Exposure and ISO

Steve on the 350d mailing list asked me "When/why would you change the ISO No.?" I thought the answer I gave him will be useful for more people, so putting it here:

ISO number is the third dimention in controlling exposure (first two being aperture and shutter speed). Consider the scenario:
1. You want a loooooooong depth of field since you are shooting a lush green field. So, you have set your aperture to, say f22. With f22 (which is a very small aperture), you have very little light entering the camera. So, you have t have a loooong shutter speed. BUT!!!! you realize you have forgotten your tripod at the hotel. So, there is no way you can hand-hold the camera at that slow a shutter speed and avoid camera shake. So, you nudge up your iso to , say 1600, which will allow you to have faster shutter speed at the same aperture level while keeping the same exposure.

2. You want a very short depth of field (and a larger aperture), WHILE keeping a slower shutter speed (to get the sense of motion, maybe?). However, there is a lot of light and you run the risk of over-exposing your photo. So, you nudge the ISO sensitivity lower, so that you can use slower shutter speed as well as larger aperture.

If you look at those charts (ev charts, in my post HERE ) , they are for ISO 100 speed. You'll have to chuck them down onfe fstop for each ISO level increase.

Also, remember, whith low light, if you use really high ISO settings (say 1600), you have a chance of getting high noise in the picture. So, I'd recommend using upto ISO800 for critical shots.

Hope it helps someone!!

Boot & Man

This was shot at San Francisco

Yosemite Trip and Photo disaster

The reason for my not posting anything for a long time is the disaster in the name of photography I had at Yosemite. Considering I can hold the camera steadily, I went for a fery low aperture (f32) for most of the landscape photos in Av mode. That translated into a very slow shutter speed, which went against the rule of 1/focal length. Result is a bunch of blurry images. I should've cranked up the ISO to 1600 or 3200 to have reasonable shutter speeds in these cases.

Some of the photos came reasonably well though. Especially the following shots:

	While driving through Glacier Point Road, we came across this small secluded meadow, which was absolutely green, and full of colorful wildflowers. The bees were having a pretty good time.
This was shot next to the valley loop road.
	View of Nevada Falls (Upper one) and Vernal Falls (Lower one) from Glacier Point
View of Nevada Falls from Glacier Point

Rebel XT and Dust on Sensor

Dust on sensor can produce spots on your photographs, especially if it's shot with a high DOF (high f-stop value/smaller aparture). They generally become visible in one-color section of photos (like the blue sky). There was a nice explanation why we don't see this problem on film cameras, and why it is Digital SLR specific problem:

It is something that just comes with photography when you change lenses.
In the old days we send the film with dust particles and all to the
development centre. When you loaded a new film in the cam the new film
started to collect dust all over again.

These days we don't replace film anymore, so the sensor accumulates the dust
over time.

Although I haven't had this problem, I'm anticipating it soon. More so since I change lenses more often, and shoot at outdoors more often. Recently there has been a discussion going on the 350d yahoo groups and someone suggested the following steps to clean a sensor. There are suggestions for this all over the web, but this is a first hand account of sensor cleaning:

1. took a test shot for reference, so that you can see if your cleaning action moved, added or removed contamination
2. used the Giotto Rocket blower to clean the mirror box and sensor
3. took a test shot, compared to 1, noticed that amount of dust particles wast decreased but some remained
4. used the sensorbrush
5. took test shot, compared to 3, noticed that amount of dust particles wast decreased but some remained
6. repeated 4 and 5 until no further improvement was possible with the brush. Some 'welded' spots remained
7. used sensorclean (the fluid cleaner)
8. took test shot, compared to last test shot and noticed that most dust is gone except for a few die-hard minor spots.

Shooting Moon

On the last Full Moon day, I tried my hands on shooting the Moon. Having no access to expensive Astro-photography equipment, I just mounted my Rebel XT with the canon 75-300mm USM on my Hakuba S4500 tripod. I first used the camera to meter the moon, and then scaled it down about 2 f-stops. The moon was still way over-exposed:

Then I started playing around with the aperture and shutter speeds and got some more reasonably decent shots. Check them out in my moon album.

They are not absolutely perfect. But then, I didn't have a L series lens at my disposal. Things learnt:
Good thing:
1. I used a tripod, and used the timer instead of pressing the shutter myself. That saved some camera shake in such long exposures.
Things I missed:
1. Unforgivable. I forgot to set the RAW mode and was shooting JPEG.
2. I should have used MLU (Mirror Lock Up).
While discussing this, someone pointed me to a neat tool named Registax. Check it out. Potentially you can take more than one shots and stack them up to increase sharpness!! Will have to try it sometime.
This is the e-mail I got from Steve Sprengel:

Besides MLU and a more steady tripod configuration, try a little faster
shutter speed, higher f-stop, and STACK 10+ images using Registax so
you'll be able to sharpen things up quite a bit after reducing the noise
via stacking:

http://www.pbase.com/ssprengel/image/39049737


The maximum number of images in the stack will be limited to about two
minutes worth, before the moon has rotated too much for the features to
be aligned between the beginning and the end frames of the sequence.

Keep in mind that the noise is reduced by the squareroot of the number
of images, so to reduce the noise by half you'll have to stack four
images, to reduce it by one fourth you'll have to stack 16.

The other thing to understand that when stacking images of a relatively
large object like the moon, the features will shift with respect to
each other due to atmospheric distortions, so a winter night moon high
in the sky will be much better than a summer night moon low in the sky.

Some interesting info on shooting moon HERE