How to check my SLR & lenses for dust/mark related probl

Joined: 04 Sep 2005 Posts: 12 Location: Nottingham, England

I just got my Canon 350D yesterday and am enjoying taking my first photographs with it. I took most yesterday evening at night and when they came out well i was happy with the results.

Today i've took some outside shots with varying success but at one point i noticed a few marks in the eyepiece, i wasn't sure if they were marks on the display (like how the focus points are etched on somewhere?) or on the lens etc.. obviously they're not on the sensor as i'll never get chance to see them apart from in images i've taken.

Anyways i started taking some shots of the the bright blue sky and then uploaded them onto my PC to investigate.. I wasn't really seeing marks in the same places but i did notice a small mark in the bottom left and top right of the frame.

I tried cleaning the front of the lens and its UV filter (the sales person recommended it to protect the front of the actual lens) and whilst doing it noticed a mark inside the lens. On closer examination i can see it on a middle piece of glass now and its quite significant, I think i'll be returning the lens to get it replaced with a defect free one.

Being a Sunday this wasn't going to happen in a hurry so i carried on taking more photos throughout the day. Only reviewing them tonight did i notice on the sky shots a small smear almost, in the centre of the frame.

Being in the centre and how it looks it doesn't seem unlikely to be caused by the mark that i will return the lens for (which is definately on the outer edge of the lens) so I am now keen to try and see if its still present and whether i can remove it by further cleaning the lens.

I also have access to a 2nd lens from my dads old canon SLR (his other 2 sigma lenses sadly not taking pictures, coming up with the error99 code) which i would like to try and see if the mark was still there, either proving its the lens or something in the camera (the sensor already?! at only 1 day old?)

The problem is with the hours i work (and the country i live in) i am going to struggle to have access in the week to bright blue skys so that i can see if the mark is still there.. I've already tried taking a photo of white wallpaper with flash, but its just picking up the detailed relief of the paper and i couldn't see the mark previewed on the camera.

Is there a standard technique for recreating that bright blue sky in the day to look for such problems? Any other pointers appreciated?

Also sorry i'll try to make my posts shorter in the future, i just wanted to be thorough :)

Premium Member Joined: 02 Apr 2005 Posts: 2020

jimbobuk wrote:

The problem is with the hours i work (and the country i live in) i am going to struggle to have access in the week to bright blue skys so that i can see if the mark is still there.. I've already tried taking a photo of white wallpaper with flash, but its just picking up the detailed relief of the paper and i couldn't see the mark previewed on the camera.

Is there a standard technique for recreating that bright blue sky in the day to look for such problems? Any other pointers appreciated?

Skip the flash.

1. Set your focus to infinity.

2. Set your exposure compensation to +1 (I've skipped this step and it still worked.)

3. Set your aperture to the smallest setting. (f22 is fine.)

4. Take a picture of a uniformly illuminated white surface. I've actually just pulled a couple of lamps together against a wall to get enough light.

If you see dark spots in that image, you've got sensor dust. It may or may not be a problem.

Posted: Mon Sep 05, 2005 4:45 am

HI Jim

It is very unlikely, that the marks you detected on your photographs are effects of dust or dirt inside your lens. But one cannot rule that possibility out completely. Dirt might be visible if it is placed at the mount-end of the lens, especially on the rear lens' surface. Otherwise it shouldn't be noticeable.

Also a bit of dust does not degrade image quality in general. To produce a measurable and perhaps noticeable decrease of contrast or sharpness, the dirt specks should cover more than perhaps 15 % of the lens's surface area - and that really is very much.

Nevertheless I would exchange the lens too, because any dirt inside the lens assembly shouldn't be there!

If the dirt specks are on the sensor - which obviously would be very visible, you can test that by taking a photograph without any lens at all. Just photograph a uniformly bright area with te lens detached and then go to your image processing software and look for dust. Increasing contrast heavily should make that visible.

regards, René

Posted: Mon Sep 05, 2005 12:48 pm

Make a ~ 0.5mm pinhole in a lens cap or body cap. Then take a photo of a well-illuminated piece of paper or wall; of course, you will need a very long exposure, unless you shoot right at a light bulb- that works too.!

In your pinhole photo, since the DOF is essentially infinite, you'll be able to see and dust/scratches etc., even on the mirror or sensor.

:) keithwms

Joined: 04 Sep 2005 Posts: 12 Location: Nottingham, England

Thanks for your replies guys.

I had a quick go at taking the lens off and shooting as René suggested (it was the easiest option to be able to try in a few spare minutes). The no lens shot had no problems in it at all, totally white.. is this "focusing" for want of a better word, with an infinite DOF as keithwms suggests or not, could this lead to you not seeing any problems or is it a good way of testing the sensor for any dust?

Cheers

Jim

Posted: Mon Sep 05, 2005 6:46 pm

All I can say is that I never would have believed how much crud was on my sensor until I used a pinhole!

Doesn't Canon have the capability to do dust-off photos? Nikons have a nice feature that you can take a "dust ref" photo, and then you can correct for sensor dust using that as a reference. In some cases it works quite well, but, of course, nothing beats a clean sensor.

Why don't you post an image, then we can make a better guess what ails you. But do try the pinhole, it will delight and amaze.

:) keithwms

Joined: 04 Sep 2005 Posts: 12 Location: Nottingham, England

I don't have any spare lens caps that i'd like to put a pinhole in at the moment..

I've took another set of sky shots today with a different lens.. they still exhibit this centre smear so its clear that its something in the camera itself.. the sensor no doubt..

I'm new here, but see that i can only put 2 pictures in a thread, and can't link to an external page/set of images..

Here are two crops of this centre smear.. its quite annoying on those shots, not really visible on "normal" shots

What do you think?

I'll be taking my whole set back to the shop on Saturday, perhaps to just get a new camera, perhaps they will have a go at cleaning it.. they'd previously said they wouldn't do the cleaning.. its less than a few days old when i spotted it.. i'd be happy to get a new one..