Lens length equivalent? Converting from a crop to a full frame....

tkeetchtkeetch Registered Users Posts: 13 Big grins
edited February 6, 2019 in Accessories

I recently upgraded from the Canon SL1 to a Canon 6D Mark II. I have the 50mm 1.8 stm and the 100mm 2.8 L.

Now I'm trying to decide which wider prime to get for a future birth photography business.

I have the Tamron 17-50mm 2.8 for my crop, and I'm looking at my settings of past photos to see what range I prefer. But I'm confused on how the numbers convert from crop to full frame.

For wide angle shots at 17mm on the crop, what lens would give me a similar point of view on the FF?

How about 24-35mm on the crop?



  • pathfinderpathfinder Super Moderators Posts: 14,679 moderator
    edited March 30, 2019

    Hi tkeetch,

    Welcome to dgrin!

    The crop factor for Canon 7D style APS-C bodies to full format bodies is 1.6. which relates to the relationship between APS-C and FF lens apparent focal lengths.

    Hence, a 17mm lens on a crop body will match the angle of view of a 1.6x17 = 27.2 mm lens on a FF body... That is to say a ~28mm lens on a FF body will photograph as wide as a 17mm lens on your crop body. Or conversely a 21mm lens on a FF body will match the width of a 21/1.6 = 13.125mm lens on a crop body - This all presumes that the lenses are designed to use the full area of the sensors of the appropriate body they are on - EF-S lenses ( lenses built for APS-C sensor bodies not FF bodies ) will not mount on a FF body AND would not utilize the full area of the FF sensor because their image circle was designed with the smaller APS-C sensor in mind.

    This makes APS-C sensor bodies at a minor disadvantage for wide type imagery, like landscapes. BUT, it also means that a 400mm lens on an APS-C BODY effectively captures the view of a 1.6x400= 640mm lens on a FF sensor body. And that 400mm lens is generally much smaller, much lighter, cheaper and much easier to handle than an equivalent FF 640mm lens. The decision to favor FF or crop is one of some subtlety and understanding the attributes of each format.

    I use both APS-C and FF bodies routinely. I tend to think that APS-C bodies are smaller, lighter, more inexpensive and have slightly smaller file sizes and smaller pixel wells - So I may favor them for wildlife if the light is good, as they are smaller and easy/easier to handle. I definitely favor them for walk around snap shooting.

    I favor FF bodies for larger files, lower noise at high ISOs, and slightly better files, and when I want to carry much larger, heavier backpacks into the field. ( not that often that I really want larger heaver backpacks 😎, but I do accept them when necessary )

    Well captured images that are correctly exposed in decent light can be quite nice from either format. For stars and Milky Way images I favor FF bodies. But a really good frame from an APS-C body will blow away a mediocre frame from a FF.

    For wildlife in good light I am almost indifferent which format I use, but long before sunrise or after sunset, I do prefer the lower noise and slightly better files of FF bodies and lenses - despite the differences in size and weight.

    You can get an idea of the differences in files from my chasing of short eared owls which I have used both formats for, in really terrible light - near sunset and long after sunset even - seen here in any of the four galleries with Short ears - you will have to click gallery and then on each image to see "I" symbol for the exif information about camera body and lens used for each image.


    Your 24-35mm on a crop body will look similar to a 38.4-56mm lens on a FF body. Actually a 24->35 mm zoom lens lens is too small a range for me I would just use a good 50mm prime on a FF body. I try to favor primes if I can - kind of old school as they make some VERY nice zooms these days, but still, good primes are usually really good too, these days.

    I own and use a Canon 24-105 a lot, and the version 1 of this lens can be found used very inexpensively these days - I owned and used one for years with a good tripod. Great lens, and the 2nd version is even better. One of my favored lenses for a crop body is the Tamron 16-300mm lens which I find much better than I had thought it might be - with lens profiles in Lightroom it performs very nicely, and at the long end ( 300mm) is still pretty sharp if a bit smaller aperture, and reaches out like a 480mm lens on a FF body. Imagine carrying almost the reach of a 500mm lens in the size of a 50mm f1,4 prime and still shooting as wide as a 24mm lens on a FF body. I have almost 12,000 frames on mine and its still going strong. I mean really, what can one expect from a travel zoom, ..... Well, this for example

    Pathfinder - www.pathfinder.smugmug.com

    Moderator of the Technique Forum and Finishing School on Dgrin
  • tkeetchtkeetch Registered Users Posts: 13 Big grins

    Thank you! I totally agree - I love the SL1 because it is so tiny and lightweight. I take it on hikes with the 24mm pancake, and hardly adds weight to my pack!

    So... if I am understanding you right... either a 24mm or a 35mm lens for the full frame would be close to the 17mm length on my crop - one a little wider, and one a little closer. Question: Is the compression similar? When I take a close up image of someone's face with the 17mm their face is really distorted and their nose looks huge! Would this be similar to the above mentioned primes?

  • pathfinderpathfinder Super Moderators Posts: 14,679 moderator

    Yes, as I said, the 17mm on a crop body will be a very close match to a 28mm lens on a FF body - the specifics of depth of field may vary slightly due to the smaller sensor image circle having slightly more depth of field than the image on a FF with a 28mm lens. But the distortion will be similar - the distortion is greatly determined by how close your subject is to your image/sensor plane.

    Pathfinder - www.pathfinder.smugmug.com

    Moderator of the Technique Forum and Finishing School on Dgrin
  • tkeetchtkeetch Registered Users Posts: 13 Big grins


  • e6filmusere6filmuser Registered Users Posts: 3,322 Major grins

    Yes, the issue of WA & UWA lenses on crop sensors is one I, having m4/3, could only solve by investing in a second body, this with a full frame sensor. I can now use my legacy UWA lenses plus some of the latest ones from Laowa. Even so, their Magic Shift Converter has a TC in it, effectively increasing FLs by x1.4 when I coose to use it.


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