Zoom or Crop?

Another random thought from a new photographer...

My camera delivers about 18 million pixels per image but it's rare that I end up needing more than about a third of them for the final output.

Would it be fair to say that an 18 million pixel image taken with my 30mm lens could be cropped to 6 million pixels without any loss in image quality? If so, isn't the "gap" between my 30mm prime and 85mm prime essentially "filled in" by cropping the 30mm image? What advantage would be gained by including a 50mm prime or a 24-70mm zoom?

If I had a 70-200mm lens, it seems logical to pick a 24-70mm for a seamless range. But, if I could fill in the gap by cropping the image, wouldn't it make more sense to carry a 16-35mm which also covers the wider angles?

Or, if I could get reasonable output by cropping a high-quality 300mm lens why should I carry a 500mm lens?

I guess I'm just coming around to the realization that there's a lot more to lens selection than covering all the numbers. I'm thinking that a practical and easy-to-carry 7D kit might consist of only my Tokina 12-24mm f/4 and my Sigma 85mm f/1.4. The other lens in my "three lens kit" would be in a high-quality wide-angle super-zoom point-and-shoot camera. Or, maybe a better kit would be a 70-200mm f/2.8 plus the point-and-shoot.

As you will quickly find out if you bother to do some testing, cropping after the fact will always give lower quality results than using the right glass to begin with.

Having said that, cropped images are often "good enough" and a 400mm lens is a lot less expensive than a 600mm lens.

There's a lot more to image quality than pixels and angle of view. As you crop your image, as well as using fewer pixels, you're also magnifying image flaws, caused by the lens and also camera technique. Then it's simply a matter of trading off image quality until you're not happy any more. Only you can make the call on where the balance falls.

Zoom defnitely...

IMO, there are few combinations which will beat the 17-55mm f/2.8 IS and 70-200mm f/4L IS on a pair of 1.6x cameras...

The basic rule of thumb should always be to fill the viewfinder as best you can. You'll get the best images doing that.

But, yes, the 15MP and 18MP cameras do allow for a bit more cropping. Enough that I don't mind using mainly the center AF point, then cropping the image later... or occasionally converting an image from horizontal to vertical or vice versa.

Still, I don't think it's a good idea to rely to heavily on cropping, whenever you can avoid it. You'll see what I mean in time, as you work with your shots and make some prints with them.

Try it out and let us know. small JPEG 85mm shot vs cropped large JPEG 30mm. My money is on the uncropped low-res shot.

'digital zoom' is never going to equal 'optical zoom' in quality

I agree about IQ... however, until one gets the money to fill-in the gaps, cropping with high IQ lenses does the job pretty well.

So, yes, I'll probably get a 50/1.4 and a 135/2 ... but for now, I'm fairly satisfied.

------ I've looked at a couple zooms; I haven't seen anything interesting yet. But I'm just a beginner.

Wilt wrote in post #12560188
'digital zoom' is never going to equal 'optical zoom' in quality

Actually, I did some tests with an old Sigma 170-500mm and a cropped Canon 55-250mm and the Canon won hands down.

I realize that your comment assumes digital zoom versus optical zoom for equal quality lenses but that's what has brought me here. I have a Canon 18-200mm zoom that is good, but not great, quality and I'm wondering if I wouldn't have more fun using my 85mm f/1.4 and just carrying a point-and-shoot for wide angle and telephoto snapshots.

As it stands, all of my gear together makes for a large and heavy kit so it's pretty rare that I'll happen to have exactly the right lens for an unexpected situation. I guess the real challenge is to figure out which lens(es) to carry under differing circumstances.

Keep in mind to "double" the equivalent focal length FOV you have to quarter the resolution. So a 18MP image will become a 4.5MP image to get a 60mm equivalent image from a 30mm lens.

Sdiver2489 wrote in post #12562011
Keep in mind to "double" the equivalent focal length FOV you have to quarter the resolution. So a 18MP image will become a 4.5MP image to get a 60mm equivalent image from a 30mm lens.

I had a feeling my math was way off. Thanks for covering for my laziness.

RPCrowe wrote in post #12559603
IMO, there are few combinations which will beat the 17-55mm f/2.8 IS and 70-200mm f/4L IS on a pair of 1.6x cameras...

I was leaning in this direction. And based on the above math, the gap of 55-70mm isn't too bad.

cacawcacaw wrote in post #12561987
Actually, I did some tests with an old Sigma 170-500mm and a cropped Canon 55-250mm and the Canon won hands down.

I realize that your comment assumes digital zoom versus optical zoom for equal quality lenses but that's what has brought me here. I have a Canon 18-200mm zoom that is good, but not great, quality and I'm wondering if I wouldn't have more fun using my 85mm f/1.4 and just carrying a point-and-shoot for wide angle and telephoto snapshots.

As it stands, all of my gear together makes for a large and heavy kit so it's pretty rare that I'll happen to have exactly the right lens for an unexpected situation. I guess the real challenge is to figure out which lens(es) to carry under differing circumstances.

i don't think a 170-500mm sigma lens is well regarded as a good lens...where as the 55-250IS is one of the better bang for the buck lenses out there...also what happens when you want to crop your shot at 500mm...cropping a shorter lens to the equivalent of that is going to lead to a 100% crop at least...

I get fairly good IQ when cropping either my 28 or my 85 to 4x. Going beyond that is a problem... but I still think I'd prefer a 10x crop of my 85 to a 15x P&S zoom.

However, my old P&S may be outdated.

bubbygator wrote in post #12562862
I get fairly good IQ when cropping either my 28 or my 85 to 4x. Going beyond that is a problem... but I still think I'd prefer a 10x crop of my 85 to a 15x P&S zoom.

However, my old P&S may be outdated.

a 4x crop would reduce your resolution by 1/16th making even the 7D about a 1MP camera. Somehow I don't think that's what you meant.

2x crop = .5*.5 = 1/4 the resolution
4x crop = .25*.25 = 1/16th the resolution

Sdiver2489 wrote in post #12563320
a 4x crop would reduce your resolution by 1/16th making even the 7D about a 1MP camera. Somehow I don't think that's what you meant.

2x crop = .5*.5 = 1/4 the resolution
4x crop = .25*.25 = 1/16th the resolution

I assumed when he said 4x crop, that he was referring to area, not dimensions. That's the way I normally think about it.