# Re: Small Apertures and Neutral Density Filters

Dudley Hanks wrote:
"Ken Hart" <kwhart1@xxxxxxxxxxx> wrote in message news:fnnh3v\$7mo\$1@xxxxxxxxxxx
"Dudley Hanks" <hanks.dudley@xxxxxxxxx> wrote in message news:jBtnj.21983\$vp3.11955@xxxxxxxxxxx
snip

Depth of field is greater at distances than close up. For example, (these numbers are made up) focus at 4 feet and DOF might be from 3.5 feet to 4.5 feet. Focus at 10 feet and DOF might be from 7 feet to infinity, for the same f/stop.

Longer focal length lens generally have less DOF at a particular distance than short focal length lenses. So for maximm DOF, use a wide angle (short) lens.

I grabbed a couple lenses off the shelf, and made these rough observations (these are 35mm film camera lenses):
35mm lens, f/16, focus at 8', DOF scale says 4' to infinity
50mm lens, f/16, focus at 8' DOF scale says 5' to 15'
135mm lens, f/16, focus at 8', DOF scale says 7.5' to 9'
Hyperfocal distance (the distance at which depth of field is the closest distance to infinity) for the 35mm at f/16 is 8' (4' to infinity), for the 50mm at f/16 is 15' (9' to infinity), and for the 125mm at f/16 is 75' (40' to infinity). These figures are not the result of testing, but rather reading the tiny scales on the lenses, so there may be some margin of error, but the general idea is there.

Moving back and zooming in (or moving in and zooming out) will change the prespective of your subject. For example, back when cars were boats and had big tail fins, advertising photos were often shot with a slight wide lens, closeup at a low angle. This made the car look even longer and the tailfins even bigger. This was not caused by the lens, but by the closeness and the angle. Conversely, a long lens tends to compress foreground and background. Again this is not because of the lens, but because the foreground and background are so far away.

As for using your zoom and/or teleconverter to increase DOF, that's another ball of wax. The construciton of a zoom lens can have effect on the DOF. Whenever possible, I like to stay with prime lenses; usually they are brighter and better quality (usually!).
Unfortunately, my Canon A720 leaves me with a limited range of choice when it comes to depth of field.

Rob Morley sent me a link to a handy on-line dof calculater, which I think will help a lot. And, the info you gave me above helps to get an over-all "perspective" of what will happen as I move my zoom around looking for just the right depth of field.

However, I'm still left wondering a bit about the attachments that are available for my particular Camera.

For instance, there is a 1.7x teleconverter available from Canon that will increase my 80 to 210 zoom by about 70%. But, if I understand teleconverters properly, that should also change the aperture as well as the focal length. This means that the minimum aperture will end up somewhere around f/13.6, which is a fair bit smaller than the minimum of f/8 I've been told will work with my camera's sensor.

Does this mean that the images produced in the longer zoom ranges will end up with a nice depth of field but be excessively soft?

Soft and less DOF.

In which case, I'd probably be better off just trying to get closer and do the best I can with whatever dof I can achieve.

Thinking mathematically for the first time in many years,
Dudley

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## Relevant Pages

• Re: Canon EOS350D/Rebel XT - Depth of field problems
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(rec.photo.digital.slr-systems)
• Re: Hyperfocal distance
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• Re: Depth of Field
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(rec.photo.digital.slr-systems)
• Re: DOF calculator
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• Re: What next when the hyperfocal is wrong?
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