Crop Factor vs Zoom Effect

Even though digital photography has grown out of its infancy stage, and most people I know own at least one digital camera, I still see a lot of reference to the zoom factor in cameras with a smaller (than full frame) sensor. Quite awhile back, I finally got my head around the notion that the "equivalent to 400mm" reach of my little point and shoot was not like having a 400mm lens on a very small camera, but was in fact, only a cropped view of what my camera could put into a frame, but which gave the appearance of an image taken with a lens zoomed to 400mm on a full frame. The question is - do I really get the zoom or just a cropped version that mimics the field of view of a zoom lens on a 35mm body?

My understanding of the crop factor tells me no, it's not more zoom, but just the perception of zoom.

I found this site which I thought explained the crop factor/multiplier effect quite well, but rarely see the artificial perception of added zoom adequately explained.

Thots?

[QUOTE=Cathathome]Even though digital photography has grown out of its infancy stage, and most people I know own at least one digital camera, I still see a lot of reference to the zoom factor in cameras with a smaller (than full frame) sensor. Quite awhile back, I finally got my head around the notion that the "equivalent to 400mm" reach of my little point and shoot was not like having a 400mm lens on a very small camera, but was in fact, only a cropped view of what my camera could put into a frame, but which gave the appearance of an image taken with a lens zoomed to 400mm on a full frame. The question is - do I really get the zoom or just a cropped version that mimics the field of view of a zoom lens on a 35mm body?

My understanding of the crop factor tells me no, it's not more zoom, but just the perception of zoom.

I found this site which I thought explained the crop factor/multiplier effect quite well, but rarely see the artificial perception of added zoom adequately explained.

Thots?[/QUOT

Yes, the "crop factor" affects the FOV. It is the same effect as if you cropped a full frame image in Photoshop. However, the argument goes that since the cropping is being done with the sensor, that smaller piece has the advantage of using all the pixels, not just the ones left over after the crop. That is an advantage.

This "equivalent focal length" nonsense was started way back when digital first started. Most people were used to 35mm film and wanted some anchor to wrap their thoughts around. Those who used other formats, such as 6x7 or LF, rarely considered equivalent focal lengths for their cameras.

Cath - For all intents and purposes you are getting the zoom. It is not as if you have a 10 MP sensor and that you are zooming into part of that - you are using the entire sensor area, not cropping part of it.

G

Well, it does show the same field of view as a 400mm, but lens behave differently at different mm angles. So, yes and no, the 400mm equivalent doesn't mean the lens behaves like a 400mm lens, just presenting the same field of view as one.

Different size formats require different focal lengths to fill them.

Like the metric system, it's best not to try to convert but, instead, think of them in their own format. Gas at $0.79/liter is like how much a gallon? Driving 120 kilometers/hour is like how many mph? A liter is a liter, a kilometer is a kilometer.

A 300mm on my 50D gives me certain FOV that I can predict which is different (and just as predictable) as the same 300mm on my 5D.

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Originally Posted by Anbesol

Well, it does show the same field of view as a 400mm, but lens behave differently at different mm angles. So, yes and no, the 400mm equivalent doesn't mean the lens behaves like a 400mm lens, just presenting the same field of view as one.

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I agree. But I think you will get the best of the lens vs. the worst, as the corner quality is often softer and darker.

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Originally Posted by Loupey

Different size formats require different focal lengths to fill them.

Like the metric system, it's best not to try to convert but, instead, think of them in their own format. Gas at $0.79/liter is like how much a gallon? Driving 120 kilometers/hour is like how many mph? A liter is a liter, a kilometer is a kilometer.

A 300mm on my 50D gives me certain FOV that I can predict which is different (and just as predictable) as the same 300mm on my 5D.

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Ok I both understand more and less.

It's mostly about field of view - got it!

FOV is relative to sensor size.

However, isn't it still false to claim that a lens will achieve a magnification equivalent to (e.g.) a 400mm lens when it's simply a narrower view?

http://www.cheesebuerger.de/images/s...onfus/c010.gif

You can get all complicated about it but a 100 mm lens on a full frame sensor magnifies to 100mm and on a not full frame sensor it will be 150mm or thereabouts.
Just like binoculars or a telescope, the more magnification the narrower field of view.
Just like when you look real close at something, your peripheral vision narrows.

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Originally Posted by Cathathome

However, isn't it still false to claim that a lens will achieve a magnification equivalent to (e.g.) a 400mm lens when it's simply a narrower view?

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No, not really misleading. As long as you maintain the same position relative to your subject, cropping (i.e. crop factor) has the same effect as "telephotoing" - DOF notwithstanding.

Take this example I did a while ago. The first shot shows a 17mm shot uncropped. Now imagine a format that has a crop factor of 2.94. The second image is a crop of the first image at this crop factor. I chose 2.94 so that the effective focal length would then be 50mm for this example.

The third image is an uncropped 50mm shot. Looks the same right? Again, DOF not being discussed here, but as long as you DO NOT MOVE, the effect is the same.

Telephotos magnify the entire image evenly. Cropping has the same effect on the FOV as long as you don't move.