Why do images look brighter at high ISO?

[trevor_newman1]

<p>Hi all,<br>

I have noticed that higher ISOs do more than just add noise to my images. They make the scene brighter. I shoot in Aperture priority mode and I noticed that when I take a picture of the same scene at two equivalent but different exposures, the one at the higher iso comes out brighter and the colors look more vivid. I wish I had a sample image but unfortunately I left my card reader and usb cable at home. At iso 1600 whites generally look more washed out and skin tones tend to look more orange when compared to the same scene at ISO 200. I noticed this both in good light and low light. I was just wondering why this occurs. My hunch is that as the iso increases the dynamic range decreases so that by iso 1600 there are fewer colors being picked up by the sensor so the many colors of a scene tend to turn into one primary color or another. I just wonder why they get brighter. Any answers would be greatly appreciated. I don't think there is anything wrong with my camera (D90)... I just have an inquisitive mind =). Thanks all!</p>

[Ray House]

<p>Posted examples would be a great help for me. Maybe some of the 200ISO are slightly under exposed.</p>

[trevor_newman1]

<p>I will post some when I am home in a couple days. Thank you Ray =)</p>

[mike_cruk]

<p>Might be done intentionally to reduce the appearance of noise, which is most visible in shadows. Could be something non-linear in an amp somewhere.</p>

<p>I, too, would be interested to see some examples of this. If you are indeed correct, it would be an interesting phenomenon</p>

[jpo3136b]

<p>Are you solving for equivalency accurately? If you are not, then it would be easy to step in a third or two somewhere; that might account for the increased brightness. A trip-up spot might be shutters in thirds against stops by factors of two.</p>

<p>If this were the error, then as you went from 200 to 1600, not only would you have hopped up through four different stops' worth of sensitivity, but also laid on 1/3 more exposure; maybe 2/3 or even 1+1/3 or 1+2/3, depending on how it started and ended. How much of an add-on by base of steps and stops would show up might depend on how in the step-stack the error occurred.</p>

<p>If you were solving for an exposure calc, and used but overlooked 1/3 steps in duration; then the settings would seem to the user that they were equivalent, but not be. There would be an odd third in there, unaccounted for in the user adjustments, but affecting the control of light entering the camera. This would show up as increased brightness.</p>

[martijn_houtman]

<p>In my experience, sensors lose their dynamic range, causing blow-outs in the lighter areas, giving more saturation. But that would not explain an overall increase in brightness, but rather at the lighter parts of the scene.</p>

[trevor_newman1]

<p>Hey all,<br>

Your responses are very interesting. I wish I could post some samples but I will post some on monday when I am able to download some images and I will post them on either this thread or another one. I think that John is onto something. With the nearly stepless shutters on these cameras the shutter speed and aperture indicated by the camera is not always the one that is actually used. When I say that the image is brighter I mean that the colors look more saturated. More like those out of camera jpgs from consumer point and shoots that make some scenes look almost comical in their candy colors. Also just for extra info I generally shoot fine jpegs using the vivid color setting on my D90. </p>

[thomas_hardy1]

<p>I've noticed that the whole scene becomes brighter, but not in a good way ( no gain in saturation, just washed out). Let's say you're shooting at ISO 1600 in bright daylight, by forgetting to lower your ISO after being in a dark museum, cathedral etc...<br>

Here is an example of what I have done. Dark museum, high ISO, f/5.6 in aperture priority using a VR type lens. Go outside in bright daylight - and forget to switch to ISO 100. Sunny 16 says ISO 1600 needs f/16 at 1/1600. <br>

At f/11, shutter speed equals 1/3200 sec,<br>

f/8=1/6400 sec, <br>

f/5.6=1/12800 sec</p>

<p>I finally look at the lcd and notice that 30 minutes of shots are really bright, too bright. I figure that I just can't get shutter speeds fast enough to properly expose the sensor, and I rarely choose apertures smaller than f/8. </p>

<p>But then I think you may be talking about something different. </p>

 

[trevor_newman1]

<p>Thomas that is exactly what I am talking about! It always happens when I forget to change iso after a night of taking pictures or taking pictures inside. The pictures do look much more washed out. They lack the tonal depth of images at a lower ISO and look almost cartoonish. It always stumps me until I look at the LCD and realize I am shooting at iso 1600. It really only starts at ISO 1000 and above. In good light ISO 800 looks almost as good as 200 a real testament to how far sensor technology has come in the last few years.</p>