PetaPixel

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.

dofcomparison

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)


 
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  • sshoihet

    The only thing that changes “compression” is a change in perspective ie moving the camera. If you stand in the same spot and shoot with a 50mm vs a 200mm, you get exactly the same “compression” because your perspective hasn’t changed. If you crop your 50mm shot to the same fov as the 200mm, you’ll have exactly the same image.

    The only way you get “compression” with your 200mm vs your 100mm is that you increase your distance from your subject to maintain the same framing. In addition, although you will have more background blur using the longer lens (because distant object are more magnified), you will have the same dof if you maintain the same f-ratio.

  • Matt O’Sullivan

    Nope, that’s exactly wrong. Compression, ie the relative size of objects at a distance from eachother, is directly tied to the focal length of the lens. If you put a camera on a tripod and took a shot with a 100 mil lens, then swapped it for a 200 mil lens, then cropped the 100mm shot to the same field of view, the resulting images would be completely different. Try it.

  • Matt O’Sullivan

    Nope, that’s exactly wrong. Compression, ie. the relative apparent size of objects to eachother, is directly tied to the focal length of the lens. If you put a camera on a tripod and took a shot with a 100 mil lens, then swapped it for a 200 mil lens, then cropped the 100mm shot to the same field of view, the resulting images would be completely different.

    Lenses in the 45-55mm range are called “normal” because the compression they render is equivalent to that of the human eye.

    “Compression”, far more than field of view, is in my mind the defining characteristic of a lens.

  • tomdavidsonjr

    Not true. By definition “Full Frame” MEANS the sensor size is 36×24 mm. The creation of a lens that is 35mm in a micro 4/3 does not magically make an 18×13.5 mm sensor full frame (36×24 mm). It simply means there is no crop factor or aspect ratio conversion that the photographer needs to do.

  • tomdavidsonjr

    ”The first thing we do, let’s kill all the lawyers,”
    After reading this comment thread, I would say that immediately after that, they’re going to line up the photographers. Jesus Christ, we are a bunch of whiney, sniping children! This is why we mostly work alone, I guess.

  • Matt O’Sullivan

    Well, yeah it kinda does. It affects depth of focus, which in turn factors into depth of field.

  • Matt O’Sullivan

    Sorry, no. It’s misleading because it doesn’t describe reality. It’s wrong.

  • Matt O’Sullivan

    Nope, this is misleading and, basically, wrong. The signal/noise ratio is not determined by total light hitting the sensor, but by the light-per-area, which is identical given identical lenses and apertures. The cropped sensor in a 7D is receiving NO LESS LIGHT PER AREA than the full frame sensor in a 6D.

    Another factor is of course the density of the photoreceptors. If they are close together, then the noise is increased.

    Think of the light like rain; one piece of sidewalk doesn’t get any “less” wet that a bigger piece of sidewalk.

  • Matt O’Sullivan

    Ahhh wait… Have I been wrong all this time? Hahahha I think I have. Shutting up now.

  • umnumnumnumnum

    This is as boring as listening to someone talk about car engine parts and how they operate. I’ll just a photographer for events, and take cheap pictures with my cell phone, for every day events. Have fun listening to the rest of his commentary. I’m out, peace!

  • http://erickwatson.me Erick Watson

    If I shine 100 lumens in to a Full Frame sensor, an APS-C sensor and a M 4/3 sensor, each sensor with 20 megapixels each, the amount of light reaching each pixel is the same. They do not collect less light.

  • ConfusedMan

    My initial point there was not a practical one, simply another way to see that sensor size does not directly impact DOF. It has to do with other practical decisions made later on (such as preserving the angle of view)…

    Also, if we crop the FF image and enlarge it to 8×10, wouldn’t it have the same DOF (and angle of view by construction) as the image from the cropped sensor enlarged to the same 8×10 (in this case, the focal length a f-number are identical)?

    In my earlier comment, I’m not comparing the cropped FF image to the original FF image, but the cropped FF image to the cropped sensor image, which, for our purposes, should be identical (ignoring pixel density, etc).

    I’m also a lit bit confused about something. The DX image is enlarged more than the FX image to get to 8×10. That means if the FX and DX image have the same DOF before enlargement, the DX image will have a final DOF that is shorter at 8×10.

    We know that increasing the focal length decreases the DOF. Hence, to maintain the same angle of view, the FX lens has a longer focal length than the DX lens. This means that, prior to enlargement, the FX image has a shorter DOF. But now, when we enlarge, the DOF of the FX image and the DX image would be the same at 8×10. But this is clearly not the case, as demonstrated by Tony. What am I missing?

  • http://www.ryanbrenizer.com/ Ryan Brenizer

    Actually, this is entirely false. It will look like a portrait taken with a 70mm. Otherwise every last photo you ever took from your phone would be amazingly, hilariously distorted, because it’s only a 4mm lens.

  • istreetshooter

    Matt is correct on this. The signal-to-noise ratio is measured at a point within the system, not across the entire system.

  • Nate

    I have not read all 114 comments so appologise if this is a repeat question. When comparing bokeh and approximate equivalent aperture he mentioned that the aperture x crop factor will give approximately the same bokeh for the fixed aperture. Is this also true for lenses produced specifically for an APS-C sensor eg a canon 50 f/1.8 vs the 17-55 f/2.8? Using his theory the bokeh using the 50mm f/1.8 on a crop sensor will be approx equal to f/2.6 on a FF sensor but on the 17-55mm f/2.8 is this also true – for example would the bokeh of the be similar to an a equivalent fixed aperture focal length on FF?

  • http://www.monsoonshower.com Saju Thomas Paul

    so, just to clarify, if im using a 50mm f1.8 lens on a 1.6x crop factor camera, would i be getting an aperture of 2.8?

  • Michael Andrew Broughton

    wrong. that compression and flattening is determined solely by distance to subject and distance to background. period.

  • Michael Andrew Broughton

    you’re completely forgetting that an image from a crop sensor has to be magnified more to be displayed at the same size, which means greater apparent noise in the final image.

  • Guest

    take your own advice. you’re wrong. period.

  • Michael Andrew Broughton

    only in terms of dof, compared to a 80mm on ff. it’ll still act like f1.8 in terms of shutter speed.

  • John P. Hess

    And only if you’re comparing it to a 35mm equivalent

  • ImagineIt

    Apertures and shutter speeds are exactly the same on crop and FF sensors. The advantage of the larger pixels on FF is that increase dynamic range (black point to white point) because the signal to noise is likely to be better.

  • SAR_Admin

    Plain wrong guys. Tony did a HUGE mistake on calculation. Focal lenght is always the same. Field ov view changes when doing equivalence calcs!

  • Silverstream

    What is super important to me is the amount of light that comes in a lens at a given focal length, not so much how much bokeh there is. As someone who does a lot of event photography, the majority of event work is about getting a great image and while bokeh is helpful and I try to maximize it when I can(with my 85mm f1.4) it does not stop me from being successful. In regards to shutter speed with my 1.6crop, I soooo wish I had IS/OS/VR on my 85!!!!!!

  • PH

    ZZzzzzzz All this slavish comparison to EXACT equivalence to FF is getting old. For starters – the vast majority of people don’t, or never will own a FF camera. The two shots above aren’t exactly the same – so what!. Is the FF one really better or just different? The FOV crop factor is useful on the SX50, but NOBODY cares about the equivalent f stop – nobody! They buy it for the reach – I know many birders who love it and don’t care what the same shot would look like on a 5D w/600mm lens which they wouldn’t buy anyway. Sure, the 45mm 1.8 MFT has the FOV of a 90mm and DOF 5.6 The 45mm is still a 1.8 lens – can capture the same low light shots as the 90 1.8 on a FF camera, but since its a 45mm it’ll have more DOF – but evidently unless it has razor thin DOF – can’t be any good? Yes, crop sensor cameras AREN’T exactly the same as FF cameras, but we don’t need Tony to feel sorry for us or badger manufacturers (I’m confident they’ll ignore his plea, as they should) to specify in gory detail how their crop cameras compare in every respect to a FF camera Just elitest drivel.

  • Seamus Warren

    Hello PetaPixel Peepers, :)

    I am still not understanding why Tony Northrup says crop factor impacts lens speed/brightness.

    My understanding is the x2 crop factor we were discussing applies to depth of field and with field of view.

    In the example of the LUMIX 12-35mm f/2.8, the lens speed/brightness remains f/2.8 but the field of view and depth of field are the equivalent – in 35mm terms – of a 24-70mm f/5.6 when the x2 crop factor is applied.

    I think the fact we use an f-stop value to describe both lens speed and depth of field and we refer to both values as “Aperture” is adding to the confusion.

    While a sensor’s physical limitations may impact it’s light sensitivity, the crop factor does not impact lens speed.

    Thanks. :)

    PS: Perhaps “undermines” used in an earlier post is too strong a term implying intentional sabotage, which I am sure is not the case.

    Tony’s application of crop factor to the focal length component of the mathematical formula by which we determine lens speed does appear erroneous.

    Again, my understanding is lens speed is not determined by so-called crop factor.

    Thank you. :)