I'm looking to get into real estate photography and I'm often advised that for full frame cameras you want something like an 18-35mm and in the same breath they also say if you have a crop sensor camera you'll want a 10-20mm lens. I have a Nikon d500 crop sensor camera and I'm looking at a dx (crop sensor) lens made by sigma that's 18-35mm f/1.8, my question is, will it have an ~equivalent/comparable~ field of view to a full-frame lens with the same specs?
The Nikon D500 sports a sensor that measures 16mm height by 24mm length. This format is called APS-C (Advanced Photo System – Classic Format). The industry likes to make comparisons to the esteemed (100 year old) full frame 35mm (FX) a format rectangle measuring 24mm height by 36mm length.
The best way to do this is to find the diagonal measures of both formats and divide. We are talking Crop Factor (magnification Factor).
The FX rectangular formant has a diagonal measure of 43.27mm. The APS-C format is a rectangle with a diagonal 28.8mm. The magnification difference is 43.27 ÷ 28.8 = 1.5.
The value 1.5 tells us, we must magnify the APS-C 1.5X because it is 1/1.5 = 0.66 = 66% of the size of the FX format.
Now the rest of the story: A 18mm – 35mm is mounted on an FX. What is the focal length range equivalent for an APS-C. Answerer 18 ÷ 1.5 = 12 and 35 ÷ 1.5 = 23. OK to round so – you want a 10mm – 25mm Zoom to do this task. The task is to deliver about the same angles of view.
“DC” is Sigma’s way of saying this lens is designed for Crop cameras, and even though it might be able to be mounted on a Full Frame camera, the image circle will not be large enough to cover the entire Full Frame image sensor.
With very few exceptions, the focal length written on the lens is the actual focal length, and does not change when mounted on different formats of sensor size.
So, the Sigma 18-35 DC(DX) will NOT have the Ultra-Wide field of view (FOV) you are looking for when used with a crop camera.