Showing posts with label aberration. Show all posts
Showing posts with label aberration. Show all posts

Thursday, March 22, 2012

What causes longitudinal chromatic aberration on Nikon 50mm f/1.4D?

Question

I have looked on my new 50mm lens photo (I have Nikon D90) and I see blue contour, blue highlight around the unfocused objects (longitudinal chromatic aberration). What's is going on? Is it common for this lens (Nikon 50mm f/1.4D) or I did something wrong?

enter image description here

Asked by garik

Answer

What you're seeing is longitudinal chromatic aberration (otherwise known as axial colour), whereby the light at the edge of the bokeh disc undergoes a colour shift depending on whether it is in front of or behind plane of focus. The reason for this is that light of different wavelengths focus at different distances along the axis of the lens.

This is very common of ultrafast lenses like your 50 f/1.4 and is exacerbated by strong contrasts such as shooting against the open sky. Unlike Lateral Chromatic Aberration (where colours are shifted outward regardless of depth) which can be fixed easily in software, occurrence of Longitudinal Chromatic aberration depends on depth information, which is absent from the image. As such it is more difficult to correct automatically (though some software such as Capture NX2 and DXO offer this feature). It does however reduce rapidly on stopping down.

In a nutshell there's nothing wrong with the lens, it's just this type of subject/shooting environment is not ideal for f/1.4 lenses.

Answered by Matt Grum

Longitudinal chromatic aberration on Nikon 50mm f/1.4D

Question

I have looked on my new 50mm lens photo (I have Nikon D90) and I see blue contour, blue highlight around the unfocused objects. What's is going on. Is it common for this lens (Nikon 50mm f/1.4D) or I did something wrong?

enter image description here

Asked by garik

Answer

What you're seeing is longitudinal chromatic aberration (otherwise known as axial colour), whereby the light at the edge of the bokeh disc undergoes a colour shift depending on whether it is in front of or behind plane of focus. The reason for this is that light of different wavelengths focus at different distances along the axis of the lens.

This is very common of ultrafast lenses like your 50 f/1.4 and is exacerbated by strong contrasts such as shooting against the open sky. Unlike Lateral Chromatic Aberration (where colours are shifted outward regardless of depth) which can be fixed easily in software, occurrence of Longitudinal Chromatic aberration depends on depth information, which is absent from the image. As such it is more difficult to correct automatically (though some software such as Capture NX2 and DXO offer this feature). It does however reduce rapidly on stopping down.

In a nutshell there's nothing wrong with the lens, it's just this type of subject/shooting environment is not ideal for f/1.4 lenses.

Answered by Matt Grum

Saturday, February 11, 2012

What is the meaning of these Photodune reasons for rejection?

Question

I wanted some advice, I am new to photography, I posted an image on Photodune for sale and it was rejected because of:

  • Element or framing composition
  • Noise, exposure, lighting or saturation
  • Chromatic aberration, moirĂ© pattern or other image quality issues

This is the photo in question.

enter image description here

Answer

I see five things wrong. Three are compositional:

  • The bowl should be centered vertically (its horizontal placement is good), with room enough to bleed the image and still have the whole bowl showing. Your margin at the top is good, but the bowl touches the bottom, is partially cut off (by about four pixels) and there's no room to print the image full-bleed. That means it has very limited editorial usefulness.

  • The counter/table should extend all the way to the bottom of the picture. Having the bowl hanging over the edge means that there's a black void at the bottom, and that's not nearly as attractive as having the granite texture extend throughout the picture. The edge of the granite is also not square with the edge of the frame, and even if you had included the blackness on purpose, it is absolutely essential that graphic elements are placed deliberately -- either keep them straight or make them leading lines.

  • Similarly, the upper chopstick just misses being a proper diagonal leading line. If it had hit the exact corner of the frame, it would have been a great element, but it just misses the mark. You have to get these things bang on, and sometimes that means creating a series of very similar images that work properly at different crops. Make yourself familiar with the proportions of full-page, page-and-a-half and double-truck layouts in trade magazines -- those proportions will make a difference to your sales.

The fourth problem is noise -- it's very noticeable in the midtones in the upper interior of the bowl. The textures everywhere else make the noise invisible, but when you have an area of flat colour (or a smooth gradient) with little or no texture, noise stands out like a sore thumb. De-noising the image isn't optional.

Finally, there are artifacts from sharpening, especially along the upper edge of the chopsticks and the upper-left edge of the bowl. That may register to some as lateral chromatic aberration, but it looks a whole lot more like an unsharp masking artifact to me -- those particular edges were already prominent, and trying to force them created a one-pixel outline.

Both of those last two can be fixed. You may not want to change the process for the entire image, but that's why we have layers and layer masks. For instance, running a noise filter over the entire image may add to the minor depth-of-field problem at the top of the noodles and make the granite look like a granite-printed laminate. So run the filter against a copy of the image on a new layer, then mask out everything except the bowl.

The same thing goes with the sharpening -- the rest of the image may need it, but you ought to mask it out where it's creating artifacts, or at least sharpen that area of the picture to a much lesser degree.

I don't see any problems with your overall exposure or saturation, by the way -- the only things clipping are specular highlights, and they're supposed to do that.

The stock market is hard, and excellence is the entry level. There's no room at all for "okay", "good enough" or even merely "really good". Real sales start at "spectacular", and that means getting absolutely everything right to begin with before you add your special sauce.

Answered by Stan Rogers

Monday, January 16, 2012

Why is there a strange white spot on all of photos from my Zenit-B SLR?

Question

I've taken some photos with my old Zenit-B camera. And I have some strange white spot on all of the photos in the upper right corner. Sometimes it is not too recognizable, but it is everywhere.

spot on a normal photo spot on a dark photo sometimes it is not recognizable

It looks like condensate, but I can't see anything like that on the lenses when I open camera. Also it is quite strange that this spot even appeared on absolutely dark photo.

Does anybody know what could it be and how could it be fixed?

Answer

I would say light leakage and possibly internal reflections of some sort.

Considering you get a "bright patch" in the top right, I would highly suspect there is light leaking in.

The "dots" look a lot like reflections to me. In the lens, on a piece - anything that is shiny and reflects light.

I'd say start by checking whether the back closes properly. Next check if you have light leakage from the viewfinder.

Lastly, have a good look if you can see a reason for reflections in the camera.

If none of the three turn up something, try a different lens and see if the issue still occurs. If no, then it is the lens, otherwise... I'm out of ideas...

A last common option for failure would be the shutter, BUT that tends to look rather different.

Saturday, December 24, 2011

Is it technically possible to build a camera body to correct for lens defects?

Question

Most lenses tend to be expensive because the lens needs to be very complicated in order to correct for various optical effects (aberration, etc). Is it possible to build a sensor and processor combination which would fix defects produced by cheap lenses? I'm assuming the processor knows exactly the parameters of the len's defects.

Answer

Not only is it possible, but it's becoming commonplace. The micro-four-thirds system makes extensive use of it, and some compact cameras now do too. (I imagine that if they don't yet, most super-zooms will within a few years.) Digital Photography Review has a good article on this at http://www.dpreview.com/articles/distortion/ , and it's worth reading even if you don't like that site's reviews in general.

Correcting distortion and lateral chromatic aberration is an option in my Pentax K-7, but although it doesn't impact shot-to-shot time (unless you fill the buffer), it takes a few extra seconds per image.