Calculating f stop of Telephoto lens

[IanG]

My ex-Military 17" Dallon has no aperture scale but is marked as having a max aperture of f5.6, the diameter of the aperture blades is 42.8mm wide open min is 5.0mm, the lenses stop down to f45.

I'm assuming the normal calculation f stop = focal length (431,8mm)/diameter doesn't apply to telephoto lenses without an additional fixed factor. Having assumed there's a factor involved the calculated theoretical scale looks to be very close to the ones on 2 similar 17" f5.6 Dallons I've found photos of on the Internet (one's here).

Ian

[Dan Fromm]

Measure the entrance pupil's diameter with the diaphragm wide open. It should be 3 1/8".

[IanG]

The front element diameter is approx 77mmmm give or take 0.5mm, it's set back in the barrel. That calculates at around f5.6 which is marked on the lens cap and sleeve the lens came stored in. This indicates a fixed factor of approx 1.8 between the aperture calculated from the diameter of the aperture & focal lengthen.

It's fairly obvious that the effective aperture of a telephoto is larger than a standard lens of the same focal length due to the inverse square law, the satandard lens at the same actual aperture diameter is further from the film plane at Infinity (and other distances)

So I need to divide the theoretical aperture by approx 1.8 to get the effective aperture with this telephoto.

Ian

[Dan Fromm]

Ian, in this case measurement beats calculation.

[Nodda Duma]

F/# is ratio of the focal length to the entrance pupil. The entrance pupil is the aperture as imaged by the lenses in front of its position in the optical path.

In many designs the entrance pupil diameter is not the same as the physical aperture diameter: the only case where they are equal is when the aperture stop is on or in front of the first lens element (sounds like the case here when the lens is wide open).

But when stopped down, the entrance pupil will be set by the physical aperture diameter and not the front lens. Very interesting.

[Dan Fromm]

Jason, Ian's lens' aperture is somewhere in the middle of the lens. The entrance pupil, as you just said, is an image of the stop viewed through the front of the lens. We know what its diameter has to be with the lens wide open, and I've given Ian the number.

His problem is one many of have faced when we've switched a lens' cells from one shutter or barrel to another that has no aperture scale. We have to make a new aperture scale. SKGrimes does this routinely. There are standard aperture scales for Copal shutters for many common lenses. They're sold in long rolls. For common lenses put into Copal shutters, the machinist just snips a scale from the roll and attaches it to the shutter.

For uncommon lenses, SKGrimes' practice is to measure the entrance pupil with the aperture wide open, mark the barrel or shutter, stop down until the entrance pupil is one stop (1/sqrt(2)) smaller, mark, ... They've done this for me with, e.g., an aerial camera lens whose aperture scale was on a sleeve that had to be removed to mount the lens on a board.

[ic-racer]

It's fairly obvious that the effective aperture of a telephoto is larger than a standard lens of the same focal length due to the inverse square law,

1) Lens construction is not part of the equation
2) Inverse square law is not part of the equation

[Effective Aperture Calculation] Multiply the marked f-stop by (1 + M), where M is the magnification on the ground glass -- From the Large Format Photography Home Page

[IanG]

Jason, Ian's lens' aperture is somewhere in the middle of the lens. The entrance pupil, as you just said, is an image of the stop viewed through the front of the lens. We know what its diameter has to be with the lens wide open, and I've given Ian the number.

His problem is one many of have faced when we've switched a lens' cells from one shutter or barrel to another that has no aperture scale. We have to make a new aperture scale. SKGrimes does this routinely. There are standard aperture scales for Copal shutters for many common lenses. They're sold in long rolls. For common lenses put into Copal shutters, the machinist just snips a scale from the roll and attaches it to the shutter.

For uncommon lenses, SKGrimes' practice is to measure the entrance pupil with the aperture wide open, mark the barrel or shutter, stop down until the entrance pupil is one stop (1/sqrt(2)) smaller, mark, ... They've done this for me with, e.g., an aerial camera lens whose aperture scale was on a sleeve that had to be removed to mount the lens on a board.

I thought you'd grasp my points Dan, and coming from different perspectives we reach exactly the same conclusions.

I think the point Jason's missing is I know at max aperture it's an f5.6 lens so half the diameter is f11 and so on, and I can calculate the apertures between.

Ian

[IanG]

1) Lens construction is not part of the equation
2) Inverse square law is not part of the equation

Actually they are key in telephoto lens designs. A telephoto lens shortens the nodal point to film distance so for a given aperture mm or inch diameter there's a big change in exposure. I suppose a simplistic way of looking at it a 12"/300mm standard lens sits on the maximum extension on my Wista or Super Graphic, the 17" Dallon sits at the end of an unextended trackbed at Infinity, roughly half the extension, now equate that to the inverse square law, it's a no brainer.. . . . . . . . it's about 2 stops` difference.Use Maths not math the plural adds the missing gaps . . . . . . . .

Ian

[Dan Fromm]

A telephoto lens shortens the nodal point to film distance

Eh? Wot? It does no such thing. The rear node-to-film plane when the lens is focused to infinity is the same for all lenses of the same focal length. Tele lenses' rear nodes are way out in front ...

Ian, I'm not sure whether you're in the UK or Turkey but either way its past your bed time. Have a glass of warm milk, sleep well and reconsider in the morning.