Q & A : Lens Size and Image Circle

To submit a question to The Photo Video Guy Q & A send an email to ross@thephotovideoguy.ca Another query from Darren;

"On a full frame camera the image circle has to be large enough to encompass the full size of the image sensor.  My question is: On a full frame mirrorless camera can the size of the lens be smaller?  On a DSLR camera the lens has allow for the mirror. On a mirror less camera the lens does not have to allow for the mirror.  Does this mean a lens like the 70-200 f2.8 can be made smaller for a mirrorless camera??"

The answer to this question is a qualified "yes".

The size of the image circle is an exercise in optical physics.  The arrangement of lens elements in a lens is designed to accomplish the goals of the lens designer, one of which is to define the placement of the image circle to the focal plane.  It's a common understanding that the size of the lens dictates the size of the image circle.

This is not necessarily so.  The size of the lens is also impacted by the distance the lens to the focal plane.  For example. the lenses on my Hasselblad have to create a larger image circle than the lens for the Canon full frame.  The Lenses are physically larger.  Back when I was shooting medium format film, the lenses for my Mamiya RX67 were much larger than for the Hasselblad 500CM.  Both arrangements had to deal with allocating enough space for a mirror.

However, on the Sinar which is a large format camera where the lens board is connected to the focal plane by a bellows, the lenses are actually quite small.

Thus one can conclude that lens size is only partially related to image circle size but also must take into account the distance from the rear element of the lens to the focal plan.

In looking at mirror less camera / lens combinations, we find that most mirror less lenses are physically smaller than their counterparts for APS-C and Full Frame.  This can be attributed to the lack of requiring space for a mirror, a considerably narrower camera body and the requirement for a much smaller image circle.

Creating a larger image circle where the lens mount distance doesn't change and the amount the rear element can enter the camera body is not limited by the presence of a mirror requires that the work must be done optically.  While this could mean elements larger in diameter, it could also mean elements with more radical curvature, the use of more dispersion managing elements and the use of more aspherical correction in the elements.  Lens speed in terms of maximum aperture is also going to be a big factor in physical lens size.  Building that 70-200/2.8 for the full frame mirror less could result in a physically smaller lens, but it may not.  That decision is going to be up to the lens designer.

My guess is that they will work towards smaller and lighter at the cost of maximum aperture.  In the case of full frame mirror less the aperture is a direct comparator when it comes to depth of field whereas f/2.8 on a full frame has less depth of field than f/2.8 on a micro four-thirds built lens when images are compared.

So the answer is a qualified "yes" but only the lens designer will have the final word.