Could you explain how much having a full body camera affects the quality of your photos versus a cropped higher megapixel camera? Specifically, how does a camera like the 5D mark1 compare with a camera like the Rebel T3i? Is it better to have a full-body 12.8 megapixel camera or a 1.6 crop factor camera that's 18 megapixels?
I have a lot of questions like this one. Crop or full frame? What does “equivalent” focal length mean? Which is better? Is a 12mp full frame better than an 18mp crop sensor? So on and so forth.
Get a full frame camera. Take the sensor out and put a smaller one in there. Same camera. Same lens. Now just a smaller sensor. Light comes through the lens and covers the full frame sensor. Now that you have put a smaller sensor in this camera (and NO… you can’t actually do this) you now see less of the image being projected by the lens.
Or… look at your computer monitor. Put strips of black paper across the top, bottom, and sides. You are now seeing less of your screen. You have now made a “crop sensor” from your “full frame” screen.
Where “equivalent focal lengths” come into play is all based on a size of 35mm film. Back in the film days a negative made by a 35mm camera was 24mm x 36mm. As digital started coming of age it was VERY expensive to make a sensor of this size. So APS sized sensors were made but put in 35mm camera bodies. You used your 35mm based lenses on these. A 50mm lens no longer had the same field of view that it had when you shot film. It was 1.5x that. Or 1.6x that. Or whatever the “crop factor” was for the sensor. We had to figure out how to speak about this so we talk about “equivalent” focal lengths. If you put a 20mm lens on a 1.5 crop factor body then it was “equivalent” to putting a 30mm lens on a 35mm film body.
The lens is still 20mm. It hasn’t changed optically. With a crop sensor you are just seeing less of it. It’s like putting the black paper around your computer monitor. Your screen resolution is the same as it was before. You’re just seeing less of it.
Now… which is better? Typically full frame cameras perform much better at higher ISO’s. This doesn’t happen with medium format cameras though. I don’t know why. We’ve been told over and over that larger sensors give better low light performance. Then you get a HUGE sensor and it sucks at high ISO. I don’t know why. If you told me the technical reasons why I’d fall asleep. Anyway…
If you love wide angle lenses then full frame is for you. If you love longer lenses then maybe crop bodies are for you. If you are shooting with a 200mm lens on a crop factor body and wanted that same field of view on a full frame then you’d have to jump to a 300mm lens. Can’t you just crop a full frame? Yes. You can. But you are throwing resolution away. Let’s say you have an 18mp full frame and an 18mp crop sensor. With the crop you get that field of view at 18mp. With the full frame you have to crop “resolution” out of your image so you may end up with a 10 or 12mp image.
At the end of the day it’s not MP numbers that count. It’s the quality of the pixels. Now we start getting into the size of the pixels and blah blah blah blah blah. OMG shoot me. If I was going to go into a car analogy I’d start talking about horsepower vs. torque. Or something. Not sure.
Full frame cameras typically perform better in low light, give you your full field of view on your lenses, and are typically found in better camera bodies. That’s not gospel. Just a wide brush view. I live in the wider range of lenses so I prefer full frame cameras. I prefer the higher ISO performance they achieve. If I had to buy a new DSLR today I would only look at full frame bodies.
I got started again on crop sensors. I shot those for five years. It’s what was available when I started again and then what I could afford until I got my D3. If I could have any new DSLR today it would be the new EOS-1D X. That is a hot camera. I have no need for a new DSLR so I’m not even going to put a finger on that 1Dx. I don’t want lust to enter my heart and cash to leave my wallet.
To the original question. For me… personally. I’d take the older 5d1 over the T3i. Better sensor. Full frame. Better body. Easier to use. More robust. Lovely camera. You could hand me a T3i and send me on a job and I’d get it done. I’d cuss at the camera. I hate not having dedicated dials for aperture and shutter but… I could do a magazine assignment with that camera. I could shoot band promos with that camera. I could shoot headshots. I’d hate it for an event but, after a lot of cussing, I’d get the job done. Overall though? I’d rather have the 5d1 if given the choice between those two.