# Lens front element diameter - Photography

Why do different lenses have differing front element diameter? Is a lens with a bigger front element inherently superior than one with a smaller front element? Does a 77mm capture more light than a 58mm? - Siddhartha...

1. ## Lens front element diameter

Why do different lenses have differing front element diameter? Is a
lens with a bigger front element inherently superior than one with a
smaller front element? Does a 77mm capture more light than a 58mm?
- Siddhartha

Siddhartha Guest

2. ## Re: Lens front element diameter

Siddhartha Jain wrote:

A number of reasons. Some lens designs need a larger front element.
Generally wide angle, zoom and larger aperture lenses call for larger front
elements. Another issue is the use of filters. Often using a larger size
filter mount will allow the use of stacked filters where a smaller mount
would not.

--
Joseph Meehan

26 + 6 = 1 It's Irish Math

Joseph Guest

3. ## Re: Lens front element diameter

Do not confuse filter diameter with front element diameter, which you do not
know in general.

Dps Guest

4. ## Re: Lens front element diameter

Dps wrote:
do not

Yes, I think I got the two mixed up and assumed that filter diameter ==
front element diameter!!

- Siddhartha

Siddhartha Guest

5. ## Re: Lens front element diameter

"Siddhartha Jain" <co.uk> wrote in message
> do not
>
> Yes, I think I got the two mixed up and assumed that filter diameter ==
> front element diameter!!
>[/ref]
Front diameter can mean a faster lens in terms of f:stop. Let's assume the
lens was a 200mm with a 58mm diameter. 200/58=3.44. There will be some type
of bezel so the glass will be a bit smaller so this could be a 200mm f:4
lens. Now the same 200mm with a 77mm diameter 200/77=2.59 this could be a
200mm f:2.8. This is using the very basic math, lens designers may decide to
use a 82mm element and a baffle to mask of the edge to correct for edge
distortions. This is a simple explanation, the actual optical designs are
more complex.\

Darrell Guest

6. ## Re: Lens front element diameter

On 31 Jan 2005 03:30:08 -0800, "Siddhartha Jain"
<co.uk> wrote:

The front element has nothing to
do with the lens' max aperture.
Nor does it say much about the
quality of the lens.

I have a 90mm f/8 lens (a wide-
angle for 4x5 format) with a
huge front element -- at least
45 or 50 mm diameter. By that
measure it should be an f/2 lens!

Similarly: a Canon zoom (for my
10D) rated 17-40/f4, with a front
element that's at least 60 mm in
diameter.

There are a zillion different
lens designs. Wide angle lenses
tend to have large front elements.

rafe b.
http://www.terrapinphoto.com
rafe Guest

7. ## Re: Lens front element diameter

rafe bustin wrote:

No no, I did not confuse front element diameter with aperture.
- Siddhartha

Siddhartha Guest

8. ## Re: Lens front element diameter

rafe bustin wrote:

>
>
>
> The front element has nothing to
> do with the lens' max aperture.
> Nor does it say much about the
> quality of the lens.
>
> I have a 90mm f/8 lens (a wide-
> angle for 4x5 format) with a
> huge front element -- at least
> 45 or 50 mm diameter. By that
> measure it should be an f/2 lens!
>
> Similarly: a Canon zoom (for my
> 10D) rated 17-40/f4, with a front
> element that's at least 60 mm in
> diameter.
>
> There are a zillion different
> lens designs. Wide angle lenses
> tend to have large front elements.
>
>
> rafe b.
> http://www.terrapinphoto.com[/ref]

Not only do wide angle ones, but so do low f/# (fast) lenses. Minimum
diameter of front element is primarily determined by these two factors
(f/# and fov). Also, distance from front element to front principal
point is a factor. Grinding glass is expensive, so no one wants
elements to be bigger than they have to be. But generally you do not
want front element to act as an aperture stop, nor do you want
vignetting. So there is a certain minimum diameter that the lens has to
be. Modern lens design software takes care of determining that, however.
Don Guest

9. ## Re: Lens front element diameter

In article <com>,
rafe bustin <net> wrote:
>
>
>The front element has nothing to
>do with the lens' max aperture.[/ref]

That's not strictly true - it does place an upper limit on the maximum
aperture. If you have a 50mm lens with a front element that's, say. 25mm
across, it can't possibly be an f/1.4 lens, for example.

As you point out though, it isn't necessarily an f/2 lens either.
Chris Guest

10. ## Re: Lens front element diameter

Siddhartha Jain wrote:

You've gotten several replies. I'll add to that that often a lens will have a
greater than absolutely neccesary filter diameter so that the photographer can
use a given set of filters. I have 3 lenses that are 72mm. I'm sure that for
the 80-200 they could have used a smaller filter size. However, as one who owns
an 80-200 f/2.8 is likely to own a 28-70 f/2.8, they've conscientiously made the
dias the same. 9 current lenses by Minolta are 72mm.

--
-- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm
-- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm
-- [SI] gallery & rulz: http://www.pbase.com/shootin
-- e-meil: there's no such thing as a FreeLunch.
Alan Guest

11. ## Re: Lens front element diameter

"Siddhartha Jain" <co.uk> wrote in message

All things being equal (which, of course, they never are!), given two lenses
of identical quality and similar focal lengths, I'd think the larger lens
would have less image-plane distortion _for_ a_given_exposure_, since more
of the refracted light reaching the image plane through the larger lens
would pass closer to the optic axis, i.e., the most accurate part of the
lens.

Of course, the choice consumers probably face is deciding between cameras
having smaller lenses of high quality and cameras with larger lenses of
somewhat lesser quality. In such cases, it's a wash, I suppose.

Paul Guest

12. ## Re: Lens front element diameter

Paul H. wrote:

The term 'distortion' has a specific meaning to a lens designer or
optical engineer. I think you mean aberrations here. Many lens
aberrations are a function of the diameter of the lens (aperture
diameter). There are many aberrations which increase rapidly with
aperture diameter. About the only 'aberration' that increased diameter
reduces is diffraction blur. Spherical aberration, coma, astigmatism,
etc. All increase with diameter.
Don Guest

13. ## Re: Lens front element diameter

"Siddhartha Jain" <co.uk> wrote in message

I think it has a lot to do with the zoom and being a wide angle. On the
other hand, anybody remember the good old days when almost all Nikon lenses
used 52mm filters? I have a 28mm 3.5, an 85mm 1.8, and an 80-200mm 4.5 zoom
that all use 52mm filters. Go figure.

Sheldon Guest

14. ## Re: Lens front element diameter

Don Stauffer in Minneapolis wrote:
>
> The term 'distortion' has a specific meaning to a lens designer or
> optical engineer. I think you mean aberrations here. Many lens
> aberrations are a function of the diameter of the lens (aperture
> diameter). There are many aberrations which increase rapidly with
> aperture diameter. About the only 'aberration' that increased diameter
> reduces is diffraction blur. Spherical aberration, coma, astigmatism,
> etc. All increase with diameter.[/ref]

To me "distortion" (wrt a lens) means that caused by the basic optical
mechanics resulting in pincushion or barrel distortion. Hence, as Paul
said a larger diameter lens design for focal-length/aperture is more
likely to be closer to flat. "Aberation" would include the others you
mentioned.

eg: distortion is geometric and aberation is most other optical shortcomigs.

I may be wrong of course, but that's my take on distortion v. aberation.

Cheers,
Alan

--
-- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm
-- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm
-- [SI] gallery & rulz: http://www.pbase.com/shootin
-- e-meil: there's no such thing as a FreeLunch.
Alan Guest

15. ## Re: Lens front element diameter

Sheldon wrote:

>
>
> I think it has a lot to do with the zoom and being a wide angle. On the
> other hand, anybody remember the good old days when almost all Nikon lenses
> used 52mm filters? I have a 28mm 3.5, an 85mm 1.8, and an 80-200mm 4.5 zoom
> that all use 52mm filters. Go figure.
>
>[/ref]

In addition it depends on the length of the lens, and the distance from
the front element to the aperture stop plane. The further from the
aperture stop the front element front surface is, the larger it needs to
be for a given FOV.
Don Guest

16. ## Re: Lens front element diameter

"Don Stauffer in Minneapolis" <net> wrote in message
news:net... [/ref]
lenses [/ref]
lens [/ref]
more [/ref]
cameras
>
> The term 'distortion' has a specific meaning to a lens designer or
> optical engineer. I think you mean aberrations here. Many lens
> aberrations are a function of the diameter of the lens (aperture
> diameter). There are many aberrations which increase rapidly with
> aperture diameter. About the only 'aberration' that increased diameter
> reduces is diffraction blur. Spherical aberration, coma, astigmatism,
> etc. All increase with diameter.[/ref]

True, but I think you're reading more into my comment than I intended to
convey. If you have a ray of light approaching a spherical refracting
surface of radius R parallel to and at distance H from the optic axis,
Snell's law obtains: n1*sin(A)=n2*sin(B) where (n1, A) is the (index of
refraction, angle to the normal in air and (n2, B) the corresponding
characteristics of the refracting medium.

sin (A)= H/R, so sin(B)= (n1/n2)*H/R, or for sufficiently small H,

B=(n1*n2)*H/R

The quality of the image at the image plane will depend greatly on the
accuracy and consistency of exit angle B. The uncertainty in B, dB, is then
roughly

dB = -dR* (n1*n2)*H/ (R^2),

i.e., it varies inversely as the *square* of the radius of curvature. By
the way, I do know that "dB" usually denotes decibels--it's just a
coincidence here, though.

Furthermore, having ground a couple of telescope mirrors in my mispent
youth, I can testify that larger radii of curvatures are easier to implement
than smaller ones and also that grinding accuracy falls off precipitously
the her one gets from the optic axis. Hence a portion of a larger, less
curved lens would be cheaper to produce than a better engineered-smaller
lens to achieve the same light gathering ability within a certain error
range. Or so my thinking went.

That's all I meant. If I had known there was going to be a test, I would
have studied. :-)

Paul Guest

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