Ugly Hedgehog® - Photography Forum
is glass size related to lens speed?
If you want to reply, then register here. Registration is free and your account is created instantly, so you can post right away.
Page: 1 2 3 next>>
Jun 29, 2014 00:25:10   #
wingnut1956
 
Hi fellow 'hoggers
I have a question that may seem stupid to some of you but it has me confused a bit. I'm not real sure how to ask this but try to bear with me. I have a few different lenses, all of these I'm referring to are Nikon lenses. I have an 18-70 zoom from an older camera, a d70 I believe, and it has a 67 mm filter size, but it's only a 3.5-4.5 speed. Yet, I have a 35 & 50 mm with only a 52 mm filter and they are both 1.8. I also have a 55-200 kit lens that has a 52mm filter. What I'm confused about, is -Wouldn't a larger filter size (i.e. More glass) mean it's a faster lens as it can allow more light to enter?-I thought the zoom range might come into play but the 55-200 ruins that theory since it's smaller than the 18-70. Also, if I had not traded my 18-55 in on my Tamron 18-270, that was also a 52 mm 3.5 lens. If all other things are equal, would the larger glass area give me a better picture than a lens of the same speed and focal length? Is there an advantage of one over the other?

| Reply
Jun 29, 2014 00:26:57   #
Nikonian72
 
The larger the aperture, the larger diameters of lenses.

Aperture is measured in "f/stops", which is a mathematical ratio of diaphragm iris diameter divided by focal length. As an example, aperture f/2 means that the iris opening is half of the lens focal length. Aperture f/1.2 means that the two distances are nearly the same, while f/64 means that the iris is miniscule compared to the lens focal length.

Aperture f/2 is larger than f/3.5, and requires more glass to gather light. Larger aperture lenses are heavier, and require bigger pieces of perfect glass for all internal lenses. Also, zooms require more internal lenses, and larger lenses, making them heavier than prime lenses of similar focal length.

| Reply
Jun 29, 2014 00:36:27   #
mcveed
 
It is the optical design of the lens that determines the diameter of the front element. You cannot compare zoom lenses with prime lenses in this manner. With a prime lens of given focal length, the larger the front element the faster the lens will be, usually. The speed (light gathering capability) of a zoom lens will be generally indicated by the zoom range and the size of the front element. The greater the zoom range the larger the front element must be to give the same speed. These are general comments because not all lenses comply with the foregoing.

| Reply
Jun 29, 2014 00:40:18   #
Shellback
 
Another thought-
The larger diameter could be because of using larger lens elements, which could have advantages with regard to sharpness and light falloff at the edges of the image circle.

| Reply
Jun 29, 2014 02:16:30   #
SharpShooter
 
Wingnut, it's also important to note that the entrance diameter is also limited by the exit diameter and ultimately becomes the limiting factor.
Since faster lenses have more glass and are thus expensive, the manufacturers use high quality glass, making them better but more expensive still.
So it's a catch 22. And very small gains become exponentially more expensive and larger. Just look at the difference between a 200mm f2.8 vs a 200mm f2.0! :lol:
SS

| Reply
Jun 29, 2014 03:39:58   #
amehta
 
wingnut1956 wrote:
Hi fellow 'hoggers
I have a question that may seem stupid to some of you but it has me confused a bit. I'm not real sure how to ask this but try to bear with me. I have a few different lenses, all of these I'm referring to are Nikon lenses. I have an 18-70 zoom from an older camera, a d70 I believe, and it has a 67 mm filter size, but it's only a 3.5-4.5 speed. Yet, I have a 35 & 50 mm with only a 52 mm filter and they are both 1.8. I also have a 55-200 kit lens that has a 52mm filter. What I'm confused about, is -Wouldn't a larger filter size (i.e. More glass) mean it's a faster lens as it can allow more light to enter?-I thought the zoom range might come into play but the 55-200 ruins that theory since it's smaller than the 18-70. Also, if I had not traded my 18-55 in on my Tamron 18-270, that was also a 52 mm 3.5 lens. If all other things are equal, would the larger glass area give me a better picture than a lens of the same speed and focal length? Is there an advantage of one over the other?
Hi fellow 'hoggers br I have a question that may s... (show quote)

Yes, the filter size is entirely related to the maximum aperture which the lens can have, though sometimes the filter size is larger than necessary for other design reasons. The maximum f-stop is basically the objective lens divided by the focal length, though this does not apply at all focal lengths of a zoom range.

The 35mm f/1.8 lens has an objective lens with a diameter of about 20mm, and the 50mm f/1.8 object lens diameter is about 30mm. But Nikon has recently standardized on a few filter sizes, especially 52mm, 67mm, and 77mm, so they made the lens with the 52mm filter size.

With the 55-200mm lens, at 200mm it is a f/5.6 lens, so the objective lens must be about 40mm. As Shellback suggested, it might be a little larger to improve image quality.

| Reply
Jun 29, 2014 05:20:46   #
Nikonian72
 
amehta wrote:
The maximum f-stop is basically the objective lens divided by the focal length.
Incorrect: The objective lens diameter does not change, nor does the focal length of a prime lens. The diameter of the diaphragm iris does change, resulting in different f/numbers for different iris sizes.

"The f-number N is given by N = f/D, where f is the focal length, and D is the diameter of the entrance pupil (effective aperture aka iris aperture)" {not objective lens} per http://en.wikipedia.org/wiki/F-number


Aperture f/4
Aperture f/4...
Aperture f/8
Aperture f/8...
Lens focal length = front element to camera sensor
Lens focal length = front element to camera sensor...

| Reply
Jun 29, 2014 10:06:11   #
amehta
 
Nikonian72 wrote:
Incorrect: The objective lens diameter does not change, nor does the focal length of a prime lens. The diameter of the diaphragm iris does change, resulting in different f/numbers for different iris sizes.

"The f-number N is given by N = f/D, where f is the focal length, and D is the diameter of the entrance pupil (effective aperture aka iris aperture)" {not objective lens} per http://en.wikipedia.org/wiki/F-number

The key words are entrance and effective.

If the pictures are of the same lens, it is almost a 200mm lens, so at f/4 the effective aperture is about 50mm (2 inches). In the top pictures, 50mm is about the size of the objective lens, not the diaphragm opening.

EDIT: 50mm, not 5mm, at f/4

| Reply
Jun 29, 2014 10:12:42   #
jimni2001
 
amehta wrote:
The key words are entrance and effective.

If the pictures are of the same lens, it is almost a 200mm lens, so at f/4 the effective aperture is about 5mm (2 inches). In the top pictures, 5mm is about the size of the objective lens, not the diaphragm opening.


50mm is almost 2 inches.

| Reply
Jun 29, 2014 10:22:31   #
amehta
 
jimni2001 wrote:
50mm is almost 2 inches.

Thanks.

| Reply
Jun 29, 2014 13:44:40   #
Nikonian72
 
amehta wrote:
The key words are entrance and effective.
No, the key word is pupil, which is the narrowest part of any optical light path, and usually variable, just like the pupil in your own eye.

Your definition does NOT jive with any internet reference, nor any text book reference. The photos are not mine, but are typical of visual examples found on the internet.

Per http://pleasemakeanote.blogspot.com/2010/10/mathematics-of-fstop-aperture-numbers.html
"In photography, the lens aperture is that opening in the lens (or on the camera body) that determines the amount of light that is to be admitted to the light sensitive medium (film or CCD ...). The surface area of this opening can be adjusted by the use of a diaphragm. The action of closing or opening the diaphragm is called stopping down the lens. We define the f-stop number as:



| Reply
Jun 30, 2014 00:43:36   #
amehta
 
Nikonian72 wrote:
No, the key word is pupil, which is the narrowest part of any optical light path, and usually variable, just like the pupil in your own eye.

Your definition does NOT jive with any internet reference, nor any text book reference. The photos are not mine, but are typical of visual examples found on the internet.

Per http://pleasemakeanote.blogspot.com/2010/10/mathematics-of-fstop-aperture-numbers.html
"In photography, the lens aperture is that opening in the lens (or on the camera body) that determines the amount of light that is to be admitted to the light sensitive medium (film or CCD ...). The surface area of this opening can be adjusted by the use of a diaphragm. The action of closing or opening the diaphragm is called stopping down the lens. We define the f-stop number as:
No, the key word is b i pupil /i /b , which is ... (show quote)

Ok, if "pupil" is also important, follow the link from the wikipedia article you posted on f-number to the one on the entrance pupil. While it is not a simple entity, it is definitely not the same as the aperture when there are lens elements involved:

"In photography, the size of the entrance pupil (rather than the size of the physical aperture itself) is used to calibrate the opening and closing of the diaphragm aperture."

The pleasemakeanote blogspot is interesting, but I would not consider it a photography authority.

| Reply
Jun 30, 2014 02:09:27   #
SharpShooter
 
I'm not gonna get in the middle of this, but, it's important to note that the physical length of any lens can be very misleading to use in any calculation, as the actual physical length is not really directly connected to the focal length of a lens.
By that I mean that the actual design of a true, "tele"- photo lens means that the lens is shorter than it's actual focal length.
For example, I have a 600mm lens and when mounted to my camera the actual 600mm mark falls easily about 6 inches behind my cameras back.
So a tele lens' view is equal to the view of a long lens with an angle of view equal to that of a 600mm lens, but doesn't have to be even close to that in actual length.
Lets not forget that lens technology is getting so complex that the materials used could render an f-stop with better light transmission than the actual physical dimensions of the lens.
So is it possible to take, say a cheap 50mm f1.8 lens, swap out the glass/plastic with $10,000 worth of elements that have a much higher ability to transfer light. It would be the same exact physical dimensions as before including the iris, but would actually have a better f-stop value than before, rendering an actually faster lens in the exact same physical body?
So many of todays lenses don't actually adhere to formulas so much as standards.
We do have an optic engineer here on the Hog now, but I don't remember who he is.
Maybe he could weigh in on this, for him, very basic concept.
Just saying. ;-)
SS

| Reply
Jun 30, 2014 08:31:30   #
Sirsnapalot
 
SharpShooter wrote:
I'm not gonna get in the middle of this, but, it's important to note that the physical length of any lens can be very misleading to use in any calculation, as the actual physical length is not really directly connected to the focal length of a lens.
By that I mean that the actual design of a true, "tele"- photo lens means that the lens is shorter than it's actual focal length.
For example, I have a 600mm lens and when mounted to my camera the actual 600mm mark falls easily about 6 inches behind my cameras back.
So a tele lens' view is equal to the view of a long lens with an angle of view equal to that of a 600mm lens, but doesn't have to be even close to that in actual length.
Lets not forget that lens technology is getting so complex that the materials used could render an f-stop with better light transmission than the actual physical dimensions of the lens.
So is it possible to take, say a cheap 50mm f1.8 lens, swap out the glass/plastic with $10,000 worth of elements that have a much higher ability to transfer light. It would be the same exact physical dimensions as before including the iris, but would actually have a better f-stop value than before, rendering an actually faster lens in the exact same physical body?
So many of todays lenses don't actually adhere to formulas so much as standards.
We do have an optic engineer here on the Hog now, but I don't remember who he is.
Maybe he could weigh in on this, for him, very basic concept.
Just saying. ;-)
SS
I'm not gonna get in the middle of this, but, it's... (show quote)



:thumbup: :thumbup:

| Reply
Jun 30, 2014 10:28:32   #
romanticf16
 
wingnut1956 wrote:
Hi fellow 'hoggers
I have a question that may seem stupid to some of you but it has me confused a bit. I'm not real sure how to ask this but try to bear with me. I have a few different lenses, all of these I'm referring to are Nikon lenses. I have an 18-70 zoom from an older camera, a d70 I believe, and it has a 67 mm filter size, but it's only a 3.5-4.5 speed. Yet, I have a 35 & 50 mm with only a 52 mm filter and they are both 1.8. I also have a 55-200 kit lens that has a 52mm filter. What I'm confused about, is -Wouldn't a larger filter size (i.e. More glass) mean it's a faster lens as it can allow more light to enter?-I thought the zoom range might come into play but the 55-200 ruins that theory since it's smaller than the 18-70. Also, if I had not traded my 18-55 in on my Tamron 18-270, that was also a 52 mm 3.5 lens. If all other things are equal, would the larger glass area give me a better picture than a lens of the same speed and focal length? Is there an advantage of one over the other?
Hi fellow 'hoggers br I have a question that may s... (show quote)

Tell the whole story- aren't there 2 f stops on these zooms, one for wide and one at tele? Like f3.5-5.6 for example? the expensive glass (70-200f2.8vrII) the f2.8 remains constant from 70mm to 200mm- that is why it's front filter size is 77mm and it costs $$$$.

| Reply
Page: 1 2 3 next>>
If you want to reply, then register here. Registration is free and your account is created instantly, so you can post right away.
UglyHedgehog.com - Forum
Copyright 2011-2019 Ugly Hedgehog, Inc.