Higher and Lower Sensitivity

Digital cameras have developed over recent years to give the user an ever increasing armoury of features to use. Chief among these is the ability to control one’s ISO (what used to be ASA in the days of 35mm film cameras). This gives me the ability to change my ISO mid-shoot (make it more or less sensitive to light), thereby controlling my ability to select an appropriate shutter speed or aperture for the situation. It is especially useful in low-light conditions.

This exercise required me to experiment with different ISO settings and accordingly different speed and aperture settings. I needed a scene which I could photograph both at ISO100 (least sensitive) and also at a higher ISO (with the sensor becoming more sensitive). I chose a crowded street scene in the morning knowing that it was just bright enough for me to take photographs at a fast enough shutter speed but also dark enough for me to be able to increase my ISO at will without the danger of over exposing the scene.

I started at ISO100 and took several shots at varying shutter speeds and apertures.

My f-stop is wide open at f/4 (my max for this lens) and the shutter speeds are at around the area that I am happy taking photographs without gaining excessive camera shake or motion blur. At 1/80 sec I am at the lower end of the shutter speed I would be happy shooting with in these circumstances. The bike rider in the background is quite blurred as is movement in the frame in general. ISO100 is the lower limit for this scene and it could not have afforded to be any darker without me having to either shoot at lower shutter speeds or increase my ISO.

I changed up to ISO400 making the sensor 4 times as sensitive to light. This enabled me to up my shutter speed meaning that my motion blur was reduced dramatically. I have also shut down slightly on my aperture giving me a bit more depth of focus.

I doubled the ISO to 800 and in aperture mode varied my depth of focus from f5.6 to f/9. Through this range my shutter speed remained high and indeed I could have closed the aperture down even more and still kept the shutter speed up. For a street scene like this a good depth of field would be my choice over a lower ISO.

Again I doubled my ISO giving me a massive range of shutter speeds and apertures to play with in the given circumstances. Even at f/11 I am still able to shoot in a range between 1/250-1/320 sec. At f/4 I am starting to be in danger of overexposing the scene as my shutter speed hits 1/2500 sec (my camera maxes out at 1/4000) and wouldn’t need much more light coming in for this to happen without closing down the aperture.

As the ISO sensitivity is increased I have more flexibility in the types of settings I am able use within the lighting constraints. This is a great asset to a photographer. However, with increased sensitivity comes increased noise (which I covered in a previous post for Digital Photographic Practice). The images below are blown up from the originals to demonstrate the difference I can expect to experience as the ISO sensitivity climbs.





When comparing the images side-by-side like this it is easy to see the noise starting to make an appearance at ISO1600 in the more shadowed areas, especially the green horizontal shape above the bottle. The longer I look the more noise I can start to see. At ISO100 there is barely any although the temperature (around 33C) at the time also plays a factor in the amount of noise the sensor will pick up on. Please click on the images to view them at 100% to get a better view of the noise I mention. In fact, ISO1600, is not a particularly high ISO these days with some cameras being able to surpass ISO204,800. At this point you will most certainly be able to see the noise. However, the tradeoff is that you will be able to shoot in near darkness. So shooting at the lowest ISO is preferable from the point of view of getting a ‘clean’ image with minimal noise. However, such is the advancement in camera technology noise is becoming much less of an issue for photographers these days.


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