Effective focal length Nikkor SW 90 f/8

[Grumium]

I am looking for the effective focal length of the Nikkor SW 90 f/8. I do not own the lens up to now but am considering it for a special DIY project (focusing helicoid).

Although the above shortcut lists many dimensions and data, the effective focal length is missing (or am I missing something?):
http://homepages.tig.com.au/~cbird/nikkor/sw.html#SW90

Does anybody know what I am looking for or is it stated on each lens?

[Nodda Duma]

Effective Focal Length, or EFL = 90mm.

Are you instead looking for a distance from a reference feature on the lens to the image plane? Typically back focal length (from vertex of last optical surface) or flange focal length (from a mounting flange on the lens body) is called out.

In the spec for the lens, flange focal distance = 97mm (understood to be at infinity focus), referenced to surface "E" in the drawing....the mounting flange on the shutter.

Hope this helps,
Jason

Edit: This pdf has a clearer drawing:

http://www.kenleegallery.com/pdf/Nikkor_LargeFormatLenses.pdf

[ic-racer]

Not sure of 'effective' focal length nomenclature if there are no diopter or accessory lenses in the system.
The flange focal is 97mm. It is not a retrofocus, that is just how the cells are mounted in the shutter. Also, the back focus is 65mm and the front focus is 72.8mm

[Dan Fromm]

Um, er, ah, Jason, that's the specification. There's no guarantee that the lens the OP buys will match it exactly. If he wants to make a helicoid with an accurate focusing scale he'll need to measure the actual focal length and design and engrave to it.

OP, there are a number of focal length concepts. There's the marketing focal length. The 90/4.5 Nikkor SW's marketing focal length is 90 mm. Its design/specification focal length may be different. And its actual focal length may also be different.

To make these ideas concrete, the 38/4.5 Biogon's marketing focal length is 38 mm. According to Zeiss, its design/specification focal length is 38.5 mm. I once bought some aerial cameras that contained 38/4.5 Biogons, ended up with 20 lenses. The cameras' maker had measured each lens' focal length and marked it on the lens. They ranged from 38.3 to 38.8 mm.

[Grumium]

Let me put the cart before the horse: I do have a focusing helicoid that is built (including distance markings) for the Schneider 90mm f/5.6 Super-Angulon XL.

The 90 XL has a flange focal distance of 103.5 mm. Thus the 90 f/8 has to sit 6.5 mm closer to the film plane at infinity. This won't be an issue.

The 90 XL has an effective focal length of 90.7 mm. Will the distance markings also be correct at all distances (assuming that they are for the 90 XL) for the 90 f/8?

;-)

[Bob Salomon]

Why not do it the easy way. Rodenstock makes a helical specifically for the 90mm 6.8 Grandagon-N and Schneider makes helicals specifically for their 90mm 5.6 and 8.0 lenses. These helical so, with the lenses they are designed for will work properly without you having to compute anything.

[Dan Fromm]

The 90 XL has an effective focal length of 90.7 mm. Will the distance markings also be correct at all distances (assuming that they are for the 90 XL) for the 90 f/8?

;-)There's no guarantee.

[Nodda Duma]

Um, er, ah, Jason, that's the specification. There's no guarantee that the lens the OP buys will match it exactly. If he wants to make a helicoid with an accurate focusing scale he'll need to measure the actual focal length and design and engrave to it.
.

You'll get no argument from me on that, Dan. In my experience he'll see the actual focal length vary by up to ~ +/- 0.5 mm due to tolerancing.

[rfesk]

The answer is: For all practical purposes your scale will be accurate.


Let me put the cart before the horse: I do have a focusing helicoid that is built (including distance markings) for the Schneider 90mm f/5.6 Super-Angulon XL.

The 90 XL has a flange focal distance of 103.5 mm. Thus the 90 f/8 has to sit 6.5 mm closer to the film plane at infinity. This won't be an issue.

The 90 XL has an effective focal length of 90.7 mm. Will the distance markings also be correct at all distances (assuming that they are for the 90 XL) for the 90 f/8?

;-)

[ic-racer]

We are talking about zone focus. The numbers on the focus ring might be off by a factor of 0.7%. Is this going to mess you up?

[Grumium]

I don't think so? How did you arrive at 0.7%?

[Grumium]

I am of a different opinion. I will compensate for the difference in FFD by machining a dedicated mount, thus bringing the whole helicoid assembly 6.5 mm forward.

As the effective focal length of both lenses differ (Do they? Initial question of EFL of the Nikkor is still unanswered), the marked distance is only matching the real distance at a single point (e.g. at infinity) and not across the full focus range.

[Dan Fromm]

I am of a different opinion. I will compensate for the difference in FFD by machining a dedicated mount, thus bringing the whole helicoid assembly 6.5 mm forward.

As the effective focal length of both lenses differ (Do they? Initial question of EFL of the Nikkor is still unanswered), the marked distance is only matching the real distance at a single point (e.g. at infinity) and not across the full focus range.

Don't you mean backwards?

I'm sorry, but you'll have to measure your 90/8's focal length. Your problem is exactly the same as that of cine lens' makers as discussed somewhere in Arthur Cox' book Photographic Optics. These lenses are focused by tape measure so their engraved focusing scales have to be very accurate. Cox discusses the computations, points out that no two lenses are exactly alike.

Comment on language: in the US dialect of photographer's English, a thick lens' focal length means its EFL.

I've seen .7% and 0.77% in several posts in this discussion. Pardon my ignorance and enlighten me. Where did those numbers come from?

[Oren Grad]

The specified FL of the 90/8 Nikkor SW is 90.1 mm.

http://www.nikon-image.com/products/lens/other_lens/sw_90mmf8s/

[Dan Fromm]

The specified FL of the 90/8 Nikkor SW is 90.1 mm.

http://www.nikon-image.com/products/lens/other_lens/sw_90mmf8s/

That's nice, Oren. What's the actual FL of the OP's lens?

Mullen, I know what Rodenstock and Schneider and, yes, Fotoman do. So what? Please don't read "So what?" as an insult, I don't mean it as an insult.

[Oren Grad]

That's nice, Oren. What's the actual FL of the OP's lens?

To state the obvious: he will have to measure it if he needs to know.

FWIW: I have an 8x10 box camera with a 120 SA in helical mount. When I need to be sure that best focus is placed on a particular spot, I use a loupe on the GG.