Chromatic Aberration

[H@rry]

I have a problem that I have not been able to figure out. When I try to take a macro of a blue or purple tip coral it always comes out with a blue cast on it that I understand is Chromatic Aberration. Other colors don't seem to be affected. It seems to be worse when shooting in RAW. Funny thing is that the image on the screen that is displayed after the shot looks good. Here's three examples.

Nikon D40
Nikkor 105mm f2.8 with hood and 36mm extension tube - manual focus
Hood was in contact with glass and perpendicular as close as I can see.
400w MH
Actinics off (although I tried it both ways and the result was the same)
No post processing other than converting the RAW to JPG.
All were resized in GIMP

Any ideas will be welcome.

RAW converted to JPG with Picasa

RAW converted to JPG with Lightroom

JPEG Fine

 

[nickb]

That doesn't look like CA. In any event, the lens you are using doesn't have a CA problem. Looking at the EXIF data, you used a 5 sec exposure at f/36 with EV=+1.33. Any particular reason? Try again using f/8 and EV=0. Plus, did you do any white balance adjustment? Did you use a tripod? And turn of the pumps?

 

[H@rry]

It's the same all the way down to 1/50 f3.

I shoot each shot at the widest aperture, at f11, and the smallest aperture and them pick the one I like the best. It's usually the smallest aperture because of the DOF.

The reason for the EV=+1.33 is because I increase the exposure until I see a blink on the highlight screen and then back off one click. That's how I was trying to determine the correct exposure. Could that be the problem?

Tripod, glass clean, pumps off.

 

[H@rry]

Here's one @ 1/320 f3.3

 

[TitusvileSurfer]

I can see CA in your images. The best way to stop this is of course before the picture is taken. Zooming in will help but your lens obviously can't zoom. I don't have a Nikon and don't know Nikon, so this is pure speculation, but you might have a bad copy of your lens? Anyway I decided to try and save the image in Photoshop as I am just waiting for my cousin to call after she buys "Left4Dead" for the Xbox 360 and have nothing better to do in the mean time. I took a small fraction of the image to work with, mainly because the CA is so easy to see when viewing a crop. Unfortunately it ended in tragedy.

Firstly I always make copy the background layer for reference and if I edit the image directly it gives me something to fall back on.

1) The first real step in this image is to make an adjustment layer and select Hue/Saturation. I have a dinosaur version of Photoshop (5.5) so bare with me. You may have more options than I do, but you should at the last be able to do everything that I can.

2) Since your chromatic abrasion is blue, I took all the blue out of the image by selecting "blue" and taking the saturation to -100.

3) Select your paint brush of choice with a *black* background color. As you paint over the adjustment layer, the color we took away will be restored.

4) As Bob Ross would say, "You just paint in wherever you think that blue lives. This is your image and that blue lives wherever you want it to." As indicated by the arrow, I decided (more or less for the purpose of instruction) that the blue in the corner indicated by the arrow was too much.

5) I selected the area I wanted to change.

6) I made another adjustment layer *select the origional layer to adjust as opposed to the adjustment layer we have been working on all along*, but changed the name to "Corner CA" so that I wouldn't confuse it with the already existing adjustment layer. I selected "Master" this time, which effects all colors, but only took the slider down to -30 as opposed to -100.

7) Next the disaster happened. I was going to remove the halo seen around the coral (the actual CA), and then restore to color in the background, but during the selection process the program froze. Because I was going to delete everything as soon as I was done anyway, I never saved my work. Except for the screen shots I took, all is lost. Phooy.

Even though I didn't finish this pet project (for reasons out of my control), I decided to post it anyway. I hope I took it far enough for you to learn something from it. Oh and i'm sorry the screen shots are bigger. I screwed that up to.

 

[nickb]

H@rry, I am confused by what you are referring to in your first post when you talk about a 'blue cast'. In your first image, several branches of the coral have sections which are blue color (the whole branch is blue). These are in sections which might be considered 'shadow' zones. Is that what you are referring to as CA?

I'm also puzzled as to why you are using extension tubes with the Nikkor 105mm which is already a macro lens.

 

[H@rry]

I'm using the tube to get a closer/bigger macro image.

Maybe I'm just overreacting to this whole thing. The color should be purple and not blue and certainly not that glaring. After reading TitusvileSurfer's post I tried to monkey with the saturation and hue on the blue scale and it comes around pretty well. I'll work with that a little more. I just thought that I shouldn't have to do all that.

What was surprising/disappointing to me was that the JPGs come out better than the RAWs right out of the camera.

 

[TitusvileSurfer]

The JPEGs are SUPPOSED to come out better than the RAWs right out of the camera. JPEGs have already been edited. Already been white balanced. Already been sharpened. Already been saturated. Already been everything. RAWs have nothing done to them at all. A RAW image is a blank editing canvas allowing you (the artist) to fill in your own coloring book after you decide what the outlines look like by taking the picture. It leaves the decision of what is best to tinker with in your hands for later, rather than the camera's for the here and now. The whole point of RAW is that you can take the time and with your far superior human brain and decide for yourself what should be what. With JPEG, the camera calls the processing shots in 1/100000 of a second. It is of course processed though (for better or worse). I would hope it would be for the better at least a little bit. If a RAW photo looks better than the same photo but in camera JPEG, that means the camera's editing actually hurt the image more than it helped it.

Think of it like ordering a stake in a restaurant vs. making it yourself. You forgot to tell the waiter how you want it cooked though. So the chef makes the stake medium well with peppercorn (which you don't like) and adds too much salt. You wanted the stake medium rare with sauted mushrooms. With a RAW image you start with the exact same stake the chef in the restaurant started with. Now your cooking it on your own grill on your own terms. You make that stake medium rare and you saute your mushrooms with a little red wine. You do the work yourself, but the final product suits your taste much better (assuming you know how to cook i.e. adjust the image yourself).

 

[nickb]

Is your lens the Nikkor 105mm f/2.8 VR Micro? If so, it can get down to full macro size (1:1 magnification) without needing any extension rings. The size of the image you posted is a long way from 1:1 magnification. The 105 micro is also one of Nikon's sharpest lenses and one which has extremely low CA (see here ). The older, non-VR version is also extremely good (see here).

As you describe it, I don't think that the blue vs purple coloration is CA. It more likely an issue with White Balance. There also appears to be a lack of shaprness, perhaps indicating a focusing issue.

JPGs are going to look better than RAW right out of the camera. The camera applies a lot of post-processing when making a JPEG. The RAW is literally that: the RAW pixel exposures. It provides you with a lot more opportunity to do post-processing. But, with no PP, the image won't look as good as the JPEG.

 

[H@rry]

Alright, now I'm confused again. The lens is Nikkor 105mm f/2.8 VR Micro. I just got it a few weeks ago from B&H.

Please bear with me and "lead me from the darkness into the light"! How can you tell it's not 1:1? What could I have done to prevent it from being 1:1? What can I do now?

 

[TitusvileSurfer]

You want the image to be as close to 1:1 as possible, which is the whole point of a macro lens to begin with. We can tell it is not 1:1 because the polyps (likely) wouldn't be too much smaller than my Photoshop example. We are assuming you are using extension tubes to get a "bigger macro image". The lens without tubes is already capable of 1:1 macro. The extension tubes allow you to merge the subject and lens even closer than before, in effect creating a 1.5:1 or 2:1 or even greater macro. If you aren't even achieving 1:1, tubes are only hurting you as they sacrifice light when used. His point is that you are literally sacrificing the light, since you are (apparently) receiving no benefit from them anyway.

 

[nickb]

As often happens, Titusville beat me to the submit button. I echo everything he said and add:

That's an excellent lens which can give 1:1 magnification with high sharpness and no CA. Good choice You should be able to take great images with the camera/lens combination you have.

 

[H@rry]

So are you guys saying the tube is what is preventing me from achieving 1:1? What else can I do to achieve 1:1?

Sorry to be so much of a pain, but I'm missing something here.

 

[nickb]

No. The tube is letting go beyond 1:1. Just remove the tube and the lens will do fine to give you 1:1 magnification.

 

[TitusvileSurfer]

Yeah what he said. The tube lets you go beyond 1:1...but if you aren't actually going beyond 1:1 its only holding you back. Is your lens so close to the coral that no matter how hard you try, you can't focus without the tube? If not don't use it. If you can't focus without the tube, then do use it.

 

[H@rry]

OK, here's a couple without the tube. The first one is the RAW converted to JPG and resized and the second one was "processed". Does this look more like 1:1?

 

[TitusvileSurfer]

Yeah that looks like it might be 1:1. Basically if you got the lens as close as it possibly could to the coral (being blocked by the glass doesn't count), you'll hit 1:1. If you can't go any closer for focus reasons but still have room between the lens and the glass, that is the time to use the tubes.

 

[Iostream]

White balance in the aquarium is a difficult beast. You can fix it in post processing, but to be exact you will need a gray card, or even better an expodisc. Using the expodisc I the camera will even set perfect white balance under pure actinics.

 

[H@rry]

Gotcha, I think I understand the tube thing now.

I'm not real satisfied that this pic is the way I want to display this coral. As I look at the coral very closely with my "naked eye" I notice that I do see the sharp break in color between the purple and the amber. The whole "stalk" on the right side is about 3/4 inch and when blown up this is readily apparent. I'm thinking it may be more visually appealing if I back off a little bit so the sharp boundary between the colors doesn't show up so well. It almost looks fake like it was "colored over".

Thanx guys for helping me understand the extension tube thing. I don't think I'll be using them again.

 

[TitusvileSurfer]

Nothing a little Photoshop can't fix.