Lenses…what is a focal length, what is field of view, why does my head hurt!?

It’s often asked and actually a lot but people get very confused when it comes to focal lengths of lenses when used on different sensors.

This is getting more and more confusing when cameras like the Sony F3 come out with kit lenses including the 35, 50 and 85mm and when I say to people you need to take into account the equivalent field of view of these lenses won’t be what you are used to if you are using a full frame camera. If you have only ever shot  super 35mm film  you will know what a 50mm will give you on the F3 or any other S35 camera (although some digital camera sensors differ slightly in size)

The thing is a lens is a lens. No matter what it is sold for or even stuck on. It’s always going to be a piece of glass that projects an image onto a sensor. It’s the field of view that matters. If I stuck a 50mm lens on a Sony Ex3 my magnification (or crop if you want) would be about 5.2x giving me an equivalent focal length on a full frame camera of 258mm….anyway, I can try and explain as much as I can but it’s best if I leave it to much cleverer people than me. AbelCineTech have done a great video and Field of View comparator. Click on the video image to see the video by Mitch Gross and the field of view calculator to see that. There are also some great iphone apps that help you with this. So remember, if you buy the Sony F3K kit your widest lens is a 35mm, this is not a very wide lens so you will need to budget for a wider lens to go with it.

My friend Mike Collins has done a simple video below these to show the difference between a 5dmk2 and 7d for FOV.

5Dmk2/7D lens comparison test from Mike Collins on Vimeo.

This is a short test with the tripod in the same spot switching between prime lenses to show how the crop affects the 7D. The subject, ace stand in Chris Clement, was roughly five feet from the camera. This isn’t meant to be an aesthetic test to show the difference in image quality between the two cameras. It’s a down and dirty field guide for myself and the other shooters we work with so we can quickly figure what lens we want to use on each camera.

We go from 20mm all the way to 100mm with a Lensbaby composer thrown in at the very end.


  1. Hi Philip, Love the site, your work is really inspiring. Im a canon 5d mark 2 shooter and love the camera but at the moment own no lenses of my own. Just wondering if you could recommend some lenses for me, Im on a budget budget but would consider buying quality lenses an investment. Is there any advantages of taking fixed focal length lenses over zooms? thanks so much for your time

    1. If I may offer a quick suggestion:

      Just pick up their cheap 50mm for now. It’s like 100 bucks. Sure, it’s plastic, feels silly, the focus ring is in the worst possible spot and it’s super thin– But the optics are fine, it shoots in fairly low light (f/1.8– although it’s not very sharp unless stopped down) and it’s the lightest lens available for that camera.

      Just use it until you can save up enough for a nice L lens. I always recommend fixed focal length due to IQ, but try out as many lenses as you can at a camera shop and find the one that looks and feels best to you.

      Two “less expensive” (under 500) lenses I’d also recommend are the 100mm macro USM. VERY sharp and the macro is lovely. The 135mm Soft Focus is also quite a unique lens that I fell in love with.

  2. Great post, crop factor is an important but often forgotten issue when purchasing a DSLR or video camera for that matter. If you have the money a director viewfinder is a great tool to use to compare different focal lengths and their FOV’s.

  3. I think the best way to learn this stuff is to simply shoot with a few different cameras and as many lenses as possible over a period of time.

    This stuff always used to make my head hurt– Now it makes perfect sense. This is due to a newfound interest. It’s near impossible to learn something that doesn’t really affect you or your shooting, but it’s easy to learn something that you see every day.

  4. Thanks for posting this Philip. So glad it could help. I just updated the FOV comparator with a new picture and some new camera additions (including the Sony F3). Barry Green and Panasonic also gave us updated info on the AF100 sensor size so I changed that as well.

    Andy Shipsides – AbelCineTech

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