Silly question about 'zoom'

Started Mar 2, 2013 | Discussions thread
Beach Bum Senior Member • Posts: 1,055
Some good points but not entirely accurate IMO...

Guidenet wrote:

uwwarhawk wrote:

Ok ok... I know from researching that comparing a P&S zoom (i.e. 4x) to a DSLR range is pointless. My question, and forgive me if this is stupid, is if a camera says xx-1200mm equivalent (such as the Canon SX50) does that mean you would need a 600mm lens on a DSLR to get 1/2 of the reach of the superzoom? Or is it not that easy either? Lol.

This is often the most misunderstood question even among seasoned photographers. Everyone knows you can't change the focal length by changing the sensor size, but so many seem to want to explain it that way regardless. The focal length does not change nor does anything to do with the focal length change when you change the format or sensor size. 100mm stays 100mm. Period.

Crop factor only changes the field of view or angle of view. It's how much of the scene you can see. Going to a smaller sensor does not get "reach" or magnification in the least. It does not get you closer to your subject at all. None. Zero. Nada. Only focal length can give you reach from an optical point of view. Any other type of reach is digital. It's like a digital zoom on a compact. The tiny sensor on a point and shoot like the Canon SX50 just crops a tiny portion of the larger overall image sitting out there. If you digitally enlarge it later, it might look like reach because you only captured that small little bit of the potential image. That's what you enlarged digitally.

As an example, lets take two fairly equivalent cameras like a 4/3rd and a Full frame because the crop factor is a nice and even 2x. Let's take a single 100mm lens for both. Let's make them both the same pixel density. The full frame will be 24mp and the smaller will be 12mp. it's more complicated than this, but I'm simplifying. Density doesn't quite scale this way.

The image that comes out of the back of that 100mm lens is exactly the same no matter what, regardless of how large a slice of it I capture. The image I grab with a full frame will be twice as large diagonally as I capture with the 4/3rd camera but everything in the image is the same size. I don't have more magnification. I just grab a smaller piece of that same image.

Let's say that above image includes a bird on a fence post. That bird will show more area around the bird with the larger sensor because I grabbed more of the latent image. The bird will have less space around it with the 4/3rd capture because it's a smaller sensor, but the bird didn't change in size with either. He didn't get closer. There is no reach. I don't get the same equivalence f a 200mm lens. The only thing I got was a crop of the same image. I got a smaller field of view. That's all. That's why it's called "Crop Factor." That's why we say FOV or field of view. You're not any closer. No additional reach, just less image.

The reason people think it's more reach is because after that smaller image is removed from the sensor, we digitally enlarge it so we can see it. We have to enlarge that smaller image more than the larger image to see it at the same size on our screen. It looks like we got more reach because there's less space around the bird, but it's nothing but digital zooming. We could also do the same by cropping around the larger image and digitally enlarging the result.

The issue lies with quality. Digital enlarging is not as clean as if you captured the image larger to begin with optically. Optical zooms are better than digital zooms. That's why a 600mm lens on a full frame camera will produce a better image than a 300mm lens on a 4/3rd. You have to digitally enlarge the 4/3rd image twice as much to catch up. With the tiny sensor on your SX50, you have to digitally enlarge it a huge amount to get up to the same size image on the screen and the quality suffers accordingly. The original image was just a tiny fingernail sized crop of the whole image. It was a tiny field of view.

The point is that smaller sensors don't give reach. They just slice smaller images and you have to enlarge them more to get them the same size. This extra digital enlarging is what you're seeing and thinking is reach. That's all there is to it.

Just for fun, here's a APS-C Crops lens on a Full Frame camera. Notice the sensor size is traced on the larger image. The image doesn't get closer with the crop. It just get's smaller. Focal length always remains the same. The laws of physics balance. Magic doesn't occur. Capture size doesn't change focal length. Small sensors don't get you closer to your subject. Professional nature and sports photographers don't prefer smaller sensors. They pretty much all use Full Frame these days because they understand this concept.

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Cheers, Craig
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I agree with your emphasis that the true focal length is extremely important. I also agree that a 300mm lens on u4/3 isn't exactly the same as 600mm lens on a full frame (except for the fact that they both share the same equivalent focal length).

But you also can't say that a 300mm lens on a u4/3 functions exactly the same as a 300mm lens on full frame (by cropping). The truth lies somewhere in between, and it's not at all easy to explain or understand.

Let me start by using an extreme example of comparing the SX50 with its 215mm lens to a 215mm lens on a full frame. Under no circumstances that I can imagine would you be able to get anywhere near as sharp of an image with the full frame by cropping the full frame sensor to 1200mm eq. focal length. The reasons are many.

First and foremost is the pixel density of the SX50 is far,far higher than the full frame sensor. In fact, I would be surprised if full frames reached 360MP within my lifetime. This is about what the full frame sensor would need to match the SX50's pixel density.

Second is the fact that smaller sensor cameras are generally more efficient per unit area than larger sensors to compensate for their miniscule size. So, indeed, you can't scale down a full frame sensor to the size of the SX50 sensor and expect it to perform anywhere near as well.

Third is the fact that small sensor cameras probably have sharper lenses than larger sensor cameras to make up for the much higher pixel density. The reason this is possible is because the smaller sensor cameras have to maintain that sharpness over a much smaller unit area. A lens for a full frame camera has to project its image over a much larger surface area, so it's probably a lot more difficult to maintain the same type of sharpness.

So, my point is, the reality is that a 300mm lens on u4/3 is neither like a 300mm lens on a full frame or a 600mm lens on a full frame. The truth is more complicated.

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