A question to all who know about lens design

[Frank Sanders]

I like the Sony Zeiss 55/1.8 very much, it sharp and its compact. My question is why cant a 35/1.8 or a 85/1.8 be built in that compact size? Is only 50 or 55mm possible because of lens physics?

Would make the addition of autofocus to Loxia lenses them that much bigger?

[Frank Sanders]

Typo: 55/2.8 I mean

[tinplater]

You can't beat the 35mm 2.8 Zeiss FE for sharpness in a miniscule size. 

[Username]

I like the Sony Zeiss 55/1.8 very much, it sharp and its compact.

My question is why cant a 35/1.8 or a 85/1.8 be built in that compact

size? Is only 50 or 55mm possible because of lens physics?

 

Would make the addition of autofocus to Loxia lenses them that

much bigger?

   

Last Q? first ... AF is often added with a verrrry minimal 

effect on size of an existing MF lens. OIS usually adds 

visibly to bulk. 

  

About the 85/1.8 you have 2 reasons for larger size. FL 

is longer so the barrel is a bit longer but greater increase 

is due to maintaining a fast max aperture in that longer 

FL. By extreme design measures, the barrel length could

be reduced somewhat ... check out the diff between "true 

telephoto" vs "long focus" lens design ... but the girth is 

mathematically locked in by the maximum-aperture : FL

ratio ... check out the numeric meaning of "f-stop ratio".   

  

Look for pictures of Leica's 90/2.0 Summicron and the 

90/4.0 Elmar for Leica's rangefinder cameras. Great

examples of what lens speed does to lens size. Same

maker, same mount, same FL, huge difference !    

    

The f/stop scale is NOT a brightness scale. Its the ratios

of the size of a variable as compared to a constant. The 

working aperture is the variable. The FL is the constant. 

The variable is always stated as "1". The resulting "right 

side" [the denominator] of the ratio is called an "f-stop". 

The numbers you usually see indicated are simply the 

commonly referenced points [stops] along a continuum. 

  

There IS a [sort of] "brightness scale". It is the "EV" or 

"Exposure Value" scale. The "whole" f-stops and "whole" 

shutters speeds are agreed on for general use cuz they 

represent changes in increments of "1 EV". Doubling or 

halving the ISO is also "1 EV". Go study ... have fun. A 

whole bunch of stuff will make sense instead of seeming 

like a mysterious mishmash of numbered settings :-) 

     

About the 35/1.8 is a similar situation to the 85. The f-stop 

ratio is NOT the problem ... do look that up and it will be 

clear to you, it's not rocket sci ! BUT modern WA lenses 

are usually retrofocus designs [another thing to look up].   

    

SLRs require that design, to leave space for a mirror, but 

live-view cameras have a problem that film cameras did 

not have. If you had film instead of a sensor, your 35/1.8 

could be very compact, as with Leica-type RF lenses and 

you CAN adapt those to live-view cameras. Users of such 

adapted WA lenses discover WHY live view cameras are 

better off with SLR/retrofocus WA deigns. Just one more 

thing to look up. Bottom line is that "retrofocus" is exactly 

the opposite of "true telephoto". Above it is noted that the  

true telephoto design reduces barrel length, so natcherly

its opposite will increase barrel length. 

  

Unfortunately, retrofocus turns some physical parameters 

on their heads, thus front element size becomes waaaay

larger than the max-aperture : FL ratio normally predicts.  

   

This was not a short post. If I explained here those 3 or 4

things that I asked you to look up ? Waaaaaay longer post.  

[Frank Sanders]

Thank you for this basic explanation. I will take a closer look at all the topics you brought up.

 

"AF is often added with a verrrry minimal 

effect on size of an existing MF lens. OIS usually adds 

visibly to bulk."

 

So there is hope for an compact autofocus 35mm/2.0 lens like the Loxia one.

[Guest all8]

Take a look at the Rx1 line of cameras ...

[Username]

..................

 

"AF is often added with a verrrry minimal 

effect on size of an existing MF lens. OIS usually adds 

visibly to bulk."

 

So there is hope for an compact autofocus 35mm/2.0

.............

    

That statement was just a direct answer to the 

question of whether AF actually causes lenses 

to be so much bigger than formerly. Again, the

answer is "No, AF needs almost no extra space 

in the lens vs older MF lenses."   

   

As for your hope that compact AF lenses will be 

widely available, don't hold your breath. Makers 

of lenses seem to put a very low priority on your 

concern. IOW, evidence is that it CAN be done, 

but the makers apparently don't much bother to.  

   

When I say "evidence is ..... " most evidence is 

older AF lenses. I have plenty of those oldies as 

useful leftovers from film-era AF cameras, and I 

don't find most newer lenses very appealing due 

largely to the "obesity" of nearly all newer lenses.  

   

My own 35/2.0 AF lenses are the same size as 

my pre-AF era 35/2.0 Nikkor. However those AF

lenses are 30 years old. Those AF lenses were

not marketed as "compact models" back when

they were new, but I think today they would be

marketed as such. Fashions change over time :-(  

   

But lens makers do know that there's at least a

niche market for pancakes and compacts and a 

few lenses are offered for that market. Whether 

they will make the particular lens you desire is 

anybody's guess. Anybody but me, anywho, cuz 

I'm not interested in new lenses.