Understanding Crop Factor

Most of today’s sensors used in digital single lens reflex cameras have a smaller area than the surface of 35mm film but focal lengths are still expressed in the 35mm format. As a result, each camera’s effective focal length will be different than the one listed on a lens.

I’ve heard and read many notes of what camera body or lens someone should first get. In one instance, I saw a post online asking if they should get a DSLR, action camera or something else to capture and record moments. Most people are quick to suggest of getting a DSLR, GoPro, or that high end smartphone everyone is touting around, while some went as far by suggesting mirrorless cameras.

Of course, a mirrorless is a great choice as it is compact and offers the same flexibility of a larger DSLR except that most mirrorless especially the basic models are equipped with a crop sensor with a factor of 2. That is way smaller than most entry level DSLR offered by Nikon and Canon at around 1.3 to 1.5 crop factor. That is because mirrorless cameras are Four Third Systems, sure there are new models that have a Full Frame sensor but good luck on getting one in exchange of your soul. Those are too expensive.

If you look at the internet what is the crop difference of entry level DSLR compared to Four Third Systems, you’ll definitely reconsider your options, and probably pee yourself when you see the size of a Medium Format.

There is a great deal of confusion surrounding crop factor, and is particularly difficult to explain but the basics are; crop factor does not affect a lens focal length and definitely crop factor does not affect a lens aperture.

The first thing to know about crop factor is that, as with all factors, we need to have a base reference from which to work. In the photography world, this reference is a piece of 35 film. In the digital photography world, full frame sensors are the same size as this film, a film frame with a width of 35mm. Cameras of this photography format are collectively known as 35mm cameras.

A round lens produces a circular image circle, not rectangular. The sensor, or film, at the back of the camera captures a rectangular portion of this image circle. When we use 35mm film as a standard, any camera with a sensor smaller than a frame of 35mm film will cover a smaller portion of the image circle produced by a given lens and will thereby change the field of view of that lens. This is the crop part of the crop factor.

For example, if you attach a 50mm lens to a camera, like I have, with a smaller than 35mm film sensor, you will have to multiply the focal length of that 50mm lens by a factor derived from the size differential of the sensor to calculate the 35mm equivalent focal length. This will then give you the means to figure out the lens’s field of view based on that new equivalent focal length. This is the factor part of crop factor.

This multiplication factor is the ratio of the size of the digital sensor to the dimensions of the 35mm film negative. So for example, if you have, let’s say, a 28mm lens, to less the confusion, and you attached it to a crop sensor camera body with a smaller than 35mm film sensor, your real focal length would be 42mm on Nikon and 36mm on Canon.

Before buying a lens or bragging of having a 50mm lens, it’s important to know the crop factor of your DSLR. The conversion chart below shows the effective focal length of a lens by crop factor.

35mm1.3X1.5X1.6X2.0X
8mm10.4mm12mm12.8mm16mm
10mm13mm15mm16mm20mm
14mm18.2mm21mm22.4mm28mm
17mm22.1mm25.5mm27.2mm34mm
20mm26mm30mm32mm40mm
28mm36.4mm42mm44.8mm56mm
35mm45.5mm52.5mm56mm70mm
50mm65mm75mm80mm100mm
85mm110.5mm127.5mm136mm170mm
105mm136.5mm157.5mm168mm210mm
135mm175.5mm202.5mm216mm270mm
200mm260mm300mm320mm400mm
400mm520mm600mm640mm800mm
500mm650mm750mm800mm1000mm
600mm780mm900mm960mm1200mm

If you have a zoom lens on a smaller than full frame camera, you can figure out the effective focal length equivalent by multiplying both focal length numbers by the crop factor. For example, a 70-200mm lens becomes a virtual 105-300mm lens on a 1.5x APS-C sensor.

Canon EF-S EF-M1.6X
Nikon DX1.5X
Nikon CX2.7X
Micro Four Thirds

Olympus

Panasonic

2.0X
Pentax DA1.5X
Sony E-Mount1.5X
Sigma DC1.5X-1.7X
Samsung NX1.5X

I hope this clear things up. Crop factor is really quite simple if you know well your camera body and what types of lens that you use. So if you are out shopping for a zoom lens with a max focal length of 300mm because you want to reach for the moon but then you only have a crop sensor DSLR body, then it is ideal to get a 70-200mm zoom lens as it is cheaper and more logical choice. The crop factor should be listed in the specifications section of a camera manual. Just do the math.