Ok so I was reading one of the other threads and I was really confused.
I know that crop factor effectively multiplies the focal length, but in regards from full frame to crop bodyhow does it effect f/stop? I was trying to figure it out looking at effective focal lengths and apertures and I could not figure it out. Can you please explain??
Originally Posted by powers_brent
Yes, you need to multiply the f-stop by the FOVCF as well. The reasonfor this is to achieve the same framing with a 35mm format camera (given the same focal length) you would need to get considerably closer to your subject. The closer you are to your subject the thinner the DOF.
DOF is dictated by: focal length, aperture, subject distance
I'll give an example. I have no idea how far or close you would need to be to achieve the same subject framing so I'll make that part up, the rest should apply though:
If you took a picture of a subject that was 10 feet away with a 100mm lens at f/2.8 on a 1.6X FOVCF camera you may need to move in to 7 feet to take that same picture (same framing) with a 35mm format camera. Since you are now closer to your subject, the DOF is now thinner. In order to achieve the same DOF as you would have with the 1.6X FOVCF camera you would need to stop down to f/4.48 (2.8 * 1.6). Make sense?
Mark's right. I would add that moving closer or further away will change a vital element of composition: perspective. Many times it's not desirable to change this element of composition, and sometimes it's even impossible. The only way to keep perspective the same between two sensor sizes is to change the focal length by the crop factor.
If any two lenses have the same iris diameter, the DoF, diffraction, and light gathering power will be the same, no matter what the focal length, f-number, or sensor size. Given the same field of view, and non-macro focus distance.
ahhhhhhhhhh ok! Now I get it. Perfect. Thanks Guys.
What do you mean, "how does it effect f/stop?" In terms of exposure, it doesn't affect it, at all. The same amount of light per square centimeter gets through the lens, regardless of the camera or sensor.
If you're concerned about depth of field, don't use the "crop factor." Instead, go to DOFMaster and check out his calculator. It takes into account the "circle of confusion," which depends somewhat upon the magnification when printed. (Depth of Field is not a linear function of the aperture NOR the focal length--you can't just multiply the DOF numbers by the crop factor.)
George Slusher
Lt Col, USAF (Ret)
Eugene, OR
Agreed. But I would argue that "total amount of light" matters far more than "light per area".The same amount of light per square centimeter gets through the lens, regardless of the camera or sensor.
F-number is the established paradigm of photographers; how most of us think and work day in and day out. This mode of thought was necessary for film, since the response curve was intrinsically tied to the exposure (no matter what the sensor size), and it continues to work just fine in any discussion of a raw digital sensor of a single size.
However, in a discussion of raw digital sensors of multiple sizes, the old paradigm is suboptimal and even misleading. Now that we have a linear capture medium, it doesn't matter what the intensity of light per area is: all that matters is the *total* amount of light.
In film, if you go from 35mm to MF, you can't reduce exposure to keep DOF the same, because the image would be underexposed; that's why many MF/LF shooters used tripods and slow shutter speeds. But that's not true of digital: the size of the sensor compensates perfectly for reduced exposure, so any larger sensor can get the same image as a smaller sensor.
If you think in terms of f-number only, then it may seem that larger sensors have less noise, thinner DOF, less diffraction, higher MTF, more weight, etc. But that's not true in all circumstances: only when f-number is kept the same.
For example, people often lament that there is no FF equivalent to the 17-55 f/2.8 IS. But there is, and it's even better! The only reason they didn't see it is because they were held back by their f-number-centric frame of mind. The 24-105 has lower light intensity per area (exposure) but it has so much larger area that it more than makes up for it and actually collects more total light and has less total noise. You actually have to stop down the 24-105 by a third stop to get images similar to the 17-55.
As another example, some think that the f/2 lenses on four thirds are a great advantage over the f/2.8 lenses on full frame: they think they can get faster shutter speeds. In fact, it only takes an f/4 lens on FF35 to get the same shutter speed, noise, DoF, and diffraction as an f/2 lens on Four Thirds.
It's not always even possible to keep f-number the same when going to a larger sensor. Most medium format lenses are only f/2.8 or slower, and actually have deeper DOF (and lower total light gathering power in low light) than the f/1.2 lenses on FF35.
A tiny 5x4mm digicam at f/2.8 has the same light intensity as a 56x41mm f/2.8 medium format digital back. But the larger camera gathers for more light in total, and when printed at the same size, it has far less noise, less diffraction, and thinner DOF. However, if we apply the crop factor to f-number: f/31 on the MFDB will result in the same total light, same noise, same diffration, and same DOF.
When comparing multiple format sizes, keeping f-number is same is not necessary, desirable, or even possible in all circumstances.
Crop factor explains equivalence between two different sized sensors. Multiply the focal length by the crop factor to get the same angle of view. But angle of view is not the only thing that may be compared between two camera systems.
Noise, DOF, diffraction, MTF, MP, weight, etc. can also be compared, and these comparisons should be done by applying the crop factor to the f-number. The effect is to keep the iris diameter the same. Then, no matter what the sensor size, noise, DOF, and diffraction remains the same.
For non-macro focus distances, I find that the crop factor does provide equivalent depth of field to a very close approximation, so I find it highly useful.
I've done the calculations from 1/3" sensors up to 6x17 cm sensors and found that crop factor precisely correlates with DOF. Have you found any examples where it does not? As Mark stated, 50mm f/2.8 has the same DOF on APS-C as 80mm f/4.5 on FF35.
To correct a bit what I wrote before, rather than editing the previous post:
What do you mean, "how does it effect f/stop?" In terms of exposure,
it doesn't affect it, at all. The same amount of light per square
centimeter gets through the lens, regardless of the camera or sensor.
If you're concerned about depth of field, don't use the "crop factor." Instead, go to DOFMaster
and check out his calculator. It takes into account the "circle of
confusion," which depends somewhat upon the magnification when printed,
if I recall correctly.
The reason I said that is that which
camera's f/stop gets multiplied by the FOVCF depends upon what you are
comparing. If you want the same framing with the same lens,
then Mark is right (or, at least, pretty close--the details are a
little bit different). Here's data from DOFmaster for the 5D (full
frame) and 30D (1.6x).
30D, 100mm, 80 ft, f/2.8: 70.8 - 92 ft, total 21.2 ft, 9.2 ft in front, 12 ft behind
5D, 100mm, 50 ft (to get the same framing), f/2.8: 44.3 - 57.4 ft, total 13.1 ft
To get the same DOF total, stop the 5D down to f/4.5:
5D, 100mm, 50 ft, f/4.5: 41.5 - 62.8 ft, total 23.1 ft, 8.5 ft in front, 12.8 ft behind
(Do note the difference in the front and behind numbers--not great, but different.)
That's only one comparison, however. What about comparing the DOF at the same distance
and focal length? In that case, it's the 30D's aperture that would have
to be stopped down--the reverse of the above. Start with the 50 ft,
f/2.8 data above.
30D, 100m, 50 ft, f/2.8: 46.2 ft - 54.4 ft, 8.19 ft total
Stop down the 30D to f/4.5 and you get 44.3 - 57.4 ft, just like the 5D at f/2.8.
Why
is that important? Distance, not focal length, determines perspective.
If you want the same perspective, you'll have to have the cameras at
the same point.
Now, get the same perspective--i.e., distance--and the same framing, by changing the focal length. We could use either camera as the standard, so choose the 5D.
5D, 100mm, 50 ft, f/2.8: 44.3 - 57.4 ft, total 13.1 ft, 5.7 ft in front, 7.4 ft behind
30D, 63mm, 50 ft, f/2.8: 41.5 - 62.9 ft, total 21.5 ft, 8.5 ft in front, 12.9 ft behind
So, to get the same DOF along with the same perspective and framing, the 5D would have to be stopped down to f/4.5, as before.
George Slusher
Lt Col, USAF (Ret)
Eugene, OR
And this is because DOF/bokeh is amplified by the focal length and thus to reduce that you stop down right?
*DOF shortened, Bokeh amplified
I can't quote Daniel's message--my browser won't show the formatting tools, etc.
I corrected my error in my second message. The crop factor is close, though one has to figure out which way to apply it, as I showed. It all depends upon what you want to be the same: framing, perspective, or both. You only have three variables: distance, focal length, and aperture.
Applying it to point-and-shoot cameras in another example, using the Canon S3 IS (6x FOVCF).
S3, 16.7mm (equivalent to 100mm full-frame), 50 ft, f/2.8: 28.1 - 228.6 ft, total 200.5 ft, 21.9 ft in front, 178.6 ft behind
That would have the same perspective and framing as my last 5D example:
5D, 100mm, 50 ft, f/2.8: 44.3 - 57.4 ft, total 13.1 ft, 5.7 ft in front, 7.4 ft behind
Now, stop down the 5D to 6 x 2 = f/16.8. DOFMaster only has f/16, at 29 - 183 ft, and f/18, 27.5 - 273 ft, so f/16.8 would probably be about right.
I can't argue with your analysis, Daniel--I certainly don't know enough to argue, except for one thing: for me, shutter speed, itself, is important--not for "camera shake," but for moving subjects. In that case, the light per unit area per unit time does matter. If all you photograph are static scenes, you're probably right, though I expect that there's some effect of the size of the "pixels." (The 1.6x bodies generally use smaller pixels, especially the high-density bodies like the 50D.) However, photograph a galloping horse with a 5x7 view camera. For the same perspective (distance) and framing, you'll need the same shutter speed as a full-frame, 1.6x body or 6x point-and-shoot to stop the action. What counts there is the angular speed of the moving parts, which translates into motion blur. Theoretically, it wouldn't depend upon focal length/framing, but only upon distance. However, in practical terms, a shorter focal length lens gives a smaller image of the horse, so the motion blur isn't as noticeable. As I recall, that's also the reason for the 1/FL guideline, essentially the angular rate as a fraction of the field of view, though correct me if I'm wrong. (It won't be the first time.)
George Slusher
Lt Col, USAF (Ret)
Eugene, OR
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