Some Hypermathematics on Focal Lengths

Started Sep 12, 2013 | Discussions thread
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fotobert Contributing Member • Posts: 910
Some Hypermathematics on Focal Lengths


I was wondering, why my Tamron 70-200 seems to be a bit to long for portraits. So I did some test shots and calculations. I compared my Tamron 70-200, Pentax-M 100, Pentax 18-135 and a Sigma 70mm macro.

The results are somewhat strange. Comparing the focal lengths for a given width of field (or just the other way round) it is:

1 Tamron mm = 1.25 Sigma mm = 1.64 Pentax mm.

Pentax and Tamron are consistant, i. e. the old SMC Pentax-M 100 fits in the Pentax 18-135 scale and both ends of the zoom range of my Tammy match to the width of field. So what does it mean?

My Tamron is either 70-189 Tamron-mm or 87.5-236 Sigma-mm or 115-310 Pentax-mm

My Pentax is either 11-82 or 15-110 or 18-135 mm.

The distance of the test shots was quite short (1200 mm, approx 4 ft), so someone might say "IF lens!!!", but the Tammy is not the only IF lens in this group.

Any ideas? At least one manufacturer needs some adjustment

 fotobert's gear list:fotobert's gear list
Pentax K-5 Pentax K-3 II Tamron SP AF 70-200mm F/2.8 Di LD (IF) MACRO Pentax smc DA 18-135mm F3.5-5.6ED AL [IF] DC WR Sigma 50mm F2.8 EX DG Macro +18 more
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