Buying my first L lens- Not an easy choice

[Brendan7]

According to Daniel the Great (Daniel Browning, duh!) f/4 on FF is brighter than f/4 on 1.6. You need a slower shutter speed on a 1.6 to get the same brightness. Daniel, correct me if I'm wrong [:P]


woops...I just hijacked another thread.

[barba]

I do not believe this to be true. I have not noticed such a radical change in exposure values going from 35mm to a crop sensor.

[neuroanatomist]

f/4 is f/4. Period. A lens set to f/4 will let in a fixed amount of light, whether that lens is attached to a 1D4, a 5D2, a 7D, a T1i, or an old Elan film camera. The subsequent comments in that thread are best summed up by Daniel's statement, "Anyway, that doesn't take away from the importance of understanding what happens when the performance per area is the same and you vary the area." Key point there is we had moved on to a discussion of sensor characteristics. What comes out of the back of the lens is totally unaffected by that. To illustrate that point, you can compare any two sensors of the same size but different properties (e.g. 7D vs. 50D/T1i). Performance and light-gathering ability differ, independent of the lens.


There's no easy calculator illustrate this with amount of light, but there is with depth of field. Try the following experiment - go to the DoF calculator atDOFMaster. With the default settings on the page (50D et al. 1.6x crop body, 55mm, f/16, 10 feet from subject), you'll see a DoF of 6.61 ft. Change just the pop-up for the body to a 5D MkII, a full frame body. Now, all else being equal, the DoF is now 12.3 ft. What?!?!? Didn't we all agree you multiply the aperture by 1.6 for a crop, meaning the DoF will be thinner on FF? Why does changing the sensor from crop to FF make the DoF wider? Because of that clause about 'same framing' that I mentioned. Because of the crop factor, you are closer to the subject for a given subject framing on a FF body. With the DoF calculator, 5D MkII, 55mm, f/16, 6.25 feet from subject = 4.02 ft DoF; a ~1.6x aperture or f/25 gives a 7.44 ft DoF (the actual 1.6x would be f/25.6, but that's not an option). 6.25 ft FF framing = 10 ft framing on a 1.6x body. Settings of50D body, 55mm, f/16, 10 feet from subject gives 6.61 ft DoF, which is what f/25.6 on a FF would yield. So, for the same framing, same lens settings as above, a crop body gives a 6.61 ft. DoF, whereas a FF body gives a thinner DoF of 4.02 ft.


So, the bottom line is that a sensor does not and cannot affect the lens properties. f/4 is f/4. f/4 on a particular FF body may be 'brighter' than f/4 on a particular crop body, but then again, f/4 on a new FF body may be 'brighter' than f/4 on an old FF body. The light projected by the lens is the same, but how the sensor detects the light depends on the sensor.


And if that's not an egregious threadjack, I've never seen one... [:P]

[Daniel Browning]

The 300 f/2.8 is really a 480 f/4.5 on your body

You've got a good point to make, but I think there is a better way to phrase it. For example:
<p style="padding-left: 30px;"]"The 300mm f/2.8 your body is really equivalent to 480 f/4.5 on full frame."


I think the difference is important. 300mm f/2.8 is always 300mm f/2.8, no matter what body you put it on. But when you compare angle of view, the 300mm on the 7D is equivalent to 480mm on the 5D2. And when you compare noise, 300mm f/2.8 on the 7D is equivalent to 480mm f/4.5 on the 5D2.

f/4 on FF is brighter than f/4 on 1.6. You need a slower shutter speed on a 1.6 to get the same brightness.

If you replaced brightness with noise, I would agree with that statement. Personally, I consider noise to be a much more important factor than brightness. (Brightness just requires a slight adjustment in the raw converter -- if that.)

[Atomhot]

f/4 on FF is brighter than f/4 on 1.6. You need a slower shutter speed on a 1.6 to get the same brightness.

If you replaced brightness with noise, I would agree with that statement. Personally, I consider noise to be a much more important factor than brightness. (Brightness just requires a slight adjustment in the raw converter -- if that.)
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hmm&hellip; The noise on a FF ( 5Dmkii ) @ &fnof;4 should be less than the noise on a 1.6 ( 7D ) @ &fnof;4 for the same lens, shutter, aperture et al.

[Kenny]

Ok time to dumb this thread down a little. I have been reading a lot of previous posts and i think some basic selections are starting to make sense. I must say some of the pictures you guys have posted just amaze me. Once you begin to shoot outside and at distance how do you know what is the correct lens or is it just a matter of learning as you gain more experience and your skills improve. I am just overwhelmed when I begin to think about the proper lens for every different subject. The only time I really cant take pictures is at an airshow I go to once a year the subjects are just to far away even on the ground. I know I cant afford anything to get these shots but I have been try to figure out how do you select the bestlens for Short/intermediate /long distance indoor vs outdoor. My guess is there is no simple answer because of all the variables but what questions do you ask yourself as you think about your subject and how you will prepare.

[Daniel Browning]

hmm&hellip; The noise on a FF ( 5Dmkii ) @ &fnof;4 should be less than the noise on a 1.6 ( 7D ) @ &fnof;4 for the same lens, shutter, aperture et al.

Yes, that's what I intended to say; sorry if I wasn't clear. In other words, f/4 on FF has less noise than f/4 on 1.6X, and f/4.5 on FF has the same noise as f/2.8 on 1.6X (if the performance per area is the same, as in the case of the 7D and 5D2).