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FORUMS Photography Talk by Genre General Photography Talk
Thread started 18 Apr 2017 (Tuesday) 20:48
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Hard to explain correctly

 
Pagman
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Apr 18, 2017 20:48 |  #1

Many folks Know I have a Nikon D7100 (24mp) anyway this camera has built in a crop mode that in effect crops away 1/3 of the outer edge leaving 15mp's, this has a few uses - larger buffer size, faster frame rate, more files on mem card and smaller files on pc, it also changes the effective eqv fov size of a 300mm lens including the crop sensor crop, to a generous 600mm.

My quiry is this - when using this would it change the lens holding situation aka lens legnth vs shutter speed (I have no stabilisation on my lens) to eliminate shake when I want to get some prop blur from helicopters or piston planes, another words in theory with a 300mm on a crop = 450mm requiring a shutter speed of 1/400 - 1/500sec, but with the extra crop mode equaling 600mm eqv, does that mean it now requires 1/600sec min to stop shake?.

I wonder if the camera calculates the extra crop after pressing the shutter or should the extra crop be taken into account with a higher shutter speed from the get go?

Sorry if I havn't made sense - just dont know how else to explain it-?


P.


Nikon D7100, Nikkor 300 f4 IF ED :-)

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MalVeauX
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Apr 18, 2017 20:53 |  #2

Heya,

Consider it EFL or "effective focal length." Forget the idea of crop this or that. Instead, think of the FOV (field of view ) in arc-degrees/minutes/second​s. If the FOV changes, the arc-degrees/minute/seconds changes and will be less tolerant of movement to keep a subject within a specific FOV range. This is why things blur at long focal length (narrow FOV) compared to short focal length (wide FOV), because the subject will pass through more/less apparent arc-degrees/minutes/second​s based on FOV. So yes, if you're at an EFL of 600mm, you will want a faster shutter speed to compensate for the FOV if you want to avoid blur.

Very best,


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Pagman
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Apr 18, 2017 21:00 |  #3

MalVeauX wrote in post #18331676 (external link)
Heya,

Consider it EFL or "effective focal length." Forget the idea of crop this or that. Instead, think of the FOV (field of view ) in arc-degrees/minutes/second​s. If the FOV changes, the arc-degrees/minute/seconds changes and will be less tolerant of movement to keep a subject within a specific FOV range. This is why things blur at long focal length (narrow FOV) compared to short focal length (wide FOV), because the subject will pass through more/less apparent arc-degrees/minutes/second​s based on FOV. So yes, if you're at an EFL of 600mm, you will want a faster shutter speed to compensate for the FOV if you want to avoid blur.

Very best,


But what gets confusing the image size does not change through the Vf only a grid to show the cropped area, the magnification through the Vf is still showing the eqv of 450mm fov, and I assume the extra crop making it 600mm only happens when the file is processed in the camera, so its not like sticking a convertor on the lens.


P.


Nikon D7100, Nikkor 300 f4 IF ED :-)

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MalVeauX
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Post has been edited 1 month ago by MalVeauX.
Apr 18, 2017 21:07 |  #4

Don't know it works in-camera for your model.

But, you can certainly test it fairly easily. If you pan with a common subject in both full FOV and your cropped mode, at similar rates, you should see significant differences, or none at all if it's truly all done in-camera after the image is created (ie, after the exposure, it culls data, bins, etc).

As it is, if the pixels recording the data are limited, the magnification will be based on diagonal of the sensor to the focal length (since your effective sensor size is reduced in crop mode, for the same focal length, the inherent concept of magnification increases (the concept of reach)). If magnification goes up, again, FOV narrows and you have less arc-degrees/minutes/second​s in the same FOV, and so a subject moving through the FOV will be perceived as moving faster through the FOV, which means it will blur with more time compared to less time (ie, you need a faster shutter speed to freeze the movement through a narrow FOV).

Very best,


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Pagman
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Apr 18, 2017 21:14 |  #5

MalVeauX wrote in post #18331682 (external link)
Don't know it works in-camera for your model.

But, you can certainly test it fairly easily. If you pan with a common subject in both full FOV and your cropped mode, at similar rates, you should see significant differences, or none at all if it's truly all done in-camera after the image is created (ie, after the exposure, it culls data, bins, etc).

As it is, if the pixels recording the data are limited, the magnification will be based on diagonal of the sensor to the focal length (since your effective sensor size is reduced in crop mode, for the same focal length, the inherent concept of magnification increases (the concept of reach)). If magnification goes up, again, FOV narrows and you have less arc-degrees/minutes/second​s in the same FOV, and so a subject moving through the FOV will be perceived as moving faster through the FOV, which means it will blur with more time compared to less time (ie, you need a faster shutter speed to freeze the movement through a narrow FOV).

Very best,


What I do know - the exposure is identicle in both modes as the metering and even AF use the same size relative to the lens and the overall sensor size(I have tried this so know this to be true) so perhaps there is no adjustment needed and just act as if it was 450mm with my 300mm lens?

Where's all the d7100 owners when you need them ;-)a

P.


Nikon D7100, Nikkor 300 f4 IF ED :-)

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Wilt
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Apr 18, 2017 21:24 |  #6

Assuming that your camera has no built in 'smarts' about the relationship of FL and crop size, when you change internal crop size to a smaller size, the camera itself would not compensate with a faster shutter speed. It would be up to the photographer to understand that 300mm on the FF format size needs 1/300 for minimum Rule of Thumb for hand holdable speed, while at the smaller crop size the same 300mm FL needs 1/450 (or 1/600 as you stated) for minimum Rule of Thumb hand holdable speed.

SOME cameras do have some logic about the relationship of FL to minimum handholdable speed built into its programming, but that relationship is assuming a constant format size rather than a user-changeable one.


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Pagman
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Apr 18, 2017 21:25 |  #7

So if Im working it correct - its not the prcessed size (from cropping a file in camera) that matters, its the physical size or magnification viewed throught the lens, that dictates the shutter speed?


P.


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Pagman
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Apr 18, 2017 21:30 |  #8

Wilt wrote in post #18331691 (external link)
Assuming that your camera has no built in 'smarts' about the relationship of FL and crop size, when you change internal crop size to a smaller size, the camera itself would not compensate with a faster shutter speed. It would be up to the photographer to understand that 300mm on the FF format size needs 1/300 for minimum Rule of Thumb for hand holdable speed, while at the smaller crop size the same 300mm FL needs 1/450 (or 1/600 as you stated) for minimum Rule of Thumb hand holdable speed.

SOME cameras do have some logic about the relationship of FL to minimum handholdable speed built into its programming, but that relationship is assuming a constant format size rather than a user-changeable one.

But the extra crop mode is not a physical aspect like we see built into different crop cameras, its an extra file crop done by the camera after pressing the shutter, the only physical sign in the vf that its in this mode is a crop frame shown in the vf, but the vf size does not change to reflect this nore does the view through the lens/vf change in size/fov, just the saved file onto card.


P.


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Wilt
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Post has been last edited 1 month ago by Wilt. 2 edits done in total.
Apr 18, 2017 21:45 |  #9

Pagman wrote in post #18331692 (external link)
So if Im working it correct - its not the prcessed size (from cropping a file in camera) that matters, its the physical size or magnification viewed throught the lens, that dictates the shutter speed?


P.

Let's understand it using this hypothesis...let us assume that if the lens+camera 'shake' displacement is more than 1/100 of the total frame height when it is enlarged to 8x10 print, we can perceive the blur from camera shake.
With 300mm lens on FF, the frame sees 4.58 degrees vertically, and the extrapoliation of our assumption is that a shake of 0.0045 degrees is detectable as shake blur. On an 8" tall print, the shake blur is 0.080" in magnitude; if I have a FF frame, that is 24mm/100 or 0.24mm at the sensor.

Now let us change to APS-C size frame...
the frame shrinks from 24mm to 15mm, but we are still enlarging that image to 8x12" print...so the blur with 300mm lens, at 0.0045 degrees on a 2.86 degree tall frame in APS-C, that amount of blur is 1/64th of the APS-C frame height (rather than 1/100 in the FF frame). So on the 8x12" print it is magnified to blur of 0.125" (1.6x larger) on the APS-C print, rather than the blur of 0.08" on FF print. And that is why a faster shutter speed is needed for the 300mm lens on the smaller format...to keep the blur less than 0.08"


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Pagman
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Apr 18, 2017 22:16 |  #10

Wilt wrote in post #18331704 (external link)
Let's understand it using this hypothesis...let us assume that if the lens+camera 'shake' displacement is more than 1/100 of the total frame height when it is enlarged to 8x10 print, we can perceive the blur from camera shake.
With 300mm lens on FF, the frame sees 4.58 degrees vertically, and the extrapoliation of our assumption is that a shake of 0.0045 degrees is detectable as shake blur. On an 8" tall print, the shake blur is 0.080" in magnitude; if I have a FF frame, that is 24mm/100 or 0.24mm at the sensor.

Now let us change to APS-C size frame...
the frame shrinks from 24mm to 15mm, but we are still enlarging that image to 8x12" print...so the blur with 300mm lens, at 0.0045 degrees on a 2.86 degree tall frame in APS-C, that amount of blur is 1/64th of the APS-C frame height (rather than 1/100 in the FF frame). So on the 8x12" print it is magnified to blur of 0.125" (1.6x larger) on the APS-C print, rather than the blur of 0.08" on FF print. And that is why a faster shutter speed is needed for the 300mm lens on the smaller format...to keep the blur less than 0.08"


You lost me between "Lets and - to keep to keep the blur less than 0.08"

Sorry just cant get my head round any of that - you know like with math when you stare and stare at figures but they just dont sink in, thats why I asked the question - I just couldn't come to an understanding.-?


P.


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Wilt
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Post has been edited 1 month ago by Wilt.
Apr 18, 2017 22:21 |  #11

Pagman wrote in post #18331724 (external link)
You lost me between "Lets and - to keep to keep the blur less than 0.08"

Sorry just cant get my head round any of that - you know like with math when you stare and stare at figures but they just dont sink in, thats why I asked the question - I just couldn't come to an understanding.-?

P.

In simple English, with no numbers...

When you mount a lens of long FL on a camera, the movement of that lens is recorded as blur. Below a certain amount of blur, our brain is sufficiently fooled into thinking 'sharp picture'.

Mount the same lens on a smaller format, and the recorded image is magnified by a larger amount simply to make the same final print size...and the blur is magnified as well! So what used to fool the brain now is perceived by the brain as 'blurry picture due to motion'. So we have the shrink the amount of recorded blur by increasing the shutter speed.


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SkipD
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Apr 18, 2017 22:51 |  #12

Pagman wrote in post #18331692 (external link)
So if Im working it correct - its not the prcessed size (from cropping a file in camera) that matters, its the physical size or magnification viewed throught the lens, that dictates the shutter speed?

What you're forgetting about is the size that you're going to print or display the image at when you're done with the image.

Assume that you print everything at 8x12 inches just to think about the issue. There's got to be greater magnification of the IN-CAMERA image to fill the 8x12 from the cropped image as opposed to a full "35mm size" image in the camera. The greater magnification not only magnifies the image itself, but it magnifies the blur created by camera/lens motion when you made the exposure. Thus, cropped in-camera images need a bit more shutter speed to further reduce the effect of blur relative to larger in-camera images.


Skip Douglas
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Tixeon
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Apr 19, 2017 11:18 |  #13

My simplified (simple minded) approach would be something like . . . making a full sensor 11x14 image then physically cutting out an 8x12 portion. My concern would be whether the resulting 8x12 would be any sharper by letting the camera do the cropping.

Any unsharpness, from shutter speed or focus, will be more noticeable the more an image is cropped & enlarged back up to the desired size - assuming all other settings are the same. Hopefully that simple description doesn't add more confusion.


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Wilt
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Apr 19, 2017 13:33 |  #14

Tixeon wrote in post #18332081 (external link)
My simplified (simple minded) approach would be something like . . . making a full sensor 11x14 image then physically cutting out an 8x12 portion. My concern would be whether the resulting 8x12 would be any sharper by letting the camera do the cropping.

Any unsharpness, from shutter speed or focus, will be more noticeable the more an image is cropped & enlarged back up to the desired size - assuming all other settings are the same. Hopefully that simple description doesn't add more confusion.

Hang on here, that model isn't right. Think of the 11" x 14" print, with 1/60" blur (a visible amount vs. the hypothetical 1/100 not-detectable blur of my post 9). Now cut out an 8"x12"...it still has 1/60" blur in it...the absolute size of the blur on print is the same in the 'cut-out' vs. the 'full size' enlargement. If 1/100" blur was the starting point, it would be equally 'undetectable' in the 11x14 full size and the 8x12 cutout print.

That is why it is better to think of captured blur initially...and when a 15x22.5mm image is magnified by 13.5x to 8x12", a captured blur of a certain size is magnified by that amount as well. So on the 11x14" print it is more visible to our eye than the same blur magnified by 8.5x to make an 11x14" print.


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mfturner
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Apr 19, 2017 14:00 |  #15

That sounds like a great mode, there are times when I know I'll be cropping that much anyway, it would be nice to get the advantages you mention.

But to your question, MalVeauX and Wilt are correct, treat it like the equivalent FL of 600mm.

Another thought if you have the time, go out and experiment by taking several shots each at different shutter speeds in the two modes. I've done this and could quickly see what I could get away with, and where I start only getting two out of three or even one out of three shots acceptably sharp for occasions that need that.




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