Light gathering of lens - why no huge wide objective lens?

[SlowSteve]

Hello all.

One of my hobbies is shooting, and with that comes the use of telescopic lenses. The scopes can be just as expensive as very good photo lenses, and are made by similar companies - for example Ziess and Nikon do top range scopes.

Rifle scopes are specified as something like "3-9x56" which is a scope offering a magnification of between 3x to 9x, with a 56mm wide objective lens - the lens right at the front of the scope - towards the barrel end.


Scopes are often used in low light, so the scope makers try to get as much light as possible to the eye. To do this, the objective lens is sometimes much wider than the lens body - for example my Sightron has a 56mm lens, but a 30mm body.

The difference this makes is significant - in the summer time, if I compare my 56mm lens to an identical 30mm lens, I can shoot around 1/2 hour earlier in the morning, and up to an hour later in the evening, as the sight picture is far brighter.


I was wondering why lens makers don't follow the same model. By having the lenses wider at the front, it would mean more light is captured. This could allow for improved performance during the day - faster shutter speeds, or more usefulness at night- gigs etc.

I don't think there are any technical reasons against doing this - although the lens would be heavier I guess.

Does anyone know if the lens makers have tried this before, and if so, what sort of results they had?


Thanks

Steve

 

[arad85]

They do... it's the F number. A lower number lets in more light....

 

[geebus]

A bit like this?

There's a very tatty used example on ebay for £2.5k. A lot more new.
Might explain why they're not your typical daily lens, never mind the weight .

 

[flossie]

I was wondering why lens makers don't follow the same model. By having the lenses wider at the front, it would mean more light is captured. This could allow for improved performance during the day - faster shutter speeds, or more usefulness at night- gigs etc.

er... I may be missing something in your question, but all front lenses are wider than the back, which is by definition 35mm - the majority of wide angle / fast lenses are around 77mm so that's more of an increase than the example rifle lens you gave...

 

[odd jim]

Hello all.

One of my hobbies is shooting, and with that comes the use of telescopic lenses. The scopes can be just as expensive as very good photo lenses, and are made by similar companies - for example Ziess and Nikon do top range scopes.

Rifle scopes are specified as something like "3-9x56" which is a scope offering a magnification of between 3x to 9x, with a 56mm wide objective lens - the lens right at the front of the scope - towards the barrel end.


Scopes are often used in low light, so the scope makers try to get as much light as possible to the eye. To do this, the objective lens is sometimes much wider than the lens body - for example my Sightron has a 56mm lens, but a 30mm body.

The difference this makes is significant - in the summer time, if I compare my 56mm lens to an identical 30mm lens, I can shoot around 1/2 hour earlier in the morning, and up to an hour later in the evening, as the sight picture is far brighter.


I was wondering why lens makers don't follow the same model. By having the lenses wider at the front, it would mean more light is captured. This could allow for improved performance during the day - faster shutter speeds, or more usefulness at night- gigs etc.

I don't think there are any technical reasons against doing this - although the lens would be heavier I guess.

Does anyone know if the lens makers have tried this before, and if so, what sort of results they had?


Thanks

Steve

As above, they do!

 

[jimmyriddle]

Stuff

They do, but I would have thought the reasons for not all lens' being larger diameter are cost and weight.

They do... it's the F number. A lower number lets in more light....

That's the aperture, he is referring to the light gathering properties of the objective lens.

er... I may be missing something in your question, but all front lenses are wider than the back, which is by definition 35mm - the majority of wide angle / fast lenses are around 77mm so that's more of an increase than the example rifle lens you gave...

I think he means larger relative to other scopes/lens', not relative to each end of the lens.

 

[AJQS]

The OP doesn't make much sense without the focal lengths and max F number - I gather the scope has no Iris so is fixed at max aperture, which presumably is large on account of the big element.

 

[HoppyUK]

Apart from the above, there are a few lenses about with much bigger front elements (objective lens) than others of similar spec - one that springs to mind is the Sigma 50mm f/1.4, which is much bigger than the otherwise identical Canon or Nikon 50 1.4 lenses.

It doesn't change the f/number, or the exposure strictly speaking, but it does reduce vignetting at lowest f/numbers (less darkening of the corners).

 

[arad85]

That's the aperture, he is referring to the light gathering properties of the objective lens.

I think you'll find the aperture is the measure of the light gathering ability of a lens

 

[StewartR]

Apart from the above, there are a few lenses about with much bigger front elements (objective lens) than others of similar spec - one that springs to mind is the Sigma 50mm f/1.4, which is much bigger than the otherwise identical Canon or Nikon 50 1.4 lenses.

It doesn't change the f/number, or the exposure strictly speaking

That's the key to it.

With SLR lenses, for any given focal length, the speed of the lens is determined by the diameter of the entrance pupil, not the diameter of the front element.

With rifle scopes (and telescopes, binoculars, etc.), there is no aperture stop so it's the diameter of the front element which is the main consideration.