A Concise Explanation of How Crop Factor Affects Both Focal Length AND Aperture

Editor’s Note: Due to some issues with the camera, this video is very shaky at times. It didn’t bother us much, but if you’re easily distracted this video might annoy more than it educates you. You’ve been warned.

If you’re just getting into the world of cameras and lenses, the term “crop factor” and phrases like “this is a 35mm equivalent lens” might still confuse you. Well, that shouldn’t be the case much longer.

The video above offers a clear, concise and simple explanation of crop factor that will hopefully clear all of this up and equip you with some important knowledge that will come in handy the next time you’re shopping for a lens or crop sensor body.

The video was put together by photographer Tony Northrup in response to some comments he’s gotten on his other videos, and although it’s not perfect, it does a great job explaining how crop factor affects not only focal length (read: field of view), but aperture (read: depth of field) as well.


We won’t go into the technical details, since that’s what the video is for, but the TL;DR goes something like this. Using Nikon as an example: a 50mm f/1.4 lens, when attached to a D7100 (1.5x crop factor), will produce about the same image as a 75mm f/2.0 lens attached to a D800 (full-frame camera).

The crop sensor affects your field of view (how close you are to your subject), your depth of field (how thin your focus plane is/how much background blur you’ll get) and the amount of TOTAL light hitting the sensor (same amount of light per square inch of sensor, but less total light because you have less sensor area) and therefore your image quality.

For a more in-depth explanation combined with visuals that really help drive the point home, check out the video at the top. And if you feel you can explain crop factor in an easier to understand way, feel free to drop that explanation in the comments.

(via Reddit)

  • Marco Harder

    I think this is misleading. Sensor size has a relationship with depth of field, NOT aperture. The f-number is a ratio between two parameters of a lens [focal length and effective iris diameter if I’m not mistaken], and is totally independent of sensor size.

  • Marc

    Yeah, I couldn’t really pay attention on what he was saying because of how bad the video was moving around…

  • Sean Mason

    I like Tony’s videos. He hit the nail on the head here.

    One analogy I like to use is a flash light. Shine a flash light on the wall and draw a circle around the bright areas. Now, move closer and draw a new circle that should be half the area. It will be twice as bright but the overall amount of light is still the same.

    This is what crop sensors do. They take a lens that outputs only so much light (per brightness of the scene) and make the image circle smaller. There is the same amount of light but they claim the lens is one or two stops brighter.

  • Jonathan Maniago

    “Increasing my f-stop number from f/2.8 to f/5.6 gave me basically the same amount of background blur, the same bokeh. So that’s why you multiply the aperture times the crop factor because it allows you to compare lenses and the amount of background blur that you’ll be able to get.”

    Similar blur comparisons, so I wouldn’t say he’s off the mark.

  • Sean Mason

    Yes, if you only want to consider shutter speed and ISO, that is right. However, shutter speed and ISO are not the same on crop vs full frame. On full frame you can hand hold a 50mm lens at 1/50. The same person would need 1/100 to hand hold a 50mm on a m43 sensor. Also, full frame ISO is one to two stops better than crop sensors. This is because a 20 MP full frame sensor has bigger pixels than a 20 MP crop sensor. Those pixels get more photons at the same apertures.

  • Mike

    Wouldn’t the Nikon 50mm f/1.4 be equivalent to a 75mm f/2.2 rather than f/2? It’s a 1.5 crop, not a 1.4 crop.

  • Sean Mason

    1.5x crop is .75 of the length and .75 of the height. That means is 50% the area and a full stop difference in terms of DOF and equivalent ISO.

    Mico 4/3 is .5 the length and .5 the height, so it is 1/4 the area of full frame and a full 2 stops difference.

  • fast eddie

    Yeah, but his lovely wife made up for it :)

    I didn’t know that the aperture is affected like that, but it makes total sense when presented this way. I just always focused on multiplying focal length for the sake of shutter speed.

    Thanks for teaching me something new!

  • Matt O’Sullivan

    This is one of the most misleading and confusing explanations I’ve ever seen about crop factor cameras.

  • David Brown

    Now I finally get it! Lost me a little on the ISO conversion though…

  • bcdouglas

    “how close you are to your subject” NO! It affects how much of your subject you see! We see less of a subject on a crop sensor, then we ‘normalize’ that view to standard photo dimensions, which gives impression of having ‘more reach’ and/or zooming. The ability of a lens to zoom in (or out) is relative to our frame of reference. The physics of the lens do not change based upon what the light traveling through the lens hits after the lens.

  • Matt O’Sullivan

    A 7D does not have “less light” hitting the sensor than a 5D. It has exactly the same amount of light-per-area hitting the sensor.

  • poops

    Not quite. Micro 4/3 is a 4/3 ratio, not 3/2.
    So, it’s a little more than half the height, and a little less than half the width of full frame.

  • poops

    It was terribly misleading.
    A f/2.8 is a f/2.8. It never changes. No matter what the sensor size is. The angle of view(crop factor) and depth of field changes with different sensor sizes. But the light gathering capabilities never change.

  • poops

    Exactly right! The f-number never changes.

  • Vitor S

    Totaly misleading. He is talking about “picture quality” when he is really talking about
    “bokeh amount”.

    He is totally missing sensor technology advancements in regards to picture quality , not considering “glass quality”, and that last plea to manufactures is so stupid…. f1.8 is f1.8 .. we should measure fStops by bokeh now? Idiot…

    Mr. Northrup: my Canon 5D mk1 makes lower “quality” pics than my Nex 7 (by large amounts).. … UR FREAKING TALKING ABOUT 35mm vs crop “bokeh” amount

    Peta Pixel… this post should ashamed your site

  • Zos Xavius

    All very true. At this point a stop difference between crop and full frame really isn’t all that much IMO when even crop sensors are producing quality iso 6400 files.

  • Zos Xavius

    And the overall area is smaller, so therefore less light is hitting the sensor ultimately. Sure aperture is the same, but the field of view is smaller also so less light is reaching the sensor.

  • Zos Xavius

    The light gathering capabilities of the lens never changes that is true, but less of the image circle and consequently less of the light it provides is reaching the sensor. That’s why smaller sensors have lower signal to noise ratios.

  • Stephen

    I find his focus on bokeh to be somewhat bizarre. More so when you consider he spends so much time talking about bokeh and yet never mentions perspective distortion. Yes, the three photos at 1:22 have different bokeh. They also show different compression. Why wouldn’t you also talk about that?

  • Matt O’Sullivan

    But I don’t understand how this is relevant to anything.

  • Zos Xavius

    What he doesn’t point out is how perspective changes between 100mm and 200mm. There is more compression in the 200mm shot as well as the background shifts position.

  • Zos Xavius

    I just wrote the same thing. =)

  • Matt O’Sullivan

    The depth of field does not change with different sensor sizes.

    DoF is dependent on these factors:
    – Focal length of lens
    – Aperture
    – Flange (distance of rear element to film plane)

  • Jonathan Maniago

    Compression is only affected if you change your field of view AND your shooting distance. Also, note that the model shifts her position slightly in between shots.

  • Matt O’Sullivan

    Cropping a sensor does not affect the depth of field.

  • Stephen

    Yes, but it’s relevant when you begin talking (as Tony is) about lenses and crop sensors in “equivalent” terms.

    My own preference would be to scrap that whole “equivalence” discussion. I don’t think it’s useful. It is certainly more confusing than helpful, to tell people they should be thinking about their camera/lens in terms of some other camera and some other lens they don’t own. But if we are going to insist on talking that way, then compression is relevant. And hey, look at Tony’s photos. You can see it at work.

  • Alex

    The video is pretty good for a layman interpretation of crop factor and lens comparison across formats. The problem is, different formats are different formats. A 135 format camera such as a Nikon D800 is considered full frame because the lenses are designed for that format. Micro four thirds lenses are designed for micro four thirds cameras and are still considered full frame even though the sensor is smaller. Micro four thirds is pretty much the digital equivalent of 110 format cameras of the past. By putting a 135 format lens on a 110 format body, you are essentially only using a portion of the lens instead of using total surface area which would cause slight variations in exposure. Tony is right in doubling the F stop for his DOF comparison but for exposure, there is more to consider than the lens like the sensor and the T stop of the lens with camera which is partially based on the F stop of the lens.

  • poops

    That’s not entirely true.
    The signal to noise ratio is based on the size of the pixels, not the sensor size. The light the sensor sees is the same, but bigger sensors generally have bigger pixels(Not always though).
    The problem people have with the video, is that the photographer states that the f-number changes according to sensor size, and that manufacturers should change their labeling methodology. That is false.
    Two images with the same f-stop, the same shutter speed, and the same ISO, will have the same exposure.

  • Jonathan Maniago

    Are we talking about the same thing here? What I’m seeing in 1:22 is a model that shifts positions in between shots and a narrower field of view.

  • Clover

    I’ll be taking pictures today…blah blah blah…….My lovely bride….micro sensor, blah blah blah……I Look like I’m 50……same amount of light per square inch of sensor, blah blah blah……she looks like she’s 14, isn’t she hot?? Let me show you another lens…can’t stop finding reasons to take head shots of my hot bride.

  • Stephen

    This reply makes a bit less sense now that you’ve edited your comment (twice) after reading mine. Which I don’t think is an especially cool thing to do, but hey, I guess my beef there is at least half with Disqus. G’day.

  • poops

    No, of course not. You’re right.
    But practically speaking, cameras with smaller sensors are going to have shorter flange distances.

  • Kaouthia

    No, 1.5x crop is 0.66 the length & height.

  • Bruce

    I agree that camera makers should provide more meaningful specs, but the assertion that the f-stop should be multiplied by the crop factor only pertains to the DOF, not the exposure, right? Sounds like a separate DOF specification of some kind would be more informative than redefining the aperture spec.

    If you really want truth in advertising, then we should abolish “full frame” as well. I’m old enough to remember when 35mm was deemed inferior because it was originally a crop factor itself – i’d prefer calling it what it is – 35mm. I know, us older people may think of film whenever 35mm is mentioned, but there are a lot of people out there today who never shot on film and simply cannot relate.

    I’d like to take it a step further and dump the EFL terminology altogether and adopt specs that stand on their own merit instead of relating to some antiquated “standard”. I usually get flamed when I mention this because us old people don’t like to remove our blinders and are used to the 35mm world, but basing everything new on an old film standard doesn’t really make sense to many who never shot 35mm.

  • Sean Mason

    You know why every time this argument comes up, it is a total S-storm? All the crop sensor owners will point out things like:

    2.8 is 2.8.
    Sensor size does not affect DOF
    FOV is not the same as Focal length…

    Yes, but it depends on your reference frame. I recall this same argument in physics with centripetal and centrifugal. If you are going to ignore how cameras are used and not be bothered comparing two very different photographs, then you are cherry picking your facts in order to justify your own purchases.

    If you assume that you are a normal person taking a normal photograph then it makes sense to normalize all of these factors in terms of achieving the same photo. That is what Tony did. He tried to take the same head and shoulders photograph of his wife from the same distance using different formats, apeartures and focal lengths.

  • Jonathan Maniago

    Personally, I’d rather find the actual field of view rather than try to think of 35mm equivalents. Granted, that requires a calculator which can do trigonometric functions, but the resulting value seems far more useful and intuitive (degrees as opposed to millimeters).

  • Seldom Scene Photography

    Crop factor only impacts aperture in the sense of DOF, not of exposure. So he’s actually overly simplifying things here…

  • Tim Worden

    Petapixel needs to stop using old material from Reddit r/photography

  • Sean Mason

    Bruce, you bring up good points. Some sort of field of view for the image circle would make more sense than focal length. There is nothing that is 50mm on your 50mm lens.

    Pick up a magnifying glass. Notice how it zooms in as you move it away from your face. This is a simple lens. The FOV depends on the distance from the glass to your face. Lens manufactures are saying that the lens has the same optical light bending properties as a simple lens that is 50mm away from the focal plane. Even the numbers on your lens are translated into a different reference frame.

  • Seldom Scene Photography

    Agreed — aperture is aperture, and doesn’t need to be “scaled” by crop factor. But people *should* be aware of how the crop factor impacts DOF. Unfortunately, the video’s producer seems to think DOF is the major determining factor in picture quality — which most photographers would likely describe as both simplistic and inaccurate.

    What’s left out of this presentation is that a crop sensor camera’s deeper DOF (for a given aperture) can be to its advantage in low-light environments. So, if all else is equal (camera body noise level at equal ISOs, lens aperture, etc.), a crop sensor camera will let you take an image with greater DOF than will a camera with a “full frame” (35mm equiv.) sensor. Given that weddings (for example) are often held in places that don’t allow flash, this phenomena is helpful if you want to make a picture that actually has more than just the bride’s nose in focus.

    As to film-based standards, I’m with you. 35mm cameras were radical when they were introduced, as they were deemed inferior to “full frame” medium format Hasselblads and such. Now it’s cropped sensor cameras’ turn in the barrel.

  • Jerome van Passel

    Alex you nailed it completely. That is where Tony Northrup explanation fails a bit. He uses all three cameras on the same full frame lens.

  • Seldom Scene Photography

    Yes, but where Tony goes off the rails is by conflating shallow DOF with picture quality. I’d argue the tradeoff really looks more like this:

    Full frame sensors
    + *Can* have better dynamic range / lower noise because the sensor size allows for (but doesn’t always result in) bigger / “better” pixels
    + For same aperture & focal length have smaller DOF — good if you really want/need razor-thin DOF in a shot

    Cropped sensors
    + Cameras / lenses *can be* (but aren’t always) smaller & more portable — as a result, you’re more likely to have a camera on hand when a good shot comes up. This is an advantage in travel / street / candid photography, not so much in studio shots.
    + For same aperture & focal length have larger DOF — good if you *don’t* want razor-thin DOF in a shot (e.g., a low-light situation that requires large apertures)

  • PentaxMan

    Full frame is CanoNikon’s version of Blu Ray: a con to get people to buy the Beatles again.

  • Katy Charlton

    Super!!! Just what I’ve been thinking about and trying to figure out for the past few months! Thank you!!

  • MichaelTapp

    Sweet! I’m currently debating on whether or not to buy the 5Dmkiii or a Gh4. What’s your micro 4/3 portrait lens?

  • manaaaa

    So get your crap “Fuji mirrorless is like having FF”! less is bore

  • tttulio

    her hairstyle split in half on M43 and to the side in FF makes a much bigger difference.

  • sascharheker

    Crop factors affect neither focal lengths nor aperture!

    Just like cropping a print using scissors does not alter the focal lengths or the Aperture used during exposure!

  • sascharheker

    Exactly! Just like a 300dpi File doesn’t have more pixels than a 72dpi one!