olympus makes a 2x crop sensor so their 300mm f/2.8 lens becomes a 600mm f/2.8!
olympus makes a 2x crop sensor so their 300mm f/2.8 lens becomes a 600mm f/2.8!
Originally Posted by bburns223
Well, more like a 600mm f/5.6, really.
Well, more like a 600mm f/5.6, really.
How does a smaller sensor decrease the maximum aperture of the lens??? bburns223 isn't talking about a teleconverter, but about the fact that Olympus uses the four-thirds sensor format, which is smaller than the APS-C format (thus, a 2.0 crop factor vs. a 1.6 crop factor).
The short answer is that it doesn't, but when comparing both the FoV and DoF of a lens/body combination where the sensoris smaller than 35mm to a 35mm format you multiply both the focal length and f number by the FOVCF multiplier. In the case of the micro four thirds format the multiplier is 2.
- 300mm x 2 = 600mm (equivalent FoV)
- f/2.8 x 2 = f/5.6 (equivalent DoF)
So, you're saying that myEF 100mm f/2.8<span>LMacro IS USMlens, when mounted on a 1.6 crop body, is really f/4.5? A crop body camera causes an across-the-board loss 1 1/3 stops on all lenses? Does that mean I require an f/1.8 lens to utilize the high-precision focus point (which requires f/2.8)? With a camera set to ISO100 and the 100mm macro lens set to f/8, if a 1.6 crop camera meters a 1/250 second exposure, the same setup with a FF body would meter a 1/640 second exposure?
I understand that sensor size affects DoF, with smaller sensors having effectively greater DoF. I have not heard that this affects the light-gathering ability of the lens, though. With respect, can you back that up? Thanks!
EDIT: I see that you deleted the "and light gathering ability" part of your response...
I removed that from my original post because I admittedly stepped beyond my comfort zone in regards to discussing the characteristicsof differentsensor sizes [:$]. I'm sure that either Daniel Browning or Jon Ruyle couldcomment in much further (and correct) detail. I was not suggesting that different sensors will meter the same scene differently. Sorry for being ambiguous.
I am saying that it "behaves" as a f/4.5 in terms of DoF on your 1.6 FOVCF body. This is why I am NEVER satisfied with the DoF I get with my f/2.8 zooms on my 50D. I could obtain a thinner DoF with an f/4 lens on a full-frame body.
It is still an f/2.8, but it has the same DOF and light gathering ability as an f/4.5 on full frame.
I can try to "back it up" with an explanation, but I do not claim to be an authority (though I do believe that authorities agree with me).
As Mark said, the size of the sensor does not change the light per unit area. But bigger sensor and same light per unit area means more light. In practice, this means that other things being equal, ISO 400 on the 2x crop factor camera will have just as much photon noise as ISO 1600 on the FF camera. I think we've all experienced the fact that cropping makes pictures appear more noisy. If you don't believe this, take a picture at high iso, but not so high that read noise is really bad (maybe iso 3200). Compare the whole picture to a heavily cropped version of it. The difference in noise level should be obvious.
No. An f/2.8 lens is still f/2.8, it just behaves like a slower one in terms of DOF and light gathering ability. When an af point requires f/2.8 or f/5.6 to work, sensor size is not an issue.
The reasons used to support the 'less light with smaller sensor' above are all dependent on the characteristics (not just size) of the sensor itself, not the amount light falling on it. Yes, croppingmakes pictures appear more noisy - it doesn't make then noisier, the noise is there (digitally), but enlarging them makes it more visible in the image. Smaller sensors have more noise, but that depends on the 'all other things being equal' clause - smaller pixels have more noise, yes, because their photon depth is shallower (lower eV) meaning they have a lower signal to noise ratio. But, if you pack 4 'shallow' pixels into the same physical area as one 'deep' pixel, you'll capture nearly the same amount amount of light (some is lost due to inter-pixel spacing), with a tradeoff of more noise and less dynamic range. Because all of these are dependent on sensor properties, not size, the 'crop factor' light loss is not going to be 1.6 here. These same issues will affect light gathering ability across the board - some FF sensors will gather more light than other FF sensors, and there are newer crop sensors that will gather more total light than older FF sensors. This is due to technological improvements, as evident from the higher ISO values achieved in newer sensors (of all sizes), even in the face of reduced pixel sizes - the 'wells' are getting deeper. It doesn't make sense (to me) to account for these pixel differences with a fixed sensor-size crop factor - you'd have to apply a correction like that when using the same lens on an old 1Ds MkI vs. the forthcoming iDs MkIV, or a 50D vs. a Rebel XT, etc., as the equal-sized sensors in each pair have very different light-gathering ability.
It makes sense that image sensor size is not an issue for AF points - the AF system uses a different sensor, and that sensor is still seeing an 'f/2.8' amount of light, because that's what is being delivered by the lens - to both sensors. That's also why the 1D series can autofocus at f/8 vs. f/5.6 like all other bodies - it has a more sensitive AF sensor.
It doesn't. Nor does it change the focal length. But cropping changes the *effective* f/ number, just like it changes the effective focal length. This is because when you crop, not only is your field of view narrower (longer focal length) but you get more DOF and less light reaches the sensor (bigger f/ number).
So when you crop, you should not only multiply the effective focal length by the FOVCF, you should also multiply the effective f/ number. The effective aperture does not change. In this way, cropping is very similar to adding a teleconverter (though of course when you use a teleconverter you keep all your resolution whereas you lose megapixels when you crop)
I mean, yeah... what Mark said
It's important to realize. Some people pay top dollar for the fastest lenses possible, then put them on crop bodies.
However, if you buy, say, a 50mm f/1.2 for your 50D, you should be aware that your $1500 lens is acting like an 80mm f/1.9, and that an 85 mm f/1.8 is more than $1000 cheaper and has much better IQ wide open (especially if the f/1.2 is cropped). The $1000 is almost enough to make up the difference in price between the 50D and the full frame 5DII.
Similar for the 85mm f/1.2 on crop body vs 135mm f/2 on full frame (smaller price difference I think, but probably bigger IQ difference)
Don't get me wrong- I am *not* saying one shouldn't put a fast lens on a crop body. There may be reasons to prefer the 50D / 85mm f/1.2 combination (and other fast lens crop body configurations), of course. But one should be aware.
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