Of Storage and ISO!

When you start shooting pictures, a lot of things are important. Lighting, colors, shutter speed and aperture are just a few variables. In digital photography, there are several others which may not directly affect the picture as such but have a bearing on what you can and cannot do with the pictures later. One such variable is the image size. Image file size is in turn dependent upon image resolution, image format, compression level, ISO setting, and the subject. The first four are something you can change while shooting a particular subject.

Depending upon the camera model, there is a highest resolution at which the images can be shot. It is the most commonly referred to resolution when we talk of cameras. For example, if we say this camera is of resolution 3 megapixels, it means that the maximum resolution of the images shot by that camera can be 3 megapixels. But the same camera can also shoot pictures at much lower resolutions as well!! Now you might ask the question, “Why do we shoot at lower resolutions when there is a higher resolution available?” There is a simple explanation to this, agreed photos taken from digital cameras are free but here is a limit to the number you can store on your memory device at a time and that is a limiting resource. If you are on a vacation taking pictures at say 9 megapixels resolution, you don’t want your memory device to be full by the second or third day when you still have a week or so yet to go. Given below here is a table from the User manual for Sony DSC H50 camera which very intuitively defines the various resolutions it supports and the purposes for which images taken by those cameras can be used (every camera will have a different resolution and image sizes, to know that “Read thy manual”) :

So unless you are intending to print your pictures on A3 size of paper, its illogical to take photos at this high a resolution because obviously its going to occupy more space on your memory card for no good reason. But at the same time if you are planning to take a picture with say some 50 odd people in it and then crop it into smaller parts while still retaining the clarity, taking the image at 9 M does make sense.

Since a lot of other factors also play a role in determining the image size, only a rough idea to what the expected image size might be can be given. A 5M image may be around 2 MB while a VGA image will be around 0.2 MB. I guess you can do your maths for the resolutions in between and how many pictures you can store on your memory device.

Coming on to the next important factor that you are likely to change and control the image size is the ISO sensitivity. ISO basically refers to the sensitivity of the image sensor to light. Lower ISO will be preferred in bright lighting while a higher ISO is required in dimly lit areas so as to make the sensor more sensitive to light. There are cameras available in the market with the ISO sensitivity ranging from 50 to 800. Some cameras do offer an ISO sensitivity of upto 3200 as well but they are rare as of now. The higher the ISO rating, the more sensitive the image sensor is to light. It is worthwhile to know and remember from now on that the lower the ISO sensitivity (50 or 100), the less digital noise (the equivalent to “grain” when using a film camera) you will have. As the ISO rating goes up, your photos will have more digital noise. But again, there is a limitation to the lowest ISO setting you can use depending upon the lighting. Plus there are some other considerations as well, some amount of digital noise (grain) may actually be artsy than a nuisance. Before making a final decision on which ISO settings are too “grainy” for your photos, shoot a few shots with different ISO settings and compare them yourself. After you’ve answered these questions, you will know what ISO setting to use.

Now coming to the remaining two factors which determine the image size. These are absolutely camera and model specific so you need to go back to your manual and find out whether your camera supposrts these or not. If not, you can conveniently skip the remaining part of this blog post. If your camera allows you to choose between different file formats and compression levels, its nice to give them try. Three basic types of file formats are offered on compact level digital cameras: .jpg, .tif, or a proprietary “raw” format. Out of these .jpg and “raw” are compressed file formats and hence take less space on the memory device, .tif is an uncompressed format and takes more space. To make the image file smaller, compressed file formats like .jpg and “raw” use a mathematical algorithm that simplifies the image, thereby making it smaller. Simplifying an image also means that there is some decrease in image-quality. Proprietary “raw” formats are file formats that are unique to a single vendor, such as Nikon’s .nef format, or Canon’s .crw format. Unlike non-raw formats, where an image is taken and the camera processes it to get optimal results, a raw format image file is written to the digital photo storage media as it was captured on the image sensor without any additional processing. The advantage to these raw files is that you can use special software to adjust the original image parameters, such as white balance, contrast, sharpening, saturation, and so on. Besides being compressed, some of the proprietary files use 16-bit images instead of 8-bit images—meaning that they contain much more picture information, which can be useful if you edit the image with an image editor that can work with 16-bit images. The downside of using a proprietary format is that you may need special software to convert the images so that you may view them or use them in other applications. Also at times, these image files can be very large (just to give you an idea, an image shot with the best settings in .jpg format may only be around 2 MB while it may be as large as 20 MB when shot with the .crw raw format at 16 bits).

My advice as far as the format is concerned, if you are not ready to manipulate big files using special softwares and want instant decent results, .jpg is the way to go.

Now coming to the compression level, some cameras may allow you to set a particular compression level for your photos. A moderate amount of JPEG compression can dramatically reduce file size while only slightly reducing imagequality; in fact, you may not notice any image degradation at all relative to a non-compressed or .tif “raw” format. As compression level increases, file size decreases, as does image-quality to some extent. However this is best used when you are facing a crunch of storage space. As long as you are having enough storage space, its advisable to store images with the least compression.

Hope these few tips would help you to take plenty of photos, always, without having to worry about memory space.

Take home message: Calculate your memory space before hand and always keep the trade off between image size and resolution, ISO, format and compression in mind.

Happy Clicking!!!

The Guild