Filter Size (Lens Diameter) and Exposure

[Apex]

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

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

 

[Ben-71]

[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.

 

[Helen B]

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.

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

 

[Garbz]

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.

 

[Helen B]

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

 

[Ben-71]

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​

 

[Helen B]

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.

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

 

[Bifurcator]

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.

 

[Bifurcator]

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

 

[SBlanca]

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?

 

[compur]

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

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

 

[frXnz kafka]

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

 

[compur]

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.

 

[prodigy2k7]

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.

 

[Bifurcator]

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