Just got the 35mm 1.8 - why is it so small.

Geoffc

Senior Member
Ok this might seem like a strange question, but here goes. Faster glass by its very nature is normally a larger diameter. For example my 70-200 and 11-16 are 77mm. In fact that seems to be a standard for a lot of Nikon fast glass.

So why is my 35mm 1.8 lens only 55mm diameter? It is longer focal range than the 11-16 so that can't be it. Is it a zoom vs prime thing? Just curious.
 

Rick M

Senior Member
Because it's shorter. Optics closer to the sensor capture a wider angle of light, so the front element does not need to be as wide. Zooms are longer to allow for the internal movement of the optics, so they require a wider front element to capture the same amount of light as a prime.
 
Last edited:

Eduard

Super Mod
Staff member
Super Mod
Because it's shorter. Optics closer to the sensor capture a wider angle of light, so the front element does not need to be as wide. Zooms are longer to allow for the internal movement of the optics, so they require a wider front element to capture the same amount of light as a prime.

I always thought they were small so you could put them in your coat pocket and travel light. Go figure. :confused:
 

Geoffc

Senior Member
Because it's shorter. Optics closer to the sensor capture a wider angle of light, so the front element does not need to be as wide. Zooms are longer to allow for the internal movement of the optics, so they require a wider front element to capture the same amount of light as a prime.

Not shorter than my 11-16, which is also plastic. I guess it must be down to a range of things. Not a big deal I was just curious.
 

Eye-level

Banned
A lot of "fast" glass have the larger front elements because they can draw in more light that way.

1.8 is fast but it is not that fast and traditionally Nikkor primes around that focal length and a lot of other focal lengths have been a standard 52mm.
 

PhotoAV8R

Senior Member
Comparing the filter size is interesting, but irrelevant

It's all math. A 40mm f/2 lens has a front lens element that is 20mm in diameter. A 200mm f/2 lens has one that is 100mm in diameter.

BWTHDIK
 

Eye-level

Banned
I beg to differ!

Compare an old 85/1.8 which is 52mm to an old 85/1.4 which is 72mm in the front...totally relevant Don. The faster lens requires the bigger front element to work. Physics not math has everything to do with it too. When you start getting into the shorter wider focal lengths it doesn't matter as much that is why the 35/1.8 is smaller than his zooms in the OP.
 
Last edited:

PhotoAV8R

Senior Member
F-stop is the focal length divided by the diameter of the lens. Division is math. At least it was way back when.

The size of the filter on a lens means nothing except it must be at least as large as the effective diameter of the lens. The difference in the diameter of the objective lens between f/1.4 and f/1.8 35mm lenses is less than 6mm.

My response does not address the difference in the physical size of the OP's lenses.

BWTHDIK - Math major 1974
 

Eye-level

Banned
OK like I said 85/1.8 is 52mm...85/1.4 is 72mm...what is the difference in physical size there?

He is talking about size of lenses relative to each other not f stop. All I said is lenses that have larger front elements tend to be faster than ones with smaller front elements but it is not always so! It also depends on the distance to sensor or film plane and probably a ton of other things like lens designs and configuration.

I agree that at any given focal length the aperture determines the amount of light that gets to the sensor regardless of the size of the front element but faster glass allows you to have a bigger aperture and that usually means a bigger lens.
 
Last edited:

Eye-level

Banned
Your are right Don! I apologize for trying to argue with you. Filter size is irrelevant unless the front element has to be so big as to increase the barrel size.

You can have two 50's say one is 1.8 and the other is 1.4. They both will have 52mm filter rings but the 1.4 will have a much bigger front glass element and the barrel will be longer.

The old 85's are weird because the 1.8 has the 52mm standard and the 1.4 has this monster light bender on the front of it. The 1.4 is as big as a softball...much larger and heavier than the 1.8.
 
Top