It's nice when nobody disagrees about the facts
Never gets in the way of an argument though...[]
It's nice when nobody disagrees about the facts
Never gets in the way of an argument though...[
I fear Daniel Browning's knowledge.
Originally Posted by Keith B
Me too. But, his obvious knowledge and willingness to share w/me is damned nice.
I printed out this thread and actually studied it. Exceptionally good information, and exactly what I was after. While not extensive, my understanding of aperture and stops of light are considerably improved. BTW, what is the unit of a "stop of light"? Anyways, I've ordered a 5d II at 17Photo and the 24-70L. It came down to what I (think) I needed/wanted. More often than not I'm shooting fast moving dogs, etc so IS will be of no benefit there. Given the explanation that by going from f/4 to f/2.8 I can double the shutter speed at any given ISO, I'm going to see how it works. My old 24-70L was mounted on a 350 Rebel. Why on earth I sold it is still beyond me other than continuing in my tradition of buy high/sell low. Anyone interested in a lightly used 50d?
You guys hankering for a 24-70L/IS version should thank me. By purchasing today, I feel I'm guaranteeing the new model will be out shortly. At which point, I'll upgrade and spend a bundle more in the process.[:P] You're welcome.
Seriously, I'd like to thank again all those who helped enlighten me on this subject. gary
Thanks Keith and Gary.
Photons. Somewhere on the order of 2e+12 photons per photograph, give or take a few orders of magnitude.
Photons. Somewhere on the order of 2e+12 photons per photograph, give or take a few orders of magnitude.
A "stop" is a factor of 2. (So a "stop", or even a "stop of light" is actually unitless)
So one stop more light means double the light. One stop less light means half the light. A lens is one stop faster if it lets in twice as much light. Moving the iso up one stop means doubling the iso. Stopping down one stop means closing the lens aperture so half as much light gets in. Etc.
I don't see how you can have a fixed number of photons and say that is a stop of light. I'm sure I'll be corrected if I'm wrong [
You're right; that wouldn't make sense. I guess I interpreted the question as "what is the unit of light?". I.e. half/double the photons per stop.
This post has created more questions than answers for me. In the end, I agree that the 5dII is the better solution, but many of the points that Daniel makes do not appear logical. I suspect it is
a misunderstanding on my part. Below is my
understanding of this thread's topics - please correct any errors as I'm very eager to learn more.
I'm attempting to compare apples to apples, here are some ground rules for my conclusion at the end:
The 1.6x 50d has half the sensor area of a FF 5dII - therefore half as much light still produces the same light intensity at the 1.6x sensor likewise the FF requires twice as much light.
A 17-55 on a 50d is effectively a 27-88mm from a full frame perspective - DoF not considered. I mention this for the 1/shutter speed rule.
The 24-105 has the same 77mm filter size as the 17-55. This is arguably the same the front element, correct? I did not understand Daniel's comparison.
On a 50d much of the 24-105's (effectively a 38-168) image would be projected off the sensor - wasted light.
In Bryan's review of the 17-55 he says it is arguably the most hand holdable lens Canon makes, this excludes camera body and its' ISO perforamnce. The combination of wide aperture, IS, and wide angle are my understanding as to why this is true. This powerful combination is unmatched by any other canon lens for static subjects.
All of this relates to optics, which I felt Daniel touched on for the most part. I believe there is an light per area advantage of the 2.8 vs the 4.0. A very specific example will clearly show my misunderstanding or my understanding of this. Assuming equilavent ISO, equal shutter speeds, equal subjects, and the 5dII at 4.0 properly exposes the image. Will the 50d at 2.8 (remember the assumptions) overexpose, underexpose or equivalently expose the image? I belive it will overexpose it.
Ultimately, the question was - will the 5dII's ISO performance compensate for the light advantage the 2.8 has? Similarly asked - Is the 5dII's ISO performance that much better than the 50d's? Now the question becomes - How much better is "that much" better?
For reference, Bryan shows the 50d vs 5dII ISO comparison well in the 5dII review.:
http://www.the-digital-picture.com/Reviews/Canon-EOS-5D-Mark-II-DSLR-Digital-Camera-Review.aspx
When comparing the 5dII and the 24-105 4.0 3 stop IS to the 50d and the 17-55 2.8 3 stop IS - I believe that the ISO of the 5dII needs to be one stop better than the 50d's. Why? Because the 2.8 vs 4.0 focal ratio, the illusive "stop". In my opinion, the charts easily show the difference between sensor noise is at least 1 stop of improvement, but you should be the judge. Compare ISO 6400 of the 5dII with ISO 3200 of the 50d - the difference in noise is substantial on both the cloth and color card. I agree with Daniel's point that the efficiency of the 50d is greater than the 5dII, but this is a minor 5dII disadvantage comparatively to the large advantage the 5dII makes up with the ISO charts.
My ISO conclusion - 5dII's ISO performance easily outperforms the 50d's even though the 24-105 4.0 IS optically has a light intensity disadvantage compared to the 17-55 2.8 IS.
If weather sealing, build quality, and expanded range the 24-105 offers are also considerations it makes the victory that much sweeter.
Yes. (It's actually two and a half times smaller area, but half is close enough: (36x24)/(22.3x14.9)=2.6X).
I was wrong. Thanks for the correction. Yes, they are the same size. The front element has to be at least as large as the aperture, but it can be any size according to the lens design. I should have left that point only to telephoto lenses where it is more often true. I should have stuck to aperture: if you hold both lenses up with the end caps off, and look through them from the front at a bright background, you'll see the same size aperture (hole) through them. With the 17-55 f/2.8 at 55mm f/2.8, for example, the hole will be the same size as the 24-105 set to 88mm f/4.5 (though f/4 is close enough to look the same).
Excluding the camera body and ISO performance, yes. If those are taken into account (as you did below), then I'd argue that the 24-105 on full frame eeks out slightly more hand-holdability (as you intimated) thanks to the wider field of view, slightly wider aperture, and the same 3-stop I.S.
Agreed: overexpose.
I'd say it's in close proportion with the difference in area, except for a small difference allowed for the slightly higher technology of the 50D. In the case of the 24-105, the aperture is 1/3 stop wider than it needs to be for this difference (2.6X area difference means 1+1/3 stop). Generally, I think we can expect it to scale with the area of the sensor, then we factor in differences in technology/performance on top of that expectation.
Great post, Matthew.
It seems to me like you understand the situation pretty well.
There is a simple way to think of all of this. It is common to say that, eg, a 100mm lens on a 1.6fovcf camera acts like a 160mm lens on a full frame camera. Ie, you multiply focal length by fovcf. What people more rarely point out is that you should also multiply f/ number by fovcf. For example, an f/2 lens on a 1.6fovcf camera acts like an f/3.2 lens on a full frame camera. Or, to put another way, an f/4 lens on a 5DII acts like an f/2.5 lens on a 50D. Cropping does not affect aperture, but it makes lenses act longer and slower.
This is true for both depth of field and exposure considerations. That is, you'll get (slightly) narrower dof and more background blur with the f/4 lens on the full frame than with the f/2.8 lens on the 50D, and, for a given amount of noisiness, you'll get (slightly) faster exposures with an f/4 lens on a full frame body than a f/2.8 lens on a 50D.
Of course I'm not saying you can shoot iso 400 on both cameras, f/2.8 on 50D and f/4 on 5DII and have a faster shutter speed on the 5DII. Obviously at a given iso, the f/4 lens will have a slower shutter speed no matter how big the sensor is. What I'm saying is that because the 5DII sensor is 2.5x as large, iso 1000 on the 5D II should be about as noisy as iso 400 with the 50D. So you can shoot f/4 at iso 1000 on the 5DII and have a faster shutter speed with about the same noise as f/2.8 at iso 400 on the 50D. (This is all just another way of saying what you said above... I think you have it exactly right).
Right. But you can say more. A 17-55 f/2.8 on a 50d is like a 27-88mm f/4.5 on a 5DII (2.8 times 1.6 is about 4.5), and this includes DOF and exposure considerations.
Exactly right. The 24-105 f/4 on the 5D II is slightly more hand holdable than even the 17-55 f/2.8 on the 50D (assuming the IS's are comparable). But since the 17-55 is incapbable of illuminating a full frame sensor, I personally would call the 24-105 a more hand-holdable lens.
I agree with you here as well.
Aha. It's amazing what one word (in this instance - factor) will do for my understanding of a subject. I've been thinking of it from the wrong perspective, so Jon's reply helps. Now, while I don't pretend to comprehend everything, I will continue to study this subject. It's like the compliment I once got from a boss/business owner, "I love hiring people smarter than me. It's so useful". gary
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