Achromatic doublets, coverage and resolution

[genotypewriter]

It's very difficult to come across information on using achromatic doublets as a taking lens.

The exact thing I'm wondering about is, if the focal length of the achromatic doublet is 1000mm (i.e. a +1 diopter) and it's stopped down to f/125 (using a ~8mm aperture stop place in front of the lens), what is the diffraction-limited angular coverage like?

I guess this also depends the diameter of the lens as well, although the opening stays the same. Any idea on how big a difference we would see if the diameter went from say 50mm to 100mm (while the stop stayed the same size at ~8mm)?

Any input would be appreciated. Thanks in advance...

G

[Dan Fromm]

A number of long focus lenses for 35 mm still cine cameras are achromatic doublets. Century Tele Athenar and TA II lenses, for example. Mine weren't outstanding. Astro Berlin's long lenses (see http://camerapedia.wikia.com/wiki/Astro_Berlin) are also achromatic doublets.

I apologize for not responding directly to your question, but if you're looking for a decent and not too expensive lens that covers at least 4x5 -- if not, why ask about doublets? -- your best bet is probably a long process lens. They're around and dialyte types, e.g., Apo-Ronars, are quite competitive with modern tele lenses for LF. Like doublets they're not teles so require extension >= focal length.

[genotypewriter]

Thanks for your response, Dan.

I was being a lot more ambitious than shooting 4x5 with one. In my defense, I wasn't planning on enlarging and hence the f/125.

The issue with enlarging lenses, as you know, is things get expensive at those focal lengths and honestly I was hoping to go beyond the ~45 degree coverage limit. Haven't come across any WA enlarging lenses around the 1000mm/40" mark.

I have a single element diopter and it seems to cover 8x10 with enough sharpness when I stop it down to ~f/125. Didn't measure lp/mm or anything. I'm expecting an achromatic doublet to be even better.

Would be good to hear from someone who has experimented with one.

[Hermes07]

Afraid I can't answer your question directly either, but a few things to think about..

Firstly, if you're thinking along the lines of getting a 1000mm lens to use for 4x5, as Dan says, there are other options - 1000mm isn't THAT extreme so there's no reason to be resorting to a doublet. Process lenses are superb and aren't that uncommon in this focal length, but they'll be fairly expensive. Igor Camera has a 1000mm Dallmeyer Dallon Telephoto which (as a genuine anastigmat) will top what an Achromat can do and is a cheaper option.

If you're looking at using doublets in general then consider that a rough rule for the circle of usable definition from a classic landscape lens (single achromat with stop positioned a good distance in front of it) is about half the focal length. Although it can be reduced by narrowing the aperture, spherical aberration in a typical achromat bought today (they're likely to be biconvex if not specifically designed for photography) will be very pronounced if covering a negative.

Ultimately, it depends on exactly what you're trying to achieve. Narrowing the field of view will reduce the effect of many of the off-axis aberrations so it all depends on exactly how much coverage is critical to you. You mention apertures of 50mm/100mm which equate to f/20 & f/10. When you think that the traditional landscape lens was used at f/30+ on a blue-sensitive medium which wasn't going to be enlarged, and still found lacking, I think you will be disappointed at the performance of a doublet at anything wider.

[genotypewriter]

Firstly, if you're thinking along the lines of getting a 1000mm lens to use for 4x5, as Dan says, there are other options - 1000mm isn't THAT extreme so there's no reason to be resorting to a doublet. Process lenses are superb and aren't that uncommon in this focal length, but they'll be fairly expensive. Igor Camera has a 1000mm Dallmeyer Dallon Telephoto which (as a genuine anastigmat) will top what an Achromat can do and is a cheaper option.

If you're looking at using doublets in general then consider that a rough rule for the circle of usable definition from a classic landscape lens (single achromat with stop positioned a good distance in front of it) is about half the focal length. Although it can be reduced by narrowing the aperture, spherical aberration in a typical achromat bought today (they're likely to be biconvex if not specifically designed for photography) will be very pronounced if covering a negative.

Ultimately, it depends on exactly what you're trying to achieve. Narrowing the field of view will reduce the effect of many of the off-axis aberrations so it all depends on exactly how much coverage is critical to you. You mention apertures of 50mm/100mm which equate to f/20 & f/10. When you think that the traditional landscape lens was used at f/30+ on a blue-sensitive medium which wasn't going to be enlarged, and still found lacking, I think you will be disappointed at the performance of a doublet at anything wider.

Appreciate your interest, Hermes, but it would've been a better use of your time if you had read what I wrote before replying.

[Maris Rusis]

It's very difficult to come across information on using achromatic doublets as a taking lens.

The exact thing I'm wondering about is, if the focal length of the achromatic doublet is 1000mm (i.e. a +1 diopter) and it's stopped down to f/125 (using a ~8mm aperture stop place in front of the lens), what is the diffraction-limited angular coverage like?

I guess this also depends the diameter of the lens as well, although the opening stays the same. Any idea on how big a difference we would see if the diameter went from say 50mm to 100mm (while the stop stayed the same size at ~8mm)?

Any input would be appreciated. Thanks in advance...

G

I have used achromatic doublet lenses as taking lenses with focal lengths from 900mm down to 100mm.

The largest aberration that severely limits sharp coverage is field curvature. An approximate rule of thumb suggests that the radius of the curved field is about equal to the focal length. In general achromats work better as long focal lengths on small formats and abominably as short lenses on big formats. Some of the other first order aberrations (spherical, astigmatism, coma, distortion, vignetting, lateral colour) can be tamed somewhat by stopping down. In practice I find the limit to stopping down is a 3mm aperture for a 8"x10" photograph. Smaller than this results in an image softened by visible diffraction. A 1000mm lens will work fine at f300 and a 100mm lens is ok at f30, and so on. Non-diffraction aberrations will continue to affect the image....more or less.

One thing is certain: no matter how you compromise on aberrations an achromat is not a cheap and versatile substitute for a fully corrected camera lens. I wasted a lot of time finding this out.

[genotypewriter]

...

Thank you, Maris. The field curvature radius ~= focal length is a bit shocking. As mentioned, I won't be enlarging the shots so I was hoping that stopping down to the point of diffraction limit nearing the naked eye's resolution might help a bit with the curvature and other aberrations. Any idea on how big a difference the ach. doublet's diameter makes if we keep the aperture fixed (at a small size)? Through a simple lens design tool I figured that a larger diameter lens is better (at the same aperture) but wasn't sure how it affected the field curvature and other aberrations.

One more question for you, if you don't mind... I can't remember the lens design name off the top of my head but I've seen a symmetrical one that looks very similar to two opposing doublets (convex sides out). Is there any room for improvement if two ach. doublets are used this way instead of just one?

Thanks, again.

[Struan Gray]

How much of a difference is made by having a physically larger lens (of the same glass and surface curvatures) depends on where you have the aperture stop, and how large it is. Bigger stops (larger holes) need bigger lenses, as do stops located further away from the glass.

To a certain extent you can trade field curvature against blurring aberrations like coma by moving the stop - this is why traditional landscape menisci have the stop some way out in front of the glass. It's no magic cure though.

Using two lenses in a symmetrical arrangement does help to squelch aberrations. It also reduces the total focal length, roughly by half. Your choice :-)

If you want to tinker with simple lenses, it's easier to play than predict what will happen. If you want to experiment with long lenses without spending a lot of money there are ways to avoid the fuzz and fiddle of singlets and achromats. One is to get a cheap off-brand tele for 35 mm. Many of them will cover LF film once you remove the tubes and baffling that otherwise vignettes them. Better, and easier, is to get a reasonably long lens and remove the front group. 420, 465 and 480 mm process lenses (apo-ronars and the likd) in barrel are still relatively cheap, and removing the front element will roughly double the focal length *and* give you an easy way to mount it on your camera. Some sort of coating comes free too.

[Hermes07]

Appreciate your interest, Hermes, but it would've been a better use of your time if you had read what I wrote before replying.

I wrote this reply before reading your second post.

Now that I have, let me be clearer.

Without knowing exactly what doublet you're talking about, no-one will be able to answer your question accurately. The covering power will depend on the shape of the lens and the stop position, and will vary significantly based on these. A bi-convex doublet with a close stop will show far more curvature of field than a meniscus doublet with a distant stop. You can improve the image by increasing the stop distance, thereby choking the coverage to eliminate the inbound off-axis rays that produce the field curvature. Adding a second doublet to make a mock rapid rectilinear/aplanat has the same effect - improving the central image but reducing the angle of illumination.

Questioning a lens' performance only becomes meaningful once you define exactly what it is you want it to do. I've used a meniscus with satisfactory performance at f/90-f/128 before but this was with my AOV and covering requirements - what are yours?

[William Whitaker]

When I hear "achromatic doublet", I keep thinking refractor telescope...