Crop factors and lens performance

Shooting in low light levels requires a good understanding of the limitations of the equipment you are using. This shot was taken with an M4/3 camera using a 12-100mm lens at 100mm with an aperture of f/8 to ensure sufficient depth of field. The exposure time was 1/10 second, made possible by effective stabilisation and ISO 4000 sensitivity. Both these parameters were within the scope of the photographer’s ability to keep the camera steady during the shot and the end use for the picture (printing no larger than A5 size).

Photographers often become obsessed with lens speed, particularly when shooting in dim lighting and especially with respect to prime lenses. There’s a good reason for this response: faster lenses allow more light to reach the image sensor, thereby reducing the need to boost ISO settings and allowing faster shutter speeds to be used. This reduces the risk of blurring due to camera shake.

But with modern improvements to sensor performance, stabilisation and autofocusing technology, lens speed becomes less important. There are other factors to take into account, especially the effects the size of the camera’s image sensor can have on the amount of light a lens admits.

The influence of crop factors

Users of digital cameras will be familiar with the influence of the size of the image sensor on the effective focal length of a lens. It’s commonly referred to as the ‘crop factor’ and we use it when expressing lens focal lengths relative to the equivalent focal length of a 35mm film frame (which has a 1x multiplier or ‘crop factor’).

This illustration shows the effect of different crop factors on the area covered at a specific focal length – and the resulting effective focal length adjustment. The main image was taken with a 35mm sensor camera, while the rectangular outlines show the area covered by a sensor with a 1.5x crop factor, a 2x crop factor and a 2.7x crop factor (working inwards).

Most photographers will be familiar with the 1.5x (or 1.6x for Canon equipment) crop factor for APS-C sized sensors and the 2x factor for Micro Four Thirds (M4/3) cameras. They’ll be less familiar with crop factors for compact cameras, where lens focal lengths are almost universally described in 35mm equivalent terms. For what it’s worth, a ‘1-inch’ sensor has a crop factor of 2.7x, which means the sensor is roughly one-eighth of the area of a 35mm frame.

Anyone who uses cameras with different sensor sizes will probably be aware that a 12-40mm lens on an APS-C camera has an effective focal length equivalent to an 18-60mm on a 35mm camera. The same focal length range on an M4/3 camera covers the equivalent of a 24-80mm lens on a 35mm camera.

How many photographers are aware that depth of field in shots is likely to be greater for a specific aperture and focal length with cropped sensor cameras? The aperture itself doesn’t change; it always remains the ratio of the distance from the lens to the focal plane (sensor) divided by the diameter of the pupil of the lens.

But, as the sensor size increases, the depth of field will decrease for a given aperture if you fill the frame with a subject of the same size and at the same distance. This is because when shooting with larger sensors you have to move closer to the subject or use a longer focal length if you want the subject to fill the frame.

Should this worry you?

Not really; just be aware that when you use a smaller-sensor camera the depth of field in shots will be that much greater than it would have been had you used a ‘full frame’ camera with a lens of the same focal length at the same aperture setting. Factor it in when you want a very shallow plane of focus in a shot, allowing for the likelihood that the maximum aperture available with a ‘full frame’ lens will probably be wider than lenses for smaller formats.

These two photographs were taken of similar-sized subjects with the same effective focal length and aperture setting. Note the much greater depth of field in the image captured with the M4/3 camera, compared with the ‘full frame’ camera.

Interestingly, crop factors can be an advantage for macro photography where you want a wide depth of focus, although you’ll still need to compensate for the reduced amount of light reaching the sensor. Increasing the exposure time is one way to compensate; increasing ISO is the other. Both have consequences.

Longer exposures risk blurring from camera shake, an issue that is addressed by stabilisation. Raising sensitivity increases image noise. Both these issues are addressed below.

Stabilisation

These days, most camera and lens manufacturers build optical stabilisation into either their camera bodies or their lenses – and increasingly, both. The exception is for very wide-angle lenses, where any blurring due to camera shake will probably be insignificant in the light of the wide angle of view recorded by the lens. If present, it would only be visible if the image was substantially enlarged.

Effective stabilisation in both the camera body and the lens enabled this wide-angle shot to be captured with the camera hand-held at an exposure of two seconds at f/5. Subjects that moved during the exposure are blurred, but the rest is pin-sharp.

Sony and Olympus were the leaders in sensor-shift stabilisation, while most other manufacturers relied on stabilised lenses, in which a ‘floating’ element (or group of elements) is used to redirect the path of the exposing light in response to camera movement detected by gyro sensors. But this is changing and, in the latest cameras, the sensor and lens stabilisers can work together to increase the effectiveness of the system.

Improvements to the gyro systems that detect motion and the motors that adjust the sensor or stabilising element in the lens have enabled many recent camera/lens combinations to offer shake correction levels of six to eight f-stops or more plus steadying across five axes: up/down, pitch/yaw and rotation. Longer lenses require more correction than lenses with wide angles of view but it should still be possible to use shutter speeds as slow as 1/50 or 1/15 second with a 200mm lens hand-held, although you can’t guarantee every shot will be pin-sharp.

This night shot of a pagoda reflected in a puddle was taken with a M4/3 camera (2x crop factor) using a 12-100mm lens set to a focal length of 34mm (68mm equivalent in 35mm format). The hand-held 1.3-second exposure was taken at f/8 at ISO 6400 to ensure sufficient depth of field.

Sensitivity

Fortunately the sensitivity of modern image sensors can be adjusted across a huge range of values, compared with the range available to film photographers. However, there are consequences associated with these adjustments that must be taken into account (although the latest cameras will be advantaged over five- or ten-year-old cameras because of improvements to noise-reduction technology).

Sensitivity is also influenced by crop factors. The point at which increasing noise reduces picture quality varies with different sensor sizes, with ‘full frame’ sensors being better able to handle high ISO settings than cameras with cropped sensors.

If you know the ISO settings at which your camera’s images begin to deteriorate it’s easy enough to avoid them by allowing the camera to adjust ISO automatically. Simply set an upper limit to the Auto ISO range to the highest ISO value you’re prepared to tolerate (which will vary depending on your end use for the images). The Auto setting will always be biased towards low ISO values, giving you the best chances of obtaining relatively noise-free shots.

Most cameras include ‘extended’ ISO settings that provide values above and below the ‘native’ range at which the sensor and image processor have been assessed to perform best. Where possible, these settings should be avoided since they will reduce overall performance. Think of them as last resorts when you have no alternatives.

Sometimes it’s worth pushing the ISO value to the highest ‘native’ setting and living with the consequences. This shot was taken with an M4/3 camera using a 25mm focal length (50mm equivalent) and aperture of f/7.1. The exposure of 1/60 second was the slowest that would allow the main subject to be sharp but to achieve it sensitivity had to be pushed up to ISO 25600. Noise-reduction processing was applied when the shot was edited.

Exposure calculation should become an issue of determining the depth of field you want and how high you can push ISO sensitivity. While today’s cameras provide much higher sensitivity settings and retain colour accuracy at high ISO settings than previous models, if you use the maximum ISO sensitivity the camera supports, you can expect a reduction in picture quality due to image noise.

Work-arounds

Some situations allow a wider range of exposure controls than others when you’re pushing the limits of your camera and lens. The simplest is to move into a better-lit place, wait for the light to change or add light with a reflector or artificial light source.

An increasing number of cameras are including multi-shot modes that can record a burst of images in quick succession and combine them to achieve a specific result. Olympus was the first manufacturer to introduce a sensor-shift, multi-shot mode to record very long exposures as well as for achieving higher resolution without increasing image noise. It was also the first to bring a multi-shot high-res mode to hand-held shooting.

The Live Composite mode in recent Olympus OM-D cameras seamlessly composites a series of exposures taken over an extended time period (up to three hours) without over-exposure and with minimal image noise. Photographers can watch the image build up on the monitor screen and end the capture sequence when the result is what they wanted.

Panasonic was the first to use a multi-shot mode for in-camera focus stacking to achieve a wider depth of field. This ‘Post Focus’ setting uses the camera’s 4K or 6K movie recording function to record roughly one second of MP4 video at 30 frames/second, changing the lens focus during the recording.

Other multi-shot long exposure modes require the camera to be tripod mounted because they operate over very long exposure times. So they’re beyond the scope of this article.

Successful shooting in low light levels requires you to understand the multitude of factors that can influence your choice of equipment and the types of images you can produce. Shooting technique will always play an important role in minimising camera shake with slow shutter speeds. Keep your finger on the shutter release and when you have the subject framed perfectly, breathe in and squeeze down on the shutter button. Don’t lift your finger until the shot is captured.

Article by Margaret Brown (see Margaret’s photography pocket guides)

Excerpt from Photo Review Issue 85

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