Speed of lens...?

[guggle]

Hi all,

I have read a few reviews of lenses here and on other sites, and I don't understand what the speed of a lens is and why it is important. Would someone please explain it to me?


Many thanks, Michael.

[William]

A fast lens does'nt mean that one goes faster than another It usually means the low light capability of the lens , It means they will have a really wide aperture EG : 1.2, 1.4. to 2.8 , They cost a lot more also

[ameerat42]

+1
And it is used as a relative term, too, like there aren't just "fast" lenses and "slow" lenses, but a lens can be "fast"-er than another one, or "slower" than some others.

Don't ask why they're called fast/slow. There are various reasons about.
Am.

[kiwi]

It's important to have "fast" lenses in two different ways, it means that you can shoot in lower light for the same ISO and shutter speed, and it also allows you to have a shallower depth of field so you can nice background blur (bokeh)

[jupiter618]

I thought the "fast" must come from the faster shutter speeds it enables you to use? A fast lens lets you use fast enough shutter speeds to get the shot closer to the optimum/base ISO.

[kiwi]

yes, true, exposure is a combination of light affecting the triangle of iso/shutter speed/and apperture.....if you can get savings in one area you can pay off the other two areas

[Lance B]

I thought the "fast" must come from the faster shutter speeds it enables you to use? A fast lens lets you use fast enough shutter speeds to get the shot closer to the optimum/base ISO.

Yes, you are correct, the term "fast" lens is in fact in reference to the result of being able to use faster shutter speeds by being able to open up to a wider aperture (read smaller aperture number) for the same ISO. So, an f4 lens may allow a shutter speed of 1/50sec, whereas a fast lens like an f2 lens will allow a shutter speed of 1/200sec for the same ISO used.

[kiwi]

Fast also relates to how speedy your pockets get emptied of cash

With the exception of the fast and cheap nifty 50's (50 1.8)

[Lance B]

Fast also relates to how speedy your pockets get emptied of cash

With the exception of the fast and cheap nifty 50's (50 1.8)

LOL. This is also a fact!

[Wayne]

LOL. This is also a fact!

Amen, all 3 of us can attest to this follow on effect of getting some fast glass.

[William W]

I have read a few reviews of lenses here and on other sites, and I don't understand what the speed of a lens is and why it is important. Would someone please explain it to me?

The history and the technical term – “Speed of a Lens”

The "Speed of a Lens" is a defined, technical term used in Photography and Lens Terminolgy.

The term does NOT have it roots founded in the resultant shutter speed required to make an image.

As one definition:
“The speed of a lens is its ability to pass light from object to image and is determined by the geometry of the lens (#1) and by its transmission (#2)”

(#1) Geometry of the Lens includes –
“the beam of light passing through a photographic objective is limited by means of a diaphragm, the aperture in this diaphragm may be fixed in size or it may be varied in size to control, the light being admitted.”

(#2) Transmission includes –
“losses of light by reflection and absorption within a lens . . . the extent of the loss of light within a lens depends primarily upon the number and the composition of the glasses employed.”

Ref: The Manual of Photography, Horder Alan( Ed) (Focal Press London) Sixth Ed.

There are ample other technical references, stating similar.

The Manual of Photography dedicates all of Chapter 5, to the topic of “The Speed of a Lens” and doesn’t seem to mention Shutter Speed, once, as "Speed of a Lens", is a technical term rooted in the transmission capacity of a lens and is not founded in the relationship to a fast (or slow) shutter speed, which might be able to be used.

That said, there is obviously a direct relationship between the Speed of a Lens, when that lens is put on a camera and a fast shutter speeds which can be used, because a fast lens is being used.
And that is easy to remember: but that relationship neither changes the history nor the Technical Term “Speed of the Lens” - which certainly stands apart from any relationship to Shutter Speed.

***

Lens development:

As technology advances; lenses became better; materials finer; production less expensive; demand greater: what is now considered a “slowish lens” – for example a 300mm F/5.6 was once considered a “fast lens”.

Constraints, especially with regard to Focal Length:
In regard to the common DSLR cameras, in general terms: it is more difficult and/or more expensive to design and make large maximum aperture lenses which containing few aberrations, in Wide Angle Focal lengths and also in the Telephoto Focal Lengths.

Also it is much easier to make quality PRIME lens with a large maximum aperture than to make a zoom lens at around the same Focal Length, with the same Maximum Aperture.
Taking these facts into consideration, when we discuss the Speed of a Lens – i.e. describing the lens as either being “fast” or “slow”: we are discussing its relationship to the other Lenses in its class.

For example (in the Canon Range) we would, say the EF50/1.4 is “a fast lens”; and the EF 50F/1.2L is “faster”; and the EF50F/1.0L is “even faster” , but they are all “fast lenses”

On the other hand – the EF 400F/2.8L is also “a fast lens” even though it is 3 stops slower than the EF50F/1.0L – BUT it is a 400mm lens, not a 50mm lens.

Also the EF24 to 70F/2.8L is “a fast lens” even though when set at 50mm it is only F/2.8 and again three stops slower than the EF50/1.0L – BUT is a ZOOM lens and not a PRIME lens.

WW

[MissionMan]

I've also heard people refer to the speed of the lens as how fast it focuses. I.e. suitability for sports. It's not the technically correct terminology but for some people the autofocus speed is important. It's also important to remember that low f stop numbers don't automatically mean fast lenses although in majority of cases they are fast.

[Lance B]

Whilst the reference used to "prove" that the term a fast lens is not related to the resultant shutter speed allowed for a large aperture lens, I have to respectfully disagree with the author. He states: "The Manual of Photography dedicates all of Chapter 5, to the topic of “The Speed of a Lens” and doesn’t seem to mention Shutter Speed, once, as "Speed of a Lens", is a technical term rooted in the transmission capacity of a lens and is not founded in the relationship to a fast (or slow) shutter speed, which might be able to be used."

The emphasis on the words, "doesn’t seem to mention Shutter Speed, once as "Speed of a Lens"...". As I see it, there is no other way to deduce that the term a "Fast Lens" is anything but the relationship of the faster shutter speed afforded a large aperture lens. To me, nothing in the linked article/information would indicate otherwise and it has always been a long accepted fact that this is the case. There really is no other reason to term it a fast lens than for the fact that it affords a faster shutter speed.

[arthurking83]

That this Manual of photography has omitted any reference to speed of the shutter, is not really relevant to the point.

The simple fact that if this faster lens is going to be used in preference to a not as fast lens, implies that a faster shutter is required given the constraints of a pre determined lighting amount and sensitivity from the capture medium.

Faster lens can mean both ultimate max aperture, and alternatively a higher T-stop even tho lenses with the same max aperture may still ultimately allow different levels of light through to the keeper(or so to speak).

Any way you look at it, given a pre defined level of light availability, and ASA/ISO rating from the sensor material .. a faster shutter speed is going to be required when using a faster lens.
There may be the situation where the author of this Manual of Photography is assuming that you have an understanding of exposure at this point and adding this reference is simply not necessary.... or it could be confusing!!
There could be the situation where if the author specified shutter speed as an advantage of a having a faster lens, that photography students will all rush out and shoot at faster shutters speeds all the time, even tho they may have also changed other exposure variables as well.

Of these three other variables, the shutter speed is generally the easiest one to work with(up to a point, of course).

-------------------

What's quite amusing in all this tho, is that in having previously made a reference to T-stops of a lens, and what I believed to be the transmission levels of some of my lenses, I'd made an error in what I believed to be the slower lenses!!

While I though that my 18-105VR kit lens was slower(and it still is in a specific set of circumstances), it turns out that my 'slowest' lens is in fact my fastest lens .. the Nikon 50mm f/1.2 Ais.
It has no ED lenses in it's construction, and I'm not sure of the coatings Nikon used in it's design, but of all the 50mm equivalent lenses I can access, this lil big bugger is the slowest lens(produces the darkest rendering of a scene) at the same aperture, ISO and shutter speeds!

AFAIK, it also has the simplest optical formula of all my lenses too.

The lenses that I have that achieve a 50mm focal length, are Nikon 50/1.2; Sigma 50/1.4; Tamron 28-75/2.8; Tamron 17-50/2.8, Nikon 18-105VR.
(I also currently have access to a Pentax 50/1.8 thanks to our generous member and mod Andrew , but I didn't include this one's results as it's a M42 lens and the results coudl be skewed via the use of extension tunes and adapaters).

In terms of brightness at the same settings the ordering from brightest to darkest rendering image given the same shutter speed, static ISO and constant aperture value of f/8(to eliminate the possibility of vignetting skewing results) are:

1. Tammy 28-75,
2. Tammy 17-50/2.8
3. (tied) Nikon 18-105 + Sigma 50/1.4
4. Nikon 50/1.2 by a fair amount too!

There are subtle differences in contrast rendering as well in all the images, the two most similar rendered images in terms of brightness(histogram, as well as image on screen) are the Sigma and Nikon 18-105.

Given the results, and knowing that the Nikon 50/1.2 ais has the simplest design and least amount of glass with it's optical make up, I can only assume that there may be an inaccuracy in the operation of the diaphragm by the camera body.

In terms of metered tho, the two most accurate are again the inseparable Sigma 50/1.4 and Nikon 18-105VR.
(which by the way for Nikon owners is probably Nikon's best ever kit lens, and highly recommended)
Both of these lenses metered and exposed neutrally given a standard reference exposure of eg. 1/6s at f/8 and ISO200 in the single bulb lit room.. and yes, room lighting was a constant)
The darkest again was the Nikon 50/1.2 and then followed by the Tammy 28-75/2.8 metering quite dimly.
But once the shutter speed was compensated for (from 1/10s) to the reference of 1/6s it then produced the brightest image.
Tammy 17-50/2.8 metered at 1/8s but still produced an image similar to the Sigma and kit Nikon

[Speedway]

Too much technical hoo ha to try to confuse normal people that the ones writing this are somehow superior to them. The average person just wants a simple answer to a simple question. A larger (smaller number) aperture does allow a faster shutter speed! Hence a faster lens is the common description.
Keith.

[William W]

Whilst the reference used to "prove" that the term a fast lens is not related to the resultant shutter speed allowed fora large aperture lens, I have to respectfully disagree with the author. .. etc

No.

There was no point in posting those comments to “prove” anything.

It was an historic reference for those interested and more importantly for background for the OP.

Your original post suggested that the term “Speed of the Lens” was rooted in the fact that a “faster” shutter speed could be used . . .

i.e. the link was between “speed” and “shutter speed”



This appears to be incorrect – in fact I would state - it is incorrect.



The term “speed of the lens” when originally coined was related to the “AMOUNT” of light transmitted.

And as a RESULT of that, a faster shutter speed could be used.

That is a small, but a significant difference in the roots of the term.

And as the OP asked for an explanation – the roots of the technical term goes to making that full explanation.

That this Manual of photography has omitted anyreference to speed of the shutter, is not really relevant to the point.

Yes it is.

See above.

Too much technical hoo ha to try to confuse normal people that the oneswriting this are somehow superior to them.

Not at all.

No one stated that they were confused.

If you do not find the history interesting, then that is fine by me.

I do not consider you inferior to me for not being interested in the History of Photography – why would you think that of me?

Theaverage person just wants a simple answer to a simple question. Alarger (smaller number) aperture does allow a faster shutter speed! Hence afaster lens is the common description.

Maybe – and it has been provided.

But that is no reason to criticize additional material that others might find interesting and or useful.



There seems to be a lot of angst in these responses of yours ? ? ?



WW

[Tommo1965]

i think in today's world most would accept that when using the term "fast glass/lenses",,it relates to large constant aperture lenses that when used in difficult lighting conditions can still deliver enough light through the lens to enable a high shutter speed....

Williams reference seems like a good alternative explanation..but most people wouldn't think that if the term Fast glass was used

its the same as if someone said they were Gay..you wouldn't immediately think they were happy would you....but Gay still means happy....but also means something else today

[ricktas]

Ultimately there are several ways to define what a fast lens is, as this thread has shown. None of the answers are incorrect, just different interpretations of the physical characteristics of a lens and the science behind how it operates. What one author chooses to use to define it, is just that author's opinion, written down.

This thread has provided some great information on why a lens might be called faster, we don't have to accept one theory over another. Hopefully some of the less experienced photographers who read through this thread, get/got a better understanding of how lenses work, and if that was achieved, then the thread is a success. However, if it gets bogged down in interpretative arguments, about who might be 'right' then most people turn off and go find something more interesting to read. So let's accept that the term 'speed of a lens' can mean a variety of things..and move on!

[arthurking83]

That this Manual of photography has omitted any reference to speed of the shutter, is not really relevant to the point.

.......

.....




Yes it is.

See above.



.....

OK then!! ... It may be if this is your only source of information but I still believe that in this instance it is irrelevant that the author of that article has omitted any reference to shutter speed.

My point with my comment was that the omission of shutter speed by the author of the manual of photography was irrelevant simply because it was coming from a specific point of view .. and not the only point of view, nor the definitive one. If time is taken to research this information from multiple sources, there will be a different interpretation of the importance of certain information.

There are many sources of information relevant to the technical aspects of photography, and there is no compulsion to believe that this Manual of Photography is the only definitive source!
This is a matter of personal preference, and while you may find a source of info without any reference to shutter speed when the topic is about faster lenses, I can easily find a source of material that does reference shutter speed in the description!

Lens speed refers to the maximum aperture diameter, or minimum f-number, of a photographic lens. A lens with a larger maximum aperture (that is, a smaller minimum f-number) is a fast lens because it delivers more light intensity (illuminance) to the focal plane, allowing a faster shutter speed. A smaller maximum aperture (larger minimum f-number) is "slow" because it delivers less light intensity and requires a slower shutter speed.

ref: wikipedia

[ameerat42]

Re:

...No.
...It was an historic reference for those interested and more importantly for background for the OP.
...Yes it is.
...Not at all.
...No one stated that they were confused.
...If you do not find the history interesting, then that is fine by me.
...But that is no reason to criticize additional material that others might find interesting and or useful.
...There seems to be a lot of angst in these responses of yours ? ? ?
...WW

and foregoing of the ilk, I submit as follows:

Look, the term arose in the Middle Ages before it had been shown that all objects fall at the same rate in the Earth's gravitational field.

It had been assumed that the larger aperture lenses would fall faster than the smaller-aperture ones because they were bigger (more glass, you know).
Hence they were called "fast lenses".

Well, after dropping a few from the towers of the time it was later found that they didn't fall any faster at all, but the term stuck, as terms often do.

So there you have it, along with "baker's dozen" and "eggs in baskets".
m.