50mm – Factoring your Crop and Multiplying your Focal Length

Every photo taken on this site thus far has been actually been shot at 75mm (with a 50mm lens). The reason this happens is because my camera, the Nikon D90, has a sensor that is smaller than one frame of film used to be. What that means is that it cuts off the full 50mm image and becomes a 75mm equivalent.

In practice, this means that if you were to take a photo with a full frame sensor camera (like most pro bodies out there) using a 50mm, I would actually have to use a wider lens (35mm) to get the same looking photo from the same spot. If I used a 50mm lens from the same spot, the photo would look like I had zoomed in compared to the full frame version.

How did I figure that out? Using Crop Factor and the Focal Length Multiplier.

Crop Factor/Focal Length Multiplier:

These two are actually the same thing. Basically every sensor out there is compared to the 35mm format.

Why? Well because 35mm was the format of film and most photographers used it (even though there are many other larger formats used by professional and serious amateur photographers).

When things went digital, none of the sensors were at the 35mm size at first. These smaller sensors meant that users who were accustomed to their 50mm having a certain angle of view were in for a big surprise. So in order for old school photographers to figure this out, they came up with a new term – crop factor – to talk about how much their sensor cropped from what they were used to in 35mm film.

Crop factor is also known as Focal Length Multiplier because you can use it to figure out the 35mm equivalent of a particular lens on a different sized sensor.

For example:

As I mentioned above my D90 shoots at a 75mm equivalent when I use my 50mm. This is because the Crop Factor/Focal Length Multiplier is 1.5x.

35vs50mm

So how can we figure out focal length equivalents using this information?

Focal Length of the lens x Focal Length Multiplier = 35mm Equivalent Focal Length

So if you want to know what a 200mm lens is on a Nikon D90 do the following:

200 x 1.5 = 300mm

And so on. Only thing is, different cameras have different Crop Factors/Focal Length Multipliers. We’ll have a look at that in the next post.

If you’re looking for more info – Wikipedia has some here!

Friday I will be posting what that means to the 50mm Experiment Challenges – a must read for anyone who’s planning on submitting an entry to the current March 2012 Water Challenge or plans on participating in future challenges.

Questions?

Something to add?

Fire away below or email me at photography@koreweb.com.

Matt

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