Team-BHP - The DSLR Thread

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Originally Posted by vina (Post 2353206)

One question[/b][/u]: for very long exposures (10min or more) in very low light, how do the sensors perform these days? Some years ago the dark current was big enough to mess up the pictures (compared to a film shoot).

Most of the sensors heat up while exposing. How much depends on their design and heat sinks used. That is why they cool the sensors for Astronomy. As the sensor heats up it generates "spurious data" - dark current. In advanced cameras there are two shots - one the normal and the next the "dark current" shot where the sensor is not exposed. Then the two shots are processed to get rid of dark current. Still there is a limit. The longest commercial camera uses a Medium Format P45+ back and can take clean pictures of around an hour, but at around zero degrees centigrade. The exposure will reduce to about 15 minutes at 30 degree.

You can use any camera to take long shots, as long as it is very-very cold (but then batteries would be the problem).

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Originally Posted by tsk1979 (Post 2353337)

I am not a beginner now. Yet most of my photographs, even good ones came from 18-55 non IS lens which is not as sharp as the new 18-55 IS.
If I just had the 50mm 1.8 I would have been severely limited in landscape photography
50mm 1.8 is 80mm on APS-C, which makes it very narrow for landscapes. Its ideal for potraits, and quite good for street photography

For at least twenty years in film era, all I had was the 50mm prime on my camera. As long as you know the lense, you can take all sorts of shots with it, provided you can move nearer or farther away from the scene. Obviously you can neither come too near a wild animal (hence telephoto), or too far away from a scene, if you hit a wall/obstacle behind you (hence wide angles), but for general photography that was all I had and I have thousands of snaps.

Though zooms are a boon, they suffer from some limitations
1. Given same build quality and optical quality, zooms will be at least three times more expensive compared to the prime lenses.
2. Zooms normally are a stop or two slower. If you want excellent low light shots, fast primes are at times the only way to go. That is why the Leica 55/f0.9 is so popular. You can take discrete shots in very low light without flash.

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Originally Posted by navin_bhp (Post 2353829)

Unfortunately, The two new copies of the lens I bought were not so sharp wide open. Hence my conclusion. I don't have the body or the lens now anyways. I used it with a 350D body long time back. I did get some amazing shots out of it at 2.5 and higher though.

There are a lot of articles and discussions on the net regarding quality issues of consumer lenses. It seems that the QA is neither thorough not 100%, resulting in wide variation lense to lense. Some of the issues are
1. Lense has too large a back focus or front focus
2. Lense assembled poorly, and elements not collimated, hence the lense will not focus across its area uniformly
3. Barrel material defective or the AF motor sluggish.

If you are particular about the quality, the best method is to patronise a shop where they have a lot of copies of the lense of your interest, and allow you to test them on your camera. If interested please look up the net for articles on how to test the lenses in the shop, and come out a winner.

:OT
With modern DSLR touching 15MP+, lense sharpness and distortions are not apparent in small prints - 5x3 or even 7x5, unless the lense is really bad. Further camera shake is prime culprit for less sharp shots than the lense it self. If you want to take a super sharp picture use
1. Sturdy Tripod
2. Windless day
3. Less than f/8 aperture, preferably f/4
4. Lower ISO - 100/200. Discernible noise usually creeps up after 400 ISO.

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Originally Posted by Aroy (Post 2354682)

Thanks.

I'm not sure why should the sensor heat up during exposure under low light conditions - there is almost no electrical activity on the sensors (CCD or CMOS, doesn't matter) and while some auxiliary circuits may be active even they wouldn't eat much power to cause any heating problems.

Also if the sensor did heat up, then the normal-shot + dark-shot method will be useless for calibration, the dark-shot will give an estmate of dark current only at a certain temperature and the current variation is highly non-linear (and unpredictable) with temperature. If the sensor does heat up then you'll probably have to go through the sequence -> dark-shot1 + normal shot + dark-shot2 and then estimate based on the two dark shots.

By the way, do the digital cameras come with a physical shutter? an electronic shutter will not be very useful for the purpose of the dark-shot above.

Good example of P45, thanks for that.

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Originally Posted by Aroy (Post 2354682)

:OT
With modern DSLR touching 15MP+, lense sharpness and distortions are not apparent in small prints - 5x3 or even 7x5, unless the lense is really bad. Further camera shake is prime culprit for less sharp shots than the lense it self. If you want to take a super sharp picture use
1. Sturdy Tripod
2. Windless day
3. Less than f/8 aperture, preferably f/4
4. Lower ISO - 100/200. Discernible noise usually creeps up after 400 ISO.

I'm a little confused here - once the focal plane (i.e. the image formed at the sensor/film) is already blurred by the lens, how will sensor resolution matter - I would have thought that no amount of sensor resolution will help that. Can you shed some light (pun unintended)?

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Originally Posted by vina (Post 2354806)

I could not understand your point. SO when sensor captures image, there is no activity?

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Also if the sensor did heat up, then the normal-shot + dark-shot method will be useless for calibration, the dark-shot will give an estmate of dark current only at a certain temperature and the current variation is highly non-linear (and unpredictable) with temperature. If the sensor does heat up then you'll probably have to go through the sequence -> dark-shot1 + normal shot + dark-shot2 and then estimate based on the two dark shots.

Actually amp glow problems cannot be fixed, but they are rare. However, as exposure time increases hot pixels increase. Dark frame noise reduction reduces hot pixels to a large extent. Most modern cameras come with option of doing dark frame reduction automatically with long exposures.

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By the way, do the digital cameras come with a physical shutter? an electronic shutter will not be very useful for the purpose of the dark-shot above.

DSLRs have a mechanical shutter

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Take an image taken with a not so sharp lens, view it at 100%. If sensor is 8MP or 20MP, both will show softness.
However, as you scale image to a size much smaller than what the sensor is capable off, your perceived sharpness will increase.

So a 5x7" print from a 20MP sensor will be cleaner than a 5x7" print from a 2MP sensor, because in case of a 2MP sensor, the 5x7" will be closer to a 100% crop.

Moreover, with more MP, you can print picture at 300DPI and still get a 8x10 print, however with less MP you may have to print at 150DPI, which will mean lesser quality

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Originally Posted by siddutta (Post 2355084)

btw , what is the difference between VR & IS . and if I have a option, what should i opt for ?
from nikon india website, Nikon3000 comes with AF-S 18-55mm VR Kit Lens

VR and IS are the same. VR is a terminology used by Nikon and IS by Canon. VR --> Vibration Reduction and IS --> Image Stabilization. Any lens which has this will provide compensation (to an extent) to camera shake if required. More useful on the long end of a zoom where even minor shakes can result in blurred pictures. A lens without IS/VR would require very steady hands to get a sharp picture.

I am not sure though of the utility it provides to the 18-55 lens as the zoom range is limited. That being said, I have an 18-55 IS lens myself :)

Gurus, correct me if I am stating something wrong

EDIT: Just saw tsk1979's post. It does help at slow shutter speeds. Have shot at upto 0.3 s shutter speed and have managed clear pictures.

Thanks Tanveer

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Originally Posted by tsk1979 (Post 2354890)

I could not understand your point. SO when sensor captures image, there is no activity?

AFAIK there is no activity on the sensing chip, in fact in the entire camera.

You will understand it easily (unless already know): The pixel sensor is just a reverse biased diode - every time a photon hits the reverse bias voltage goes down. Overall sensor is an array of pixel sensors + read circuit (row selectors, ADCs etc.).

During exposure, the read circuit is off completely. Dark current is the reverse saturation current of the pixel diode (and in CMOS sensors, the leakage current of the pixel selector and other transistors)

No power is expended in the process, there is nothing to heat the sensor up - dark current is so small it can not cause any appreciable heating. A bad sensor design may have toggling clock nets (though those are gated too) but even that is not much power.

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Originally Posted by tsk1979 (Post 2354890)

I didn't know about this - will have to read up. What is "amp glow" and what kind of behaviour do "hot pixels" exhibit?

Dark frame reduction will be a technique of calibrating dark current of each pixel and then subtracting the resulting spurious signal from proper exposure signal. It will still leave shot noise behind, and will double the inherent pixel noise (i.e. reduce the pixel resolution by one bit)

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Originally Posted by tsk1979 (Post 2354890)

DSLRs have a mechanical shutter

good to know - electronics shutter is not a good idea in general except for video.

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Originally Posted by tsk1979 (Post 2354890)

I'm not denying the advantages of higher resolution - e.g almost all algorithms you'll use for image distortion correction will benefit from higher spacial resolution. Was just trying to clarify whether softness present in the image plane can be reduced by using more pixels (i.e. is there some new technology built into the cameras).

In late 90s people were working on super-resolution (the sensors in those days had relatively poor resolution - both in MP and bits/pixel) where they would take multiple shots and then correlate them - worked on stationary objects very well and would help reduce equipment induced softness too. I was wondering whether that has found its way in commercial DSLRs.

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Originally Posted by vina (Post 2355185)

Thanks Tanveer

AFAIK there is no activity on the sensing chip, in fact in the entire camera.

You will understand it easily (unless already know): The pixel sensor is just a reverse biased diode - every time a photon hits the reverse bias voltage goes down. Overall sensor is an array of pixel sensors + read circuit (row selectors, ADCs etc.).

So when does the CMOS/CCD sensor show activity?

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During exposure, the read circuit is off completely. Dark current is the reverse saturation current of the pixel diode (and in CMOS sensors, the leakage current of the pixel selector and other transistors)

No power is expended in the process, there is nothing to heat the sensor up - dark current is so small it can not cause any appreciable heating. A bad sensor design may have toggling clock nets (though those are gated too) but even that is not much power.

When you open the shutter, the sensor cavities start collecting "photons". I am sure this process will require power consumption. I

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I didn't know about this - will have to read up. What is "amp glow" and what kind of behaviour do "hot pixels" exhibit?

Hot pixels are colored pixels which come on the picture. As you increase exposure time, their number increases. Amp glow is a purple/green tinge corners of some older sensors get. Google for images, you will find many

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Dark frame reduction will be a technique of calibrating dark current of each pixel and then subtracting the resulting spurious signal from proper exposure signal. It will still leave shot noise behind, and will double the inherent pixel noise (i.e. reduce the pixel resolution by one bit)

Dark frame simple takes and exposure with shutter closed. Its like shooting with lens cap on.
So you take a picture, lets say 30 seconds long. Then you close shutter and again take a picture for 30 seconds. Hot pixles from 2 are subtracted from 1 leading to a cleaner image.

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When you downscale a image, your "apparent" sharpness increases.
Take a P&S, and view the picture on a 15" laptop screen. It will look wonderful.
Now view it on a 60" TV, you will find it soft because its downsampled much less!