Why do they say PRIME lenses are good for LOW LIGHT?

[Bend The Light]

Is it true that, as there is less "stuff" in a prime lens, fewer elements, fewer moving-around bits, and so on, that the lens can be made with a wider maximum aperture? And that might lead to the idea that a prime lens has better light handling, where it's only really because they can be made with wider apertures more easily/cost effectively?

 

[dxqcanada]

SIGMA 18-35mm F1.8 DC HSM | Art Lens | sigmaphoto.com

 

[Overread]

It's most likely the fact that better quality AF motors also tend to go hand in hand with being quieter ones*; since few people would pay more for a better operating quality with increased functional noise levels. So its not the fact that the motor are quieter, but that they give a smoother, better controlled flow over the focus range.

That said its my experience that the quality of AF often relates to the lenses designed focusing setup; macro lenses for example often have very tiny motions for big shifts in focus at their longer ranges which means even with good focusing motors they can tend to be a bit more hit and miss since small shifts in the focusing give a noticeable shift in the focus.


As for lenses there is some truth in that some will let more light through than others. Even at the same aperture the amount of light transmission through a lens can vary. Far as I know this has a minimal or no effect on the exposure as far as the camera and photographer are concerned (ergo external light metering still works). It can mean, however, that one lens will perform better in lower light conditions for AF or appear brighter through the viewfinder over another.



*build in sound dampening within the lens structure might also play a part in reducing motor noise

 

[tigidig4x]

Thanks for your inputs guys! I really saw the different kinds of answers to my question! Much appreciated! =)

 

[HughGuessWho]

Whether a lens can be heard when it focuses has nothing to do with whether or not it will perform well in a low light situation. A lens could sound like a farm tractor and still perform well in low light.

I would be happy to hear an explanation to the contrary, though...

I don't read Buckster's comment to say it did.

 

[Derrel]

Some have better/faster/quieter motors for faster/quieter auto-focus as well, but mainly it's the wider aperture diameter that gives better performance in low light.

Whether a lens can be heard when it focuses has nothing to do with whether or not it will perform well in a low light situation. A lens could sound like a farm tractor and still perform well in low light.

I would be happy to hear an explanation to the contrary, though...

Explanation to the contrary. Two examples come immediately to mind. The Canon 50mm f/1.8 EF-II, the current Canon economy 50mm lens. It has the MM or micro-motor focusing system, Canon's cheapest and noisiest, and has no full-time manual focus override. The lens fairly well scritches and screeches when it has focusing difficulty...it makes quite a racket in fact, and is LOUD. Annoyingly, embarrassingly loud in situations where quietness and decorum are paramount, like at a wedding ceremony, or a quiet dinner event, or whatever. With the plastic-mount 50/1.8 EF-II, if the lens has difficulty achieving focus in low light, which mine very often did, the user has to mechanically disengage the lens's on-board focusing motor, and then focus by hand, using a small, front-mounted focusing ring.

On the flip side, a lens that has a better, faster, and quieter motor: the Canon 50mm f/1.4 EF, their current mid-line 50mm. It has Canon's better Micro USM, or micro ultrasonic motor, with switchless, full-time manual focusing override. This is quite simply, a better, faster, quieter focusing motor (like Buckster mentioned), and is one of the MAJOR differences between the 50/1.8 and the 50/1.4; this focusing motor actually WORKS reliably in poor light, and it is quieter than their micro motor, by a great amount. Now, bear in mind, the MSM is NOT Canon's "top-grade" focusing motor; that is their Ring USM motor, found in Canon's very-best lenses.

In the Nikon realm,things are a bit different,since technically the focusing "motor" is in the body of better cameras, and in the LENS on the newer, better lens models. Nikon's noisy-focusing lenses are virtually all "screw-drive" lenses that use the camera body's internal AF motor, and a screw-and screw-head slot system of fairly primitive mechanical linkages that are used to drive a lens to focus by a process of basically, trial and error and evaluation of degree of focus error. Most Nikkor screw-drive lenses are fairly noisy in operation, and in really noisy environments, the sound can be obscured. You can actually FEEL the focusing action occurring many times...and I mean literally feel it. And you can hear it too. Nikon's 50mm 1.8 AF and AF-D lenses date back to the 1980's and are fairly noisy focusers. The new 50mm 1.8 AF-S G has Nikon's Auto Focus-Silent-wave motor system in it. Unlike screw-driven focusing, which Pentax and Sony and Nikon still use in many lenses, the AF-S system uses multiple types of computer-gathered, computer-analyzed data to predict focus, and can take one, initial reading of focus, compute the degree of OOF,and then drive the lens to perfect focus in ONE, single, discrete movement. Unlike screw-driven focusing that uses phase detection to repeatedly analyze, and then compare front- and then back-focus in dim lighting, the AF-S protocol is not a mechanically-dependent, trial-and-error protocol, but is instead, coded to color-aware, color-sensing, color-analyzing, and Distance-aware data analysis. This is the real reason that Nikkor AF-S lenses of wide-aperture, like f/2.8 and wider, can follow focus so perfectly and easily and so reliably, even under bad lighting conditions, and why older screw-driver focusing lenses are less-reliably able to focus in the same way, or with the same speed, or with the same degree of predictability. When focus distances change very rapidly, or erratically, the older screw-drive motor system often falls behind, whereas the newer AF-S system can track, and can PREDICT movement, and in low-light scenarios, the ability of an ultrasonic motor versus a simple, mechanical system is pretty well won most times by the ultrasonic system.

A good example here is the 85mm 1.4 AF-D Nikkor, a fabulous portrait lensm, but which used old-school screw-driven focusing; in things like indoor high school basektball, this lens has trouble following continuous action in dimmer gyms; the newer, cheaper, slower aperture 85mm 1.8 AF-S G OTOH, has AF-S, so it can follow low-light action, basically seamlessly, and not get confused, and NOT need to rack back to minimum focus distance, and then back out when it encounters a tough focusing scenario.

 

[Josh66]

Isn't that more a side effect of having a crappy focusing motor though? The noise doesn't make it poor in low light - the poor motor makes it loud.

Granted, better focusing motors will pretty much always be quieter - but the noise is a result of poor design, not the other way around.

 

[Derrel]

The crappier system makes a LOT more noise. The 50/1.8 is a LOUD, high-pitched focuser. It has their absolute bottom of the barrel focusing system. The 50/1.4 EF-II has their mid-grade focusing motor, and while it is a lot better than their 1.8 in terms of low-light focus, as me, and many others have noted, there are times when the 1.4 model's mid-grade focusing motor simply does not want to focus....it just seems to "stop" or "freeze"...and then the user needs to grab the ring, and start it again. I've mentioned this a few times here on TPF, as has Big Mike, and others.

I do not think people are equating the presence of the noise per se as what causes the lens to be poor in low-light situations. it is not the noise, per se, but rather the cheaper, noisier lenses tend not to perform as well. Seems like Steve5D was just spoiling for a fight, or cannot read too well. What Buckster wrote was this: "Some have better/faster/quieter motors for faster/quieter auto-focus as well, but mainly it's the wider aperture diameter that gives better performance in low light."

Lenses that have better/faster/quieter focusing motors >>>>>gain the benefit of faster/quieter autofocusing>>>>>>

and related to the zooms vs primes aspect: wider-aperture lenses perform better in low light.

A good number of TPF users have little or no experience with prime lenses. Buckster has a lot of experience with gear, and his comment as stated made perfect sense to me. But, as we've seen, there's often a lot of confusion about things. Bottom line is that Canon's prime lenses, for the most part have wide aperture values that, combined with decent focusing systems, make them pretty good choices when the light level is low. BUT....and this is my experience: the 50/1.8 EF-II has poor low-light focusing. Noisy, slow, indecisive at times, and has no manual override without switching for AF to M, and so, despite the wide f/stop of 1.8, it is NOT a very good focuser, and I personally attribute that to the cheapie micromotor focus. The Canon 50/1.4 I replaced it with focuses

1.more quietly
2.faster and
3.more-reliably
4. but is still not perfectly reliable, due to the occasional inexplicable "stoppages" where it just does not seem to realize it needs to focus!

 

[Josh66]

I have not read through the entire thread yet. I think I agree with everything you said, but it seems to me that Steve was just saying that 'noise' does not equal poor focusing ability. And that I agree with. But, I also agree that poor focusing ability usually does equal noise. The noise is a symptom, not the actual problem.

 

[Derrel]

Look at Canon's lens specification chart here: http://www.usa.canon.com/app/pdf/lens/EFLensChart.pdf

What you will see is that there are a small number of older Canon prime lenses that use the AFD focusing motor system: 15/2.8 Fisheye; 24mm f/2.8; 28mm f/2.8; 35mm f/2; 135mm f/2.8 Soft Focus; 50mm f/2.5 Compact Macro. This is a noisey motor, and not nearly as good as their Ring USM motor system.

I own the 135/2.8 Soft Focus and the Canon 135/2 L, with the Ring USM motor. A number of these older lenses have AF performance that's not really up to par compared against ultrasonic motor focusing lenses made by Canon or Nikon, in my experience. I agree, the noise goes with the cheaper focusing motor...and...the lenses that use the cheaper focusing technology focus more-slowly and may also force the user to switch out of AF mode to make corrections, and so on.

The MM or micro motor is reserved for their bottom of the line stuff. AFD appears to be legacy. Ring USM is now their best-performing technology.

 

[Josh66]

I own the 135/2.8 Soft Focus [...]

Me too - I think we might be the only ones on this forum with that lens, lol. And I agree with your assessment.

Although the new STM motor seems to be fairly quiet. Not quite as quick as ring USM, but near equally quiet (slightly more noisy).