Crop sensor question

drmaxx wrote in post #14454654
I still don't get it. If I take a picture with an FF camera and just crop it - then it has no influence on the sensitivity of the lens for movements. Isn't a crop sensor just exactly that: A crop? But no change in optical behaviour?

A 30 mm P&S might have the same frame as a 180mm lens on a FF. But isn't is still a 30mm optical lens with the same optical behaviour? With a 180mm lens you would stand way farther back to frame the same picture then with a 30 mm P&S lense - therefore you won't have the same optical behaviour.

I might just be too dumb - but in my view a 50 mm should always show the same optical behaviour -- independent of the crop I choose to convert into a picture.

As I said in my first answer in the thread:

Camera shake is a twisting of the camera, that makes the projected image smear over multiple pixels on the sensor.

If you twist the camera a fixed angle, the image projected by the 50mm lens will move as much on the sensor if you have a large-format camera, a "full frame", a 1.6x crop-factor camera or even a P&S camera.

But a shift of the image of 0.01 mm on a sensor that is 36mm wide represents 0.027% of the sensor width. If your sensor have 5000 pixels horisontally, it represents a smear of 1.4 pixels.

A shift of the image of 0.01 mm on a sensor that is 22mm wide represents 0.045% of the sensor width. If this sensor also have 5000 pixels horisontally, it represents a smear of 2.3 pixels.

A shift of the image of 0.01 mm on a sensor that is 10mm wide represents 0.1% of the sensor width. If that sensor also have 5000 pixels horisontally, the smear would be 5 pixels.

So as you can see - a fixed amount of camera shake with the same focal length will hurt more for a camera with a smaller sensor. All because the smaller sensor requires a larger magnification to get to the same-size print. So 50mm is a tele lens for a P&S. But it's a wide lens for a large-format camera.

And with a tele lens, a small shake give big fuzz in the image.
While a wide lens allows much bigger camera shake before it hurts the image.
Hence the rule of adjusting the shutter time to the focal length of the lens - and the rule is specifically for a camera with 36x20 mm film. A LF camera with 50mm lens can manage with much larger shutter times while a crop-factor camera requires correspondingly shorter shutter times.

And since the projected smear maps to physical pixels, it does matter how large you view the image or how large you print. The larger you print, or the closer you view the image, the bigger the individual pixels on the sensor will be magnified.

So photos that looks great on the web can have lots of camera shake that isn't visible in a photo that is 1024 pixels on the long side. But that becomes apparent if you make a big print of the image.

@AJSJones and @pwm2: That does make sense! Thanks for the clarification.

All I know is that with image stabilisation and moving subjects, things are even more complicated so that the old rule of thumb is exactly what it is: Something to use, if you don't have any other information.

In my case (60D as well) 80 mm and with IS I can get away with 1/30 handheld and have a decent picture. However, when ever people are involved I typically use 1/160 for crisp results.

drmaxx wrote in post #14454425
Why should that be? A 50 mm is a 50 mm - independent how much of the picture is captured by the sensor. It is still the same 'magnification' of a movement. So why should crop sensor cameras be more sensitive then FF? I don't get it.

The shutter speed rule-of-thumb has been applied to 35mm film cameras for decades. As said above, different photographers can shoot slower than the rule-of-thumb shutter speeds and get nice sharp images. Others have to double or triple the shutter speed to avoid blur due to camera motion. On average, though, the rule-of-thumb is a reasonable starting place for most photographers.

That said - for an APS-C camera, it is important to know that the 35mm film rule-of-thumb does not apply directly. There's a simple reason for this:

Let's assume that the camera motion with a 35mm film camera with a 100mm lens on it and a shutter speed of 1/100 second produced blur on the negative that was 0.1mm long. Enlarge the negative (24mm X 36mm) to an 8" X 12" print. The motion blur now becomes 0.846mm long on the print because the 8X12 print is 8.46 times larger than the 35mm negative. This .846mm blur is a noticeable size if one is critical about examining the print.

Now, put the same 100mm lens on a 30D camera (with its 15.0mm X 22.5mm sensor), use the same 1/100 second shutter speed, and have precisely the same motion of the camera, producing a blur of 0.1mm on the sensor. Now we make an 8" X 12" print from the image produces in the 30D camera. The 8X12 print is 13.54 times larger than the sensor in the 30D, so we magnify the 0.1mm blur by 13.54 and it becomes 1.354mm on the print. This is significantly larger than the blur on the print made from the 35mm negative. In fact, if you divide the 1.354 by .846, you will get the "magic number" of 1.6.

Had we shot the image in the 30D with a faster shutter speed such as 1/160 second (the divisor 100 multiplied by 1.6), the .1mm blur on the sensor would have been reduced to .0625mm instead of 0.1mm. Then, when enlarging that image to an 8X12 print (again using the 13.54X enlargement) we would have the same .846mm blur on the print as we have on the print made from the 35mm film negative.

So you see, if everything else is identical (the amount of movement of the camera, the focal length, etc.), there is more motion blur from the APS-C format camera than from the 35mm film (or "full-frame" DSLR) in the final print. We can remove that difference if we use a faster shutter speed with the APS-C camera.

The bottom line is that to keep the results of the 1/focal length rule-of-thumb (for a 35mm film camera) consistent for an APS-C camera we must modify the formula to 1/(focal length X 1.6). This is an absolute fact which is indisputable when one does the math.

Pixel density has NOTHING to do with this. It's purely the size of the in-camera image and the size of the final viewed image or print that matters.

SkipD wrote in post #14455386
Pixel density has NOTHING to do with this. It's purely the size of the in-camera image and the size of the final viewed image or print that matters.

It's a bit of yes and no.

For prints, the blur is a percentage of the sensor and the same percentage of the print (unless the user performs a crop of the sensor data, in which case the blur will be magnified).

But a big issue here is that this forum is full of pixel peepers. People who always expect/assume that every pixel in their image should matter. And then it's important to note that 10 MP for a "full frame" represents larger pixels so the same-size smear will cover fewer pixels than it would on a crop-factor or a P&S camera.

So it's important to that the computational example also mentions pixels, making people aware that if they want to use a camera with more but smaller pixels, they need also spend more care when taking their photos. And that in many situations, we just can't get full use of the sensor resolution. Because we can't use shorter shutter time with available light. Or we do need a small aperture for the depth-of-field and have to accept a diffraction-limited photo.

SkipD wrote in post #14455386
Pixel density has NOTHING to do with this. It's purely the size of the in-camera image and the size of the final viewed image or print that matters.

pwm2 wrote in post #14455667
It's a bit of yes and no.

For prints, the blur is a percentage of the sensor and the same percentage of the print (unless the user performs a crop of the sensor data, in which case the blur will be magnified).

But a big issue here is that this forum is full of pixel peepers. People who always expect/assume that every pixel in their image should matter. And then it's important to note that 10 MP for a "full frame" represents larger pixels so the same-size smear will cover fewer pixels than it would on a crop-factor or a P&S camera.

So it's important to that the computational example also mentions pixels, making people aware that if they want to use a camera with more but smaller pixels, they need also spend more care when taking their photos. And that in many situations, we just can't get full use of the sensor resolution. Because we can't use shorter shutter time with available light. Or we do need a small aperture for the depth-of-field and have to accept a diffraction-limited photo.

One has to realize what the effective size of an images is when viewed at 100% on the computer. If that dimension is understood, then my statement (in the top quote here) still applies perfectly.

SkipD wrote in post #14455689
One has to realize what the effective size of an images is when viewed at 100% on the computer. If that dimension is understood, then my statement (in the top quote here) still applies perfectly.

And do you think people really consider the actual magnification involved?

SkipD wrote in post #14455689
One has to realize what the effective size of an images is when viewed at 100% on the computer. If that dimension is understood, then my statement (in the top quote here) still applies perfectly.

The problem with this though is that, judging by many of the posts on this forum, there are very many photographers out there that just cannot comprehend this facet of the science of photography. It is right up there with "crop factor and reach" as things that seem to get people confused with digital imageing.
Please note that I am hoping that this comment will not kick the usual arguments off again, taking the thread off topic.

Alan

pwm2 wrote in post #14455740
And do you think people really consider the actual magnification involved?

BigAl007 wrote in post #14455784
The problem with this though is that, judging by many of the posts on this forum, there are very many photographers out there that just cannot comprehend this facet of the science of photography. It is right up there with "crop factor and reach" as things that seem to get people confused with digital imageing.
Please note that I am hoping that this comment will not kick the usual arguments off again, taking the thread off topic.

The question that started this thread was about the use of a rule-of-thumb for slowest shutter speed to use with any particular focal length on an APS-C ("crop") format camera.

Everything that I wrote in my posts here simply supports the reason for adding the "crop factor" into the basic "1/focal length" formula.

Changing the pixel density of a camera does absolutely nothing that would affect this formula. Besides, a rule-of-thumb formula is only for the "average" photographer and any individual photographer may find that he/she needs a different factor anyway.

Skip I wasn't disaggreing with you over anything that you said. In fact I pretty much agree with everything that you have said. There are though a large number of folks out there that seem to struggle with basic optical theoey, or think that because it is now mostly recorded digitally that this somehow changes things.
Back in the film days I only used MF cameras occasionally but found for me that with 6x4.5 1/f seemed to work best for me, but the way you use the camera in that situation is so different than for a 35mm style SLR.

what lens are you using? if it's stabilized, you can throw that old rule out the window. a stabilized lens will let you shoot at least a couple of stops slower and still get sharp photos. i've gotten nice, sharp photos at 400mm, shooting handheld at 1/125, with a stabilized lens.

if your lens is NOT stabilized, then yes, you probably should try to shoot at LEAST the reciprocal of the focal length, if not a bit faster.

Like any good rule- you know the rule so that you can revert back to it when you need to. Otherwise, like a lot of other rules, it does not always apply to me; feel free to break it when it's convenient.

If I go into the "rules" with someone, it's because they violated a rule that they didn't know about.. Then you let them know the rule, and how it works- and that like all rules- you can break them- when need be.

Thank you all for all of the helpful information. Interestingly enough, all the different points of view reinforce what I always loved about photography: That it is an individual pursuit.