KevinL1011
Posts: 2858
Joined: Fri Mar 04, 2005 3:48 pm

Fundamental Question About Digital Images

Mon Mar 13, 2006 2:31 pm

I've been shooting with a digital SLR a little over a year now. So far the fundamentals of shooting film have applied to digital except there's one thing I don't understand.

With film, shutter speed and choice of film ASA affects image "saturation" and grain. More of a concern with monochrome film, low light / low contrast images or fast shutter speeds.

Does this apply to digital photography? Does a digital image get "grainier" as the ASA setting goes up? Does changing ASA effect the photocell's sensitivity or only boost the amplification?(gain) How does ASA affect contrast? Does choosing a higher ASA setting to achieve the fastest possible shutter speed degrade the image? Is there such a thing as Image Saturation with digital?

I've been a member of A.net over a year and this is my first post in this forum. I've shot motion, stills, film, video, belonged to IATSE for over 25 years and I consider this forum intimidating. Members who post here and members whose photos have been accepted by A.net make me feel like a beginner again. Very impressive work.

Thanks in advance for your help!
474218, Carl, You will be missed.
 
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JeffM
Posts: 7569
Joined: Sat May 07, 2005 3:32 am

RE: Fundamental Question About Digital Images

Mon Mar 13, 2006 2:36 pm

Welcome Kevin, and relax. You are far more experienced then most in here.

Quoting Kevinl1011 (Thread starter):
Does a digital image get "grainier" as the ASA setting goes up?

Yes, in most cases.

But you have a camera, you could answer all these questions yourself. ASA is not used....ISO is.

-Jeff
 
KevinL1011
Posts: 2858
Joined: Fri Mar 04, 2005 3:48 pm

RE: Fundamental Question About Digital Images

Mon Mar 13, 2006 2:53 pm

Quoting JeffM (Reply 1):
ASA is not used....ISO is.

Excuse me, thinking film again. Isn't ISO the digital equivalent of ASA? If so, does it have the same effect?
474218, Carl, You will be missed.
 
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JeffM
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RE: Fundamental Question About Digital Images

Mon Mar 13, 2006 3:07 pm

When increasing ISO sensitivity on a digital camera, the output of the sensor is amplified, so less light is needed. That also amplifies any undesired noise, this creates more grain in the picture, just like in conventional photography, but because of different reasons. It is similar to turning up the volume of a radio with poor reception. Doing so will not only amplify the music but also the hiss and crackle or "noise".

Your best bet Kevin is to try it yourself and see.
 
KevinL1011
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RE: Fundamental Question About Digital Images

Mon Mar 13, 2006 7:41 pm

Quoting JeffM (Reply 3):
Your best bet Kevin is to try it yourself and see.

I have, but the difference between say a 100 ISO setting and a 200 ISO setting does not appear as drastic as with film. I'd expect double the grain so I could work in half the light. Digital does not seem to react this way. And then there's contrast ratio. Film is about 1000:1 and video about 300:1. I think the human eye is around 800:1 , or somewhere between film and video. Where does a digital camera fit in?

Does a 7 megapixel camera show noticeably less grain than a 6 megapixel camera? Is a camera with more megapixels less sensitive to ISO change?

With video, bumping up the camera gain amplifies image and noise. You have already said It's the same for digital SLRs. Therefore, the relationship between sensitivity and noise when changing ISO should be more like video than film.

I guess what I'm asking is, with a digital SLR, should I employ the rules of exposure for video or film? It seems that changes in ISO on a digital camera does not appear to effect F-stop increment as much as film does.

BTW, my 15" monitor is set to 1680x1150. Maybe I just can't see the difference on this equipment?
474218, Carl, You will be missed.
 
javibi
Posts: 1295
Joined: Sat Oct 02, 2004 5:55 pm

RE: Fundamental Question About Digital Images

Mon Mar 13, 2006 8:45 pm

Quoting Kevinl1011 (Reply 4):
Does a 7 megapixel camera show noticeably less grain than a 6 megapixel camera? Is a camera with more megapixels less sensitive to ISO change?

Not necessarily, it'll depend on the sensor's "quality" and (something we sometimes forget) size (small digital cameras cameras usually have smaller sensors which usually produce much more noise when cranking the ISO that a camera with a bigger sensor, even if it may have less pixels).

Quoting Kevinl1011 (Reply 4):
It seems that changes in ISO on a digital camera does not appear to effect F-stop increment as much as film does.

AFAIK it should work the same way as in film, but there is no set standard between manufactures to define what ISO "n" is in a digital camera, so I guess you might perceive some small difference...

Cheers

j
 
IL76
Posts: 2237
Joined: Mon Jan 05, 2004 5:43 am

RE: Fundamental Question About Digital Images

Mon Mar 13, 2006 9:21 pm

Quoting Kevinl1011 (Reply 4):
Does a 7 megapixel camera show noticeably less grain than a 6 megapixel camera? Is a camera with more megapixels less sensitive to ISO change?

As Javier said, it has a lot to do with the size of the sensor. Basically like the size of the film. Large size film allows you to get a lot more quality/detail than a little APS film. In small sensors, like little point-and-shoot cameras have, the pixels are crammed into a smaller space than for example a DSLR. Because of putting so many pixels into a small space (every pixel is like an "electronic circuit") they tend to interfere with eachother. This interference causes noise. Leaving more space between the pixels and thus having a larger sensor reduces noise tremendously, but as a side effect results in bulkier cameras. You will see that the high-end digital cameras will have large sensors (full frame sometimes, the same size as 35mm film), not to mention the medium-format digital backs which have a rediculously high resolution and image quality (like the medium-format film cameras).
E
 
C133
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Joined: Wed Jan 12, 2005 7:34 am

RE: Fundamental Question About Digital Images

Tue Mar 14, 2006 12:38 am

All sensors are not created equal, either. Look at the camera review sites and I think you will see noise an important item in the review of all or most cameras. It's all about the electronics.

I believe that most photographers here always shoot with the lowest ISO possible for the conditions, to minimize noise.

Terry
Fine: Tax for doing wrong. Tax: Fine for doing well.
 
KevinL1011
Posts: 2858
Joined: Fri Mar 04, 2005 3:48 pm

RE: Fundamental Question About Digital Images

Tue Mar 14, 2006 5:48 am

Quoting IL76 (Reply 6):
As Javier said, it has a lot to do with the size of the sensor. Basically like the size of the film. Large size film allows you to get a lot more quality/detail than a little APS film.

I wondered about that but didn't want to confuse myself further. Does this also mean that what is considered as a "normal" focal length lense will vary from one DSLR to another? Or does the image plain remain the same and only pixel count differs?

I'm starting to think that I should treat exposure changes with repect to ISO and F-stop, a little more like video ( w/o shutter changes) and all else remains the same.

Reading other threads in this forum gives me impression that lense speed is as important (maybe more) than ISO setting. Of course I realize that a faster lense allows a lower ISO, but it seems that the lense grind is more important than ISO setting.  confused 
474218, Carl, You will be missed.
 
C133
Posts: 205
Joined: Wed Jan 12, 2005 7:34 am

RE: Fundamental Question About Digital Images

Tue Mar 14, 2006 7:43 am

Improved lens speed often goes with high quality, but speed isn't critical, most of the time. It's all about quality, quality, quality, on A.net.

T.
Fine: Tax for doing wrong. Tax: Fine for doing well.

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