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Crop Factor: Why you multiply the aperture by the crop factor when comparing lenses

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  1. Chelsea is the wow factor in this video. She is incredibly beautiful and patient. She never moved while Tony was making the video. Impressive on many counts.

  2. No. This is just completely wrong on *so many levels*. If you change the working distance to compensate for magnification then the depth of field will be the same. The issue is that the CoC is larger relative to the sensor. The aperture *does not change*. This is just ridiculous.

    Also a larger sensor does not result in "more light" being gathered, because, as you pointed out, this depends on the sensel size, and not the size of the sensor. This though has NOTHING AT ALL to do with DOF or aperture. In fact, it doesn't change sensitivity at all, only provides more signal relative to intrinsic noise.

    Oh. And going from f/2.8 to f/5.6 is a decrease in aperture.

  3. Basically what's going on is this:

    Lenses focus light into discreet "packets" called the "Circles of Confusion", or "CoC". When you look at OOF portion of an image, these circles are sufficiently large that they read as being "blurry". That is the reason why you get "bokeh balls" through things like leaves.

    So when we're talking about sharp focus, what we really are saying is that the CoC is sufficiently small enough to read as being "sharp". This is also why larger apertures give less DOF, because the CoC is larger. But the focus point is always a 2D plane, there is no real "DOF" only a range of space where the CoC is going to be small enough for us and resolving power of our devices to consider it "sharp".

    So when we change the format size the CoC is relatively larger on smaller sensors, resulting in a more blurry background. But this also goes for the whole of the image. In fact, if we compensate the magnification by changing our working distance, the bokeh will be the same, though the DOF will increase. Likewise we can decrease the aperture (say from f/2.8 to f/5.6) and the bokeh will be the same. This latter point is what he was getting at, but it's a pretty weird way of talking about it. Bokeh should be seen as a function of magnification and working distance, and not a function of aperture. Because we tend to have longer working distances with a crop sensor to frame the whole image, it is more or less a non-issue.

    But absolutely the aperture does not change with sensor size, only the relative magnification.

  4. So here’s the thing I’m new to all this camera stuff and I understand the crop factor and multiplying and dividing both focal length and f/stops. If a full frame camera lens works at 50mm at f1.8 and the same lens get placed on a M43 sensor it will now act like 100mm at f3.6. So are you saying in theory if I wanted my M43 to work at 50mm at f1.8 I would need a lens that’s capable of 25mm at f0.9??? Now I don’t think I’ve ever seen a lens with those specs. Is there where speed boosters come in?

  5. If you use the same focal length and the same aperture on every camera you get the same dof. Obviously the field of you is changing because of the crop factor.
    You cannot compare shots taken with a different focal length because that does obviously effect the angle of the light that hits the sensor which does obviously effect the dof. It's just physics Tony….God damn it. Stopping down to 5.6 at 200mm just gives you the same dof like if you would shoot at 100mm 2.8.

  6. Tony >> This video tutorial is absolutely top of the rank. For all my thinking I was still not applying the crop factor times the aperture for comparison.
    As you say I had been trained to think crop factor alone by myriads of the junk produced by manufacturers.
    Just to complicate the subject further, I never really care too much about the focal situation behind the subject, because I operate photoshop to manipulate everything outside the subject.
    But still, an ace is an ace and this comparison video is a and must view and think for all serious photographers. paul scott NZ

  7. Well my head just exploded. Great video and explanation. I'm going to have to process let my mind process this since it's the first time anyone has explained the relationship of crop factor to aperture and ISO.

  8. The F stop measures the amount of light entering the sensor . The depth of field looks different for sure but it’s 1.8 ? So why would manufacturer multiply crop factor to to the aperture. Just clarify this for me please ?

    Ok depth of field looks different

    But is exposure/brightness different .

    We aren’t talking smaller pixels less light im asking in a low light situation would they both be at same brightness and not depth of field .

  9. Good video, I knew about this.
    I want to know 2 more comparisons between full frame body/full frame lens 50mm or 85mm/f1.8 vs aps-c body/aps-c lens 35mm or 50mm/f1.8 which makes very close distances match. Will the back ground blur be the same or aps-c f1.8 lens will acts as 2.7/2.8 compared to full frame body & full frame lens?
    Another comparison is if aps-c f1.8 lens is used on full frame body even though it only uses crop factor center sensor area only and loses megapixels but will it acts same as f1.8? or f2.8 after crop factor.
    Thanks, if anybody know..

  10. What about when you're using a full frame camera in APSc mode? the photo info tells me that my 85mm f/1.4 is 127mm f/1.4 when I shoot in crop mode.

  11. If u shoot with F1.4 lens on both Crop & Full sensor & compare the bokeh, it would be a fair comparison. My answer is … bokeh is the SAME!! Not 1.5times worse!!! Which makes your theory total crap.

  12. Thank you, Tony & Chelsea, for your informative and entertaining tutorials! I realize this video is already from 5 years ago, but I am hoping you could answer the following question: does the crop factor also need to be applied to the aperture when the lens is designed for an asp-c sensor? E.g. I enjoy the results I get with my Sony a6000 and 55-210mm kit lens at 210mm f6.3, but if I were to use the full frame 70-300mm lens with f5.4 at 300mm on the a6000, this would mean the maximum aperture then becomes f7.9, thus slowing down my shutter speed and creating less bokeh—or is the aperture on my 55-210mm aps-c lens already, technically, f9.3 at 210mm (meaning 315mm when applying the crop factor)? In other words, would the results with the full frame lens—notwithstanding the additional "reach"/crop—be more disappointing? I apologize if this sounds confusing, and thanks again for sharing your knowledge and encouragement!

  13. I was looking at upgrading from my dx Nikon 55-300mm (crop sensor lens) to the Tamron 70-200 (full frame lens). Does this mean that the 200mm on the full frame lens is equivalent to the 300mm on the from crop sensor lens? Thank you to anyone who can explain this to me.

  14. Dear Tony and Chelsea,

    Thank you so much for your video, I keep on learning a lot every time. The crop factor as a multiplier for the aperture, in order to compare lenses in an accurate way, is almost untold in the whole Internet.
    I wish I could make you a technical question about comparing lenses on different sensor size. When I use a 35mm on a aps-c camera, the field of view can be compared to a 50mm of a full frame, but what about lens distortion? Do I have to expect the same lens distortion of a 50mm on ff if we consider the same field of view?
    Thank you SO MUCH for your time and kindness.
    Best wishes,


  15. This is an amazing tutorial for what it explains that so many others don't tell you. Everything about this video was spot on and makes a world of difference. Thank you so much.

  16. does that mean if you wanna buy a digitalcamera only for the purpose of maximum magnification, lets say you want to see as far as possible, you are stuck with the smallest sensors?

  17. You're right in almost every word. But your analysis miss the point, I really don't care about lossing a f-stop of DOF between formats. I'm surprised the way M43 cameras fullfill every pixel and bring images to life. Something that Canon's "color science" doesn't achieve straight from camera. I own a Panasonic G85 from 2 weeks ago and I am astonished by its extraordinary color rendering, its low light capabilities and image quality. BTW, I haven't met a client worried about any of the issues mentioned about DOF differences, but I know the freedom and flexibility I won with M43 for accomplishing a good work. Thanks anyway.

  18. Very clearly and concise delivery. I appreciate it as I only somewhat grasped the concept. Will make me really think about how far I want to go down the M4/3 rabbit hole of lenses for my G7. Will be getting either the 15 1.7 or 25 1.7… but how many more than that?

  19. What about if you make your money from landscape photography,or you are just shooting for fun? I think Olympus or Panasonic are much better then,due its size, lenses weight and biggest dept of field.Not to mention they have much better image stabilization up to 6-7 stops. I guess nothing can beat Olympus and Panasonic's M43 if you are hiking and traveling photographer. Going back in the past, probably that is why Leica format get so popular,even with much lower quality, compare to 8×10, 6×6 format. I own fast lenses for my full frame cameras,but most of the time I use them for they faster and easier focusing capabilities at low light and shoot wedding portraits stopped at around 5.6,because is pretty sad if you have bride at focus but blurry groom,not to mention it can be a tragic event if groom is at focus but bride and Mom are with blurry smiles. At the end I think there is nothing wrong with Micro 4/3 format, they are amazing cameras with amazing video and high tech capabilities. It is the skills of photographer not the camera or camera format. Honestly in the future, I don't see much benefit of bigger sensors, if you are not highly paid studio photographer. Seeing how smart phones are killing the camera's market I am not very optimistic at all. Soon we are going to be able to shoot the entire wedding with a smart phone, mark my words.

  20. A 100mm f2 lens is a 100mm f2 lens regardless of the body it is used on. This is very confusing for new photographers. An f2 100mm lens has a focal length of 100mm and the f2 means it has a focal ratio of 2, the max aperture of this lens is 50mm in diameter, 100mm/50mm = 2 hence f2. This applies to the lens regardless of the sensor size.

  21. Basically, the closer you stand to your subject, the "better" the bokeh. With a crop sensor, you have to physically take steps backwards away from your subject in order to obtain the equivalent field of view to the full frame which changes depth, which changes bokeh.

    Conversely, if I didn't physically take steps backwards, I would have to zoom in on the APSC camera to obtain the equivalent field of view of the full frame. The shorter the focal length, less the bokeh due to less compression.

    Did I get this right? lol

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