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Why All the Concern with Lens Speed?

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geezer52 said:
I've seen several threads lately with complaints about how slow a 2.8 or 2.4 lens is and I'm not sure I understand why. I know back in the films days lens speed was an issue for low light shooting because film ISOs rarely went above 400 but that is hardly the case with modern DSLRs. Modern DSLRs frequently have ISOs over 3200 producing quality images plus they have the advantage of image stabilization. Film lenses with f-stops ranging from 1.2-2.0 often had their own design compromises and didn't achieve maximum performance till 2.8 anyway. The Pentax DA-40 is wide open at 2.8 but achieves edge to edge sharpens from the git-go. And this is all before you venture into the ease and control of computer based post processing.

I realize some of these old, fast, film-era, lenses have their uniques charms (the Pentax 50mm, 1.4 comes to mind). But it seems to me, the only valid argument for lenses wider than about 2.0 is greater control over depth-of-field. The lens speed itself seems to be a non-issue.
Click to expand...
Well, DOF is a huge issue. That aside, if I'm using a 24mm f2.8 lens, shifting that over to and f1.2 lens means 2.5 stops more light. Rather than 6400 iso, I can now use 2000....a big difference in quality for a print.

Currently, the K-x offers the best 3200. But with a faster lens, you could shoot at 1000 in the same light....which is virtually noise free.

For me though, it's about the DOF. At f2.8, you're looking at f4.5 or so in equivalent for a FF body.....not exactly a shallow DOF.
 
Fast lenses don't have to be poor wide open. They offer superb background separation, making the subject pop out in a magical way, and they allow the use of much lower ISO.

Shallow DoF is not just simple byproduct of not being able to use f/8, but a sought after and hard paid effect. it's all a matter of style. And calling the 50 @ 1.4 on APSC DoF razor thin and unusable ? Seriously :)

85mm @ f/1.2 on FF



85mm @ f/1.2 on FF



85mm @ f/1.2 on FF



35mm @ f/1.4 on FF



35mm @ f/1.4 on FF



85mm @ f/1.2 on FF panorama

 
Nice photographs! And yes, a 50 f1.4 is about the same as f2.5 on a FF....hardly razor thin by any measure!
 
The first three are very nice photos, you are a very capable photographer, but truth be told i have seen some absolutley terrible images from that particular lens. There is more to those photos then the back ground, they are nicely processed, taken under very nice lighting conditions and well composed. These most likely would have been nice images shot out to f2.8.
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Nice!

Did u use flash? The eyes say you probably did.

DOF was a bigger factor than speed in your case.
 
I've shot where ISO 3200 f/1.4 was too slow to get the shutter speed I wanted to freeze moving subjects.

--
Lee Jay
(see profile for equipment)
 
Will I buy it? Probably.
Will I use it? Probably just for a couple street shots.

I have the DA*16-50, and it is probably a better choice for shooting most things.
If it had 1.8, then it would mean something different. It is not the case.
--
Alan Schamber

Progress is not possible without deviation from the norm - Frank Zappa
 
however, the new 35 f/2.4 isn't about low light or DoF, just like a 400mm f/5.6 or a 50/100 f/2.8 macro. primes used to be either cheap, small and good (think zeiss tessar 50mm f/2.8) or more expensive, but offering just a last resort low light option. most of the popular primes today follow the latter route, some premium ones go fast with high IQ, and others perfected the first route. the new 35er is most likely an awesome lil lens, very cheap, small and great IQ wise. if i was to buy something small and well priced, i would certainly pick the 35 f/2.4 over the 18-55 kit lens. any day. it's an awesome alternative to that lens. and that's what i believe it was meant to be. it might be slightly slower then the nikon or sony but its probably smaller and at least just as good at the same apperture, and the street price will probably be lower eventually
 
Aristophanes said:
But too thin a DOF makes AF accuracy worse. To improve AF you need to stop down.
Click to expand...
I'd prefer to restate that:

a thin DOF makes whatever focusing you do, whether AF or manual, more critical. It does not vary the accuracy itself; only the need for accuracy .

MF: When manual focusing, an old style split-image prism will AFAIK work the same geometrically, regardless of the speed of the lens, until too slow a lens starts to shade it completely. However the contrastiness may be higher when a fast lens is fitted.

But - most manual focusing on digital SLRs happens through a "bright" treated plain focus screen. The "bright" effect comes from a specially textured surface, and this means that light is not simply transmitted according to the quantity of light entering; it is prismatically redirected and this imposes in effect a lowest common denominator - you often see the DOF of a slower aperture in the viewfinder, than the sensor will record. This is one reason why manual focusing of fast lenses without split-image or microprism aids, is such a lottery.

There is in practice no great improvement in focus discrimination or brightness at f/1.8 compared to f/2.8, though there will be some between f/2.8 and f/3.2, then more and more obvious after that. This can be verified easily for oneself using optical DOF preview.

AF: Except on those cameras with an additional central AF sensor (that comes into play only when a fast lens is fitted) - the AF system does NOT necessarily make use of all the light that comes through the lens. So f/3.2 or f/2.8 or f/1.8 will probably act equivalently - although these lenses may have different levels of scattering, sharpness etc, which are separate considerations. The extra light from the faster lenses follows ray paths that, simply put, will not hit the AF module's sensors.

As far as I can make out, the AF module is designed to a virtual aperture , in effect around f/5 or so typically. This is a "lowest common denominator" that almost any lens can deliver when wide open; even a slower kit zoom at its longest extent.

If AF were to be designed with a long baseline, with a fast (large max aperture) lens in mind, then when slow lenses were fitted instead, these would vignette the AF sensors and that would confuse their accurate functioning.

The extra f/2.8-compatible AF sensor found on some semipro and pro bodies (not yet by Pentax IIRC), can give better low-light and geometric accuracy - but needs to be automatically enabled or disabled depending on the lens used.

RP
 
geezer52 said:
I've seen several threads lately with complaints about how slow a 2.8 or 2.4 lens is and I'm not sure I understand why.
Click to expand...
Because if a prime is slow, there's no reason not to just use a zoom!
 
A Schamber said:
Will I buy it? Probably.
Will I use it? Probably just for a couple street shots.

I have the DA*16-50, and it is probably a better choice for shooting most things.
If it had 1.8, then it would mean something different. It is not the case.
--
Alan Schamber

Progress is not possible without deviation from the norm - Frank Zappa
Click to expand...
Now if only the 16-50 was an f1.8 :-)
 
paulkienitz said:
Because if a prime is slow, there's no reason not to just use a zoom!
Click to expand...
Exactly..

Many primes are there mostly due to speed.
Who'd buy a 50mm f2.8 (non macro) for example? Waste of time

I agree with the comments about the 35mm f2.4 it's too slow for a prime
 
nope. the 35mm f/2.4 still has a niche because it's far cheaper then a respectable f/2.8 zoom. As for those that are disappointed in it being relatively slow, don't forget you're lucky enough to have inbody stabilization.

Some of us may not be too thrilled about it, but that just means we're not exactly the target market because we don't mind spending far more on quality glass. IMO this lens was meant to be an alternative to the kit lens, not to expensive f/2.8 zooms, nor as a solution for DoF/speed maniacs (myself included).
 
paulkienitz said:
geezer52 said:
I've seen several threads lately with complaints about how slow a 2.8 or 2.4 lens is and I'm not sure I understand why.
Click to expand...
Because if a prime is slow, there's no reason not to just use a zoom!
Click to expand...
My point too. Or rather, what's the point? Like the Tamron 28-75mm f/2.8, which for most purposes is just as sharp but so much cheaper and inifinitely more flexible than as a number of slow primes like the DA 40mm, this 35mm, etc..
 
There is a nice thread inside a Leica forum where a performance of a 40 years old Leica prime lens is compared with a latest Canon fast zoom at the same focal length (was it 35mm?)

That 40 year old glass, with old coatings etc. has delivered so much more detail at a same f-stop it's almost unbelievable.

Then, of course, people with Canon equipment have complained it's not fair to compare primes with zooms, because zooms will never deliver same amount of detail, sharpness, etc. I have argued it is fair comparison because that is what photographers have available and at hand: Leica doesn't make any zoom, so prime lens is a necessary equipment of all photographers who uses Leica cameras. They don't have a choice and other advantages Canon users take for granted with their fast zooms.

There's also another problem in the argument that DA35/2.4 is slow. "Slow" f-stop is a relative term. Because any reference to speed, or length, size, etc. must be taken relative to "what exactly".

People are saying DA35 f/2.4 is "slow" only because C, N and S (?) have lenses in that similar price category that have wider apertures -- as their specs claim -- and are thus "faster", have a head-start advantage.

But for a truly concerned photographer, that measure will be taken with a grain of salt, because the final output -- the image -- is a much greater sum than all its parts together, including the maximum f-size of the lens used to capture the photo. It is just one part of the equation, and its relative strength can be counterbalanced -- if necessary -- by other variables, including sensor sensitivity, photographer's skill, knowledge, etc.

I'm concerned that people tend to omit all these factors equally crucial in the creation of a good photograph, which makes me conclude they're not concerned at all about the art of photography itself, but only about the comparable commodities, -- a mere f-stop value -- which if taken out from the equation of photography means absolutely nothing.
 
Because most photographers these days hate noise and grain. I don't understand that as I really like grainy moody b/w photography. I guess color photos do not look good if they are noisy, so the faster the lens the better, as you don't need such high ISO.

I think there's a very dominating taste for clean, smooth slick imagery. Blame the stock photo industry.

AA

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Uluru said:
There is a nice thread inside a Leica forum where a performance of a 40 years old Leica prime lens is compared with a latest Canon fast zoom at the same focal length (was it 35mm?)

That 40 year old glass, with old coatings etc. has delivered so much more detail at a same f-stop it's almost unbelievable.

Then, of course, people with Canon equipment have complained it's not fair to compare primes with zooms, because zooms will never deliver same amount of detail, sharpness, etc. I have argued it is fair comparison because that is what photographers have available and at hand: Leica doesn't make any zoom, so prime lens is a necessary equipment of all photographers who uses Leica cameras. They don't have a choice and other advantages Canon users take for granted with their fast zooms.
Click to expand...
I did not know Canon users were prohibited from using primes on their cameras.
Uluru said:
There's also another problem in the argument that DA35/2.4 is slow. "Slow" f-stop is a relative term. Because any reference to speed, or length, size, etc. must be taken relative to "what exactly".

People are saying DA35 f/2.4 is "slow" only because C, N and S (?) have lenses in that similar price category that have wider apertures -- as their specs claim -- and are thus "faster", have a head-start advantage.
Click to expand...
No, it is a pretty slow prime no matter how you look at it. If that is a problem depends on what the photographer wants to do with it.
Uluru said:
But for a truly concerned photographer, that measure will be taken with a grain of salt, because the final output -- the image -- is a much greater sum than all its parts together, including the maximum f-size of the lens used to capture the photo. It is just one part of the equation, and its relative strength can be counterbalanced -- if necessary -- by other variables, including sensor sensitivity, photographer's skill, knowledge, etc.

I'm concerned that people tend to omit all these factors equally crucial in the creation of a good photograph, which makes me conclude they're not concerned at all about the art of photography itself, but only about the comparable commodities, -- a mere f-stop value -- which if taken out from the equation of photography means absolutely nothing.
Click to expand...
So you conclude that people are not concerned about photography based on the lenses they want? You seem obsessed with judging photography based on gear.

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http://www.flickr.com/photos/36164047@N06/
 
richardplondon said:
Aristophanes said:
But too thin a DOF makes AF accuracy worse. To improve AF you need to stop down.
Click to expand...
I'd prefer to restate that:

a thin DOF makes whatever focusing you do, whether AF or manual, more critical. It does not vary the accuracy itself; only the need for accuracy .

MF: When manual focusing, an old style split-image prism will AFAIK work the same geometrically, regardless of the speed of the lens, until too slow a lens starts to shade it completely. However the contrastiness may be higher when a fast lens is fitted.

But - most manual focusing on digital SLRs happens through a "bright" treated plain focus screen. The "bright" effect comes from a specially textured surface, and this means that light is not simply transmitted according to the quantity of light entering; it is prismatically redirected and this imposes in effect a lowest common denominator - you often see the DOF of a slower aperture in the viewfinder, than the sensor will record. This is one reason why manual focusing of fast lenses without split-image or microprism aids, is such a lottery.

There is in practice no great improvement in focus discrimination or brightness at f/1.8 compared to f/2.8, though there will be some between f/2.8 and f/3.2, then more and more obvious after that. This can be verified easily for oneself using optical DOF preview.

AF: Except on those cameras with an additional central AF sensor (that comes into play only when a fast lens is fitted) - the AF system does NOT necessarily make use of all the light that comes through the lens. So f/3.2 or f/2.8 or f/1.8 will probably act equivalently - although these lenses may have different levels of scattering, sharpness etc, which are separate considerations. The extra light from the faster lenses follows ray paths that, simply put, will not hit the AF module's sensors.

As far as I can make out, the AF module is designed to a virtual aperture , in effect around f/5 or so typically. This is a "lowest common denominator" that almost any lens can deliver when wide open; even a slower kit zoom at its longest extent.

If AF were to be designed with a long baseline, with a fast (large max aperture) lens in mind, then when slow lenses were fitted instead, these would vignette the AF sensors and that would confuse their accurate functioning.

The extra f/2.8-compatible AF sensor found on some semipro and pro bodies (not yet by Pentax IIRC), can give better low-light and geometric accuracy - but needs to be automatically enabled or disabled depending on the lens used.
Click to expand...
Everything you say is true but you've missed the crux issue.

In Pentax's implementation what evidence is there that there selected baseline and algorithms do not achieve adequate AF accuracy for lens

Why would they need an extra long baseline sensor when the existing solution delivers AF accuracy down below f1.4..?

Don't apply Canon Kludge solution to other manufacturers . :)
richardplondon said:
RP
Click to expand...
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Uluru said:
There's also another problem in the argument that DA35/2.4 is slow. "Slow" f-stop is a relative term. Because any reference to speed, or length, size, etc. must be taken relative to "what exactly".

People are saying DA35 f/2.4 is "slow" only because C, N and S (?) have lenses in that similar price category that have wider apertures -- as their specs claim -- and are thus "faster", have a head-start advantage.
Click to expand...
It is slow, relative to all the millions of 35mm prime or fixed focal length lenses ever made for SLRs and DSLRs. Maybe not slow to the Pentax users who are used to the speed of the Pentax Limited lenses. Even some compact fixed lenses are faster. I don't understand how this point can be argued.
 
awaldram said:
In Pentax's implementation what evidence is there that there selected baseline and algorithms do not achieve adequate AF accuracy for lens
Click to expand...
My point was: when one is comparing a f/1.8 lens with an f/2.4 lens, it is not helpful to assert that the f/1.8 lens would allow the user to manually focus any better , nor is it right to suggest that it will allow the camera to autofocus any better . It will be substantially the same. Of course the pictures will be different, if these fastest apertures are used; but not what happens in the viewfinder.

The AF module we now have may or may not give adequate accuracy - that is a matter of opinion. I personally think it does, being realistic about how much these cameras cost and about the limitations inherent in the technology. We must have sensible expectations.

People may justifiably complain that a camera has focused slowly , or has focused onto the wrong thing , but those are not imprecision .
awaldram said:
Don't apply Canon Kludge solution to other manufacturers . :)
Click to expand...
If you mean the central AF sensor for fast lenses, that has some justification IMO for very particular usages, but most of us get by perfectly without it. Especially if our lenses are not so fast, in which case it would be useless to us anyway. So, not really a kludge; but we can maybe call it expensive over-engineering for the mass marketplace - which is why we don't see it in most cameras.

RP
 
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