens I used, a 70-200 f4 zoom, is very sharp, and, provided I caught the action, I was fairly confident the images would be sharp, that is, if I hit focus.

The lens choice proved to be the correct one, as the cheetahs chased the reedbuck almost into our vehicle, taking the antelope down within twenty yards of our landrover.

Afterwards, I momentarily fretted that I could have had a larger image size, and the potential of a more dramatic image if I had used my 500mm lens. But as I thought about this I realized that I really did make the right choice, and here's why.

As we teach in our Complete Nature Photo Course, as image size increases, depth of field decreases. So, although I may have filled the frame with just the heads or faces of the antelope and cheetahs at the moment of the take-down, it's quite unlikely that more than one character in that drama would be sharply in focus.

Also, it's quite likely that my longer lens would have been too much, that I would have only had a portion of all of the action within my frame. This was especially true as the chase began, as I've shown in the first image (above).And, as I said, my image size would have larger with the 500mm and my depth of field would have been reduced. So, either the cheetah(s) or the reed buck would have been in focus, but not both, and that's IF I was lucky enough to hit focus at the critical moment.

However, with the smaller image size of the 70-200 (zoomed, I believe, to approximately 110mm) the depth of field was increased, and covered all of the subjects in the frame. Provided the image holds up -- based upon the pixel count of the camera, I could crop down to a achieve a strong image while enjoying the depth of field the smaller image size originally provided.

cheetah and redbuck

Here I've enlarged a portion of one of the images in the series, taken just after the shot shown above. You can see that the sharpness extends from the face of the reedbuck to the faces of the cheetahs. I didn't do the math, but I recon that this crop approximates the field of view of my 500, but the depth is dramatically different. At 500, I'd be lucky to have even one of the three subjects in focus.

You might argue that closing down the aperture may have given me the required depth of field, but doing so, the shutter speed would suffer. Of course, I could then raise the ISO but then I'd be potentially dealing with degraded image quality or noise, and I still doubt if I'd have the sharpness via depth. So, I'll still stand by recommendation of using a shorter lens.

This same principle would hold true for macro work, where, as image size increases depth of field decreases, it would be prudent to refrain from going for maximum magnification and instead seek better depth of field by framing for a smaller image size or less magnification. Cropping, afterwards, would bring back the full-frame image but with the enhanced depth of field captured originally with the smaller image size.


Previous Tips, July 2009 onward

Get Professional Help!
Mini-Molar Bag
Access America Trip Insurance
Bogen Base for Macro Work

Archived Tips of the Month
prior to July 2009
Most of my original Tips of the Month for the last several
years are available through this link. The 'look' is from my
original web site, although if I ever have enough time I might redo these pages to match the new web site But that's not a high priority.