Smaller format zooms more efficient

Benjamin G · March 30, 2011

I've noticed that the smaller the format the faster the zooms are.

For example the Fujinon 5.5-88mm that came with my 1/3" camera is a T1.4. I've seen 16mm Angenieux zooms under T2. Yet when you get to 35mm T2 is about as fast as you can get for a zoom.

Is it just because more glass is needed to cover the sensor and more glass equals more light loss? Or is there something more to it than that?

M Joel W · March 30, 2011

More glass is one factor, but not to the extent that more glass means more light loss (with modern coatings you lose very little light and contrast even with complex designs); it's mostly a factor of more glass costing more money and resulting in an enormous lens. The front element has to be as large as the maximum aperture and so a 240mm f1.4 lens would need a 171mm-wide front element, which is HUGE and very expensive and would likely result in tons of flare. Image circle is another factor. I shoot some large format and lens designs list coverage in degrees (so the longer a lens the more coverage) and the word is that the widest lenses of a given design are also the sharpest over their (more limited) image circle. So it gets really hard designing fast, wide, sharp lenses for bigger sensors. I have a 135mm f5.6 plasmat that weighs half a pound (and is disgustingly amazing); its 240mm f5.6 brother weighs more than four times as much and seems even bigger than that.

Equally important is the fact that you need some depth of field to shoot. Imagine pulling focus at ultra-telephoto t1.4 on super35. No one could do it! I assume it would be possible to make a t1.4 zoom for super35, but it would be incredibly expensive and probably have rather poor image quality (flare, spherical aberration, etc.) and be almost unusable in terms of size.

You'll notice it's the same with stills. For 35mm still cameras you usually get primes in the f1.4 range; for 6x7 and 6x6 cameras f2.8 is about as fast as it gets; for 4x5 and 8x10 f5.6 is fast (f2.8 exists and by most accounts is amazing), for ULF Schneider makes f11-f22 lenses. The limitation seems to be related to the size of the front element and depth of field. Despite normal aperture values of f16-f64, focusing in 4x5 is an art and a science.

This is mostly second hand and maybe full of fuzzy science. I have a book on lens design but am scared to read it. It's full of real science. I'm just repeating what I've been told and have found from experience.

Edited March 30, 2011 by M Joel Wauhkonen

Benjamin G · April 4, 2011

I had noticed it in still lenses as well as I recently got a 35mm adaptor with a Nikon Mount (and a PL mount). This is what actually led to me noticing the big difference as I was comparing cine lenses and still lenses as well as lenses from smaller formats.

What is the title of that book on lens design? I would like (to try) and read it and see if I can get a better understanding of why things are the way they are.

Thanks