Small Fast Optics - what is possible, what is reality

[PentHassyKon]

Interesting question - I don't know the answer, but who really needs one given the way fast films have evolved, and digital sensors are gradually developing?. This current obsession with fast lenses goes hand in hand with the craze of 'Bokeh', and in stead of being a necessary low-light tool, has IMO become an overused cliche, or unused 'status symbol'. 🙂
Dave.

Well to quote the instructions given at the top of this sub-forum:

This is the forum to discuss optics or lenses in general, to learn about the tech behind the lenses and images.

I for one am just interested in what is possible, independent of bokeh. Given the size of the fast optics I've seen I was just wondering how small one can make for example a 40/0.75? My though on this is that given certain constraints placed on the lens designer - physics will dictate what the smallest possible size will be for a particular focal length and aperture. What are the variables the lens designer must consider? What are the comprises that must be made? We've seen what a 35/1.4 vs a 35/1.2 looks like or even a 50/2.0 vs 50/1.0. My interest is in educating myself (and possibly others here) on what is possible theoretically. Is the main driver the size of the lens mount?, image circle?, focal length? distance to focus/film plane?

If I compare lens size to focal lenghts, with SLR lenses: 85/1.4 vs 50/1.4 it seems the larger the focal length for a given aperture, the larger the lens. So my "small picture" question that really started this thought process is 50/1.4 vs 40/1.4 vs 35/1.4 - let's go to a 1.0 aperture: will the 40mm be smaller the either the 50 or 35 for a 24x36mm film size? Going to the "big picture" question - what is the smallest lens with the largest aperture given the 35mm format for an M or LTM camera?

 

[Cutly]

SLR : zeiss 55 1.2 c/y

 

[Dave Wilkinson]

Fast lenses are necessary for anyone that shoots available light and doesn't want to restrict themselves to ultra fast films, which can be very expensive and yield much lower quality images than their slower, more readily available counterparts. The evolution of digital sensors is somewhat irrelevant to those who prefer film, and we don't all enjoy being forced to shoot at an eighth of a second every time the lights dim.

I've been shooting velvia lately. I love the look of the stuff, but find the speed to be limiting. Without lenses like the CV 35mm 1.2, there's no way I'd be able to shoot half of what I'd like to. Here's an example from a few weeks ago:

Fast lenses are invaluable tools, and it disappoints me to see this intriguing thread turn so quickly to obnoxious bokeh commentary.

Nice portrait, but a shame about the sickly color caste-that always seems to appear inthis type of work! 🙁

 

[Finder]

First lets start with a little basics of what the f-number is. Properly it is a ratio: f/2.0. f is focal length. So f/2 is the focal length divided by two. So a 50mm f/2 lens would need an aperture of 25mm and a 100mm f/2 would need a 50mm aperture. f/1 brings that to 50mm and 100mm respectively.

However, it is not that simple. Lenses tend to be complex--more then one element--and so light must pass through a relatively long optical path in the lens. The angle of view will dictate the size. Since a wide-angle lens must collect light entering the optics from a large angle, the elements must be large enough to capture the light. This is why wide-angle lenses have large front elements compared to their focal length where telephoto lens have front elements close to the size the focal ratio/f-number would suggest. So there is this balance between focal length, the longer the focal length, the larger the elements required for a given f-number, the the angle of view which means short focal length lenses also require larger elements. Normal lenses are the best compromise. Also, the larger the format, the larger the lens of a particular angle of view. So 35mm has been able to capitalize on fast optics compared with larger formats. Theoretically, lenses for smaller sensors could be made faster with smaller elements, but there tends to be resolution problems to making a lens match the sensor.

Then there is image quality. With a simple lens design, the fastest lens is f/0.5. Simply take a glass sphere and cut it in half and then place it on the film/sensor. You can imaging what a wonderful image you will get--it could put Lensbaby and Lomo out of business. Now, lenses are more complicated than that, but it is hard to manufacture such large optics and control aberrations.

I will let you on to a little secret, lenses are not designed for their largest aperture. Designers will allow some vignetting and aberrations at maximum aperture in order to keep costs down.

So you are right in your hunch that the normal lens tends to be the fastest. What is the fastest lens possible? Hard to say? How much money do your have? And what is the state of lens design and manufacturing? So far, around f/1 is where manufactures have come to a halt. Whether that is because things like depth of field becomes horrible at larger apertures, or there is not way to control aberrations, or perhaps manufacturing is just too expensive, it is hard to say. Or it could be they think f/1 is fast enough.

 

[rxmd]

Is the main driver the size of the lens mount?, image circle?, focal length? distance to focus/film plane?

All of them. 🙂

You can do part of the math yourself. To get the aperture value, you have to divide the focal length by the size of the image of the aperture. (Not the aperture itself, but that already gives you an idea.). So for a 50/f1.0 lens, the image of the aperture must be about 50mm across. The diameter of the M mount is 38mm, which is not a lot for fast lenses because you can't fit that huge 50 in there, meaning you need a retrofocus construction, where the aperture must sit quite far outside and you need a rear lens block afterwards to edge the image through the bayonet throat. At least the lens register is small so that you have some room in the back for that lens block, unlike SLR lenses where it all has to happen outside. Already a 35/f1.2 needs some consideration, you want some metal around your lens and the image of the aperture has ca. 30mm diameter. Wider lenses are a problem because of the lens register; at 28 or so mm lens register, you just can't build a fast 28 or 24 or 21 without some optical tricks (read:retrofocus) that move the aperture outwards, which is why the Leica 21/1.4 is quite huge by rangefinder standards. A 75/f1.4 likewise needs an aperture about 50mm. Leica doesn't build anything bigger, unlike Canon, for example, where you get a 85/f1.2. Imagine the aperture of that thing (or of a 200/f1.8 which they also sold, or a 400/2.8 which is available from a number of manufacturers).

Now figure in that a large image circle needs large lenses (a 21/f1.4 for Leica is certainly more impressive than a 25/f0.95 whose image circle is barely 21mm or so, the bayonet has 25mm diameter, and lens register is only 17mm). And figure in some extra glass for correcting aberrations etc, because otherwise people will complain about purple fringing and bokeh.

 

[Dave Wilkinson]

Slightly away from the subject....but in threads like this we usually are inundated with examples of 'keepers' that could not possibly have been obtained without that ultra fast, ultra expensive lens. Without wishing to offend any specific person - I often wonder why a particular shot was so urgently needed, or deemed a keeper, when - if mine - it would be an immediate candidate for the bin!. Of course - you will tell me - people shoot what they like, and that's fine, just my two pennies, and shows how our tastes differ. 🙂
Dave.

 

[Finder]

Without wishing to offend any specific person - I often wonder why a particular shot was so urgently needed, or deemed a keeper, when - if mine - it would be an immediate candidate for the bin!.

Well, I am glad you don't want to be offensive.

 

[NickTrop]

Nice portrait, but a shame about the sickly color caste-that always seems to appear inthis type of work! 🙁

It's a great portrait, and I find the color cast appealing - not "sickly" at all, and easily corrected in post if you want to. To the point about lens speed vs film speed. I initially was disappointed that Yashica capped film speed on their Electro CC at 400 iso. It was a later model, high speed film was out there... didn't get it. It couldn't have been done to save costs. Yet with its 1.8/35mm lens spec, you really didn't need anything higher than 400. After using that camera a while, I realized why. Great low-light shooter.

 

[ferider]

My, probably boring, thoughts on the subject:

For the true need for speed:

.) half a stop is just that.
.) I rather have fast longer than fast shorter lenses. For example, in a 35/75 combo, I like the 75 to be one stop faster. WRT hand-holding, a 35/1.4 is "faster" than a 50/1.4.
.) for M mount, except maybe for the newest Leica lenses, in terms of IQ you always pay when going to f1.4 or faster. Distortion, curvature of field, etc. Name it character, bokeh, whatever.

A fast lens on an SLR is a very different beast since speed facilitates use.

And then of course, for some, the bigger the lens, the better 🙂

 

[PentHassyKon]

All of them. 🙂

You can do part of the math yourself. To get the aperture value, you have to divide the focal length by the size of the image of the aperture. (Not the aperture itself, but that already gives you an idea.). So for a 50/f1.0 lens, the image of the aperture must be about 50mm across. The diameter of the M mount is 38mm, ................snipped......

So for a 40/1.0 then the diameter of the lens would be at a minimum 40mm? I guess my question, or rather the answer I'm trying to fit into the question is- will a 40/0.95 be smaller and "easier" to mfg than a 50/0.95? From this wiki: http://en.wikipedia.org/wiki/Lens_mount, I see that the throat dia of the M mount is 44mm - will the optimum focal length for an f1.0 lens then be 44mm?

 

[Finder]

So for a 40/1.0 then the diameter of the lens would be at a minimum 40mm? I guess my question, or rather the answer I'm trying to fit into the question is- will a 40/0.95 be smaller and "easier" to mfg than a 50/0.95? From this wiki: http://en.wikipedia.org/wiki/Lens_mount, I see that the throat dia of the M mount is 44mm - will the optimum focal length for an f1.0 lens then be 44mm?

Optical design does not have to be so linear. The Noctilux 50mm f/0.95 has a 52.5mm aperture. There are 18mm SLR lenses that have the lens further away from the image plane than the focal length. And you can focus a lens at different object distances without having to move the optics closer or further from the image plane. I do not think you can come up with anymore exact answer than it would probably be a normal lens.

 

[not_in_good_order]

Nice portrait, but a shame about the sickly color caste-that always seems to appear inthis type of work! 🙁

I think it is an excellent portrait. After experimenting in lightroom on some of my own photos with green color casts from florescent lighting, such a cast seems pretty easy to remove if the artist so desires, though only if florescent lights are the sole light source.

 

[Krosya]

Slightly away from the subject....but in threads like this we usually are inundated with examples of 'keepers' that could not possibly have been obtained without that ultra fast, ultra expensive lens. Without wishing to offend any specific person - I often wonder why a particular shot was so urgently needed, or deemed a keeper, when - if mine - it would be an immediate candidate for the bin!. Of course - you will tell me - people shoot what they like, and that's fine, just my two pennies, and shows how our tastes differ. 🙂
Dave.

So this is why you dont show any of your photos? I wonder what you would consider a keeper?

 

[PentHassyKon]

Optical design does not have to be so linear. The Noctilux 50mm f/0.95 has a 52.5mm aperture. There are 18mm SLR lenses that have the lens further away from the image plane than the focal length. And you can focus a lens at different object distances without having to move the optics closer or further from the image plane. I do not think you can come up with anymore exact answer than it would probably be a normal lens.

I guess this is why I'm still confused - need to understand ratio rule as it tells me that a fast 28mm can be made smaller than a fast 50. I'm missing something here - i suspect that a lens with a focal length smaller than the image circle requires a larger aperture dia?

 

[LeicaFoReVer]

I like my Leica M Summilux 50/1.4 pre-asph, which is not that big in size. Faster lenses, like 50/1.0, or 50/1.2 will be bigger and will block part of the viewfinder, although more expensive.

Canon 50mm f1.2 is the smallest one I believe. It is shorter than lux but must be more fat...It is cheaper too. Give it a try...

 

[Finder]

I guess this is why I'm still confused - need to understand ratio rule as it tells me that a fast 28mm can be made smaller than a fast 50. I'm missing something here - i suspect that a lens with a focal length smaller than the image circle requires a larger aperture dia?

If it were a simple lens (a single element construction), then you would be right. However, as you know, wide angles use quite a few elements to control aberrations. That makes the lens physically thick from front to back. To catch the light entering at the large angle of view, you need large, exterior elements--the internal elements can be quite small.

 

[Rob-F]

Also, I'm wondering, based on optical theory at what focal length can one get to the fastest at the smallest/lightest? Assuming this is for LTM or M mount.

I'm thinking of some of the very early Leitz wide-angle lenses. The 35mm Elmar was a tiny little thing, as was the 28mm Hektor. These examples would seem to argue that you can have a very compact wide-angle lens. That is, until you realize that these were fairly slow lenses. The Elmar was f/3.5 (an even earlier version was f/4.5); the Hektor was only f/6.3. By modern standards, they were not too well corrected, either.

Making the lens faster requires more correction. More correction requires more elements (though the use of aspherical ones can help to hold down the number required). And more elements require more room. So, the lens that is faster and/or more highly corrected has to be larger. It's just a matter of making room for all the elements. There's nor reason why a wide angle lens has to be physically short. Just compare the length of the 35mm Summilux ASPH with that 35/3.5 Elmar. Or the 28mm Summicron with the 28/6.3 Hektor.

A lens of more moderate/nearly normal focal length won't need as much correction as a wide angle lens of the same speed, which explains why the normal 50mm lens may wind up being no bulkier than a fast wide-angle lens. When the 50mm Summicron had 7 elements, the contemporary 35mm Summicron needed 8. When the 50mm Elmar needed 4 elements, the 35mm Summaron needed 6. More elements take up more room. (Though the Summaron, having a shorter focal length, has its rear element placed just about even with the bayonet mount. There's a lot of air between the Elmar's rear element and the bayonet, because of the longer infinity focus distance.)

My last point: A wider lens doesn't have to have a bigger front element. But if it doesn't, there will be more vignetting--light falloff at the corners. Enlarged front and rear elements are used with wide angle lenses to reduce mechanical vignetting. If you look through the rear element of a lens and then slowly angle the lens so you are no longer looking straight through it, you will see the round circle of light becoming more and more of an oval. The more you turn it, the less light can pass through. That's mechanical vignetting. The price you pay for that large and deeply curved front element though, is greater risk of flare. This might have something to do with Leica's use, in recent years, of concave front surfaces on some lenses!

Why doesn't the lens get faster when you increase the front element size? After all: the speed is supposed to be focal length divided by the lens opening, right? It's because the diameter of the diaphragm, as observed through the front element, is what really determines the speed. It's not the diameter of the front glass itself. Look into a monster wide-angle lens and you can see that the diaphragm looks a lot smaller than the front glass!