Lens Test: Voigtländer Nokton 25mm ƒ/0.95 for Micro 4/3

By Ctein

John Camp loaned me this lens to play with because he thought it would amuse me. Indeed, it did. Understand that this is a $1,200 toy, a specialized bauble for them what demands fractional aperture numbers (although ƒ/0.95 is a bare 1/6th stop beyond ƒ/1, so for the rest of this article, I'm just gonna say ƒ/1). Idiosyncratic barely begins to describe this optic; I don't think I've ever encountered a lens with more character, and that's both good and bad.

The lens is compact and dense. Only about two and a half inches in diameter and length, it weighs almost a pound. The build impresses. Please understand that I am not especially sensitive to build quality, which means my threshold for caring is extremely high. I've handled and used my share of Leica equipment. I'd say it's nice. That's it. Meh, so sue me. This lens, though, it's like a work of art.

The 300-degree helical runs from infinity down to about 1:4—essentially macro on a sensor as small as 4/3 (and I will get back to that). It needs that wide swing; at ƒ/1, submillimeter movements produce observable changes in sharpness. But, oh, does it swing. The focusing action is not "smooth as silk"—silk has a perceivable friction. This is silk, slathered with fresh creamery butter, gliding on a bed of frozen, polished teflon. There is no sense of mechanical contact whatsoever, just a preternaturally smooth resistance that accommodates precise focusing movements.

The aperture ring is almost as remarkable. It feels like one barely has to nudge the ring when changing apertures, yet each half-stop detent locks so firmly into place you'll never feel there's a risk of accidentally moving the ring.

OK, enough hardware fetishism. Does this thing actually make decent photos? Well, umm, kinda, sorta depends.

Light falloff, wide open, is, as you'd expect, substantial. This is an ƒ/1 lens on-axis, but by the time you get to the near edge, exposure's more like ƒ/1.4. You'll be down 1 2/3 stops from center to corner. No shock, really; just be aware that underexposure when using this lens wide-open may prove hazardous. Falloff improves rapidly: at ƒ/2 brightness is quite uniform over the entire field, down less than 1/2 stop at the most extreme corners.

Figure 1. Central crops from 3kx4k-pixel photographs show that on-axis the Nokton cleans up very rapidly. Wide-open, even a few score pixels off-axis you start to see aberrations. (click to see at 100% scale)

Wide open, the lens is sharp-but-soft in the very center. Central image quality picks up very rapidly with smaller apertures; stopping down just to ƒ/1.2 makes a substantial difference (fig. 1). By the time you hit ƒ/2 the image quality on-axis is excellent and is as good as it gets—very impressive. Even normal lenses rarely peak out at just two stops below maximum aperture.

Off-axis? Whole 'nuther story. Wide open, you don't have to move more than a few millimeters from dead center to start seeing coma tails. That's observable even in the very small central sections shown in figure 1, which are cropped from 3kx4k-pixel images.

Figure 2. 80% out to corner, image quality is simply horrid wide open. At ƒ/5.6 it's decent, ƒ/8–ƒ/11 is optimal. (click to see at 100% scale)

The quality gets dramatically worse the further one moves off axis. Eighty percent of the way out (figure 2) it's horrendous. At ƒ/1 and ƒ/1.2 there's very little difference. It's so awful that if Coca-Cola made this lens, it would be giving "Coke bottle bottom lenses" a bad name.

Some reviewers describe this quality as being "dreamy" or "glowing." I'd say that's much the same way a realtor will describe a broom-closet-sized studio apartment as "cozy" or "intimate." Not me, sorry. Try "schmeary," as in what you'd see if you wiped off your fingers off on the lens after eating a nice bagel with plenty schmear.

Get beyond ƒ/1.2 and the image cleans up rapidly. The problem is the image quality has such a long way to go that you don't really clean up the 80% zone until you hit ƒ/5.6, and the extreme corners require ƒ/8–ƒ/11.

This is the upper left quadrant of an ƒ/1.6 photo at 50% scale (when you click on the image). That gives a fair impression, on screen, of what an 8x10 or 11x14 print would look like. Observe the fairly sharp boundary between good and poor image quality at about 75% of the way out from center.

The falloff in quality as you move off-axis is unusual. It's abrupt; there's a well-defined circle of decent image quality outside of which the quality plummets like Columbus sailing off the edge of the world, as fig. 3 illustrates. At ƒ/5.6, you've got 90% coverage with decent image quality. If you're inclined to a bit of cropping, you'd be set. ƒ/8 is probably your best choice for overall quality (figures 4 and 5), while ƒ/11 gives you uniformly sharp photos all the way to the corners.

Figure 4. A full frame, photographed at ƒ/8. Image quality's pretty uniform, except at the very most extreme corners.
100% center and corner snippets below.

Figure 5.

This lens lets you have it good or have it fast, but ya gotta choose one.

It'll also let you have it close. As I mentioned, it focuses down to about 1:4. With a sensor that measures only 13x17mm, that's a pretty tight closeup. Is it any good there? Well, it's better than I expected. There's a huge amount of barrel distortion (figure 6), no big surprise in a lens that isn't specifically corrected for extremely close work. But, it's nice clean barrel distortion that Photoshop can get rid of perfectly (figure 7). You'll need to stop down, of course, for anything resembling good quality corner-to-corner. At the macro end of things, I'd probably set it as ƒ/11 and forget it. Do that, though, and you'll be pleasantly surprised. Yeah, better at the center than the corners, but acceptable everywhere (figure 8).

Figure 6. ƒ/11 is the ideal "macro" aperture for this lens, which can focus down to about 1:4 magnification. This is a photograph of an iPad screen. The funny plaid pattern is an artifact of sampling this image down to fit in 800 pixels; it's not in the original photo.

Figure 7. The photo from Fig. 6 after barrel-distortion correction in Photoshop. Pay no attention to the plaid.

Figure 8. Center and corner portions of figure 7 at 100% scale.

If I were still doing lots of nightclub photography, I'd snap this lens up instanter. It's maximum-aperture flaws wouldn't be obvious in that kind of setting and stopped (way!) down it's a credible performer in normal situations.

Ctein

Ctein looks into interesting things on a weekly basis on TOP. His column usually appears on Wednesdays—this week the Laz. Ed. delayed it.

Send this post to a friend

Please help support TOP by patronizing our sponsors B&H Photo and Amazon

Note: Links in this post may be to our affiliates; sales through affiliate links may benefit this site. More...
Original contents copyright 2011 by Michael C. Johnston and/or the bylined author. All Rights Reserved.

Featured Comment by Noons: "Been using this baby on my E-PL1 since it came out. Rarely use any other fixed lens with this camera, with the possible exception of a very old Tamron Adaptall 28mm that is absolutely sublime on Micro 4/3 and a Canon FD 200mm ƒ/2.8 that has incredible bokeh. If I have to take interior portraits on available light, this ƒ/0.95 lens is the first grab. It is amazing for what it was designed for. And therein lies the rub: it is not a lens for landscape shots! [Just for the record, Ctein didn't say it was. He merely used a landscape shot to illustrate the qualities of the lens he was examining. It's just an illustration, not a recommendation of what you should shoot. —Ed.] Nor do I think anyone in their right mind would try such with a ƒ/0.95 aperture, whoever the maker might be! Thanks, Voigtländer, for putting out an affordable kick-ass-aperture normal lens."

Featured Comment by Min Wei: "Wow, my flickr view went out of the roof today because Mike Plews' comment mentioned my Canon 50mm ƒ/0.95 with Sony Nex-5N combo, ha ha. [Mike linked to a picture of Min's camera and lens yesterday. —Ed.] For me, the ƒ/0.95 is for night shots and bokeh shots. I mainly use my Canon 50mm ƒ/0.95 with my Canon 7 rangefinder. It allows me to shoot low ISO films in low light situations without pushing the films.

"And of course the bokeh! I just love it! Here is one shot I took with an ISO 64 film at night at ƒ/0.95. Here is the one taken with the Sony Nex-3:

Photo by Min Wei

"And about using the Voigtländer lens for landscape shots. I say why not. Yes, ƒ/0.95 is the key point of this lens. It's good for low light shooting and bokeh shots. At same time, many people will attach this lens to their Micro 4/3 camera and keep it on most of the time. The performance at other aperture other than at ƒ/0.95 is definitely worth checking! And, there is at least one use for shooting landscape at infinity, wide open—shooting landscape scenes with stars and the Milky Way at night! How cool is that! Just my two cents."

Comments

[Mike Plews]

On my first TV news gig I had a Bell and Howell 70DR with a 25mm Angenieux f0.95.
I think I only shot it wide open twice as this required rack over focusing through a peephole which did not work so well in breaking news situations.
I suspect that camera ended up in the dumpster when they went ENG. Who would have guessed that those odd little lenses would command over a grand on Ebay now?

[Jeff]

Interesting lens. The schmeariness and sharpness fall-off remind me of sample images I've seen online from the Pentax 25mm 1.4 c-mount lens. Then again, I would expect that in a $120 lens (B&H price) while not so much in a $1200 lens. The big question is do you think it is worth the price difference over the other native 1.4 Micro 4/3 lenses unless you absolutely NEED f1?

[Benjamin Marks]

Yup. That's the lens to a "T". It is definitely a specialty optic. But when you are scraping for photons, it is nice to have a lens that will do what this one does. And, hey, it is 1/10 the price of the current Leica Noctilux! Now where'd I put those photons?

Ben Marks

[Libby]

Many thanks for the test Ctein. This lens is on the consideration list for the future, so looking for as many independent reviews as I can. You always do a nice job.

[Chris Y.]

You did lots of nightclub photography? That is, (or was) a great and instructive discipline. Might you post something about that?

[beuler]

Sooo... It could be nice to have it mounted on a Nikon 1. The far corner ugliness would be gone and the close focus would be very handy.

[Ed]

Essentially.......like so much these days, nice in theory, shame about the performance. I own 1,7 20.......Pana.....no problems at 1,7. So why bother....I'm sorry to say. If I would try to imp on my in the dark performance (coming from GF1) I would invest 1000 euro in second hand d700 and use its high iso capability and my 50 1,4......better results guarantied, same money spend.

Greetings, Ed (www.ekdfr.com)

[Darko Hristov]

In these day and age of ridicoulous high ISO capabilities of digital, extremly fast aperture lenses are interesting mainly because of their extremely shalow DOF when shot wide open. Unfortunately your photos do not show that aspect.

[ctein]

Dear Jeff,

A broadly-useful rule of thumb is don't ever buy an ultra-fast lens over one of more normal aperture unless you truly need that maximum aperture. Rarely will the ultrafast lens be as well-corrected at smaller apertures as a slower one, although this is not unheard of. It certainly won't be significantly better, and it will cost you all whole lot more and be a lot bigger and heavier.

The only reason for buying an ultra-fast lens is because you need apertures of f/1.2 or wider, or you like lording it over other equipment geeks.

pax \ Ctein
[ Please excuse any word-salad. MacSpeech in training! ]
======================================
-- Ctein's Online Gallery http://ctein.com 
-- Digital Restorations http://photo-repair.com 
======================================

[Kenneth Tanaka]

Thanks for looking at this, Ctein. (And for the support, John.) I wondered how this lens would fare since I saw its announcement.

I admit that I'm a bit surprised that its corners stay soft so long, given that the optics were designed specifically for the four-thirds sensor.

You've prompted me to take a look at my Leica Noctilux f/1 on my E-P3 this weekend, something I've not yet tried.

These f/1 and now f/0.95 lenses are fun but very impractical (and expensive!) trophy pieces, particularly in this day of insanely high ISO sensor sensitivities selectable on each frame.

[Carl L]

Years ago I purchased a Canon 85mm ƒ/1.2 thinking I could focus it easer. But alas it was so soft, focusing was worse then my ƒ1.8 85mm.
I believe this lens will have the same problem, probably worst.
With todays hi ISO how many time would you need an ƒ/.95 lens anyway.

[Karl]

I've been thinking about an older lens I could use on 4/3 to approximate the look I get with the 165 2.5 Cooke on my Graflex Series C's. This looks like a possibility, albeit with more contrast. Just add in a bit of fake spherical aberration and it would be close. Only $1,200 for a close approximation to an 80 year old uncoated lens. Isn't technology grand?

[John Holland]

Oooh, that coma is eye-wateringly bad. I mean that literally: it looks like what you'd see at night if your eyes were watering.

OTOH, I don't think people would use this lens wide-open in situations where the coma would predominate. Horses for courses.

You don't say much about contrast and flare, although the pictures look pretty good.

Thanks for the review; I've been curious about this particular lens since it was announced. At 1200 bucks, it'll be awhile before I attach one to my Panasonic, but it improves my confidence in the m4/3 format when exotic gear like this gets made.

[Dozen Meda]

What... no "bokeh" shots? If I want sharp from center to the corners I can take the kit lens. This baby is about speed!

I think Ctein quite missed the point here.

[Mike Plews]

http://www.flickr.com/photos/minweiphoto/6263063657/

Anything worth doing is worth overdoing

[Richard Ward]

Wow!
Only three comments.
Lenses such as these usually generate much more comment, discussion, if not plain vitriol. Mike is probably backlogged on comment approval being off making tea using an ocd methodology that entails bowing to the sun goddess of sweetners for eight to ten hours before 'brewing a cuppa' :-D
RLW

[John Robison]

Back in the 70's while working at a camera store in the Loop in Chicago the owner bought just 1 of the then newish 50mm f1 Noctilux lenses. He just had to try it out and at f1 the coma, even on 3.5 X 5.5 inch album size prints,(this was before 4 X 6 was standard folks) was memorable and very visible. So, in some ways, not much has changed in almost 40 years of lens design.

[hugh crawford]

That most definitely would fit the Psycho Girlfriend Lens profile.

Just curious does it have a wickedly curved field or is simply unsharp in the corners.

I can do some cool stuff with a lens with a paper thin curved depth of field that some of the big aperture 16mm cine lenses have. I have a lens that is sharp in the center at infinity and it will be sharp in the corner at about 8 feet (230mm Century Tele-Athenar on a full frame Canon if you are interested).

If it's just fuzz and coma , then it's cool stuff potential is greatly diminished.

[ctein]

Dear Carl,

I didn't find focusing a problem at any aperture (aside from no margin for error at f/1). It's not that kind of soft.

~~~~~~~~

Dear John,

You're right, contrast and flare are not particular problems, as you kinda inferred from the the test photos. As for the coma and other aberrations, I showed "star target" type test photos because they most clearly show the causes of the shmeariness. Look at figure 4 and compare it to the f/1.6 section of figure 2. That'll give you a good feel for the effect these aberrations have on "continuous tone" subjects. Just 'cause you can't see an individual aberration doesn't mean it isn't still a problem. The coma and other problems are there clobbering the image whether or not you can make out individual tails and streaks.

~~~~~~~~

Dear Richard,

No, I am afraid Mike has failed to see the light and actually much prefers coffee. The heathen.

He's probably fiddling with his infernal roaster.

~~~~~~~~

Dear Dozen & Darko,

I write about what interests ME. And I try not to write about what everyone else has already written about; why waste my time and yours? A minute with Google will find you plenty of other reviews and posted photos showing you bokeh to your heart's content.

pax / Ctein
==========================================
-- Ctein's Online Gallery http://ctein.com
-- Digital Restorations http://photo-repair.com
==========================================

[ctein]

Dear Hugh,

Sorry, I didn't measure curvature of field, but it can't be very much. Even at f/11, DoF at 1:4X is pretty shallow, and the corners of the photo are resolving the iPad's LCD pixels that are only 3 camera-pixels high and gaps between the columns that are only 2 camera-pixels wide.

It's not up to stellar macro levels, but it doesn't look unusually unsharp to me. Are you maybe visually confusing the individual tri-color elements in the LCD with chromatic aberration or color fringing? There's almost none of that.

pax / Ctein

[Ramón Acosta]

I used to do night club magazine for a small magazine in a small city. Being a college student with not enough money or equipment, (I rode my bicycle to the night clubs until it was stolen, then I had to take the bus), I used an old minolta SRT and a tiny flash. I learned about guide numbers and managed decent pics most of the time; my main problem was making sure I had the right distance set on the lens, so I could try to see what f number was needed. After a while I just had people stand at 5 feet from me.

[Darko Hristov]

Dear Ctein,
we all here cherish YOUR interests here. My concern is that this lens is not suited for such pictures. But shoot a portrait on near minimum focusing distance and all the mentioned shortcomings become irrelevant. With all respect, shooting landscapes @ infinity, wide open with this lens is like riding downhill offroad with a street racing bicycle and finding that it has shortcomings.

[Wayne]

Please excuse the following question if inappropriate. It comes from pure ignorance.

Is this a lens that would perform in a markedly higher manner if it were used on a stable base such as a heavy tripod?

Carl's previous writing on the significant improvement of equipment performance, related to use of a solid base, has been a real eye opener for me. Being a novice, I wonder: Does the point Carl illustrated and explaned apply equally to all lenses? or, is it a more significant point with certain lenses? Is this Voigtlander a Jekyll and Hyde on and off a heavy tripod?

Years ago I purchased a massive ProMaster tripod to use with my telescope. It is a monster and I have shunned its use with my camera. Now I wonder.............

[Craig Norris]

Very fast lenses bring two important benefits to the table even though one may never actually use the extreme large end of the aperture range as the chosen aperture for a photo.

I'm an F8-F16 kinda guy, but I really appreciate how a fast prime provides a very bright view in the viewfinder in dim lighting conditions, and I especially appreciate how fast and decisive the focussing becomes, in both manual and auto focus modes (thanks to the extremely shallow DOF).

So to those who deride the wide open performance of lenses like this F0.95 job, I'd suggest that they might still enjoy a lens like that for the two important benefits mentioned above. Especially in this day and age of EVF's and contrast detection focus, the camera needs all the help it can get in dim light, even if it's on a tripod and set for F8.

[Gunny]

So, it's a $1200 Holga optic?

[Yu-Lin Chan]

I have been using the very cheap JML 25mm f0.95 TV Lens (and its sister lens, 50mm f0.95) and I quite like it. For sure it's not a Voigtlander, but the weird bokeh when shot at f0.95 is what gives me the pleasure. It's amazingly sharp at the center wide open for such a cheap lens, but the edges, doesn't matter how much I stop down, are always bad, not to mention that it vignettes heavily regardless of aperture settings. Link to the JML 25mm f0.95 Flickr set: http://www.flickr.com/photos/84303132@N00/sets/72157622333468515/with/6771458729/

[martsharm]

I am prepared to be shot down in flames in the usual way by Ctein ("You are entirely incorrect, and here's why..."), but may I enter into a dialogue regarding the phrase:

"No shock, really; just be aware that underexposure when using this lens wide-open may prove hazardous..."

Assuming one is using the matrix-style metering that my G1 and GF1 allow, and whilst agreeing that vignetting would lead to corners underexposed in comparison with the centre... is there not a risk of *overexposure* of the subject with lenses that vignette in combination with matrix-style metering? In that, vignetting reduces the overall brightness of a scene, so a meter that looks at the whole frame will over-compensate the centre. Assuming one's subject is in the central area, this can lead to overexposure of the subject. Personally, I would be dialling in some negative compensation - if I hadn’t already resigned myself to hair-shirt spot metering with such a purist lens.

By the way, I had been pondering trying this lens myself, but it is just that little bit too expensive, and I have a lens just that little bit too similar (the ubiquitous 20mm f/1.7, which seems able to gather just as good results so simply in comparison) that I had been holding off. This review hasn’t convinced me otherwise especially since the Nikkor 50mm f/1.2 is almost half the price here in the UK. Combine the Nikkor with full-frame ISO capability and one should really be able to see in the dark. One must ponder: whither the purpose of this Voigtlander?

[Avi Joshi]

I love this lens! Makes my gh2 images look like it was shot on a FF. Of course YMMV.

[Ludovic]

A question I've had for the longest time and which is suddenly topical: how on Earth can a lens actually be f0.95?

If, as my personal sort of common sense and Wikipedia both think that "the aperture determines how many of the incoming rays are actually admitted and thus how much light reaches the image plane", how can an f0.95 lens work? By adding 5% to the available light?

(I'm actually not trying to be facetious: I'm genuinely puzzled.)

[ctein]

Dear Darko,

There are SO many good and pointed questions embedded in your reply that I'm going to have to answer at some length. Apologies.

To begin with, please do not confuse test photos with intended pictorial use. Test photos are designed to elicit the characteristics of the lens. Occasionally good test photos and good use photos can be the same thing, but it's not the rule. For example, the pseudo-star target photographs I did are extremely useful for seeing what a lens is actually doing, but don't correspond to any real pictorial situation (unless you're into astrophotography, of course). One of the main reasons I felt the need to include figure 3 is that you can't really look at star target photographs and intuit how continuous tone photographs will look. But, the star target photographs are really good for understanding why the continuous tone photographs look the way they do.

Conversely, even if this were a lens specifically designed for large aperture, close-in portraiture (something I will dispute), just about all the photographs I've seen made under those circumstances and used in reviews are completely uninformative. They're nice eye candy and are usually accompanied by lengthy descriptions by the author describing the specific ways in which such and such lens is wonderful. But, were a malicious editor to go in and mix up all those portraits, you'd never notice the difference. It's a kind of placebo affect, or maybe like horoscopes. You think you're seeing something meaningful in the photograph, but it's actually so vague that if someone were to tell you an entirely different one applied, then that would seem equally… and vaguely… plausible.

There also severe technical problems with testing a lens like this under more typical use situations. For example, depth of field truly is amazingly shallow at f/1, and absent working on an optical bench it's just about impossible to ensure sufficient camera/lens/subject plane parallelism in close work (you don't want to know how much time I spent on those macro photographs). Even in figure 3, with the subject tens of meters away, I had to show you the quality falloff at f/1.6 because at f/1 there was too much difference in sharpness between the tree and the house slightly behind it.

As for the purpose of this lens, you'd have to read the mind of the manufacturer, but I think the arguments that it's not about low-light photon gathering are weak. In micro 4/3 format, high-quality ISOs are not yet stratospheric. I am not happy going above ISO 800, finicky fellow that I am, and I can state with first-hand experience that f/1.4, on occasion, had me feeling a bit constrained for shutter speed. It's also hard to reconcile a 1:4 close focus magnification with a lens intended for portraiture. I think the designer really was trying to design a super-fast, general-purpose lens.

As if that ever stopped any of us from using a lens as we damn well pleased [grin]. But, as for thinking of this as primarily a portraiture lens?

I am not convinced this is even the right approach to be taking for portraiture. I suspect this may not be a very good portrait lens-- too short a focal length and uneven, asymmetric image quality across the field wide open. There are much, much better f/1.4 lenses out there, and for less money. If someone is that desperate for an extra stop of shallow focus for portraiture, I might argue they're into the wrong camera system. A couple-of-generation-old full-frame/APS DSLR, combined with a decent (not great) 85mm f/2-f/2.8 lens is going to produce photos that look a lot better overall and give you about the same DoF. For that matter, that 85mm f/1.4 Rokinon I reviewed a few months back is cheap, will cover up to full frame (it was merely remounted and relabeled for m4/3 by the manufacturer) and will give you incredibly shallow depth of field (like having a 25mm f/0.6 lens in m4/3) with a pleasing overall look.

I think trying to shoehorn m4/3 into extreme shallow-focus portraiture may simply be a subotpimal strategy. Like trying to extract view camera quality from 35mm film. You can do it-- but it's usually costly and always very hard and limiting.

Anyway, my thoughts, for such as they're worth.

pax \ Ctein
[ Please excuse any word-salad. MacSpeech in training! ]
======================================
-- Ctein's Online Gallery http://ctein.com 
-- Digital Restorations http://photo-repair.com 
======================================

[Kenneth Tanaka]

@ Ludovic: A perfectly logical question. I think that this is about as good of a concise explanation of the f-stop as I've read.

[ctein]

Dear Martsharm,

NOT a silly idea! You're wondering how today's complicated metering algorithms would deal with the lens with severe vignetting and light falloff. I have no idea!

But what I was thinking of in that comment was not how the meter would perform but simple, physical underexposure, whether by meter error or by photographer intent. I know that under available-darkness situations, I have on occasion been sufficiently tempted to set the exposure compensation to -1 stop or even more just to ensure I got something resembling a decent photograph. With this lens, that may really clobber image quality towards the edges when it's used wide open.

~~~~~~

Dear Wayne,

I don't think it's at all an inappropriate question, but I couldn't give you an answer. That's because it did all my testing on a tripod. I do that for several reasons. The first is that I really do want to minimize external sources of unsharpness (and sometimes this means turning off image stabilization and sometimes it means leaving it on. Life is complicated). The second is that I'm making comparison photographs and need the framing to be the same in the different photos. The third is that when I'm testing a lens with a wide range of apertures, it's often impossible to handhold it over the entire range and get good results. For example, the transmission range of this lens is 250:1. Even if I were to find a perfect lighting situation where I could use the maximum shutter speed on my camera at f/1, I would be trying to hand hold at a 15th of a second by the time I got down to f/16.

pax \ Ctein
[ Please excuse any word-salad. MacSpeech in training!]
======================================
-- Ctein's Online Gallery http://ctein.com 
-- Digital Restorations http://photo-repair.com 
======================================

[ctein]

Dear Ludovic,

Ahhh, now we're into physics. (run away, run away!)

The Wikipedia entry is correct, but it doesn't tell you how many photons are actually being admitted! An f/1 lens has an aperture that is equal to its focal length, but that doesn't mean it's relaying 100% of the light to the image plane. In fact, the equation is:

(Illuminance in image plane) = 1/4 * (f^2) * (subject luminance).

Examples: the illuminance produced by an f/1 lens is one quarter the subject luminance. That produced by an f/2 lens is 1/8th the subject luminance.

So, where your head is going next, I bet, is, “So, what happens if you build a lens that is faster than f/0.5?" The answer is that you can't. That is the theoretical limiting maximum aperture for an imaging system. If you could build a lens faster than that, it would violate at least one of the laws of thermodynamics. If you could build a 1:1 relay lens faster than that, you could build a perpetual motion machine (think about it).

10 or so years back, some physicists did figure out how to build a non-imaging optical system that could produce an image plane illuminance considerably greater than the subject luminance. This was a major achievement and got them a very nice paper in Nature. Because you couldn't use it as an imaging system, you didn't end up violating any physical laws, but it made for one dandy light concentrator; you could make a solar furnace with it where the black body temperature in the plane of focus was HOTTER than the surface of the sun.

Very cool. (for a non-thermal value of "cool")

pax \ Ctein
[ Please excuse any word-salad. MacSpeech in training! ]
======================================
-- Ctein's Online Gallery http://ctein.com 
-- Digital Restorations http://photo-repair.com 
======================================

[ctein]

P.S. Actually, I should have been talking about T-stops rather than F-stops, but otherwise the physics is the same.

[Wayne]

Thanks for the response Ctein. I did not realize you did all of your testing on a tripod.

[hugh crawford]

It's not that tricky to make a really fast lens, they are all over ebay freshly ripped from old xray machines. They aren't very much use for imaging unless you want to take a picture of something that's almost touching the front element of the lens. If you take a look at the links , you will notice that these lenses have huge front elements (diameter is four thirds focal length of course) and tiny rear elements as well as tiny image circles.

http://www.ebay.com/itm/220923852268
or
http://www.ebay.com/itm/280664843091

BTW, unless you know what you are doing and what you are doing is out of focus macrophotography with a little machinist work on the side, stay away from these lenses!

[Ludovic]

@ Kenneth: I'm afraid the link doesn't work from my side of the internet - I just hope my asking questions hasn't broken it :)

@ Ctein: I re-read your answer twice, and at the very least it reminded me of why I wasn't that good at school in physics and math (for instance, "re-read twice" probably equals to "four times", which wasn't what I meant.) But what I did get is what matters: that it actually makes sense to speak of lenses faster than f1. I had no idea that an f1 lens relayed "only" a quarter of the luminance, so I can see now how it's feasible. Thanks for an illuminating (no pun intended) intended! (And any word-soup in my post is, sadly, entirely mine.)

[ctein]

Dear Ludovic,

Oops, a mistype in my reply to you:

Change: "That produced by an f/2 lens is 1/8th the subject luminance."

To: "That produced by an f/1.4 lens is 1/8th the subject luminance."

Sorry, 'bout that.

pax / Ctein