Reducing the problem of chromatic aberration *requires* the use of at least two different materials of different dispersion. That is the basic principle of an achromat. It's not simply that a given design might have one or more low-dispersion elements, UD or fluorite. For example, the traditional achromatic doublet uses crown and flint glass, neither of which is particularly "low dispersion." But to achieve even better correction of secondary spectrum we need to use another material and this is where fluorite or fluorite-doped glass comes into play.
That is to say, low dispersion glass in itself doesn't entirely fix CA. If you made the entire lens out of it, you would certainly see less CA than if you made it out of normal glass (assuming refractive index is the same but it won't be), but it still won't be an achromat or apochromat. The key to correction is different materials, not necessarily a material with low dispersion. You take one element with one dispersion characteristic and positive refractive power, cement it to another element with another dispersion characteristic and negative refractive power, and if calculated correctly, you get a doublet with net refractive power (positive or negative) and some degree of correction where red/blue wavelengths are made to focus.
Personally, I believe there is still much reason to continue R&D in optical glass to reduce CA of all kinds. First, the amount of CA depends strongly on the design of the lens, with retrofocus and telephoto designs being particularly prone to CA. You can see this especially in UWA zooms, in which the corners are almost inevitably marred by strong transverse CA. We would see a lot more CA in the supertelephotos were it not for the use of low-dispersion or fluorite glass. What is not as obvious is that CA is the primary reason for differences in sagittal and meridional MTF in superteles, not astigmatism which is inherently small for long focal length (due to small angle of view -> low field curvature).
So when we talk about wanting to keep things simple and not use fancy constructions, that's well and good for a 50mm prime, but it just isn't going to fly with a 16/2.8 or 500/4. You can't rely on software to correct this because CA manifests in a variety and combination of forms, not just the familiar transverse CA in which the image is sharp but magnification varies by wavelength. It is undesirable to have to software correct for longitudinal CA and overall haze/loss of contrast due to CA. Distortion is probably the easiest aberration to correct in post--even the "mustache" distortion can be corrected if one collects the relevant data. But spherical, chromatic, astigmatic, and comatic aberrations are best corrected in the lens.
wickerprints wrote:
What is not as obvious is that CA is the primary reason for differences in sagittal and meridional MTF in superteles, not astigmatism which is inherently small for long focal length (due to small angle of view -> low field curvature).
This is interesting, I've seen it mentioned before on the forum. So can we tell by looking at the MTF curve of a lens just how much CA it's likely to have and whereabouts in the field it will occur?
Some of the tests and anacdotal information I have seen on imaging star fields with camera lenses seems to indicate camera lens designers are not fully correcting for monochromatic aberations such as coma in addition to astigmatism which would also probably give measurable differences in differences in the sagittal and meridional MTF results. I would suspect that the introduction of aspheric elements in optic mix would be the primary contributor but I am quickly getting out of my depth in optical design.
Here is an interseting blog on using camera lenses to image star field, some pretty high end APO designs are used and evaluated that indicate astigmatic and coma aberations.
Some of the tests and anacdotal information I have seen on imaging star fields with camera lenses seems to indicate camera lens designers are not fully correcting for monochromatic aberations such as coma in addition to astigmatism which would also probably give measurable differences in differences in the sagittal and meridional MTF results. I would suspect that the introduction of aspheric elements in optic mix would be the primary contributor but I am quickly getting out of my depth in optical design.
Here is an interseting blog on using camera lenses to image star field, some pretty high end APO designs are used and evaluated that indicate astigmatic and coma aberations.
Correct. Many designs do not correct coma to a great degree, but rather make a compromise. Spherical aberration is often (but not always) considered the more important issue to correct, thus the use of aspherical elements. Because comatic aberration is a function of the angle of the principal ray with the optical axis, long focal length lenses generally do not exhibit significant coma. Conversely, wide and ultra-wide lenses are very much prone to this type of aberration, which is why it is a common concern for star field imaging. Indeed, wide-angle astrophotography is perhaps the worst-case scenario for coma, because (1) you want to capture near-point light sources against a dark background throughout the frame; (2) light levels are extremely low thus discouraging use of small apertures which would reduce coma; (3) focus is generally at or near infinity thus stopping down to increase DOF is often unnecessary; and (4) coma tends to be prominent in wide-angle designs.
So are optics a hobby or profession? Started out for me as a career ambition but I got sidetracked from Physics and Optical Engineering to Engineering Mechanics and a career in Structural Engineering and now building Project Management. Bottom line - wussed out for the money.
Thanks Gary, Philber, Yes it was taken a couple of days ago during the frost. Decided it was time to take my Fujifilm S5 off the shelf and do some shooting with it.
180 f/3.4 APO panorama (zooming in ~2/3 up on the scale is around full resolution on this site). Had this up earlier but not a full resolution version.:
Burningheart that S5pro performs pretty well at ISO800! I've always been interested in that camera since acquiring my f31fd, their superCCD technology is very interesting. I am a big fan of the Coastal optics 60 as well
thrice wrote:
Great pano Richard! I Dig the colours.
Burningheart that S5pro performs pretty well at ISO800! I've always been interested in that camera since acquiring my f31fd, their superCCD technology is very interesting. I am a big fan of the Coastal optics 60 as well
That's what I forgot to change the ISO. I usually shoot the S5 at ISO 200, they S5 has great dynamic range and color. I just wish they would update the S5 but Fuji seems to have abandoned the market for dslr's.
The coastal optics what more can be said, except it is great for UV and IR, that's the reason I bought it for my Fujifilm S3 UV/IR. Bonus it works on other cameras too and is APO.
Shortly I will be able to use my Leica R's on the Fujifilm, I just ordered the Leitax mounts to convert all the Leicas to Nikon mount, that way I'll be able to use them on all my cameras .
I usually shoot the S5 at ISO 200, they S5 has great dynamic range and color. I just wish they would update the S5 but Fuji seems to have abandoned the market for dslr's.