Large format lens, why are they so Convex?

Shaggysk8 · Jan 26, 2011

This is probably stupid question. But large format lenses look more Convex then medium format and 35mm, I might be wrong in this, but if so why?

Paul

Ian Grant · Jan 26, 2011

They cover far larger image circles to allow for movements so need to be wider particularly the front & rear sets of elements.

Ian

Dan Fromm · Jan 26, 2011

Um, er, ah, Ian, within a prescription every dimension -- radii, elements' thickness, spacing between elements -- scales with focal length. So, for example, the front surface of a 150/6.3 Tessar is relatively flatter than than the front surface of an 85/6.3 Tessar.

Paul, be more specific. Which lenses are you comparing? There are many many lenses for every format and some are quite, um, bulbous. Others with the the same or greater coverage aren't.

For example, Ian, I have an extremely bulbous 260/10 Nikkor-Q -- this lens, in fact: Dead Link Removed ; Akiyan's image circles are at 1:1 -- that covers 75 degrees at infinity. I also have a 38/4.5 Biogon that covers 90 degrees. Its front surface is relatively flat.

Shaggysk8 · Jan 26, 2011

It was more just a general question really, I can't be more specific as I only have 3 lenses and have only ever seen those 3 lenses. I would like to understand more about lens design, but not sure my brain could handle it.

John Kasaian · Jan 26, 2011

Shaggysk8 said:
It was more just a general question really, I can't be more specific as I only have 3 lenses and have only ever seen those 3 lenses. I would like to understand more about lens design, but not sure my brain could handle it.

If they cover your format then whats the diff?
Now git out there and shoot up some film!

Shaggysk8 · Jan 26, 2011

Haha it's dark!

Ian Grant · Jan 27, 2011

I think the answer needed to be a bit sweeping Dan

Ian

Dan Fromm said:
Um, er, ah, Ian, within a prescription every dimension -- radii, elements' thickness, spacing between elements -- scales with focal length. So, for example, the front surface of a 150/6.3 Tessar is relatively flatter than than the front surface of an 85/6.3 Tessar.

Paul, be more specific. Which lenses are you comparing? There are many many lenses for every format and some are quite, um, bulbous. Others with the the same or greater coverage aren't.

For example, Ian, I have an extremely bulbous 260/10 Nikkor-Q -- this lens, in fact: Dead Link Removed ; Akiyan's image circles are at 1:1 -- that covers 75 degrees at infinity. I also have a 38/4.5 Biogon that covers 90 degrees. Its front surface is relatively flat.

doughowk · Jan 27, 2011

I also like to understand what I'm using. So I bought a couple of Rudolf Kingslake books on lenses. They haven't improved my photography, but they help me with gear-talk ;-)

Christopher Walrath · Jan 27, 2011

I have his book on the shelf.

But I wanna know, do you like your new toy, Shags?

Shaggysk8 · Jan 27, 2011

I am in love with my new toy!!! it sleeps with me at the moment. Although I will get closer to it at the weekend when we can really get to business

Bruce Watson · Jan 27, 2011

Shaggysk8 said:
This is probably stupid question. But large format lenses look more Convex then medium format and 35mm, I might be wrong in this, but if so why?

Different designs for different requirements. Small format users differ from large format users in a few key requirements. First, small format cameras have a fixed relationship between the lens plane and the film plane. This changes lens designs markedly (especially when you have to clear a swinging mirror).

Second, most small format users are handholding. This implies the need for speed, which in turn implies the need to shoot wide open and at apertures near wide open. Most LFers are bound to their tripods and tend to shoot at apertures in the f/16 and smaller range. This requirement changes lens designs too, in that the designer can't count on smaller apertures to minimize aberrations, and has to design the lens to be sharper wide open and have better off-axis light fall-off characteristics.

Third, LF use requires much larger image circles to allow for view camera movements.

What you end up with is that LF lenses tend to be simpler, and tend to be closer to symmetric designs. Small format lenses tend to be asymmetric and considerably more complicated.

Ian Grant · Jan 27, 2011

Bruce Watson said:
What you end up with is that LF lenses tend to be simpler, and tend to be closer to symmetric designs. Small format lenses tend to be asymmetric and considerably more complicated.

That also goes for 35mm & 120 Range-finder camera lenses as well, particularly wide angle lenses, and these lenses often have better performance and less distortion, and are cheaper to manufacture.

Ian

TheFlyingCamera · Jan 27, 2011

Yes- compare the price of two lenses with comparable fields of view - which costs more, a 75mm f6.8 for 4x5 or a 21mm f2.8 for 35mm? I want to say I paid about $450 for my Rodenstock 75 f6.8 and about $600 for my Contax 21mm f2.8. Both used. Both in EX+ condition.

Large format lens, why are they so Convex?

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