Is my thinking about equivalence right?

Started 3 months ago | Questions thread
James Stirling
James Stirling Senior Member • Posts: 8,232
Re: Un-equivalence

Bassam Guy wrote:

jwilliams wrote:

Muster Mark wrote:

Hi all, I realize this is NOT the topic everyone loves to discuss and I totally understand. I hope no-one feels annoyed by this post. That said, after some investigation I have come to the conclusion that "traditional" equivalence math is only useful as a rough heuristic to get a sense of what DOF will be and NOT a reasonable way to compare individual lenses or judge cost/performance of lenses in different systems.

No, not really. Equivalence is more than DOF or even mainly about DOF. I'll not go into any long winded discussion, but here's an article on this site that is a good read on the subject.

Equivalence is useful if you have any interest in light (and as a photographer, you probably should): Digital Photography Review (

That is not true. 'Equivalence' implies at least two things. If 43 is your only sensor size then equivalence is meaningless.

Facts are facts whether you use them or not and it applies equally to all systems . People who buy the the £1100 40-150mm F/2.8 pro as opposed to the £195 40-150mm f4-5.6 , presumably appreciate the abilities that two extra stops of light gathering bring at the long end. Or perhaps they just like carrying larger heavier gear.

The problem here is not those who understand equivalence and conclude not unreasonably that it is not of much importance to them . But those who endlessly post factually inaccurate statements about it some of whom amusingly get stroppy when they are called on it. There are some posters here who have been posting BS about equivalence going all the way back to the start of FT despite being corrected literally 100's if not 1000's of times.

That demonstrates an impressive degree of ignorance , denial or straight up lying. As I find it hard to accept that anyone could struggle to comprehend what is after all a simple concept.

For anyone actually interested in the facts about equivalence this post below by Bob covers it very nicely

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Jim Stirling:
It is undesirable to believe a proposition when there is no ground whatever for supposing it true” Russell
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