Comments on: Understanding Canon EF-S, Nikon DX and similar lenses for small sensor digital SLR cameras

By: Wayne J. Cosshall

Wayne J. Cosshall — Sat, 19 Nov 2011 03:21:00 +0000

I’d say, basically yes. And in actual practice probably yes too. It might be possible to make the EF lens fit the same diameter but it is probably easier from an optical design perspective to go larger diameter.

By: Carl Oxwall

Carl Oxwall — Thu, 27 Oct 2011 23:01:00 +0000

Thanks (again) Wayne!

This explanation was somewhat complicated, still.

However, earlier this very evening I found this really satisfying answer to my main question; this by reading on another web board. The following theoretical hypothesis was stated:

Take two 50mm f/2 lenses. One with an EF design; whereas the other one has an EF-S design. Note that the max aperture and focal length are exactly the same for both lenses, only thing that differs is that one casts a 36x24mm light circle on film/sensor; whereas the other cast a APS-C sized light circle. Now, by these two lenses, the EF version is — by it’s raw nature(!) — thicker (in terms of lens barrel diameter) than the EF-S variant.

Correct, or wrong? Guys in that other thread verified this as correct. I was really pleased, since it was sort of a great relief to me as well. Now, your verification is the “final nail in the coffin” I need to let go of this much painful and/or mind-itching question.

By: Wayne J. Cosshall

Wayne J. Cosshall — Thu, 27 Oct 2011 21:14:00 +0000

Hi Carl.
Ok, two parts to this answer.
Remember that a lens consists of many elements. To greatly simplify, the front element gathers light from the field of view, the internal elements focus and shape the light front and the rear element ‘fans’ this out to hit the image circle. Now there are two issues with image circles: the actual spread out of light from the back of the lens and how much of this area is well corrected optically and so can be usefully used in a camera. The smaller the area that needs to be sharp, the simpler the lens can be or the more compact, etc it can be made. Small sensor cameras only require a small, sharp and well corrected image circle. The light may well spread beyond that but at a degree of image correction that can’t be used for normal photographic purposes. It is not really the distance of the sensor from the back element, but, if you like, the rear field of view or angle that needs to be covered sharply.

By: Carl Oxwall

Carl Oxwall — Thu, 27 Oct 2011 15:42:00 +0000

Thanks for a great article; easy to understand. Now, only thing I question myself is, how are lenses “designed” to throw either a large or a small image circle? Someone told me that it basically came down to the simplicity of the sensor being closer to the lens producing a smaller image circle (like DX/EF-S lenses behave), or sensor being more far away from the lens (like FX/EF lenses behave). Can you verify this fact Wayne? Cheers!

By: Wayne

Wayne — Mon, 25 May 2009 00:19:43 +0000

Some manufacturers did produce 40mm lenses, but it never seemed to catch on. The original Leica used a 50mm as the normal lens because it was easier that the time to make a sharp 50mm than a sharp 43mm.

By: Wayne

Wayne — Mon, 25 May 2009 00:10:41 +0000

Hi Anne,

Glad it was useful to you.

By: Ashok Kandimalla

Ashok Kandimalla — Sun, 24 May 2009 16:37:57 +0000

Greetings:

You said “This is why a 50mm lens is considered ‘normal’ on a full frame 35mm camera, as it is the closest common focal length to the diagonal size”

I could not undersatnd the words “closest common focal length” – Why was 50mm chosen rather than 40mm which is closer to 43mm.

Thanks and regards

Ashok

By: Anne Bellenger

Anne Bellenger — Sun, 24 May 2009 14:13:44 +0000

Very useful article since I shoot Canon and Nikon. Your comment on Canon’t TS lens was interesting. I just bought a Nikkor PC (perspective control) 85mm f2.8D tilt-shift-rotate for my D700. I’ll remember what you said about tilting off center. Very helpful.