Learn How to Shoot in Manual Mode Pt.3: ISO
Notice the grain on the left half of the image. This is an example of noise, similar to what you’d see in a photo taken at a high ISO value.
So far we’ve covered two parts of the exposure triangle, or as I’ve coined it "The Trinity” 😎 The final and third setting to understand is your ISO.
W-ISO Sensitive?
ISO is a function of electrically boosting the pre-amplifiers of the sensor, so the light received via the lens is amplified on the sensors photosites. Think of photosites (photon sensing site) as micro-sensors that make up your entire sensor. Each photosite captures the light but can’t identify color, therefore one of three (RGB) color filters is placed over it. Each photosite is limited to the color they can interpret, however because there are millions of them, our brains translate these tightly packed clusters of red, green and blue as colors and tones. The amount of light a photosite can gather is dependent on its physical size. The greater the size of the photosite, the more light (signal) it’s able to gather.
When a photosite is larger it can gather more light. When amplifying your ISO your sensor has a stronger light signal to work with. This results in a higher signal to noise ratio yielding a cleaner image at the heightened sensor sensitivity.
This introduces a signal to noise (S/N) ratio at any ISO because the sensor is digital, it’s electronic. All sensors have a base ISO or base sensitivity with the highest S/N ratio. Excellent S/N ratios are 40:1 meaning for every 40bits of signal there is 1 bit of noise. As you amplify the signal, effectively boosting the sensors sensitivity, you introduce a lower noise ratio, like 10:1, resulting in a grainier image.
The signal is light. The light being gathered by your lens, that peaks through your shutter at whatever speed you set. Light itself is an electrical signal.
The less light your sensor receives the lower the signal strength. The sensor introduces noise to achieve a “normalized” exposure.
Sensors have come a LONG way, in the past decade alone, with their high S/N ratio at was once “impossibly high” ISO settings. A digital camera wouldn’t break ISO 6400 and if it did the noise was so bad the image was unusable. Now with the likes of cameras like the Sony A7S III we have full frame sensors capable of ISO 409,000! Still, not an ideal setting but getting a clean image at ISO 25,600 was once unimaginable, and is now possible.
That being said, if you’re a beginner then chances are you haven’t casually spent over four thousand dollars on a camera system. Most low-to-mid range prosumer DSLR’s and mirrorless cameras perform well between ISO 100-3200 depending on the size of your sensor and the megapixel count. So before we dive into ISO let’s talk about the sensor itself.
Size Matters
Unfortunately, not all sensors are created equal. If you’re starting out with your first interchangeable lens system then it’s likely your camera is packing one of three sensor sizes. The largest you’re likely to have is a full frame sensor. The size is modeled after the size of 35mm film, to digitally replicate the chemical process of analog (film) photography.
When we refer to focal lengths and their respective fields of view, we are basing these characteristics off of what’s projected on to a full frame (35mm) sensor. Things get a bit complicated when your sensor is smaller than full frame. Because the image casts onto a smaller sensor, your field of view is essentially cropped in, creating the issue of your sensors crop factor.
Sensor size changes everything! The focal length printed on your lens will not have the same field of view you would come to expect from a full frame sensor. Identify your sensor size, then its crop factor, and multiply it by your lens’ focal length. This yields your effective focal length.
Crop Factor
Most consumer cameras with interchangeable lenses have a crop factor. The most popular sensors found in these cameras are APS-C and Micro Four Thirds (MFT). Sony, Nikon and Pentax tend to use APS-C in their crop sensor cameras. while Canon has their own rendition of APS-C that’s comparable albeit slightly smaller. Then you have Panasonic and Olympus who cater to the MFT audience. “But full frame is the best! Why even bother with a crop sensor?” There really is no BEST. Best is subjective. You can capture comparable images on ANY sensor. That being said, there are advantages and disadvantages across all sensor sizes and there are nuances you should be aware of.
There are a myriad of factors as to why these smaller sensor cameras are popular.
Price: People starting out in photography tend to gravitate towards attractive price points. With favorable lighting situations a crop sensor camera can produce comparable images to a full frame camera for a fraction of the cost. Where you pay the price is in limitations, such as poor high ISO. lower resolution, bit depth, etc..
Physical Size and Weight: Smaller sensor, smaller housing, and less cumbersome lenses make these cameras attractive to enthusiasts who need portability. Much lighter in weight and smaller in dimension than full frame cameras and lenses.
Resolution: Crop sensor cameras generally sport sensors upwards of 20 megapixels (MP), which is more than enough for the general user. Honestly, even for professionals 20MP is more than enough. Think of it this way, an Instagram photo at it’s full resolution is only two megapixels. Keep in mind, for their to be more megapixels (non-physical digital packet of data) you need to have more photosites. Those photosites have to be smaller in order to fit more within the size of the sensor.
Video Capabilities: Crops sensors have broken the barrier on attractive video specs relative to how “cheap” they are. Due to the smaller sensor, it requires less energy to operate, allowing it to function at higher capacity without overheating. This allows for smaller sensor cameras to capture 4K video at high framerates since the sensor readout has to travel far less distance when it’s physically smaller.
Lens First, Then Body: Many APS-C cameras accept the same lens as their brands full frame counterparts. Your lens is arguably more important than your camera body. A great lens will yield beautiful results on any camera body. Some choose to invest in a lens first, and upgrade their camera body later. Glass stands the test of time far beyond that of camera bodies.
Enough Tech Talk…
What’s the bottom line? Every stop you boost your ISO above your camera’s base ISO expect a degradation in image quality. Image noise can be pleasant to a certain point, what’s acceptable is up to you (or your client). If you want to shoot the cleanest image your camera is capable of then shoot at its base ISO. As you raise the ISO sensitivity you introduce more grain, less dynamic range, and poorer detail.
Your ISO performance suffers when you decrease the sensor size and increase megapixels. Really ask yourself if shooting in low light environments is a deal breaker. If you mainly shoot in bright, day light situations then go for more megapixels to enhance your details at the base ISO.
That being said, most modern cameras have quite clean ISO performance well above their base sensitivity. Learn your camera’s capabilities and work within them to achieve your image.
