Lens comparisons re DOF a bit simplistic or is there more to it?

Started Jun 8, 2014 | Discussions thread
Rod McD Veteran Member • Posts: 5,752
Re: Lens comparisons re DOF a bit simplistic or is there more to it?

Hi again Deed,

There have been a great many threads about the 'equivalence' of lenses on different formats and all that it implies for light gathering, DOF, etc.

There is a basis for all of this - we calculate DOF based on FL, distance, aperture and a certain circle of confusion.  If you use a DOF calculator for a given lens, and then change the sensor size, the DOF output for the very same lens changes, despite the fact that the lens doesn't "know" what sensor lies behind it.  The way the formulae do this is to change the COC, and the argument runs that you magnify the image in printing it to a different degree from each sensor size.  The distance also has to change for the same FOV.  So, yes, one can get shallower DOF by using a large sensor.

As an aside, there is an assumption in all these debates that never gets mentioned.  The calculations are based on having a plane of focus that is vertical and parallel to the film/sensor.  If you happen to use movements, you can chuck the whole lot out the window because you can adjust the plane of focus if the subject is suitable. (Some aren't). You can choose where the optimal focus points lie.  It's one reason that view cameras or TS lenses can deliver such fabulously sharp landscape results.  And then the larger sensor wins.  (I guess a stalwart might respond that one could equally use TS lenses on a small sensor, but it's a moot point because no-one makes them for MFT or APSC).

All that aside.  I don't think there are any 'equivalent' lenses.  One buys into a system, not a lens, and systems render in different ways for many reasons - lenses have many characteristics, not just one, as do the sensors and as do in-camera processing systems.  Even with RAW.  There is no true RAW - every camera brand produces a unique file, as Fuji owners know only too well.  The decision to buy and use one system over another is complex and quite personal.

Fast lenses?  I agree that fast means the aperture, and the same effect isn't simply achieved by upping the ISO.  OTOH, I'm personally more interested in landscape and spend more time stopped down than wide open.  I understand that fast lenses are good for subject isolation.  I just don't do a lot of that type of photography.  OTOH, I've never really understood why so many people rave about their use for low light.  Fast lenses are no panacea for low light. Their wide apertures are useful only if the subject warrants very shallow DOF, and mine typically don't.  Horses for courses.

Cheers, Rod

 Rod McD's gear list:Rod McD's gear list
Canon PowerShot G1 X Olympus Tough TG-4 Fujifilm X-T1 Voigtlander 90mm F3.5 APO-Lanthar SL II Fujifilm XF 35mm F1.4 R +9 more
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