Frame size vs magnification - Page 2

thegableguy (Chris) · #21 01-10-2016, 03:04 PM

Quote:

Originally Posted by tempestwizz

From what I can understand, everybody is right. It's just terminologies and semantics making things seem more complicated than they are.
For the same final sized picture dimensions, an APS sized sensor of 22 by 15 mm will produce what appears to be a magnified image over what a full frame 35 by 24 mm produces.

If both sensors are 24 megapixel, then the pixel dimensions for the APS must be smaller than for the full frame sensor. Accordingly, the apparent resolution for a particular object on the output pictures would seem better for the APS than for the full frame.

For a full frame sensor to produce the same resolution of a particular object, a lens with a focal length 1.6 times longer would need to be used.

Magnification is a relative term. An APS image appears to be magnified by 1.6 times more than a full frame image through the same lens. But, the APS image does not show as wide a field of view as the full frame.

The practical ramifications of the above:

The apparent increase in resolution for the APS sensor for a particular target object comes at a cost. The number of photons per unit of time hitting each of the smaller pixels is less. Hence more exposure is required to achieve the same apparent brightness. The longer exposure can increase noise produced in the sensor, and or allow tracking errors to blur the potentially higher resolution image.
One more compromise amongst the many others in this field.

HTH

Brian

Brian FTW.

Great answer mate. Thank you. Helped clarify a few small points I'd not considered, like pixels being smaller means they'll be inherently noisier.

Cheers again. Happy to let it rest there.

barx1963 (Malcolm) · #22 01-10-2016, 03:05 PM

I think the word magnification should only be used in relation to visual. If you think about an object in the sky it has a "size" which is expressed in terms of angles. So the moon for example is 30' wide. If I look at it in a scope and it appears to be 10 degrees wide, then it is reasonable to talk about it being magnified 20x. Personally I dislike using the term as it conveys the idea to non astronomy people that the purpose of a scope is to magnify, which of course is only a partial answer.
If I stick a camera on the scope, take an image and then put it my computer screen in what sense is it "magnified". The image is not bigger than the moon (unless my screen is a few thousand kms wide) and you cannot really quantify the image size in terms of angle, unless I can state the conditions under which it is being viewed ie. I could state that is this image is viewed on a at such and such a size and I am sitting at such and such distance from the screen then the image is so many degrees wide. That is obviously silly o that is why for imaging we are usually more interested in talking about true field of view and resolution. At least that is I think about it!!
When I joined the local camera club here I was totally confused when they all talked about using a crop factor camera to get more "magnification" or increase their focal length, which simply made no sense to me. Using a smaller sensor, to my way of thinking made no more difference than loading an image into PS and cropping it. And the resolution is set by the combination of focal length and pixel size, sensor size is irrelevant. Sensor size is only relevant in discussing the size of the field of view.

Note: as I always say when offering my silly opinions in imaging matters, I am happy to be contradicted by more experienced people!!

Malcolm

thegableguy (Chris) · #23 01-10-2016, 04:31 PM

Look, I really do get the whole magnification-only-applies-to-visual stance. I know where you're coming from. I know the actual magnification of any given image depends on the screen or print size, and that resolution & cropping further complicates it.

All I'm talking about is the example of two cameras with the same megapixel resolution, but different size sensors. By any test, in any context, you can reasonably say that the crop sensor DSLR will produce a more magnified image than the full frame.

That's all I'm saying. Say the term magnification doesn't apply if you like. I respectfully disagree. It's close enough to the definition of what magnification is for me.

Merlin66 (Ken) · #24 01-10-2016, 04:37 PM

Chris,
OK.
Next time you're at the telescope take a image of say a galaxy (or planet) with both cameras ,the full frame and the APS.
Measure the image size of the galaxy/ planet (in micron) - you'll find it to be the same.

thegableguy (Chris) · #25 01-10-2016, 04:44 PM

Quote:

Originally Posted by Merlin66

Chris,
OK.
Next time you're at the telescope take a image of say a galaxy (or planet) with both cameras ,the full frame and the APS.
Measure the image size of the galaxy/ planet (in micron) - you'll find it to be the same.

Of course it will be. Never suggested otherwise.

What do you think is easier to say and understand:

"This photo is a magnification of that photo"

"This photo is a higher-resolution but cropped version of that photo"

I'm going with the first one. Because whatever tiny difference there is absolutely doesn't matter.

Camelopardalis (Dunk) · #26 01-10-2016, 04:58 PM

Quote:

Originally Posted by thegableguy

What do you think is easier to say and understand:

"This photo is a magnification of that photo"

"This photo is a higher-resolution but cropped version of that photo"

I'm going with the first one. Because whatever tiny difference there is absolutely doesn't matter.

Referring to your earlier reply and this...it's not so much about the "wrong" answer, it's just more a case of "accepted" or commonly used terminology. When referring to deep space objects, folk tend to reference them by resolution i.e. 1.5"/pixel rather than the M-word. Even when it comes to planetary, few refer to "Jupiter captured at 1000x", even though it is calculable, most just list the optical configuration used to capture it.

In your A or B example, all things being equal, they'd give the same result, but it's just not as simple as that. As has been mentioned, pixels are not equal. The resolution of the bigger scope is better than the smaller scope. Then there's the atmospheric conditions to consider, which could make all the difference or make them practically indistinguishable. My money would be on the bigger scope and bigger pixels every time though

thegableguy (Chris) · #27 01-10-2016, 06:29 PM

Again, I'm not using the M word as a description of what's being imaged. I really do fully appreciate the myriad reasons why that doesn't work. Honest! I really, really do.

I'm using it to describe as simply as possible the difference between the two cameras, saying one camera will produce an image that is essentially a 1.6x magnification of the other.

Meh. I'll stick with this terminology until I get sick of people correcting me. Gimme another six months and I'm sure I'll come around...

bojan · #28 01-10-2016, 06:37 PM

Quote:

Originally Posted by thegableguy

Okay, cool - I think that's where we finally fetch up against the wall. I'd say the term magnification is the perfect way to describe the difference, as the image from one is identical to the other except, well, magnified 1.5x; you disagree. I can't say I fully understand why, but maybe I don't need to.

Well that was a fun way to procrastinate from today's admin..! Cheers all.

Yes it was..

The physical image formed on FF sensor from 1,5m FL lens is bigger compared to smaller sensor and shorter lens.
However, after sampling, it has exactly the same number of pixels.. So it's not magnified, it is exactly the same.
Or magnification is exactly 1.

However, if we had a film photography, we could have been talking about magnification of 1.5x

thegableguy (Chris) · #29 01-10-2016, 07:32 PM

Quote:

Originally Posted by bojan

Yes it was..

The physical image formed on FF sensor from 1,5m FL lens is bigger compared to smaller sensor and shorter lens.
However, after sampling, it has exactly the same number of pixels.. So it's not magnified, it is exactly the same.
Or magnification is exactly 1.

However, if we had a film photography, we could have been talking about magnification of 1.5x

Yes, the image would be the same, 1:1, with the two different focal length telescopes. With the same telescope, the image from the smaller (but same resolution) sensor would, for all intents and......

Ahhhh,surely you get my point by now. Broken record time. All good. I think we understand each other, we just express ourselves differently.

bojan · #30 01-10-2016, 07:35 PM

Quote:

Originally Posted by thegableguy

Yes, the image would be the same, 1:1, with the two different focal length telescopes. With the same telescope, the image from the smaller (but same resolution) sensor would, for all intents and......

Ahhhh,surely you get my point by now. Broken record time. All good. I think we understand each other, we just express ourselves differently.

Yes