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Old 15th August 2011, 23:28   #8326
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Originally Posted by amitk26 View Post
OK let me tell the cure first which may be of more interest and then the cause of purple fringing
1. Avoid shooting very high contrast boundaries in bright sunlight. Like a subject wearing black and white striped shirt.

2. When you have high contrast boundaries like a dark subject against bright blue sky do not use open wide aperture/

3. Use post processing in GIMP / Photoshop to remove purple fringe if you can't avoid.


4.Make sure your subjects do not wear fluorescent / phosphorent print T Shirts even best lens can not avoid CA in bright sunlight. If someone needs proof I can post picture.

Now here is the cause :

Purple fringing ( and also green fringing ) is the result of chromatic aberration that is all the wavelengths do not converge at same point. It is actually property of the glass ( and not coating as some people think) ,
It can happen due to many reasons.

(a) You might remember from high-school physics that frequency of light is constant in every medium whereas speed changes. Now f = V/ wavelength thus the wavelength must change. So in denser medium ( glass) speed decreases thus to keep frequency same wavelength must decrease so in the captured image you see some color which were never there.

If the color is continuous you just do not perceive this change and make general comment the colors of XYZ lens are not vibrant as uber-expensive bla-bla lens but at the sharp contrast boundary you can detect this as purple or green fringe.
When you see fringe at the boundary it is this kind of CA.

(b) Many times lens fail to converge the light of different color at same point and thus you see fringing. ( reason is related to point (a) above itself)

(c) This one is not applicable to DSLR as built in fiters on sensor filter out IR and UV but in film the UV in spectrum shifts and gets captured.
Thank you for the explanation Amit. That kinda cleared it up in my head. Next question would be, why do I see fringing to be more pronounced with legacy lenses in high contrast situation than with the modern day lenses? Does that mean the quality of glass has either deteriorated or it just wasn't good enough to begin with?
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Old 15th August 2011, 23:35   #8327
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Thank you for the explanation Amit. That kinda cleared it up in my head. Next question would be, why do I see fringing to be more pronounced with legacy lenses in high contrast situation than with the modern day lenses? Does that mean the quality of glass has either deteriorated or it just wasn't good enough to begin with?
Well your observation is correct years of research on use of ultra low dispersion element , fluorite elements etc is aimed at just one point controlling Chromatic aberration. Actually before the color era it was not at all important to control chromatic aberration.

Legacy lenses are good in macro and normal but not for tele-photo because of this reason ( you are more likely to incur high contrast in bright sun for TF)

Another reason I can think is that even with legacy lenses you can not normally detect PF untill you are blowing the image to 100% , So before pixel-peeping era of digital no one actually saw fringing until it was really really drastic. but today first thing we see is to check the boundary at 100%.
If you carefully look at color film photograph you will observe that they were never so sharp ( you can't see eyelashes , just eye for example) and they always had small amount of fringe.

But all this is covered up today by nostalgia and famed film look , If you utter these things it will be blasphemy and film enthusiasts will bay for blood on online forums.

Last edited by amitk26 : 15th August 2011 at 23:41.
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Old 15th August 2011, 23:40   #8328
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Thank you for the explanation Amit. That kinda cleared it up in my head. Next question would be, why do I see fringing to be more pronounced with legacy lenses in high contrast situation than with the modern day lenses? Does that mean the quality of glass has either deteriorated or it just wasn't good enough to begin with?
Its a lot to do with the glass being used. I've had issues with it on some older lenses but negligible problem on the pro-grade glasses. Again, you may still see it in some cases when you blow it at 100%, but thats a problem only when you'd want a 18mp full size print

The hood saves it sometimes as well i feel, but the quality of glass is more predominant in causing/preventing the purple borders.
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Old 15th August 2011, 23:40   #8329
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Originally Posted by amitk26 View Post
Well your observation is correct years of research on use of ultra low dispersion element , fluorite elements etc is aimed at just one point controlling Chromatic aberration. Actually before the color era it was not at all important to control chromatic aberration.

Legacy lenses are good in macro and normal but not for tele-photo because of this reason ( you are more likely to incur high contrast in bright sun for TF)
Yeah, I figured if I made black and white images, CA wouldn't be a big problem. Hmm, I got Olympus OM 50mm f1.8 with 7mm, 14mm (2 of these) and 25mm extension tubes for macro work (for about $110). Next on my list is a canon FD 200mm f2.8 lens, but when I saw the sample images, it had high CA. Hence, I was wondering if it was worth the investment. That should clear it up, I'll get some extension tubes and use the 200mm for macro work as well. Thank you for your response .

Amol,
Thanks. Yeah, I don't see CA with my newer lenses as much as I do with legacy lenses.
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Old 16th August 2011, 09:46   #8330
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Originally Posted by amitk26 View Post
Well your observation is correct years of research on use of ultra low dispersion element , fluorite elements etc is aimed at just one point controlling Chromatic aberration. Actually before the color era it was not at all important to control chromatic aberration.
Actually this is not correct. Chromatic aberration causes fringes at the image edge, which are as annoying in B&W as in colour. A lot of research went into making the lenses with minimum chromatic aberration. Some of the classical low aberration lenses by Zeiss were designed to have a range from near UV to IR. The control of aberration was more important in the film era compared to digital era, as we can correct most of them in the computer, something not possible for lay public before the advent of PC.
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Old 16th August 2011, 10:59   #8331
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hi, my first post to the dslr thread.

I recently bought a DSLR, canon 1100d. I have noticed that most of the times i am missing the sharp focus on my photos. I am setting the focal points manually, when looking in the small lcd the photo looks sharp and nice but when viewed on a large monitor i can make out that its not in focus.

I checked in DPP and the focal point shows up where i intend it to but still the sharpness in the photo is not there. Any pointers are appreciated.

Please keep in mind that i am a novice in this subject and maybe missing some basics here.
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Old 16th August 2011, 11:23   #8332
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I don't own a tripod......Why can't we use a fixed 35 mm or a fixed 70 mm for portraits?....I was considering the fixed 50 mm 1.4 because I was hoping to avoid going beyond ISO 800 when I shoot people in the evening...... If I buy a 120-300 f/2.8 (assuming I can afford it), will I be missing out anything if I have nothing between that and the 18-55 kit? .....How good are the third party lenses as compared to Canon lenses in these things?
P.S. I wanted to ask you these question on facebook, Shaju, but I thought other members could benefit from this discussion.
Tripod is a one time investment that can last a life and I would say, get the habit of carrying it, you wont regret a bit. A carbon fiber will cost more but will ease your shoulder on a regular basis.

You can use fixed primes (50/85/105 etc) for portraits as the 1.4/1.8/2.8 apertures will bring very crisp and sharp images with perfect Bokeh.

Getting a 50mm 1.4 lens will not make any difference in shooting group of people in low light as you will be able to get only one person in focus. You will need f/5.6 or f/8 if the target is to get everyone in focus. Its better to invest in a good quality flash and bounce it towards the ceiling for such restaurant shoots. Even a kit lens will do very well if flash is used and you dont need to bump up the ISO too.

Now about the lens distance gap, its all in the mind. I have 20mm, 50mm and a 70-200. I dont even think about 10-19, 21-49 and 51-69 at any point of time But 55 to 120 might seem to be a wider gap, for that you can either get a 85mm prime or physically move ahead with the kit or backwards with the 120-300, hope I am not confusing you.

Third party lenses save a lot of money, you have Flickr albums where you can search for images shot by each lenses in consideration, its a good way to convince yourself about the lens in question.

Are we connected in FB ? Usernames here and actual names really confuse me everywhere !! Hope I have successfully added more items to your list
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Old 16th August 2011, 12:29   #8333
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Originally Posted by josh.machine View Post
hi, my first post to the dslr thread.

I recently bought a DSLR, canon 1100d. I have noticed that most of the times i am missing the sharp focus on my photos. I am setting the focal points manually, when looking in the small lcd the photo looks sharp and nice but when viewed on a large monitor i can make out that its not in focus.

I checked in DPP and the focal point shows up where i intend it to but still the sharpness in the photo is not there. Any pointers are appreciated.

Please keep in mind that i am a novice in this subject and maybe missing some basics here.
Check your shutter speeds
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Old 16th August 2011, 15:03   #8334
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Hi,

i use the aperture priority mode and it automatically selects the shutter speed. Shouldn't that work out ideally?
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Old 16th August 2011, 15:28   #8335
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Originally Posted by josh.machine View Post
hi, my first post to the dslr thread.

I recently bought a DSLR, canon 1100d. I have noticed that most of the times i am missing the sharp focus on my photos. I am setting the focal points manually, when looking in the small lcd the photo looks sharp and nice but when viewed on a large monitor i can make out that its not in focus.

I checked in DPP and the focal point shows up where i intend it to but still the sharpness in the photo is not there. Any pointers are appreciated.

Please keep in mind that i am a novice in this subject and maybe missing some basics here.
Hi Josh,

There's no clear answer to your problem, but I can help you with some questions that can help with understanding the issue.

- Are you shooting still or moving objects? This makes a big difference in getting sharp focus where you'd want it to be. Use AI focus or AI servo for moving objects.

- Are you using USM or HSM lenses or they are the older focusing mechanism? The lens speed in focusing makes a difference when you're shooting moving objects. I'm assuming you have the kit lens which isnt USM.

- Are all pictures looking out of focus and is it OOF by a big margin or its only visible when you blow the picture to 100%? There may be back focusing or front focusing issue with the lens, this need a lot more testing to exactly say it is. Lets keep this as the last option.

- Have you tried any test shots to understand if this is really a problem with the camera or yourself? Put the camera on a fixed solid surface (table etc) or on a tripod and let it focus on an object, then put it on timer and take the shot. See if this is also OOF, if not, the problem is with you . Learn how to hold your camera steady while you shoot, just google for techniques and you'll find plenty of advise.

Hope this helps.
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Old 16th August 2011, 15:40   #8336
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Hi Josh,

There's no clear answer to your problem, but I can help you with some questions that can help with understanding the issue.

- Are you shooting still or moving objects? This makes a big difference in getting sharp focus where you'd want it to be. Use AI focus or AI servo for moving objects.

I am shooting still subjects.

- Are you using USM or HSM lenses or they are the older focusing mechanism? The lens speed in focusing makes a difference when you're shooting moving objects. I'm assuming you have the kit lens which isnt USM.

I only have the kit lens.

- Are all pictures looking out of focus and is it OOF by a big margin or its only visible when you blow the picture to 100%? There may be back focusing or front focusing issue with the lens, this need a lot more testing to exactly say it is. Lets keep this as the last option.

It is only visible when the picture is 100%(but i think to the expert eye it may be visible right away) and not by a big margin but definitely you can make out that the focus is not where it was intended to be.

- Have you tried any test shots to understand if this is really a problem with the camera or yourself? Put the camera on a fixed solid surface (table etc) or on a tripod and let it focus on an object, then put it on timer and take the shot. See if this is also OOF, if not, the problem is with you . Learn how to hold your camera steady while you shoot, just google for techniques and you'll find plenty of advise.

I am trying very hard to keep the camera still but then again i will try my best. Most of the pics which have come OOF are the ones where i have tried taking photos of small objects like flowers.


Hope this helps.
Thanks for the reply, i have added my updates inline. Can i post a photo here or share the link to a photo?

Last edited by Technocrat : 17th August 2011 at 03:06. Reason: highlighted answers for better reading, thanks
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Old 16th August 2011, 15:55   #8337
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How do I eliminate the shadows cast by the lens hood? Whenever I have used the hood, I have noticed my corners to be blackened.
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Old 16th August 2011, 17:36   #8338
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How do I eliminate the shadows cast by the lens hood? Whenever I have used the hood, I have noticed my corners to be blackened.
There is only one solution: Take it off
Curious to know what lens though !
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Old 16th August 2011, 17:38   #8339
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Thanks for the reply, i have added my updates inline. Can i post a photo here or share the link to a photo?
I'd advise you not to look at 100% magnification all the time. It causes a lot of worry each time and while you never would print it at that resolution, the pleasure of making good images is lost. For now, just try resizing your images to 1600px on the longer side and see if the issue still bothers you, if not, you have a solution there.

You may also want to check if the lens perenially focuses wrongly by doing the simple "camera on timer test". If you are trying to get to very fine details with the kit lens, you are probably expecting too much of it. Always, know the limitation of your gear and then shoot with it, that ways you can compensate for its shortcomings by making adjustments while you shoot.

In your case, if you're shooting flowers, have them in a nicely lit place (eg. by a window). Make sure you have enough shutter speed for the shot. I saw you mentioned about using Av mode, but that doesnt guarantee you everything. You will continously have to keep checking the shutter speeds and for your lens, keep it atleast faster than 1/50s. You can always compensate the shutter with increased ISO, but that will also show noise deterioration in your final image.

As I said, the last thing could be that your lens has a focusing issue, but there is no micro adjustment provision in the 1100D so you basically need to sweat it out yourself.

Post a link showing a shot where you think its missed and also include the exif info on aperture, shutter and iso.
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Old 16th August 2011, 18:01   #8340
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I'd advise you not to look at 100% magnification all the time.
=================
Post a link showing a shot where you think its missed and also include the exif info on aperture, shutter and iso.
Hi,

I would try and resize the photos and see how they look. Will also try out Tv mode to set higher shutter speed.

Meanwhile the below link points to some photos that i have taken using my new dslr and their respective details are present as well if you select each photo individually.

aayushjoshi: Galleries: Digital Photography Review
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