Why Is The MM Range On A Film Lens Different On Digital? :: Digital Photo Secrets

Why Is The MM Range On A Film Lens Different On Digital?

by David Peterson 9 comments

It’s one of the weird relics from the transition to digital. We’ve still got the old lenses, but the cameras we’re attaching them to are no longer the same. When you use an old lens with a digital camera, you have to multiply the lens’ focal length by a number known as the crop factor. I’d like to take a quick moment to tell you what a crop factor is, how it affects your photography, and why it’s important.


The image at normal size with no crop factor
Photo By: Frank Kovalchek

If there is any big takeaway, it’s this. Digital camera sensors are smaller than 35 millimeter film. When you attach a film lens to a digital camera, only a portion of the image gets projected onto the sensor. The rest is cut off around the edges.

Imagine using a flashlight to shine the famous Bat Signal onto a square piece of paper. If you were to cut some of the edges off of the paper and discard them, less of the Bat Signal would appear on the paper, and more would appear on the surface surrounding the paper. That’s what’s going on in your digital camera when you attach a film lens. In effect, what you get is an image that looks a little more zoomed in. The degree to which it is zoomed in is known as the crop factor.

So, what you do to get the Bat Signal to fit into the now smaller piece of paper? That’s right, you’d have to take a few steps back until it fits. We also know this as zooming out. When you know the crop factor, you can figure out how much to zoom in or zoom out to get the image to fit in the way you want. That’s why it’s important. The following math will help you use it. (Update: Changed 'zoom in' to 'zoom out'. Thanks to Armond in the comments for noticing).

How to figure out how much your lens crops the image

The crop factor is a useful number because it tells us what our image will look like on the sensor. We can multiply our crop factor by the focal length of the lens to determine how much more zoomed in we’ll be.

It goes like this:

Lens Focal Length * Crop Factor = Cropped Picture Focal Length (what it is zoomed to)

So, if you were to have a 50mm Nikon lens with a crop factor of 1.5, you’d end up with a picture that appears as though it were taken with a 75mm lens. It would be zoomed in to that focal length.

The above picture would look like this:


The same image with a crop factor of 1.5. This is what it would look like if you were to shoot using a Nikon film lens.
Original Photo By: Frank Kovalchek

Some film lenses crop a little more than others

That’s where the 1.5 crop factor for Nikons and the 1.6 crop factor for Canon lenses comes from. The sensors inside the cameras are slightly different from one another, so they produce slightly different results. As long as you know the math above, you’ll be fine.

What about DX and digital lenses?

You don’t have to worry about anything with these kinds of lenses. They’re designed for digital, so there is no crop factor. Just keep shooting away as you normally would, and what you see is what you will get.

In fact, I highly recommend purchasing and using digital camera specific lenses. It removes a ton of hassle.

If you have any more questions or anything else you’re curious about, don’t hesitate to ask. Feel free to leave a comment below or send me an email.

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Comments

  1. Devin says:

    I think the confusion is, you'd have to step backward with your camera to capture the whole symbol, but step forward with the flashlight.

  2. Devin says:

    Armond is wrong, if you're using a flashlight to project the symbol, you'd have to get closer to the paper to shrink the symbol.

  3. Geo says:

    Hi, David, a little late to say !! Congratulation for your birthday!!, so, I think that wit crop factor we must take advantage of it. It is same like use lens in digital camera wit crop factor, in some time I used it to get more distance like the canon 7D with 70-200mm 2.8, it is amazing conmbinations, yes or not..

  4. Jerry Van Dyk says:

    Hi David.
    Sorry I missed your birthday I spend some time in hospital,but I did really learn and relearn a lot.I have a small group of about eight people who I teach photography, done this for some years .I am an old Hasselblad man
    ,but than digital was born .Meaning I had to start all over again .
    Presently I use a Olympus e500 and i believe that the lens factor is not 1.5
    but 2 .I have been told that this makes my 180mm lens a 360mm
    is this correct or wrong .I am just getting to old for this ,things used to be so simple read the light set the settings shoot and head for the dark room .
    Thanks for all your help Jerry.

  5. Ronny MF Ho says:

    i think the article is too much simplified. I would emphysis that a 50mm focal lengh lens is always a 50mm lens, no matter it is attached to a film camera, an entry grade DSLR or a Full-frame DSLR. The only differences are: 1. when attached to a film camera or a Full-frame DSLR, the field of view is the same. If it is attached to an non-Full-frame camera, the field of view is similar to a lens with that focal length mutiplying the crop factor. 2. even the field of view is similar, their perspecitves are different.

  6. David Peterson says:

    Thanks Armond. I've updated the text to fix it.

  7. Armond says:

    So, what you do to get the Bat Signal to fit into the now smaller piece of paper? Thats right, youd have to take a few steps forward until it fits ???
    Shouldn't it be few steps back until it fits???

  8. Elvin says:

    Hi,
    Actually (at least for all of the lenses I use, Pentax), the 50mm on digital is exactly the same as 50mm on analog lenses.
    Only: if I use the lens on a analog camera, it looks 'wider'.
    The biggest difference is that I need for example a 300mm telelens on my analog camera and only a 200mm for my digital camera (or: I don't need the 300mm tele to get the same bird in picture).
    On the other hand I need an 18mm wide-ange zoom lens on digital to get about the same picture as with an 28 mm lens on my analog systems.
    Since the digital lenses don't provide the good pictures on analog, I cannot get the 'pancake'- effect I have with an 24mm or 18 mm on an analog camera though with swapping the digital lens on my analog system: then I see a lot 'falling off'.
    The digital sensor-cut takes the best part of the analog lens for its picture though; even with 'bad lenses' I get a good picture on digital which had 'edges' on analog.
    And yes... I still use analog equipment together with digital.

    Elvin

  9. Jason says:

    Hi

    I am still only an amateur photographer, however i think this article should also have a included the difference between full frame and APS-C sensors. Both have there advantages and disadvantages.

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