Steve

I own a Sigma 18-125mm f/3.5-5.6 DC, I use it on a Nikon D80 and have found that at 18mm I get very bad veneting in the corners. Is this normal for this lens or did I get a dud lens? This is what I mean. This is the original full size and no Photoshop work done

Charles

I own a Sigma 18-125mm f/3.5-5.6 DC, I use it on a Nikon D80 and have found that at 18mm I get very bad veneting in the corners. Is this normal for this lens or did I get a dud lens? This is what I mean. This is the original full size and no Photoshop work done

Charles –

Great to hear from you!

Yes, vignetting is a fact of life on all lenses to some degree and this shot appears to have it at the top to a very small degree, and on the bottom more.. which I think is because the flash is unevenly covering the subject.  This will be more apparent at wider apertures and wider focal lengths (your sample was shot at 21mm and wide open at F3.5).  More expensive lenses ‘should’ exhibit vignetting to a lesser degree.  In any case this sort of thing is easily correctable in post processing.  What program do you use for post processing?

Looking at your image I’d be more concerned with the sharpness issue.  Wide open the lens appears to be about average for its price range, but possibly it would give you a sharper image if you were in single point AF and placed the AF point on her eye.  If the eyes are sharp the rest of the image will always look right.  With a decent lens you should be able to see definition in the eyebrows and eyelashes - they should be clearly definable.. and a great lens very detailed.  In this shot the eyebrows blur together and the eyelashes muddled.  This could be the lens, or it could be your focal point was slightly off.

Also, it appears you were using the popup flash at a very close distance.. resulting in a bright top half, and a less bright bottom half.  Try backing up a bit, using a longer focal length, and the popup flash will give you better coverage.  Popup flashes are never great, but they can be manipulated for decent results.

I hope this helps.

Take care

Steve

 

 

Steve

I shot from about 2 meters away, I do have a proper flash but was too lazy to use it that night. But even in daylight I get the same thing. Is 5.6 a good F stop to use for flash? I use Photoshop 9.

Thank you.

Charles

 

Charles –

Two meters away for the focal length indicated seems about right.  This is very close for a popup flash and explains the brighter top half of the frame.  Flash, even a nice external flash, is always difficult to use for the best effect.

Yes, you’d get the same vignetting even in daylight.  Vignetting is caused by inherent defects in the lens.. usually through design and is linked to barrel distortion.  The more barrel distortion you have (common at wider angles) the more vignetting.  Better lenses will do this less, but with today’s easy fixes in post-processing even an inexpensive lens can be easily corrected.

In Photoshop go to “filters”, then “distort” and then “lens correction.”  You’ll see the vignetting slider about ¾ of the way down.  Adjust until gone.

Flash and f-stop aren’t really related where it comes to vignetting.  They are related where it comes to distance vs. available light vs. flash power.  Aperture (F-stop) controls how much light is let through the lens.  The smaller the number (F3.5 in your case) the more light it lets in.  The bigger the number (F11-F16) the smaller the aperture and the less light it lets in.  A smaller aperture generally means the lens will be operating in the sharper part of its range and will have more depth of field, but it also means you’ll need more light for proper exposure.  If close to the subject and the flash has the power then great, the only downside would be the more power the flash uses the more recycle time is needed until the next picture can be taken.  But.. the farther you get from the subject and the more power you need.. you might exceed the output capabilities of your flash.. and to compensate you can then open up the aperture (smaller number) to let in more light.

There is a lot of theory to this stuff, much to understand.  You might want to try using your Program mode and letting the camera handle the settings for you and then correcting in post processing any vignetting there might be.

I hope this helps.

Steve
 

Steve

Thank you very much for your help in regards to Vignetting. I have attached 2 version of a photo for you, the first is the original and the second is with same but with the Vignetting fixed.

Charles

Thank you very much for your help in regards to Vignetting. I have attached 2 version of a photo for you, the first is the original and the second is with same but with the Vignetting fixed.

Please submit your questions to QandA@Bkkimages.com   All questions will be answered and most will show up in the weekly column.

Please submit your questions to QandA@Bkkimages.com   All questions will be answered and most will show up in the weekly column.