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Forum Elite
Posts: 14,334
Registered: ‎12-07-2012

Re: wide angle converter

" An f/1.8 50 mm lens with a subject at 3 m has a DOF of 2.8 - 3.2 m

 An f/1.8 21 mm lens with a subject at 3 m has a DOF of 2.3 to 4.3 m"

                                                                                           = 1.6 meter

"I used a depth of field calculator."

 

Not correctly it seems!  Because the DOF is increased with the shorter FL. A 20mm lens at f1.8 and a subject 3 meters away has a DOF of 1.6 meters.  A 50mm lens at f1.8 and a subject 3 meters away has a DOF of only .7 meter. 

EB
EOS 1DX and 1D Mk IV and several lenses!
Esteemed Contributor
Posts: 4,935
Registered: ‎02-17-2016

Re: wide angle converter

I just checked with another DOF calculator:

http://www.dofmaster.com/dofjs.html

 

For an APS-C at f/1.8 and a subject distance of 3m:

 

50 mm DOF is .24 (I had .32)

21mm DOF is 1.45 (I had 2.01)

 

I certainly had the increased DOF at shorter focal lengths* in there, so I am not sure what your beef is.

 

*Though I misstated it in the text.

 

Esteemed Contributor
Posts: 3,854
Registered: ‎06-11-2013

Re: wide angle converter

The whole point of an interchangeable lens camera is that you can change the whole lens... and there are many benefits from doing so.

 

When you put a focal length multiplier/reducer on a lens, you can create a lot of problems.  Not least... is that the camera doesn’t know you put this on the front of the lens.  It alters the focal ratio of the lens so there’s no free lunch (you no longer get the DoF of a 50mm lens when you put a reducer on it).  But since the lens is going to report what it is (and doesn’t know about the focal length reducer/multiplier) it can screw up it’s notion of metering and focus.  And of course the optics aren’t optimized for it.

 

MUCH better idea to get a lens with the focal length and focal ratios you need for the type of photography you want to do.  That’s why the lenses are removeable.

 

Tim Campbell
5D III, 5D IV, 60Da
Honored Contributor
Posts: 5,558
Registered: ‎06-25-2014

Re: wide angle converter


@TCampbell wrote:

The whole point of an interchangeable lens camera is that you can change the whole lens... and there are many benefits from doing so.

 

When you put a focal length multiplier/reducer on a lens, you can create a lot of problems.  Not least... is that the camera doesn’t know you put this on the front of the lens.  It alters the focal ratio of the lens so there’s no free lunch (you no longer get the DoF of a 50mm lens when you put a reducer on it).  But since the lens is going to report what it is (and doesn’t know about the focal length reducer/multiplier) it can screw up it’s notion of metering and focus.  And of course the optics aren’t optimized for it.

 

MUCH better idea to get a lens with the focal length and focal ratios you need for the type of photography you want to do.  That’s why the lenses are removeable.

 


Those adapters are relics of a time when zoom lenses were mediocre and relatively expensive. The adapter was a cheaper alternative that didn't cost you all that much in image quality.

 

Zoom lenses are still relatively expensive, but now range from good to excellent in quality. Adapters are still a cheaper alternative, but usually cost you dearly in image quality. And as Tim pointed out, the modern symbiotic interactions between camera and lens are not supported by the adapters. (Note that their purveyors are fond of calling the adapters "lenses", but that's really inconsistent with the current meaning of the term.) Tim, Ernie, and others have advised against their use, and that advice is best heeded.

Bob
Philadelphia, Pennsylvania USA
Forum Elite
Posts: 14,334
Registered: ‎12-07-2012

Re: wide angle converter

"I certainly had the increased DOF at shorter focal lengths* in there, so I am not sure what your beef is."

 

 

There is no "beef" !  I simply reacted to this statement.  Smiley Frustrated

 

"You can't get a wide DOF with a wide angle lens."

 

Obviously you can.  I certainly had no intention to offend you.

EB
EOS 1DX and 1D Mk IV and several lenses!
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