Joan, I can't recall any recent discussions about the effect of sensor size, so I'll take the opportunity here. If you're going to be switching back and forth between P&S and DSLR, maybe it'll make your life a little simpler. I know I have to have this conversation with myself every time I switch between P&S and DSLR, even though I've been doing that switch regularly for more than a year.JoanYoung wrote:I see that from now on I will have to add in which camera I take the pics with to avoid confusion.
Very briefly, when you change sensor size, you also change the f-number that you need to get any particular DOF. Using a larger sensor requires a larger f-number, by the same ratio as the sensor sizes. So if your DSLR has a sensor that is 3X bigger than the sensor in your P&S (linear dimensions, e.g. 7 mm x 5 mm versus 21 mm x 15 mm), then the DOF that you're used to getting at f/8 on the P&S will require more like f/24 on the DSLR.
The explanation for why this happens can get pretty convoluted, but basically, you just look into the front of the lens to see how big the aperture appears to the subject. If the subject sees the same size hole, then the picture will have the same DOF. But because the DSLR will be using a longer lens (to have the same proportion with the larger sensor), the same size hole gives a larger f-number (again in proportion to the sensor size).
To get the same shutter speed, using a larger f-number means using a higher ISO setting, and here the ratio is the square of the f-numbers. So going from f/8 to f/24 will require increasing your ISO number by a factor of 24/8 squared. That turns ISO 64 on the small P&S into something like ISO 400 on the DSLR. (Yes, this is an approximate calculation, but it's close enough for the discussion here.)
When you're used to the high noise levels of a small sensor, setting ISO 400 sounds pretty scary. But it turns out that noise level also scales with sensor size -- bigger is quieter, and by just the right amount to compensate for the higher ISO number. (This assumes similar sensor technology and has to do with the statistics of photon counting; we don't need to get into the details.)
These effects counterbalance each other in an interesting and useful way: whatever DOF, shutter speed, and noise level you can get with the small P&S, you can also get with the DSLR, but the settings to get it will be different. With the DSLR, you need to set a larger f-number and a larger ISO. (Remember that, when you're taking a picture of the same flower with the 400D!)
But wait, you ask, if all I can do with the DSLR is get the same DOF, shutter speed, and noise level as with the P&S, why bother with the DSLR??
The answer to that question is "more flexibility". The larger sensor of the DSLR gives you a wider range of options. If you can live with less DOF, then the larger lenses of the DSLR let you use a larger aperture (wider hole). That lets you reduce the ISO setting at same shutter speed and illumination level. The combination captures more light, and produces a correspondingly quieter image. If you can live with a longer exposure time, or if you can supply brighter illumination (flash!), then the DSLR lets you get the same DOF while fiddling with the ISO setting to capture more light, again producing a quieter image. And finally, you may actually want less DOF for artistic purposes, say to give better separation between subject and background. The larger lenses of the DSLR allow much larger apertures, and a wider range of apertures between aberration limited and diffraction limited.
Within the range of what the P&S can do, the DSLR offers little if any improvement, but the DSLR has a much wider range of possibilities.
I went through the sequence of sensor sizes in a different order: mostly 35mm and some 4"x5" to start, then APS-sized DSLR, then small P&S.
When I came to the forum here, it was very confusing to me to see somebody else's picture of a small fly with good DOF at f/5.6, when all my experience screamed that I would need f/16 to do that.
Eventually, I figured out how sensor size affected DOF versus f-number, and the discrepency went away. In fact, their f/5.6 and my f/16 really are the very same diameter apertures, but labeled with different f-numbers because they are associated with short and long lenses in proportion to the sensor sizes.
By now, all this makes enough sense to me that the conversation is short --- but still I have to go through it with myself whenever I switch cameras.
I hope this discussion is helpful. With luck, maybe we can get you through the learning curve a little quicker than me!
--Rik
