# Just bought an XSi , have some questions



## LCARSx32 (Sep 1, 2011)

I just picked up an XSi (450D) on ebay for $295.  It should be a considerable upgrade from my current 300D.  I'd like to know how the 450D does with long exposures (star trails, mostly).  My first gen 300D is understandably noisy when exposures are longer than 10-15 seconds.  Should the 450D handle long exposures better?  I know from reviews that it handles high ISO better, but I'm not sure if high ISO handling translates to good long exposures.

Also, is there anything I should know about?  I just read another post about how the XS needs a firmware update to fix a potentially camera-bricking flaw.  I've checked the firmware updates and they seem to fix minor things so I think I'm ok there.  I just want to make sure there isn't something else.

Can't wait for it to get here!


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## PJL (Sep 1, 2011)

LCARSx32 said:


> I just picked up an XSi (450D) on ebay for $295. It should be a considerable upgrade from my current 300D. I'd like to know how the 450D does with long exposures (star trails, mostly). My first gen 300D is understandably noisy when exposures are longer than 10-15 seconds. Should the 450D handle long exposures better? I know from reviews that it handles high ISO better, but I'm not sure if high ISO handling translates to good long exposures.


You should not be using high ISO numbers when doing long exposures. In fact, you should be using the lowest ISO possible. If you've been using higher ISOs, that might explain the noise you've been experiencing.


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## McNugget801 (Sep 1, 2011)

PJL said:


> You should not be using high ISO numbers when doing long exposures. In fact, you should be using the lowest ISO possible. If you've been using higher ISOs, that might explain the noise you've been experiencing.



Thats not true at all,  High ISO star trails are the best.
High ISO + wide open aperture = more stars


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## LCARSx32 (Sep 1, 2011)

Sorry, I didn't make that more obvious...  I shoot long exposures at ISO100-400.  I was just saying that my 300D is noisy at low ISO, long exposures AND high ISO, short exposures.  So if the XSi is lower noise at high ISO, I'm assuming it would be lower noise at low ISO, long exposures.

I'd like to know if that assumption is correct.


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## KmH (Sep 2, 2011)

Any digital camera is noisy if there is little light, because of the way digital photographs work.

The pixels that record the low light levels have little signal to offset the shot and thermal noise the image sensor generates. in othe words those pixels have a low SNR - Signal-To-Noise-Ratio.

Fully 1/2 of the luminance data a digital image has, is contained in the brightest stop of exposure. Each dimmer stop of exposure has only 1/2 of the luminance data left by the previous brighter stop of exposure.

For example, suppose your image sensor can record 6 stops of exposure, and the brightest stop has a value of 4096. By the above progression the darkest stop of exposure has a value of 64.

4096 - 2048 - 1024 - 512 - 256 - 128 - 64.

Using the lowest native ISO heats up the image sensor less during long exposures than using a higher ISO that makes the signal amplifiers work harder and get hotter.

By the same token, CMOS image sensor heat up less than CCD image sensors do, because CMOS uses less power, which generates less heat.

Many DSLR cameas offer a long exposure noise reduction feature, but if you take a 600 second exposure, the Long exposure NR takes an additional 600 seconds after the exposure is stopped.


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## LCARSx32 (Sep 2, 2011)

Hey KmH,

Thanks for the info.  Do you know if the XSi handles this better than a 300D?   Will I notice a difference?  I guess I'll find out tomorrow night when it gets here


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