I did know all this - and I am not trying to make out being a know-all as it is just common sense when you look at apertures, focal lengths and cropping etc, but it is great to point this out to people who may not realize it.
The Fast Glass TRAP - Don't Make This Mistake!
- Thread starter Steve
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Good video but what would be more interesting is a 400f2.8 vs say a 600f4 and a 600f6.3. To me those are more often than not debated if a 400f2.8 with teles vs a 600 prime. Personally if I need a 600 I’ve never thought it’s getting dark so I better grab a 70-200.
I get the question all the time, but it's more like a chain of thought where someone had the long glass but it's on the slow side. The light gets dim or the background is too busy and they start thinking that maybe they should opt for the faster 200mm or 300mm 2.8 instead of the slower 600mm 6.3 since the 2.8 is so much faster.
I never really bought into that either…and DxO/Topaz/LrC have gotten so good at noise reduction that I don’t worry about it much. But then…I’m not a pixel peeper except while doing processing and most of the ‘better just a little at 2:1 in LrC” goes away when downsampled for screen or print. And the lens blur filter along with darkening the BG and cutting down on its contrast helps with that…not as much or as well as the expensive f4 teles do maybe…but then I also believe in better is the enemy of good enough…and for a non pro hobbyist good enough is well…good enough.
It's interesting to take another look at the formula for estimating depth of field in light of this video. Of course the variables are focal length, subject distance, aperture, and acceptable circle of confusion.
One interesting thing is that acceptable circle of confusion is not a fixed size but varies with sensor size/crop factor as well as image size, viewing distance and even visual acuity of the viewer. So the part about cropping having an impact on dof makes sense. In fact if cropping was the only thing that changed then the cropped image would have less dof when viewed the same size as uncropped.
But in the video the subject distance as well as focal length are also changing. In the dof formula subject distance and focal length are the only things that are squared, while aperture and circle of confusion are not squared. To me this means these two have mudh more of an impact than aperture or the change in the coc from cropping.
Also in the formula the focal length's impact is inverse to the subject distance. This also makes sense since these two work together to decide the size of the image on the sensor -aka the magnification. As in the greater the subject distance the smaller the image on the sensor but the greater the focal length the larger the image on the sensor. So the great revelation that if you double the focal length and double the subject distance the image on the sensor remains unchanged, since the effect of subject distance making objects smaller is countered by the focal length getting larger.
So many moving parts in the video but it all seems to fit.
One interesting thing is that acceptable circle of confusion is not a fixed size but varies with sensor size/crop factor as well as image size, viewing distance and even visual acuity of the viewer. So the part about cropping having an impact on dof makes sense. In fact if cropping was the only thing that changed then the cropped image would have less dof when viewed the same size as uncropped.
But in the video the subject distance as well as focal length are also changing. In the dof formula subject distance and focal length are the only things that are squared, while aperture and circle of confusion are not squared. To me this means these two have mudh more of an impact than aperture or the change in the coc from cropping.
Also in the formula the focal length's impact is inverse to the subject distance. This also makes sense since these two work together to decide the size of the image on the sensor -aka the magnification. As in the greater the subject distance the smaller the image on the sensor but the greater the focal length the larger the image on the sensor. So the great revelation that if you double the focal length and double the subject distance the image on the sensor remains unchanged, since the effect of subject distance making objects smaller is countered by the focal length getting larger.
So many moving parts in the video but it all seems to fit.
FWIW, the subject distance in the comparisons never changed. The tripod was in the same spot for both shots
Also, I agree, there are a lot of moving parts and a LOT I could have added as well, but the trick with these videos is to get the point across without getting too bogged down in an quagmire of technicalities. So, although there was a lot of potential for side discussion about physical aperture diameter, DOF and reproduction ratio, CoC for foreground and background, etc., it would have really bogged things down and I think the viewer would have come away confused. LOL, I sometimes come away confused from this stuff!!
My goal was simply to point out that it's not strictly F/stop that makes a difference and that cropping the heck out of shot fast lens isn't the answer when there's a longer lens in the bag, even if it's slower
Doug Herr
Well-known member
photons per duck.My goal was simply to point out that it's not strictly F/stop that makes a difference and that cropping the heck out of shot fast lens isn't the answer when there's a longer lens in the bag, even if it's slower
