# Filter Size (Lens Diameter) and Exposure



## frXnz kafka (Jun 23, 2008)

Been looking to upgrade my walk-about lens lately, and I've been trying to wrap my head around filter sizes. Google was no help, so I thought I'd bring it here. 

What is the difference, in terms of the amount of light coming in, between say a lens with a 58mm filter size and a lens with a 77mm filter size? Does the 77mm let in more light, allowing for a faster exposure? Or is there some other advantage I'm missing?


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## Big Mike (Jun 23, 2008)

You are sort of on the right track...but looking at the wrong thing and putting the cart before the horse.

A lens with a bigger maximum *aperture* will let in more light...and that will be in the name of the lens as an F number.  

Now, in order to get that larger maximum aperture, lenses often have to be designed with a larger front element and therefore a larger filter diameter.


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## brileyphotog (Jun 23, 2008)

Often is the key word...the Nikon 50mm f/1.8, for instance, has a 52mm filter size. 

Little number behind the "f/" = big aperture = faster lens...easy as pie


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## AndrewG (Jun 23, 2008)

frXnz kafka said:


> Been looking to upgrade my walk-about lens lately, and I've been trying to wrap my head around filter sizes. Google was no help, so I thought I'd bring it here.
> 
> What is the difference, in terms of the amount of light coming in, between say a lens with a 58mm filter size and a lens with a 77mm filter size? Does the 77mm let in more light, allowing for a faster exposure? Or is there some other advantage I'm missing?


 
The amount of light a lens will admit is governed by its maximum aperture given by the f number. The filter size (diameter across the filter thread) has no influence on exposure. Thus an exposure of, say, 125th and f8 will be the same whether the lens has a filter diameter of 39mm or 77mm.


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## astrostu (Jun 23, 2008)

Math-wise, the amount of light that comes through the lens goes as the diameter-squared.  The f/number is _directly_ related to the aperture (aperture/focal length or vice versa, can't remember off-hand).

So, if you have an f/2.8 lens vs. an f/4.0 lens, then you could feasibly take photographs that are (2.8/4.0)^2 = (0.7)^2 = 0.49 => 49% as long with the f/2.8 lens vs. the f/4.0 lens and still get the same amount of light.  This becomes very important when shooting in low-light situations where you need a faster shutter speed.

As a rule of thumb, ever successively smaller aperture (so like 6.3 -> 5.4, or 1.6 -> 1.4) will allow you to capture the same amount of light in approximately 73% of the amount of time.


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## frXnz kafka (Jun 23, 2008)

I know all about aperture and F-stops, but surely there is a difference between an f/4 lens with a 58mm filter and an f/4 with a 77mm filter, right?


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## Big Mike (Jun 23, 2008)

> I know all about aperture and F-stops, but surely there is a difference between an f/4 lens with a 58mm filter and an f/4 with a 77mm filter, right?


Not in terms of light gathering ability or exposure.  F4 is F4.

Sometimes, the choice of filter size is more of a convenience.  I believe that most of Nikon's pro level lenses are 77mm, even though some of them might not need to be that wide.  This is good because the filters and lens caps are interchangeable between them.


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## astrostu (Jun 23, 2008)

frXnz kafka said:


> I know all about aperture and F-stops, but surely there is a difference between an f/4 lens with a 58mm filter and an f/4 with a 77mm filter, right?



What Big Mike said.  I would also think that the smaller filter lens would be poorer quality, just 'cause if you're going to make big glass, usually it's higher quality.

Do you have an example ofan f/4 58mm aperture vs. f/4 77mm aperture?  If you have a specific example, we might be able to tell you better why that may be the case.


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## christopher walrath (Jun 23, 2008)

The 77 is probably on something like an UBER-zoom (28-200 or more).  But beware, the front element sticks out far enough that you may need a spacer for a thread on lens.  A filter threaded right on the front might make contact with the front glass causing scratching, a fault that the chap who sold me my 28-200 apparently didn't know before he shipped it out.  Lesson learned and all . . .


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## Mike_E (Jun 23, 2008)

There is also the matter of the sweet spot on a lens.  The bit in the middle is the best for taking photos.  So, the more in the middle the better the lens.


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## frXnz kafka (Jun 23, 2008)

christopher walrath said:


> The 77 is probably on something like an UBER-zoom (28-200 or more).  But beware, the front element sticks out far enough that you may need a spacer for a thread on lens.  A filter threaded right on the front might make contact with the front glass causing scratching, a fault that the chap who sold me my 28-200 apparently didn't know before he shipped it out.  Lesson learned and all . . .


Huh? 77mm is the standard for most pro glass, it's not just for super-zooms.


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## christopher walrath (Jun 23, 2008)

Sorry.  Film photographer here and never saw a 77mm on any lens I had for the first twenty years.  'Course I just had basic MD Rokkor's and Sekor C's.


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## ann (Jun 23, 2008)

i have several lenses that take 77mm filters., all are 2.8 lens either film or digital makes no differences 77mm is 77mm


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## Samriel (Jun 23, 2008)

Mike_E said:


> There is also the matter of the sweet spot on a lens.  The bit in the middle is the best for taking photos.  So, the more in the middle the better the lens.



Not sure if this statement is a joke or for real. It does make sense to me though. Else I don't get why the Nikkor 50mm F/1.4 has a 52mm filter and the Sigma 50mm F/1.4 has a 77mm filter. Trying to use the advantage of this sweet spot?


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## compur (Jun 24, 2008)

It may seem like the diameter of the front of a lens ought to be related to its
light transmission but it isn't.  It all depends on the lens design. For example,
some wide angle lenses have quite large front elements but their light
transmission may be relatively low.  The same with some zooms that have long
focal length ranges, like 28-200mm, etc.  They can have very large front
elements but a relatively small max aperture.

Also, manufacturers try to keep filter size the same as much as possible for
their lenses so they may design them with the same front dimensions when
they can, even though these lenses may have differing maximum apertures.
This saves the owner from having to buy many sizes of filters.


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## Apex (Jun 24, 2008)

> This saves the owner from having to buy many sizes of filters


 
Youd think it would be the other way around :roll:


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## Ben-71 (Jun 24, 2008)

[frXnz kafka]
What is the difference, in terms of the amount of light 
coming in, between say a lens with a 58mm filter size 
and a lens with a 77mm filter size? 
The filter diameter is irrelevant.
​Only the *focal length*, the *diameter of the objective* (the front 
element of the lens) and the *duration of the exposure* determine 
how much light is gathered during a shot.

*Focal length* divided by *Objective Diameter* (in millimeters) = *f/Stop*

The diameter of the lens barrel can be much larger than the objective, 
and therefore require a larger filter, but this has no influence on how much 
light goes in.


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## Helen B (Jun 24, 2008)

Ben-71 said:


> Only the *focal length*, the *diameter of the objective* (the front
> element of the lens) and the *duration of the exposure* determine
> how much light is gathered during a shot.
> 
> ...


 
Ben,

The f-number is the focal length divided by the diameter of the entrance pupil, not the diameter of the front element. The two may be very different. Here is a link to an earlier thread that discusses how the f-number is calculated.

Edit: Here's a comparison between two 28 mm f/2 lenses. You can see that the entrance pupil (the image of the 'hole' in the iris) is smaller than the front element of both lenses, that the diameter of the entrance pupil of both lenses is the same, and that the diameter of the front elements is different. They are both designed to cover 24 mm x 36 mm, but the Nikkor is designed for an SLR and the Summicron is designed for a rangefinder camera. One design has to allow the mirror to swing up, one doesn't. The Summicron can be a more compact, more symmetrical design.

The lens with the smaller front element, and smaller filter size (46 mm vs 52 mm) produces significantly higher image quality than the lens with the larger front element, especially at f/2 and f/2.8. There is, however, a very large difference in price.







Best,
Helen


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## Garbz (Jun 24, 2008)

But that smaller front element is sharper idea is not universal in this case is it?

Compare say the 105mm MicroNikkor f/2.8 G to it's older D version. The old one isn't sharper than the new one as far as I am aware. Though it is still damn sharp.


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## Helen B (Jun 25, 2008)

Garbz said:


> But that smaller front element is sharper idea is not universal in this case is it?



Not at all universal. In this particular case it's mainly because the Nikkor has to have a greater clearance between the rear element than the Summicron - as I mentioned in the post.

Best,
Helen


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## Ben-71 (Jun 27, 2008)

Quote - Helen -
Ben,
The f-number is the focal length divided by the diameter of
the entrance pupil, not the diameter of the front element.​ ​Yes... 
I see I shouldn't write while 3% of my attention is to the post, and the
rest is focused on a business mail and on the phone calls that keep​coming in... ;-) You're of course right about the objective.

Here's a comparison between two 28 mm f/2 lenses....
....They are both designed to cover 24 mm x 36 mm, but the
Nikkor is designed for an SLR and the Summicron is designed
for a rangefinder camera.
One design has to allow the mirror to swing up, one doesn't.
The Summicron can be a more compact, more symmetrical design.​ 
Unless I misread you, this implies that the Summicron is optically better
because of its more symmetrical design.​ 
When more advanced lens developing tools and manufacturing technology
became available, Leica (among others) could offer yet better lenses by
replacing the older Double Gauss, symmetrical design & spherical lens
elements, with more sophisticated asymmetric design & aspheric lens​elements.

One design has to allow the mirror to swing up, one doesn't.

The Summicron can be a more compact, more symmetrical
design.​The Leica viewfinder design prohibits the use of larger lenses, which 
would block part of the viewfinders' image.
This is one of quite a few limitations that the non-SLR design imposes 
on the camera system.​ 
However, the SLR's larger distance between the lens and the film/sensor
does not prevent the lens from being symmetrical. The 50mm I had for a​Nikon F3, was very symmetrical.

The lens with the smaller front element, and smaller filter size
(46 mm vs 52 mm) produces significantly higher image quality
than the lens with the larger front element, especially at f/2 and f/2.8.
There is, however, a very large difference in price.​ ​A smaller diameter lens is not necessarily an (optical) advantage. 
It is just easier (less costly) to make a top quality small lens than a 
top quality larger lens.​ 
I have seen better quality from a large lens of an aerial camera, 
compared to any lens that has ever been offered for small, medium 
or large format cameras. But the price tag on that lens...​ 
As for the price difference between Leica and Nikon (or other's) lenses,
a meaningful part of it stems from the gap in manufacturing costs, from
the kind of reputation a company decides to create by fixing a certain price
for the product, from the quantity sold, etc'.​ 
Best,
Ben​


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## Helen B (Jun 27, 2008)

The general purpose of my original post was to give an actual example that showed two lenses that had the same maximum aperture but different front element diameters, and that there is no fixed rule about the larger lens being the better lens. I wasn't trying to suggest that smaller lenses are always superior, but I was trying to give an brief and simple indication of why that could be the case.




Ben-71 said:


> Unless I misread you, this implies that the Summicron is optically better
> because of its more symmetrical design.​




I don't think that I wrote that. These are 28 mm lenses. The  Nikkor's mirror clearance requirement means that the lens has to be a retrofocus lens, and this usually means that the front elements tend to be larger than non-retrofocus lenses. When I wrote the post I did hesitate about the use of 'symmetrical' because of the specific optical meaning of the word, but I couldn't think of a better word in the time I had available. With further reflection 'balanced' is a much more appropriate word. Going into the specific reasons why these two lenses are different would take a fair while, and I cannot always justify the time it would take to write a comprehensive, clear, correct and concise post. These subsequent comments, such as yours, help to spread the load.

In general a retrofocus design needs to have lager front elements because the whole front group has to be divergent - ie it needs to spread the ray pencil. In this particular case the situation is not so easy to explain, because the front element of the Summicron is divergent. I will post the lens diagrams later.​ 



> However, the SLR's larger distance between the lens and the film/sensor
> does not prevent the lens from being symmetrical. The 50mm I had for a Nikon F3, was very symmetrical.



Indeed. The 50 mm Nikkor does not have to be retrofocus. The flange focal depth of the Nikon mount is 46.50 mm, and the rear element may be closer to the image plane than that.​ 


> A smaller diameter lens is not necessarily an (optical) advantage.



I hope that I did not imply any general rue about optical quality. That's the problem with giving actaul examples, I guess. Maybe it would have been better not to show an example of the issue under discussion, or to try to explain why there are differences. I'd appreciate other people's thoughts about this.

Best,
Helen


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## Bifurcator (Jun 27, 2008)

Helen B said:


> I hope that I did not imply any general rue about optical quality. That's the problem with giving actaul examples, I guess. Maybe it would have been better not to show an example of the issue under discussion, or to try to explain why there are differences. I'd appreciate other people's thoughts about this.
> 
> Best,
> Helen



I like examples. That it was a single case to illustrate a specific point in the general discussion (a case in point) was understood. Or I thought so anyway.


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## Bifurcator (Jun 27, 2008)

I think the original confusion in this discussion came from confusing "Light-Gathering Power" with aperture. LGP is of interest and considering the diameter of the objective lens (or mirror in some cases) of for example a telescope, is a correlation of aperture.


EDIT:
Here we go: http://www.astronomynotes.com/telescop/s6.htm


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## SBlanca (Jun 27, 2008)

brileyphotog said:


> Often is the key word...the Nikon 50mm f/1.8, for instance, has a 52mm filter size.
> 
> Little number behind the "f/" = big aperture = faster lens...easy as pie



is the Nikon 50mm exactly the same as Canon's 50mm?


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## compur (Jun 27, 2008)

SBlanca said:


> is the Nikon 50mm exactly the same as Canon's 50mm?



Which Nikon 50mm and which Canon 50mm?  Both have made numerous 50mm lenses.


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## frXnz kafka (Jun 27, 2008)

compur said:


> Which Nikon 50mm and which Canon 50mm?  Both have made numerous 50mm lenses.


Does it matter? None of them are exactly the same as the other brand's counterpart


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## compur (Jun 28, 2008)

frXnz kafka said:


> Does it matter? None of them are exactly the same as the other brand's counterpart




Yes, it matters to me which lenses he was asking about because I have
reference diagrams on many of them and many 50s are very, very similar
in terms of optical design.


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## prodigy2k7 (Jun 29, 2008)

frXnz kafka said:


> I know all about aperture and F-stops, but surely there is a difference between an f/4 lens with a 58mm filter and an f/4 with a 77mm filter, right?



Nope, since all the excess light by a bigger front element is getting blocked out by the aperature.


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## Bifurcator (Jun 29, 2008)

Yeah prodigy, that's the crux of it right there.


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## AndrewG (Jun 30, 2008)

prodigy2k7 said:


> Nope, since all the excess light by a bigger front element is getting blocked out by the aperature.


 
<aperture>:thumbup:


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## prodigy2k7 (Jun 30, 2008)

AndrewG said:


> <aperture>:thumbup:


haha, i knew i did that i should have checked. I remembered that I kept doing that and I was changing it, except i got it reversed this time, made the typo on purpose thinking it was the right way


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## Battou (Jun 30, 2008)

Helen B said:


> I hope that I did not imply any general rue about optical quality. That's the problem with giving actaul examples, I guess. Maybe it would have been better not to show an example of the issue under discussion, or to try to explain why there are differences. I'd appreciate other people's thoughts about this.
> 
> Best,
> Helen



Personally I thought yout comparison was slightly inappropriate due to lens manufacturer variation.

So I took the liberty to photograph my Canon FD 50mm 1.4s, I also took four sample shots for direct comparison but I won't have those back until wensday at the least.

Both lenses are made by the same manufacturer of the same focal leingth and maximum aperture, useable on the same body with out modification.






On the left is the Canon FD 50mm 1.4 Ø - 52mm and the right is a Canon FD 50mm 1.4 Ø - 55mm.

The only differences in these lenses is the front element diameter and the lenses minimum aperture (Ø - 52mm - f/22 and Ø - 55mm - f/16) and the body/lens interface (bayonet/breech lock both Canon FD mount). 

I have not seen any variation in performance, However I had not ever done a direct comparison via tripod multi-shoot. The results of that will be back later this week.


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## AndrewG (Jul 1, 2008)

Battou said:


> Personally I thought yout comparison was slightly inappropriate due to lens manufacturer variation.
> 
> So I took the liberty to photograph my Canon FD 50mm 1.4s, I also took four sample shots for direct comparison but I won't have those back until wensday at the least.
> 
> ...


 
<aperture>:thumbup:


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## Mav (Jul 1, 2008)

compur said:


> Yes, it matters to me which lenses he was asking about because I have
> reference diagrams on many of them and many 50s are very, very similar
> in terms of optical design.


Yes, this is true, and not just with the 50mm lenses either as you implied.  Any patents that existed especially for these legacy designs would have expired long ago.  After that, there's really nothing stopping any company from reverse engineering one and making their own version.


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## Battou (Jul 1, 2008)

AndrewG said:


> <aperture>:thumbup:



Fixed


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