How to deal with the central bright spot

Just wanted to throw up an example of how I sometimes eliminate the central bright spot which occurs in many macro lenses: the flat field. To make a flat field, I took a photo of an evenly-illuminated out of focus white surface at the same camera settings as my target photo.

Here's what my flat for f/22 looked like. Central bright spot clearly visible.

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Flat applied, overall exposure adjusted, white balance neutralized to paper, and sharpened.

So my question to you all is how do you deal with the central bright spot, and if possible can you show an example of what works efficiently for you.

Have you tried any other macro lens?

To the point of trying out a different lens...
Here is a series of shots taken with a Tamron 90mm f/2.5 macro lens at its closest focus distance (1.3') to record a 1:2 repro ratio image on a 5D camera. Subject area is white bond paper illuminated under CFL known by me to have a color temp of 2700K. Shots were recorded in RAW and imported into Lightroom.

Two points I'd like to raise to your attention:

1. Shots 1 & 3 are portrayed 'as shot', with #1 at f/2.5 and #3 at f/11. One can see that at f/2.5 there is noticeable non-uniformity of field, whereas at f/11 the field looks quite uniform. So brightness of center is NOT inherent to macro lenses.
   
2. Shots 2 & 4 are Lightroom virtual copies of #1 and #3, respectively. But exposure has been altered (in postprocessing) by +3EV, to make the white bond LOOK WHITE. One of my pet peeves on POTN are shots taken by amateurs of white objects, underexposed so that the 'white' is visibly gray. That is not 'good photography' to underexpose an image, especially when it is pointed out that underexposure leads to inferior signal:noise characteristics in the shots and so many folks have a beef already about Canon inferiority in s/n vs. Sony and changing brands for that reason. PLEASE...Let's not portray the world darker than it is, and make our cameras look noisier than they need to be.
   Your meter tries to take what it sees as a target, and suggests an exposure which CAPTURES IT GREY, so use EC to tell the meter your subject area is NOT grey, and to give more exposure to the scene so that white records as white. [Stepping down from soapbox]
   

Hmm, I'm confused.

The listed metadata shows the lens as FD 50mm/1.8. Is that an adapted lens from the old (pre-1987) mount? If so, is the adapter using the "proper" flange distance to get proper infinity focus or is that not relevant because you're actually looking for extension for higher magnification?

If it's not proper flange distance then the brightness of the image circle might not evenly project onto the sensor. But then again, the FD mount was for full frame and with it now mounted on a crop sensor, it seems that shouldn't make a difference because of the smaller cross section being taken from the circle.

On one of Will's points, I've not heard of, nor have I experienced that Canon's S/N ratio was bad or rather any worse than any other manufacturer in general. From my limited access to a controlled comparison, my Sony a6000 is certainly not any better than any of my Canon sensors (FF or crop). Now, I know that the a6000 at 24MP has smaller pixels than my 60D, with smaller pixels getting less light to work with. (I'm not going to compare the noise on a6000 small pixels to my FF Canon sensors with larger pixels; not a fair comparison.) I'd be curious to be pointed in a direction to help me understand the S/N issue.

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

The listed metadata shows the lens as FD 50mm/1.8. Is that an adapted lens from the old (pre-1987) mount? If so, is the adapter using the "proper" flange distance to get proper infinity focus or is that not relevant because you're actually looking for extension for higher magnification?

If it's not proper flange distance then the brightness of the image circle might not evenly project onto the sensor. But then again, the FD mount was for full frame and with it now mounted on a crop sensor, it seems that shouldn't make a difference because of the smaller cross section being taken from the circle.

On one of Will's points, I've not heard of, nor have I experienced that Canon's S/N ratio was bad or rather any worse than any other manufacturer in general. From my limited access to a controlled comparison, my Sony a6000 is certainly not any better than any of my Canon sensors (FF or crop). Now, I know that the a6000 at 24MP has smaller pixels than my 60D, with smaller pixels getting less light to work with. (I'm not going to compare the noise on a6000 small pixels to my FF Canon sensors with larger pixels; not a fair comparison.) I'd be curious to be pointed in a direction to help me understand the S/N issue.

And any optics in the mount adapter might itself be solely responsible for the non-uniformity!

There are threads on POTN, in which certain members considered changing from Canon to Sony simply because of high ISO 'high' noise of Canon limiting its dynamic range compared to Sony. Only the intro of the 5DIV seemed to have satisfield some.

Wilt wrote in post #18227600
There are threads on POTN, in which certain members considered changing from Canon to Sony simply because of high ISO 'high' noise of Canon limiting its dynamic range compared to Sony. Only the intro of the 5DIV seemed to have satisfield some.

Thanks for the perspective. I (still?) assume that there are folks that complain for subjective reasons (also covered in the "Canon Bashing" thread), and others that have gotten into specifics and complain for objective reasons, even if the specs behind the objective reasons are too small to be realized in practical application.

I shoot nearly every day. It's either my corporate work or outside work which includes personal enjoyment, teaching workshops, seniors and weddings. Noise is so easily fixed in post-processing and the range of usable ISO speeds is pretty amazing when I look at the big picture (and over time). I could get really upset about getting noise at 128000000 or realize that I don't actually need that speed most of the time. And if those times that I would wish for that causes me to chase more gear...? Heck, I already have enough gear and lost that urge. I guess I'm just tired of the gear chase...

But I think I just digressed into a soapbox.

Yes it's an old FD on a non lens adapter and some extension tubes. To date I've only tried moving camera and lights around to reduce the spot but the flat field image may help when I can't reduce it enough.

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

The listed metadata shows the lens as FD 50mm/1.8. Is that an adapted lens from the old (pre-1987) mount? If so, is the adapter using the "proper" flange distance to get proper infinity focus or is that not relevant because you're actually looking for extension for higher magnification?

If it's not proper flange distance then the brightness of the image circle might not evenly project onto the sensor. But then again, the FD mount was for full frame and with it now mounted on a crop sensor, it seems that shouldn't make a difference because of the smaller cross section being taken from the circle.

On one of Will's points, I've not heard of, nor have I experienced that Canon's S/N ratio was bad or rather any worse than any other manufacturer in general. From my limited access to a controlled comparison, my Sony a6000 is certainly not any better than any of my Canon sensors (FF or crop). Now, I know that the a6000 at 24MP has smaller pixels than my 60D, with smaller pixels getting less light to work with. (I'm not going to compare the noise on a6000 small pixels to my FF Canon sensors with larger pixels; not a fair comparison.) I'd be curious to be pointed in a direction to help me understand the S/N issue.

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

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.

This might be a dead end, but is the position of the light or the angle of the light relative to the lens consistent? Do you get different "light areas" with the (outside the camera) light being in a different position?

I ask because I get weird lens flares when using an enlarger lens on a bellows when the light is coming from certain angles, and it's not that much different than what you showed here. It's essentially a manual lens on an adapter, like your description.

Don't really have a clue other than I wonder if it is due to stray light hitting the front element?

I'm not a macro guy but what little I've done shows no such issue. I'd make a long hood/snoot for the front of the lens and see what happens.

Just to address wilts point that I captured white as grey... well a shot of something clear on white is a challenge because the reflections on the sugar highlight brighter than the matte paper surface. I want to max out the sugar crystals dynamic range and that puts a matte surface in between the shadow and the reflections on the crystals. I shot with the needle pointing straight up at zero as it were. I know that's messed up to some people, but it works for my sensibilities.

dasmith232 wrote in post #18227880
This might be a dead end, but is the position of the light or the angle of the light relative to the lens consistent? Do you get different "light areas" with the (outside the camera) light being in a different position?

I ask because I get weird lens flares when using an enlarger lens on a bellows when the light is coming from certain angles, and it's not that much different than what you showed here. It's essentially a manual lens on an adapter, like your description.

The lights definitely interact with this central bright spot, the flat field shows sort of a best-case scenario. With an actual subject, the light spot in the center washes out the exposure. I just know there is a lot of reflecting and refracting also going on that is not solved entirely by flat-field imaging, but it seems like it can help in my case where my system is thusly flawed.

Benitoite wrote in post #18227858
Yes it's an old FD on a non lens adapter and some extension tubes. To date I've only tried moving camera and lights around to reduce the spot but the flat field image may help when I can't reduce it enough.

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. Compare '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 focused at very close distances, its focus plane was indeed planar!

http://photo.stackexch​ange.com …oes-flat-field-focus-mean

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