How to deal with the central bright spot

Wilt wrote in post #18227906
Methinks you misunderstand the purpose of 'flat field'. It has nothing to do with illumination, nothing to do with brightness across the frame.

Flat field correction means that a lens has been specially designed so that if you had a postage stamp adhered to the wall and were attempting to photograph it, all parts of the postage stamp would be in precise focus simultaneous. Contrast 'flat field' surface of a poster vs 'curved field' like the surface of a basketball. As macro lenses were often used to photograph documents, and documents are flat, lens designers needed to ensure that when the lens was focued at very close distances, its focus plane was indeed planar!

I'm not using flat field imaging to adjust geometry for lens distortion. Look up "flat-field correction" and realize I am not convolving a curved field onto a flat plane, just compensating for other aberrations besides geometric lens distortion.

Benitoite wrote in post #18227918
I'm not using flat field imaging to adjust geometry for lens distortion. Look up "flat-field correction" and realize I am not convolving a curved field onto a flat plane, just compensating for other aberrations besides geometric lens distortion.

You misunderstand. Flat field correction does not try to mold the curved surface of a basketball so it is as if it were on a flat plane.

Flat field correction tried to put all POINTS OF FOCUS so they all are on a flat plane...so if you have a newspaper on the wall, when you focus on the wall the print is equally in focus at the center of the frame as well as at the edges of the frame.

If a lens were NOT 'flat field corrected', when you focused on the newsprint on the wall, the image closer to the frame edges might be out of focus!

Wilt wrote in post #18227942
You misunderstand. Flat field correction does not try to mold the curved surface of a basketball so it is as if it were on a flat plane.

Flat field correction tried to put all POINTS OF FOCUS so they all are on a flat plane...so if you have a newspaper on the wall, when you focus on the wall the print is equally in focus at the center of the frame as well as at the edges of the frame.

If a lens were NOT 'flat field corrected', when you focused on the newsprint on the wall, the image closer to the frame edges might be out of focus!

I do see what you are saying. This is different.

Wilt wrote in post #18227573
To the point of trying out a different lens... [...]

TeamSpeed wrote in post #18227484
Have you tried any other macro lens?

dasmith232 wrote in post #18227597
Hmm, I'm confused.

The listed metadata shows [...]

Left Handed Brisket wrote in post #18227890
Don't really have a clue other than I wonder if it is due to stray light hitting the front element? [...]

This is just a link to the RAW images in case.

https://photography-on-the.net …showthread.php?​p=18228144

Benitoite wrote in post #18227873
My only setup is the FD 50 on tubes giving 1.45:1 magnification. Lens costed $20, tubes and adapter the same. Not looking to spend money just record photons.

Well I have purchased several different 1:1 macro lenses over the years, and have never seen anything like that in any of my macros, thus my question. I suspect that what you are getting is a result of that lens, perhaps a coating that is coming off, or something inside defracting light, or is just a characteristic of that lens. There are other lenses that work very well that cost very little, or you can try your EF 50 1.8 with tubes, or the 18-55 IS kit lens. There are tricks too where you can simply reverse the lens around, and there are cheap adapters that will allow you to do this electronically. Without something like this, you have to lock your aperture first using the DOF, then dismount the lens, then rotate. These might get you even more magnification than you are getting now.

There is something else wrong with the lens. I had a look at the raw and there are some strange colour unevenness (in my LR color correction in purple) especially around the fringe - and yes, it is severely underexposed.

Not surprised to see the color unevenness without the flat applied, the pattern is very familiar to me after using the setup for a couple of years now.
I'm sure if I normalized the colors we'd see some pretty drastic things. It's an elderly lens but I like it's character.
Applying the flat I would probably have to use a higher blur radius to help the color, which is less useful for sensor dust.
As to the cause of the chromaticism I always assumed it was the first wave trough surrounding the airy disk.

As far as gear goes, this lens, a 5x 20mm f/2 (13x on tubes) caught my eye recently.
http://www.zyoptics.ne​t …f2-4-5x-super-macro-lens/
Not looking for a luxury lens at this point... a reversing adapter is probably in my price range as well, but haven't bothered since assembling the chromatismatic FD-50mm piece.

TeamSpeed wrote in post #18228303
Well I have purchased several different 1:1 macro lenses over the years, and have never seen anything like that in any of my macros, thus my question. I suspect that what you are getting is a result of that lens, perhaps a coating that is coming off, or something inside defracting light, or is just a characteristic of that lens. There are other lenses that work very well that cost very little, or you can try your EF 50 1.8 with tubes, or the 18-55 IS kit lens. There are tricks too where you can simply reverse the lens around, and there are cheap adapters that will allow you to do this electronically. Without something like this, you have to lock your aperture first using the DOF, then dismount the lens, then rotate. These might get you even more magnification than you are getting now.

That same lens caught my eye as well. The magnification range is pretty close to the Canon MP-E 65mm which I already have. However, the 20mm focal length would do two things. Obviously, a wider angle of view, but a shorter focal length would provide a lot more depth of field.

It's cheap enough (for a specialty lens) that I'm considering it, but it's certainly not something I need. I'd be curious to see if anyone else gets it or has experience with it.

Update: I just looked at this lens again and it does not range from 1x to 4.5x but it's nearly fixed at the high end ranging only from 4x to 4.5x. That limits its functionality...

DOF at macro magnifications makes NO difference as to the FL of the macro optic. It has ALL to do with the capture magnification of the subject!

The bright centre would be entirely expected: "an old FD on a non lens adapter and some extension tubes. " will have all sorts of internal reflections off the sides of the tubes and adapters.
Cheapest ones are very shiny indeed. They invariably add strange colours too.

You need "flocking".
Black paper in the tube can help, or judiciously positioned discs with a central hole, which may need to be rectangular .
Even black pvc tape over chrome can fix a problem.
Materials like Protostar are better.

It's actually fogging that you're getting, so it's not just bright, it's losing you contrast and therefore image quality.

If you look on a macro forum, it's a "FAQ".


Lenses like this, when not in their design range, also very often exhibit the purplish chromatic aberration you 're seeing.
If you focus stack, the lateral CA tends to get lost, but not the longitudinal colour - usually purple, especially around out-of focus highlights.

Wilt wrote in post #18228801
DOF at macro magnifications makes NO difference as to the FL of the macro optic. It has ALL to do with the capture magnification of the subject!

Hi Wilt, I don't disagree with your point. But this is getting off topic from the original thread, so I'll stop with this point. I acknowledge and respect your statement. Where it starts to get foggy for me is that magnification is affected by focal length (but also with subject distance). I think that with multiple variables, it's still magnification that's dominant, and focal length becomes negated by the changing subject position. But I'll take this question elsewhere, and eventually get this straight in my head.

Dave - macro depth of field is not quite like at greater distances. The "Lefkowitz" formula works well:
DOF = 2*C*f_r*(m+1)/(m^2)
Where C is the diameter of the Circle of Confusion you choose
f_r is the f number marked on the lens ring
and m is magnification.
Note, no focal length, sensor size, extension, or anything else. And it's equal each side of the subject. With all these formulae, it depends which parameter you "hold still".

The other odd thing is that if you're at say 2x magnification, the FL of the lens doesn't alter the angle of view as much as you might think. You just don't get a wide angle perspective. The lens is a long way from the sensor so you get a "long focus" view.
(To get a more dramatic effect you have to use a very short lens and a relay optic to project its image onto the sensor).

By the way with your bright blob problem, take the lens & tubes off the camera , and look into the back end towards something bright. You'll probably see lots of reflections off the insides of the tubes. If you can, it ALWAYS causes trouble.

dasmith232 wrote in post #18229042
Hi Wilt, I don't disagree with your point. But this is getting off topic from the original thread, so I'll stop with this point. I acknowledge and respect your statement. Where it starts to get foggy for me is that magnification is affected by focal length (but also with subject distance). I think that with multiple variables, it's still magnification that's dominant, and focal length becomes negated by the changing subject position. But I'll take this question elsewhere, and eventually get this straight in my head.

Magnification is not affected by FL. Whether you use a 100mm lens with focal plane 400mm from the subject, or whether you use 20mm lens with focal plane 80mm from the subject, you will photograph the subject at 1X magnification (1:1)

And as Chris explains, DOF is not affected either.

Thanks guys (Chris and Wilt). Chris, with the specific reference to Lefkowitz, I was able to find the formulae and follow when the "standard" thinking and traditional formulae break down and (new) macro rules take effect. (It happens as the subject distance approaches the focal length of the lens.)

Also, that the DOF is evenly distributed (50/50) around the focus distance is good confirmation. Even in "normal" photography, the DOF shifts from 33/67 to 50/50 as the subject distance drops. So DOF distribution doesn't break down in macro ranges.