Re: MACRO SHOTS QUESTION

In article <e8j1hg06s2@xxxxxxxxxxxxxxxxx>, J. Clarke <jclarke.usenet@xxxxxxxxxxxxxxxx> writes
David Littlewood wrote:

In article <e8h0fm072s@xxxxxxxxxxxxxxxxx>, J. Clarke
<jclarke.usenet@xxxxxxxxxxxxxxxx> writes
David Littlewood wrote:

You are of course correct - language develops. However, where the
development involves degrading a usefully precise technical term into
meaningless mass-market advertising-speak, it is to be regretted, and
IMO it is right to try to resist it.

The problem is that with sensor size varying all over the place "1:1" is
no longer as "usefully precise" as "filling the frame with a field of x
dimensions or smaller".

Many of the equations important for photomacrography involve
"magnification" as key parameter. Given that it is a precisely defined
term, and is useful, I have to disagree with your conclusion.

Sensor size is of course important - but the only thing it affects in
photomacrography is field of view. Depth of field, for example, is
determined only by magnification.

And these equations no longer apply if the magnification is less than 1:1?
I'm sorry, but I don't see your point.

The precise equation for DoF is T = 2*f^2*u^2*N*C/(f^4-N^2*C^2*u^2), where N is f-number, f is focal length and C is the circle of confusion you have decided is acceptable.

Clearly this is quite unusable in practice. However, for distant scenes (m<<1) the N^2**C^2*u^2 term is very small compared to the f^4 term and the equation simplifies to T = 2*u^2*NC/f^2.

This is the equation you will most often see in discussions, but people often forget it is an approximation only valid at distances where u >> f (i.e. it's a long way away).

For close-up photography, where m is say >0.1, this approximation does not work, but some different ones become valid, and

T = 2CN(1+m)/m^2

Thus in this region, DoF does not depend on lens focal length, but only on magnification and lens aperture (and, as always, on the size of the CoC you decide is acceptable).

In any case, how often do you solve equations before taking a shot?

If you are using a film camera with no built-in metering, always for exposure measurement (true exposure = external meter exposure*(m+1)^2). For DoF calculations, you need to do it unless you have a digital camera and can review the results (at full resolution, not on the weedy little preview screen) at once.

I agree, less essential today than 10 years ago, but still happens. I feel the argument that "automation means you do not need to know how it works" argument is dangerous - like believing that calculators mean you don't need to know arithmetic.

In truth, I think this digression into arcane mathematics is obscuring the real point (but you did ask....). This is that the generally accepted definitions recognise these very real distinctions:

Close up photography (m = 0.1 to 1.0) is what you can do reasonably well with conventional photographic gear (normal lenses plus close-up lenses or extension tubes). Camera handling is fairly normal, and sunlight or fairly simple artificial lighting works well.

Photomacrography (m >1.0) is what you can only reasonably do with specialised gear (specialised macro lenses, bellows, rigid stands etc). While hand-held work may be possible at the lower end, most work requires very rigid stands (an order of magnitude better than most people are used to using) and specialised lighting will normally be required.

This is in fact the most useful definition, since it gives those coming to the field a clear clue to what they need.

David
--
David Littlewood
.

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