A Really Dumb Question

[HWest]

I know that when someone tells me, I'll slap my forehead and cry, "Of course!" like Sherlock Holmes puzzling out a three-pipe problem.

I understand why a 300mm lens would cost more than a 150mm - it's physically bigger, there's more time and materials that goes into the construction. But why are lenses with bigger apertures so much more expensive? Surely the aperture diaphragm costs more or less the same regardless of width?

Is it a difference in lenses or some such?

 

[Phil V]

The larger the aperture the larger the lenses. Look at a 2.8 zoom compared to a slow zoom.
Also, once you start along that route, you have to put more care into the optics, to reduce aberrations and other optical distortions.

For the camera manufacturers this then leads to better focus motors and build quality too.

So, the lens is no longer built to a price, but built to a high quality standard.

 

[HWest]

Ahhhh, cheers.

 

[Canon Bob]

....But why are lenses with bigger apertures so much more expensive? Surely the aperture diaphragm costs more or less the same regardless of width?

It's not the diaphragm that determines the f-stop ratio, it's the apparent aperture....the size that the aperture appears when viewing it through the front element (in simple terms). It's feasible (but not certain) that a 300/4 and a 300/2.8 could have the same size diaphram.

Bob

 

[HWest]

It's not the diaphragm that determines the f-stop ratio, it's the apparent aperture....the size that the aperture appears when viewing it through the front element (in simple terms). It's feasible (but not certain) that a 300/4 and a 300/2.8 could have the same size diaphram.

Bob

Yeah... my brain just melted. can I have that one more time for the cheap seats at the back?

 

[drexyl]

I guess bigger elements require more time to polish which in terms takes more time which = more money.

Lots more.

 

[tezmed]

if you go on to this
http://tv.cse7.org/2009/fall/
and go to optics you can see how many components there are to a lenses..... its eye opening!!

 

[Canon Bob]

Yeah... my brain just melted. can I have that one more time for the cheap seats at the back?

It's not the physical diameter of the diaphragm that determines the f-stop ratio. The light entering the front element will likely pass through several elements/groups before reaching the diaphragm and then onwards to the rear element. Taking a 600mm f/4 lens then the apparent aperture needs to be 150mm diameter ( 600 / 4 ). The front element is 150mm diamter and the rear element is likely to be around 40mm diameter. The light path through the lens gradually gets smaller in diameter and it depends at what point on this cone (not actually a cone!) the diaphragm is placed that will determine the required size.

That doesn't sound much better eh?

Bob

 

[Dale_tem]

The aperture isn't 150mm, it appears 150mm when looking through the front of the lens.

 

[Canon Bob]

The aperture isn't 150mm, it appears 150mm when looking through the front of the lens.

Precisely, Dale...the point I was trying to get through in post #4 above but maybe poorly worded.

Bob