Is f2.8 really f2.8?
GadgetRick
Registered Users Posts: 787 Major grins
Ok, I know, f2.8 is actually f2.8, however, I have been using a friend's 70-200 f2.8 L (IS). I'm 2nd shooting a wedding for him and I wanted to become comfortable with the lens before we shot the wedding. I've found, with a new lens, I have a bit of a period before I figure out it's, "sweet," spot if you know what I mean. I didn't want to be figuring this out at the expense of his paying customer.
Anyway, back to my original question/thought. I shoot (regularly) with my 28-70 f2.8 L so I'm used to shooting with an f2.8 lens. However, it seems the 70-200 is, well, more sensitive to light. Seems like it gathers more light at a given aperture than my 28-70. It may be in my head which is why I'm asking here but it sure does seem like it gathers more light at a specific aperture than my lens. Ok, I haven't checked specific focal lengths but does that matter in this situation?
I realize the 70-200 is probably a better lens (glass-wise) than the 28-70 but neither is a slouch.
Am I imagining this or is there something to this?
Thanks.
Anyway, back to my original question/thought. I shoot (regularly) with my 28-70 f2.8 L so I'm used to shooting with an f2.8 lens. However, it seems the 70-200 is, well, more sensitive to light. Seems like it gathers more light at a given aperture than my 28-70. It may be in my head which is why I'm asking here but it sure does seem like it gathers more light at a specific aperture than my lens. Ok, I haven't checked specific focal lengths but does that matter in this situation?
I realize the 70-200 is probably a better lens (glass-wise) than the 28-70 but neither is a slouch.
Am I imagining this or is there something to this?
Thanks.
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I know the Canon guys will chime in soon, hope I helped a little...
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Focal length shouldn't really matter because f/numbers are a ratio between focal length and aperture, but one test would be to take the exact same photo (as close as possible) with both lenses. Set the camera in manual mode and take both images at 70mm f/2.8, at the same ISO (not auto ISO), in RAW mode. Process both images with exactly the same profile to produce JPEGs. Are the two visibly different? Do their histograms look different?
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I agree with you, which is why I posted this question.
I'm noticing it grabbing more light in low-light situations than my 28-70 seems to do. Since I shoot with my 28-70 so often, I'm familiar with how it performs in these situations.
I did not try doing a test (yet) because I was busy at BBQs, etc this past weekend. Going to see if I can make some time to try it out today (giving the lens back tomorrow).
I will say the 70-200 is a stupid sharp lens (even at f2.8). Once I have the funds one will wind up in my slowly growing glass collection.
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Did you simply try a shootout? Camera on triplod, both lenses on 70mm, same shutter/aperture and compare the results?
The focal lengths are quite different for the most part, so it seems hard to compare the two without some sort of test.
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Well, the focal lengths shouldn't matter (I don't believe) with these lenses as it's a fixed aperture. Going to try and do a real test today.
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I've only vaguely heard of this term before though, so I don't know how much difference is normal between f and t values and whether it would be noticeable but it might be part of the reason.
Also, I'm guessing there are different coatings on the different lenses which may also account for it.
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Sorry to alarm anyone...
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Damn you! I was already planning on gutting that lens and turning it into a super large coffee mug.
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Uh, I already gutted it looking for the cause of my madness...
Putting it back together. Looks like I'll only have a few extra parts left over.
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Unfortunately that's not how it works. The maximum aperture "f" number is a relationship of the focal length and the diameter of the front element. New lens coatings have made some gains and they work particularly well in complicated zooms by reducing internal reflections "and" absorption, meaning that complicated zooms can have nearly the same efficiency as simpler lenses.
There are no special glasses that allow better efficiency than the theoretical/calculated "f" but the best lenses do use specialized elements made to reduce color aberrations.
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Ok - I lied...Sorry...No gatekeeper..Continue on :-P
Err here's what I was thinking of, it's more efficient for shorter* lenses + smaller diameter, not longer... although I bet on a large lens they'd be able to do the same thing. Wonder if they already made one with that glass... I'll check
http://www.tamron.com/lenses/technology.asp#XR
You confuse me a bit. Focal length is always important.
A fixed aperture simply means that the lens is capable of shooting wide-open at all focal lengths.
A variable aperture means that the lens is not capable of the same. The biggest aperture possible changes according to the focal length. This is not normally a problem until conditions are on the margin.
The benefits of a fixed aperture lens are a bit more flexibility in the margin. They are normally much more expensive because they are much more complicated, and "better" because they give even more possibilities in the margins of what you want to shoot. The market is sometimes confused because the expense is usually justified by combining a very wide aperture with the fixed property - so these lenses are much more expensive than than their variable cousins. People think they are always better at taking normal pictures in normal conditions - this is not necessarily true.
Second point: I really doubt the value of testing two lenses on the extreme margins of what either is capable of - 70mm in your example. If 70mm is your sweet spot then you better find a lens where 70 is in the middle or look for a prime around that number.
Sorry if I cause offense. Probably I am telling stuff you already know for years.
Huh? Aperture = diameter of the diaphragm/iris/pupil, whatever you want to call it. The "f" refers to the focal length. So the diameter of your aperture at 70mm and f/2.8 = 70/2.8 = 25mm. It's got nothing to do with the front element.
A lens is a light funnel, and the aperture is the throttle.
As for f-stops, 1 stop = twice the light, or twice the area of the aperture. Going from f/2.8 to f/4 is one stop. 70/4 = 17.5mm. pi * (17.5/2)<sup>2</sup> = 240.5mm<sup>2</sup>. pi * (25/2)<sup>2</sup> = 490.9mm<sup>2</sup>. The 10mm<sup>2</sup> error is due to rounding.
An "accurate" reproduction of a scene and a good photograph are often two different things.
Wait, you just made me lose what I thought was correct in my brain. I thought stops were measured by starting at 1 and multiplying by 1.4?
1
1.4
2 (1.96)
2.8
4 (3.92)
5.6
etc...
I'm extremely simple so I'm sorry if I just made this confusing(!)
EDIT: Never mind, you did it the actual way and not the cheater way that I learned it.
http://en.wikipedia.org/wiki/Aperture
Just as long as we all accept that each stop is halving the light
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We're both right. Those are the stops. You can just remember 1 and 1.4 and keep doubling them. Technically you start at 1 and multiply by root-2 (about 1.41421356)
An "accurate" reproduction of a scene and a good photograph are often two different things.
Note that I said, "maximum aperture "f" number", which is indeed the opening aperture (diameter of the lens) divided by the focal length (for a simple lens). For a more complex lens it may be the "effective" opening aperture divided by the focal length. Note that the f-number may be expressed as "f2", "f/2" or "f:2", depending upon the source. (I generally use the simpler "f2" notation myself.)
http://en.wikipedia.org/wiki/F-number
http://en.wikipedia.org/wiki/F-number#Notation
http://www.photography.com/articles/basic-concepts/f-number/
http://www.yourdictionary.com/f-number
Note also that the "maximum aperture "f" number" is indeed moderated and reduced by the lens diaphragm, if the lens has one.
Yes, sometimes "f" can refer to the focal length of the lens. I recommend using the context and numerical value to discern between the two meanings.
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For a single lens element yes. But you said the front element. The front element of my 24-70 is about 60mm in diameter.
I assume this would be close to the diameter of the smallest element, which in the 24-70's case would be 25mm.
An "accurate" reproduction of a scene and a good photograph are often two different things.
What I said was, The EF 24-70mm, f2.8L USM certainly qualifies for a more complicated design.
The largest rated "f-number" for a zoom lens is generally a function of the shortest focal length of the lens and the front element diameter (or effective diameter). (The smaller rated "f-number" in a variable aperture zoom lens is either calculated or determined empirically.)
There is a lot of mis-information on the Internet about how "constant aperture" zooms work, but the most lucid explanation comes from Bob Shell:
"An f-stop is the ratio between the focal length of the lens and the *apparent* size of the lens opening as viewed through the front. It must take into account the magnification factor of all lens elements in front of the diaphragm, because it is the size of the opening that the light "sees" as it passes through the lens, not the actual physical diameter of the diaphragm opening.
It is this fact that allows companies to make constant aperture zoom lenses which maintain a constant f-stop when the focal length changes, because such lenses are designed so that the magnification factor (diopter value) of all elements in front of the diaphragm changes as focal length is changed to hold the aperture value constant."
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a number that indicates how much light actually passes through it. In the end this idea
was overthrown because providing the aperature was much easier and precise enough
for photographers to calculate the correct exposure for a photograph (back then everything
was manual). In summary: f-stop is only a number indicating the ratio of focal length to
the lens opening which translates to a light-pass-value "well" but not 100% accurate.
Thats why you can get different exposures with different lenses at identical settings.
― Edward Weston
I shoot three different cameras, all with 2.8 lenses and sometimes studio lights. I find that with a different camera, same lens, (switching from one camera to another) and same iso, shutter speed, I get different results. I am suspicious that the sensors are not exactly the same sensitivity. ISO 400 varies slightly from camera to camera. I am using a eos 5d mII, an eos 1dMIII, and eos 1d MIV.
In the same way that different model light meters, even from the same manufacturer, rarely match readings, different camera models will likewise rarely match exactly. The format difference between the 1D MKIII/1D MKIV (crop 1.3x/APS-H) and the full-frame 5D MKII can also make a difference in the usable focal lengths of zoom lenses, which has an impact on exposure.
In other words, both image sensor and exposure sensor sensitivities may vary by model and ISO setting. Calibration should take care of the differences, but it rarely does so perfectly.
The important take-away point is to take the time to understand how each particular system operates, use the combination of histogram and "blinkies" correctly for the system in use, and make the correct exposure corrections for the combination of equipment and scene requirements at hand.
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